Calendaring extensions                                        N. Jenkins
Internet-Draft                                               R. Stepanek
Intended status: Standards Track                                FastMail                                Fastmail
Expires: December 31, 2019                                 June 29, January 24, 2020                                  July 23, 2019

           JSCalendar: A JSON representation of calendar data
                    draft-ietf-calext-jscalendar-17
                    draft-ietf-calext-jscalendar-18

Abstract

   This specification defines a data model and JSON representation of
   calendar data that can be used for storage and data exchange in a
   calendaring and scheduling environment.  It aims to be an alternative
   to
   alternative, and over time successor to, the widely deployed
   iCalendar data format and to be unambiguous, extendable and simple to
   process.  In contrast to the JSON-based jCal format, it is not a
   direct mapping from iCalendar and expands semantics where
   appropriate.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 31, 2019. January 24, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Motivation and Relation to the iCalendar format and jCal . . . . . .   5
     1.2.  Notational Conventions  . . . . . .   4
     1.2.  Relation to the jCal format . . . . . . . . . . .   5
     1.3.  Type Signatures . . . .   5
     1.3.  Notational Conventions . . . . . . . . . . . . . . . . .   5
   2.  JSCalendar objects   6
     1.4.  Data Types  . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  JSEvent . .   6
       1.4.1.  Int . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  JSTask .   6
       1.4.2.  UnsignedInt . . . . . . . . . . . . . . . . . . . . .   6
       1.4.3.  UTCDateTime . . .   6
     2.3.  JSGroup . . . . . . . . . . . . . . . . . .   7
       1.4.4.  LocalDateTime . . . . . . .   6
   3.  Structure of JSCalendar objects . . . . . . . . . . . . .   7
       1.4.5.  Duration  . .   6
     3.1.  Type signatures . . . . . . . . . . . . . . . . . . . .   7
       1.4.6.  SignedDuration  . .   6
     3.2.  Data Types . . . . . . . . . . . . . . . . .   8
       1.4.7.  Id  . . . . . .   7
       3.2.1.  UTCDateTime . . . . . . . . . . . . . . . . . . .   8
       1.4.8.  PatchObject . .   7
       3.2.2.  LocalDateTime . . . . . . . . . . . . . . . . . . .   8
       1.4.9.  Time Zones  . .   7
       3.2.3.  Duration . . . . . . . . . . . . . . . . . . .   9
   2.  JSCalendar Objects  . . . .   7
       3.2.4.  PatchObject . . . . . . . . . . . . . . . . .   9
     2.1.  JSEvent . . . .   8
       3.2.5.  Identifiers . . . . . . . . . . . . . . . . . . . . .   9
       3.2.6.  Time Zones
     2.2.  JSTask  . . . . . . . . . . . . . . . . . . . . . . . . .   9
       3.2.7.
     2.3.  JSGroup . . . . . . . . . . . . . . . . . . . . . . . . .  10
   3.  Structure of JSCalendar Objects . . . . . . . . . . . . . . .  10
     3.1.  Normalization and equivalence Equivalence . . . . . . . . . . . .   9
     3.3. . .  10
     3.2.  Custom property extensions Property Extensions and values Values . . . . . . . . . .  10  11
   4.  Common JSCalendar properties Properties  . . . . . . . . . . . . . . . .  10  11
     4.1.  Metadata properties Properties . . . . . . . . . . . . . . . . . . .  10  11
       4.1.1.  @type . . . . . . . . . . . . . . . . . . . . . . . .  10  11
       4.1.2.  uid . . . . . . . . . . . . . . . . . . . . . . . . .  11  12
       4.1.3.  relatedTo . . . . . . . . . . . . . . . . . . . . . .  11  12
       4.1.4.  prodId  . . . . . . . . . . . . . . . . . . . . . . .  12  13
       4.1.5.  created . . . . . . . . . . . . . . . . . . . . . . .  12  13
       4.1.6.  updated . . . . . . . . . . . . . . . . . . . . . . .  12  13
       4.1.7.  sequence  . . . . . . . . . . . . . . . . . . . . . .  12  13
       4.1.8.  method  . . . . . . . . . . . . . . . . . . . . . . .  13  14
     4.2.  What and where properties Where Properties . . . . . . . . . . . . . . . .  13  14
       4.2.1.  title . . . . . . . . . . . . . . . . . . . . . . . .  13  14
       4.2.2.  description . . . . . . . . . . . . . . . . . . . . .  13  14
       4.2.3.  descriptionContentType  . . . . . . . . . . . . . . .  13  14
       4.2.4.  showWithoutTime . . . . . . . . . . . . . . . . . . .  13  14
       4.2.5.  locations . . . . . . . . . . . . . . . . . . . . . .  14  15
       4.2.6.  virtualLocations  . . . . . . . . . . . . . . . . . .  15  16
       4.2.7.  links . . . . . . . . . . . . . . . . . . . . . . . .  15  16
       4.2.8.  locale  . . . . . . . . . . . . . . . . . . . . . . .  16  18
       4.2.9.  keywords  . . . . . . . . . . . . . . . . . . . . . .  17  18
       4.2.10. categories  . . . . . . . . . . . . . . . . . . . . .  17  18
       4.2.11. color . . . . . . . . . . . . . . . . . . . . . . . .  17  18
     4.3.  Recurrence properties Properties . . . . . . . . . . . . . . . . . .  17  19
       4.3.1.  recurrenceRule  recurrenceId  . . . . . . . . . . . . . . . . . . . .  17  19
       4.3.2.  recurrenceOverrides  recurrenceRule  . . . . . . . . . . . . . . . . .  23 . .  19
       4.3.3.  recurrenceOverrides . . . . . . . . . . . . . . . . .  27
       4.3.4.  excluded  . . . . . . . . . . . . . . . . . . . . . .  24  28
     4.4.  Sharing and scheduling properties Scheduling Properties . . . . . . . . . . . .  24  28
       4.4.1.  priority  . . . . . . . . . . . . . . . . . . . . . .  24  28
       4.4.2.  freeBusyStatus  . . . . . . . . . . . . . . . . . . .  25  29
       4.4.3.  privacy . . . . . . . . . . . . . . . . . . . . . . .  25  29
       4.4.4.  replyTo . . . . . . . . . . . . . . . . . . . . . . .  26  30
       4.4.5.  participants  . . . . . . . . . . . . . . . . . . . .  27  31
     4.5.  Alerts properties Properties . . . . . . . . . . . . . . . . . . . .  30  35
       4.5.1.  useDefaultAlerts  . . . . . . . . . . . . . . . . . .  30  35
       4.5.2.  alerts  . . . . . . . . . . . . . . . . . . . . . . .  30  35
     4.6.  Multilingual properties Properties . . . . . . . . . . . . . . . . .  32  37
       4.6.1.  localizations . . . . . . . . . . . . . . . . . . . .  32  37
     4.7.  Time zone properties Zone Properties  . . . . . . . . . . . . . . . . . .  33  37
       4.7.1.  timeZones  timeZone  . . . . . . . . . . . . . . . . . . . . . .  33
   5.  Type-specific JSCalendar properties  38
       4.7.2.  timeZones . . . . . . . . . . . . .  35
     5.1.  JSEvent properties . . . . . . . . .  38
   5.  Type-specific JSCalendar Properties . . . . . . . . . .  35
       5.1.1.  start . . .  40
     5.1.  JSEvent Properties  . . . . . . . . . . . . . . . . . . .  40
       5.1.1.  start . .  35
       5.1.2.  timeZone . . . . . . . . . . . . . . . . . . . . . .  35
       5.1.3.  40
       5.1.2.  duration  . . . . . . . . . . . . . . . . . . . . . .  35
       5.1.4.  40
       5.1.3.  status  . . . . . . . . . . . . . . . . . . . . . . .  36  41
     5.2.  JSTask properties Properties . . . . . . . . . . . . . . . . . . . .  36  41
       5.2.1.  due . . . . . . . . . . . . . . . . . . . . . . . . .  36  41
       5.2.2.  start . . . . . . . . . . . . . . . . . . . . . . . .  36  41
       5.2.3.  timeZone  . . . . . . . . . . . . . . . . . . . . . .  36
       5.2.4.  estimatedDuration . . . . . . . . . . . . . . . . . .  36
       5.2.5.  41
       5.2.4.  statusUpdatedAt . . . . . . . . . . . . . . . . . . .  37
       5.2.6.  41
       5.2.5.  progress  . . . . . . . . . . . . . . . . . . . . . .  37
       5.2.7.  42
       5.2.6.  status  . . . . . . . . . . . . . . . . . . . . . . .  38  42
     5.3.  JSGroup properties Properties  . . . . . . . . . . . . . . . . . . .  38  43
       5.3.1.  entries . . . . . . . . . . . . . . . . . . . . . . .  39  44
       5.3.2.  source  . . . . . . . . . . . . . . . . . . . . . . .  39  44
   6.  JSCalendar object examples  Examples  . . . . . . . . . . . . . . . . . . . . . .  39 . . . .  44
     6.1.  Simple event Event  . . . . . . . . . . . . . . . . . . . . . .  39  44
     6.2.  Simple task Task . . . . . . . . . . . . . . . . . . . . . . .  40  44
     6.3.  Simple group Group  . . . . . . . . . . . . . . . . . . . . . .  40  45
     6.4.  All-day event Event . . . . . . . . . . . . . . . . . . . . . .  41  45
     6.5.  Task with a due date Due Date  . . . . . . . . . . . . . . . . . .  41  46
     6.6.  Event with end time-zone End Time Zone  . . . . . . . . . . . . . . . .  42  46
     6.7.  Floating-time event Event (with recurrence) Recurrence) . . . . . . . . . .  42  47
     6.8.  Event with multiple locations Multiple Locations and localization Localization  . . . . .  43  47
     6.9.  Recurring event Event with overrides Overrides  . . . . . . . . . . . . .  44  48
     6.10. Recurring event Event with participants Participants . . . . . . . . . . . .  45  49
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  47  51
     7.1.  Expanding Recurrences . . . . . . . . . . . . . . . . . .  51
     7.2.  JSON Parsing  . . . . . . . . . . . . . . . . . . . . . .  51
     7.3.  URI Values  . . . . . . . . . . . . . . . . . . . . . . .  52
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  47  52
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  48  53
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  48  53
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  48  53
     10.2.  Informative References . . . . . . . . . . . . . . . . .  51  55
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  51  55
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  51  56

1.  Introduction

   This document defines a data model for calendar event and task
   objects, or groups of such objects, in electronic calendar
   applications and systems.  It aims to be unambiguous, extendable and
   simple to process.

   The key design considerations for this data model are as follows:

   o  The attributes of the calendar entry represented must be described
      as a simple key-value pair, pair.  Simple events are simple to
      represent, complex events can be modelled accurately.

   o  Wherever possible, there should be only one way to express the
      desired semantics, reducing complexity of its
      representation. complexity.

   o  The data model should avoid all ambiguities and make it difficult to
      make mistakes during implementation.

   o  Most of the initial set of attributes  The data model should be taken from compatible with the iCalendar data format
      [RFC5545] and [RFC7986] and extensions, but the specification should
      add new attributes or value types, or not
      support existing ones, where appropriate.  Conversion between the
      data formats need iCalendar format currently lacks
      expressivity, and drop widely unused, obsolete or redundant
      properties.  This means translation with no loss of semantics
      should be easy with most common iCalendar files but is not fully preserve semantic meaning.
      guaranteed with the full specification.

   o  Extensions, such as new properties and components, MUST NOT lead
      to requiring an update to this document.

   The representation of this data model is defined in the I-JSON format
   [RFC7493], which is a strict subset of the JavaScript Object Notation
   (JSON) Data Interchange Format [RFC8259].  Using JSON is mostly a
   pragmatic choice: its widespread use makes JSCalendar easier to
   adopt, and the ready availability of production-ready JSON
   implementations eliminates a whole category of parser-related
   interoperability issues. issues, which iCalendar has often suffered from.

1.1.  Motivation and Relation to the iCalendar format and jCal

   The iCalendar data format [RFC5545], a widely deployed interchange
   format for calendaring and scheduling data, has served calendaring
   vendors for a long while, but contains some ambiguities and pitfalls
   that can not be overcome without backward-incompatible changes.

   For example, iCalendar defines various formats for local times, UTC
   time and dates, which confuses new users. users and often leads to
   implementation errors.  Other sources for errors are the requirement
   for custom time zone definitions within a single calendar component,
   as well as the iCalendar format itself; the latter causing
   interoperability issues due to misuse of CR LF terminated strings,
   line continuations and subtle differences between iCalendar parsers.  Lastly, up until recently the iCalendar format
   did not
   The definition of recurrence rules is ambiguous and has resulted in
   differing understandings even between experienced calendar
   developers.

   In recent years, many new products and services have a way appeared that
   wish to express use a concise difference between two JSON representation of calendar components, which results in verbose exchanges during
   scheduling.

1.2.  Relation to the jCal format data within their API.
   The JSON format for iCalendar data, jCal [RFC7265], is a direct
   mapping between iCalendar and JSON.  It does not attempt  In its effort to extend or
   update represent full
   iCalendar semantics, and consequently does not address it inherits all the
   issues outlined in Section 1.1.

   Since same pitfalls and uses a
   complicated JSON structure unlike most common JSON data
   representations.

   As a consequence, since the standardization of jCal, the majority of
   implementations and service providers either kept using iCalendar, or
   came up with their own proprietary JSON representation, representations, which often are
   incompatible with each other. other and often suffer from common pitfalls,
   such as storing event start times in UTC (which become incorrect if
   the timezone's rules change in the future).  JSCalendar is intended
   to meet this demand for JSON formatted JSON-formatted calendar data, and to provide
   a standard standard, elegant representation as an alternative to new
   proprietary formats.

1.3.

1.2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The underlying format used for this specification is JSON.
   Consequently, the terms "object" and "array" as well as the four
   primitive types (strings, numbers, booleans, and null) are to be
   interpreted as described in Section 1 of [RFC8259].

   Some examples in this document contain "partial" JSON documents used
   for illustrative purposes.  In these examples, three periods "..."
   are used to indicate a portion of the document that has been removed
   for compactness.

2.  JSCalendar objects

   This section describes the calendar object types specified by
   JSCalendar.

2.1.  JSEvent

   MIME type: "application/jscalendar+json;type=jsevent"
   A JSEvent represents a scheduled amount of time on a calendar,
   typically a meeting, appointment, reminder or anniversary.  Multiple
   participants may partake

1.3.  Type Signatures

   Type signatures are given for all JSON values in the event at multiple locations. this document.  The @type (Section 4.1.1) property
   following conventions are used:

   o  "*" - The type is undefined (the value MUST could be "jsevent".

2.2.  JSTask

   MIME type: "application/jscalendar+json;type=jstask"

   A JSTask represents an action-item, assignment, to-do or work item.

   The @type (Section 4.1.1) property value MUST be "jstask".

   A JSTask any type, although
      permitted values may start and be due at certain points in time, may take
   some estimated time to complete and may recur; none of which is
   required.  This notably differs from JSEvent (Section 2.1) which is
   required to start at a certain point in time and typically takes some
   non-zero duration to complete.

2.3.  JSGroup

   MIME type: "application/jscalendar+json;type=jsgroup"

   A JSGroup is a collection of JSEvent (Section 2.1) and JSTask
   (Section 2.2) objects.  Typically, objects are grouped by topic (e.g. constrained by keywords) or calendar membership.

   The @type (Section 4.1.1) property value MUST be "jsgroup".

3.  Structure of JSCalendar objects

   A JSCalendar object is a JSON object, which MUST be valid I-JSON (a
   stricter subset of JSON), as specified in [RFC8259].  Property names
   and values are case-sensitive.

   The object has a collection of properties, as specified in the
   following sections.  Properties are specified as being either
   mandatory or optional.  Optional properties may have a default value,
   if explicitly specified in the property definition.

3.1.  Type signatures

   Types signatures are given for all JSON objects in context of this document. value).

   o  "String" - The following conventions are used: JSON string type.

   o  "Boolean|String":  "Number" - The value is either a JSON number type.

   o  "Boolean" value, or a - The JSON "String" value. boolean type.

   o  "Foo": Any name that is not a native  "A[B]" - A JSON type means an object for
      which where the properties (and their types) keys are defined elsewhere
      within this document.

   o  "Foo[]": An array of objects all of type "Foo".

   o  "String[Foo]": A JSON "Object" being used as a map (associative
      array), where all "A", and the
      values are all of type "Foo".

3.2.  Data Types

   In addition to "B".

   o  "A[]" - An array of values of type "A".

   o  "A|B" - The value is either of type "A" or of type "B".

   Other types may also be given, with their representation defined
   elsewhere in this document.

1.4.  Data Types

   In addition to the standard JSON data types, the following data types
   are used in this specification:

3.2.1.

1.4.1.  Int

   Where "Int" is given as a data type, it means an integer in the range
   -2^53+1 <= value <= 2^53-1, the safe range for integers stored in a
   floating-point double, represented as a JSON "Number".

1.4.2.  UnsignedInt

   Where "UnsignedInt" is given as a data type, it means an "Int" where
   the value MUST be in the range 0 <= value <= 2^53-1.

1.4.3.  UTCDateTime

   This is a string in [RFC3339] "date-time" format, with the further
   restrictions that any letters MUST be in upper-case, uppercase, the time
   component MUST be included and the time offset MUST be the character
   "Z".  Fractional second values MUST NOT be included unless non-zero
   and MUST NOT have trailing zeros, to ensure there is only a single
   representation for each date-time.

   For example "2010-10-10T10:10:10.003Z" is OK, but
   "2010-10-10T10:10:10.000Z" is invalid and MUST be encoded as
   "2010-10-10T10:10:10Z".

   In common notation, it should be of the form "YYYY-MM-DDTHH:MM:SSZ".

3.2.2.

1.4.4.  LocalDateTime

   This is a date-time string _with with no time zone/offset information_. information.  It
   is otherwise in the same format as UTCDateTime, including fractional
   seconds.  For example "2006-01-02T15:04:05" and
   "2006-01-02T15:04:05.003" are both valid.  The time zone to associate
   the LocalDateTime with comes from an associated property, or if no
   time zone is associated it defines _floating time_. *floating time*. Floating date-
   times are not tied to any specific time zone.  Instead, they occur in
   every time zone at the same wall-clock time (as opposed to the same
   instant point in time).

3.2.3.

1.4.5.  Duration

   A

   Where Duration object is given as a type, it means a length of time
   represented by a subset of ISO8601 duration format, as specified by
   the following ABNF:

       dur-secfrac = "." 1*DIGIT
       dur-second  = 1*DIGIT [dur-secfrac] "S"
       dur-minute  = 1*DIGIT "M" [dur-second]
       dur-hour    = 1*DIGIT "H" [dur-minute]
       dur-time    = "T" (dur-hour / dur-minute / dur-second)
       dur-day     = 1*DIGIT "D"
       dur-week    = 1*DIGIT "W"

       duration    = "P" (dur-day [dur-time] / dur-time / dur-week)

   In addition, the duration MUST NOT include fractional second values
   unless the fraction is non-zero.

1.4.6.  SignedDuration

   A SignedDuration object is represented as a duration, optionally
   preceded by represents a sign character.  It typically length of time that may be positive or
   negative and is typically used to express the offset of a point in
   time relative to an associated time.  It is represented as a
   Duration, optionally preceded by a sign character.  It is specified
   by the following ABNF:

       signed-duration = (["+"] / "-") duration

   A negative sign indicates a point in time at or before the associated
   time, a positive or no sign a time at or after the associated time.

3.2.4.

1.4.7.  Id

   Where "Id" is given as a data type, it means a "String" of at least 1
   and a maximum of 255 octets in size, and it MUST only contain
   characters from the "URL and Filename Safe" base64 alphabet, as
   defined in Section 5 of [RFC4648], excluding the pad character ("=").
   This means the allowed characters are the ASCII alphanumeric
   characters ("A-Za-z0-9"), hyphen ("-"), and underscore ("_").

   Unless otherwise specified, Ids are arbitrary and only have meaning
   within the object where they are being used.  Ids need not be unique
   between different objects.  For example, two JSEvent objects MAY use
   the same ids in their respective "links" properties.  Or within the
   same JSEvent object the same Id could appear in the "participants"
   and "alerts" properties.  This does not imply any semantic connection
   between the two.

   Nevertheless, a UUID is typically a good choice.

1.4.8.  PatchObject

   A PatchObject is of type "String[*|null]", "String[*]", and represents an unordered set
   of patches on a JSON object.  The keys are a path in a subset of
   [RFC6901] JSON pointer format, with an implicit leading "/" (i.e.
   prefix each key with "/" before applying the JSON pointer evaluation
   algorithm).

   A patch within a PatchObject is only valid, valid if all of the following
   conditions apply:

   1.  The pointer MUST NOT reference inside an array (i.e. it MUST NOT
       insert/delete from an array; the array MUST be replaced in its
       entirety instead).

   2.  When evaluating a path, all parts prior to the last (i.e. the
       value after the final slash) MUST exist.

   3.  There MUST NOT be two patches in the PatchObject where the
       pointer of one is the prefix of the pointer of the other, e.g.
       "alerts/foo/offset" and "alerts".

   The value associated with each pointer is either:

   o  "null":  null: Remove the property from the patched object.  If not present
      in the parent, this a no-op.

   o  Anything else: The value to replace the inherited set for this property on the
      patch object with (if present) or add (this may be a
      replacement or addition to the property (if not
      present). object being patched).

   Implementations MUST reject a PatchObject if any of its patches are
   invalid.

3.2.5.  Identifiers

   If not stated otherwise

1.4.9.  Time Zones

   By default, time zones in JSCalendar are identified by their name in
   the IANA Time Zone Database [1], and the zone rules of the respective
   zone record apply.

   Implementations MAY embed the definition of custom time zones in the
   "timeZones" property definition,
   properties and (see Section 4.7.2).

2.  JSCalendar Objects

   This section describes the calendar object keys that define identifiers types specified by
   JSCalendar.

2.1.  JSEvent

   MIME type: "application/jscalendar+json;type=jsevent"

   A JSEvent represents a scheduled amount of time on a calendar,
   typically a meeting, appointment, reminder or anniversary.  Multiple
   participants may partake in the event at multiple locations.

   The @type (Section 4.1.1) property value MUST be string
   values, "jsevent".

2.2.  JSTask

   MIME type: "application/jscalendar+json;type=jstask"

   A JSTask represents an action-item, assignment, to-do or work item.

   The @type (Section 4.1.1) property value MUST be at least 1 character "jstask".

   A JSTask may start and maximum 256 characters be due at certain points in
   size, and MUST only contain characters from the "URL time, may take
   some estimated time to complete and Filename
   safe" Base 64 Alphabet, as defined in section 5 may recur; none of [RFC4648]. which is
   required.  This notably differs from JSEvent (Section 2.1) which is the ASCII alphanumeric characters (A-Za-z0-9), hyphen (-), and
   underscore (_).  Note that [RFC7493] requires string values be
   encoded in UTF-8, so the maximum size of an identifier according
   required to
   this definition is 256 octets.

   . Identifiers in object maps need not be universally unique, e.g. two
   calendar objects MAY use the same identifiers in their respective
   "links" properties.

   Nevertheless, start at a UUID certain point in time and typically takes some
   non-zero duration to complete.

2.3.  JSGroup

   MIME type: "application/jscalendar+json;type=jsgroup"

   A JSGroup is a good choice.

3.2.6.  Time Zones

   By default, time zones in JSCalendar collection of JSEvent (Section 2.1) and/or JSTask
   (Section 2.2) objects.  Typically, objects are identified grouped by their name in
   the IANA Time Zone Database [1], topic (e.g.
   by keywords) or calendar membership.

   The @type (Section 4.1.1) property value MUST be "jsgroup".

3.  Structure of JSCalendar Objects

   A JSCalendar object is a JSON object, which MUST be valid I-JSON (a
   stricter subset of JSON), as specified in [RFC8259].  Property names
   and the zone rules values are case-sensitive.

   The object has a collection of properties, as specified in the respective
   zone record apply.

   Implementations MAY embed the definition of custom time zones
   following sections.  Properties are specified as being either
   mandatory or optional.  Optional properties may have a default value,
   if explicitly specified in the
   "timeZones" property (see Section 4.7.1).

3.2.7. definition.

3.1.  Normalization and equivalence Equivalence

   JSCalendar aims to provide unambiguous definitions for value types
   and properties, but does not define a general normalization or
   equivalence method for JSCalendar objects and types.  This is because
   the notion of equivalence might range from byte-level equivalence to
   semantic equivalence, depending on the respective use case (for
   example, the CalDAV protocol [RFC4791] requires octet equivalence of
   the encoded calendar object to determine ETag equivalence).

   Normalization of JSCalendar objects is hindered because of the
   following reasons:

   o  Custom JSCalendar properties may contain arbitrary JSON values,
      including arrays.  However, equivalence of arrays might or might
      not depend on the order of elements, depending on the respective
      property definition.

   o  Several JSCalendar property values are defined as URIs and MIME
      types, but normalization of these types is inherently protocol and
      scheme-specific, depending on the use-case of the equivalence
      definition (see section Section 6 of [RFC3986]).

   Considering this, the definition of equivalence and normalization is
   left to client and server implementations and to be negotiated by a
   calendar exchange protocol or defined by another RFC.

3.3.

3.2.  Custom property extensions Property Extensions and values Values

   Vendors MAY add additional properties to the calendar object to
   support their custom features.  The names of these properties MUST be
   prefixed with a domain name controlled by the vendor to avoid
   conflict, e.g. "example.com/customprop".

   Some JSCalendar properties allow vendor-specific value extensions.
   If so, vendor specific values MUST be prefixed with a domain name
   controlled by the vendor, e.g. "example.com/customrel", unless
   otherwise noted.

   Vendors are strongly encouraged to standardize any new property
   values or extensions that are useful to other systems as well, rather
   than use a vendor-specific prefix.

4.  Common JSCalendar properties Properties

   This section describes the properties that are common to the various
   JSCalendar object types.  Specific JSCalendar object types may only
   support a subset of these properties.  The object type definitions in
   Section 5 describe the set of supported properties per type.

4.1.  Metadata properties Properties

4.1.1.  @type

   Type: String "String" (mandatory).

   Specifies the type which this object represents.  This MUST be one of
   the following values, registered in a future RFC, or a vendor-
   specific value:

   o  "jsevent": a JSCalendar event (Section 2.1).

   o  "jstask": a JSCalendar task (Section 2.2).

   o  "jsgroup": a JSCalendar group (Section 2.3).

4.1.2.  uid

   Type: String "String" (mandatory).

   A globally unique identifier, used to associate the object as the
   same across different systems, calendars and views.  The value of
   this property MUST be unique across all JSCalendar objects, even if
   they are of different type.  [RFC4122] describes a range of
   established algorithms to generate universally unique identifiers
   (UUID), and the random or pseudo-random version is recommended.

   For compatibility with [RFC5545] UIDs, implementations MUST be able
   to receive and persist values of at least 255 octets for this
   property, but they MUST NOT truncate values in the middle of a UTF-8
   multi-octet sequence.

4.1.3.  relatedTo

   Type: String[Relation] "String[Relation]" (optional).

   Relates the object to other JSCalendar objects.  This is represented
   as a map of the UIDs of the related objects to information about the
   relation.

   A Relation object has the following properties: property:

   o  relation: String[Boolean] (optional). "String[Boolean]" (optional)

      Describes how the linked object is related to this object as a set
      of relation types.  If
      not null, specified, the set MUST NOT be empty.  If
      ommitted, the relationship between the two objects is unspecified.

      Keys in the set MUST be one of the following values, defined in a
      future specification or a vendor-specific value:

      *  "first": The linked object is the first in the series this
         object is part of.

      *  "next": The linked object is the next in the series this object
         is part of.

      *  "child": The linked object is a subpart of this object.

      *  "parent": This object is part of the overall linked object.

      The value for each key in the set MUST be "true". true.

   Note, the Relation object only has one property; it is specified as
   an object with a single property rather than mapping directly from
   the UID to relation types to allow for extension in the future.

   If an object is split to make a "this and future" change to a
   recurrence, the original object MUST be truncated to end at the
   previous occurrence before this split, and a new object created to
   represent all the objects occurrences after the split.  A "next" relation
   MUST be set on the original object's relatedTo property for the UID
   of the new object.  A "first" relation for the UID of the first
   object in the series MUST be set on the new object.  Clients can then
   follow these UIDs to get the complete set of objects if the user
   wishes to modify them all at once.

4.1.4.  prodId

   Type: String "String" (optional).

   The identifier for the product that created the JSCalendar object.

   The vendor of the implementation SHOULD ensure that this is a
   globally unique identifier, using some technique such as an FPI
   value, as defined in [ISO.9070.1991].  It MUST only use characters of
   an iCalendar TEXT data value (see section Section 3.3.11 in of [RFC5545]).

   This property SHOULD NOT be used to alter the interpretation of an a
   JSCalendar object beyond the semantics specified in this document.
   For example, it is not to be used to further the understanding of
   non-standard properties.

4.1.5.  created

   Type: UTCDateTime "UTCDateTime" (optional).

   The date and time this object was initially created.

4.1.6.  updated

   Type: UTCDateTime "UTCDateTime" (mandatory).

   The date and time the data in this object was last modified.

4.1.7.  sequence

   Type: Number "UnsignedInt" (optional, default: "0"). 0).

   Initially zero, this MUST be a non-negative integer that is
   monotonically incremented each by one every time a change
   is made to the object. object, except if the change only modifies the
   "participants" property (see Section 4.4.5).

   This is used as part of iTIP [RFC5546] to know which version of the
   object a scheduling message relates to.

4.1.8.  method

   Type: String "String" (optional).

   The iTIP ([RFC5546]) [RFC5546] method, in lower-case.  Used for scheduling. lowercase.  This MUST only be present
   if the JSCalendar object represents an iTIP scheduling message.

4.2.  What and where properties Where Properties

4.2.1.  title

   Type: String "String" (optional, default: empty String).

   A short summary of the object.

4.2.2.  description

   Type: String "String" (optional, default: empty String).

   A longer-form text description of the object.  The content is
   formatted according to the "descriptionContentType" property.

4.2.3.  descriptionContentType

   Type: String "String" (optional, default: "text/plain").

   Describes the media type ([RFC6838]) [RFC6838] of the contents of the
   "description" property.  Media types MUST be sub-types of type
   "text", and SHOULD be "text/plain" or "text/html" ([MIME]). [MIME].  They MAY
   define parameters and the "charset" parameter value MUST be "utf-8",
   if specified.  Descriptions of type "text/html" MAY contain "cid"
   URLs ([RFC2392]) [RFC2392] to reference links in the calendar object by use of
   the "cid" property of the Link object.

4.2.4.  showWithoutTime

   Type: Boolean "Boolean" (optional, default: "false"). false).

   Indicates the time is not important to display to the user when
   rendering this calendar object, for example an event that
   conceptually occurs all day or across multiple days, such as "New
   Year's Day" or "Italy Vacation".  While the time component is
   important for free-busy calculations and checking for scheduling
   clashes, calendars may choose to omit displaying it and/or display
   the object separately to other objects to enhance the user's view of
   their schedule.

   Such events are also commonly known as "all-day" events.

4.2.5.  locations

   Type: String[Location] "Id[Location]" (optional).

   A map of location identifiers ids to Location objects, representing locations
   associated with the object.

   A Location object has the following properties.  It must define MUST have at
   least one other property other than the "relativeTo" property.

   o  name: String (optional). "String" (optional)

      The human-readable name of the location.

   o  description: String (optional). "String" (optional)

      Human-readable, plain-text instructions for accessing this
      location.  This may be an address, set of directions, door access
      code, etc.

   o  relativeTo: String (optional). "String" (optional)

      The relation type of this location to the JSCalendar object.

      This MUST be either one of the following values, registered in a
      future RFC, or a vendor-specific value.  Any value the client or
      server doesn't understand should be treated the same as if this
      property is omitted.

      *  "start": The JSCalendar object starts at this location.

      *  "end": The JSCalendar object ends at this location.

   o  timeZone: String (optional). "String" (optional)

      A time zone for this location.  Also
      see  See also Section 3.2.6. 1.4.9.

   o  coordinates: String (optional).  An [RFC5870] "String" (optional)

      A "geo:" URI [RFC5870] for the location.

   o  linkIds: String[Boolean] (optional). "Id[Boolean]" (optional)

      A set of link ids for links to alternate representations of this
      location.  Each key in the set MUST be the identifier id of a Link object
      defined in the "links" property of this calendar object.  The
      value for each key in the set MUST be "true". true.  This MUST be omitted
      if none (rather than an empty set).

      For example, an alternative representation could be in vCard
      format.

4.2.6.  virtualLocations

   Type: String[VirtualLocation] "Id[VirtualLocation]" (optional).

   A map of identifiers ids to VirtualLocation objects, representing virtual
   locations, such as video conferences or chat rooms, associated with
   the object.

   A VirtualLocation object has the following properties.

   o  name: String "String" (optional, default: empty String). String)

      The human-
      readable human-readable name of the virtual location.

   o  description: String (optional). "String" (optional)

      Human-readable plain-text instructions for accessing this
      location.  This may be an address, set of directions, door access
      code, etc.

   o  uri: String (mandatory). "String" (mandatory)

      A URI that represents how to connect to this virtual location.

      This may be a telephone number (represented as using the "tel:"
      scheme, e.g., "tel:+1-555-555-555") for a teleconference, a web
      address for online chat, or any custom URI.

4.2.7.  links

   Type: String[Link] "Id[Link]" (optional).

   A map of link identifiers ids to Link objects, representing external resources
   associated with the object.

   A Link object has the following properties:

   o  href: String (mandatory). "String" (mandatory)

      A URI from which the resource may be fetched.

      This MAY be a "data:" URL, but it is recommended that the file be
      hosted on a server to avoid embedding arbitrarily large data in
      JSCalendar object instances.

   o  cid: String (optional). "String" (optional)

      This MUST be a valid "content-id" value according to the
      definition of section Section 2 in [RFC2392].  The
      identifier value MUST be unique
      within this JSCalendar object Link
      objects object but has no meaning beyond that.  Specifically, it  It MAY be
      different from the link identifier in the enclosing "links"
      property. id for this Link object.

   o  type: String (optional). "String" (optional)

      The content-type [RFC6838] of the resource, if known.

   o  size: Number (optional). "UnsignedInt" (optional)

      The size, in bytes, octets, of the resource when fully decoded (i.e. the
      number of bytes octets in the file the user would download), if known.

   o  rel: String (optional). "String" (optional)

      Identifies the relation of the linked resource to the object.  If
      set, the value MUST be a registered relation type (see [RFC8288]
      and IANA Link Relations [2]).

      Links with a rel of "enclosure" SHOULD be considered by the client
      as attachments for download.

      Links with a rel of "describedby" SHOULD be considered by the
      client to be an alternate representation of the description.

      Links with a rel of "icon" SHOULD be considered by the client to
      be an image that it MAY use when presenting the calendar data to a
      user.  The "display" property MAY be set to indicate the purpose
      of this image.

   o  display: String (optional). "String" (optional)

      Describes the intended purpose of a link to an image.  If set, the
      "rel" property MUST be set to "icon".  The value MUST be either
      one of the following values, registered in a future RFC, or a
      vendor-specific value:

      *  "badge": an image inline with the title of the object object.

      *  "graphic": a full image replacement for the object itself itself.

      *  "fullsize": an image that is used to enhance the object object.

      *  "thumbnail": a smaller variant of "fullsize" to be used when
         space for the image is constrained constrained.

   o  title: String (optional). "String" (optional)

      A human-readable plain-text description of the resource.

4.2.8.  locale

   Type: String "String" (optional).

   The [RFC5646] language tag as defined in [RFC5646] that best describes the
   locale used for the text in the calendar object, if known.

4.2.9.  keywords

   Type: String[Boolean] "String[Boolean]" (optional).

   A set of keywords or tags that relate to the object.  The set is
   represented as a map, with the keys being the keywords.  The value
   for each key in the map MUST be "true". true.

4.2.10.  categories

   Type: String[Boolean] "String[Boolean]" (optional).

   A set of categories that relate to the calendar object.  The set is
   represented as a map, with the keys being the categories specified as
   URIs.  The value for each key in the map MUST be "true". true.

   In contrast to keywords, categories typically are structured.  For
   example, a vendor owning the domain "example.com" might define the
   categories "http://example.com/categories/sports/american-football""
   and "http://example.com/categories/music/r-b".

4.2.11.  color

   Type: String "String" (optional).

   Specifies a

   A color clients MAY use when displaying this calendar object.  The
   value is a case-insensitive color name taken from the CSS3 set of
   names, defined in Section 4.3 of W3C.REC-
   css3-color-20110607 W3C.REC-css3-color-20110607 [3] or a
   CSS3 RGB color hex value.

4.3.  Recurrence properties Properties

   Some events and tasks occur at regular, or indeed irregular,
   intervals.  Rather than having to copy the data for every occurrence,
   you can instead have a master event with a recurrence rule generating
   the occurrences, and/or overrides that add extra dates or exceptions
   to the rule.

4.3.1.  recurrenceId

   Type: "LocalDateTime" (optional).

   If present, this JSCalendar object represents one occurrence of a
   recurring JSCalendar object.  If present the "recurrenceRule" and
   "recurrenceOverrides" properties MUST NOT be present.

   The value is a date-time either produced by the "recurrenceRule" of
   the master event, or added as a key to the "recurrenceOverrides"
   property of the master event.

4.3.2.  recurrenceRule

   Type: Recurrence "Recurrence" (optional).

   Defines a recurrence rule (repeating pattern) for recurring calendar
   objects.

   A Recurrence object is a JSON object mapping of a RECUR value type in
   iCalendar, see [RFC5545] and[RFC7529].  A JSEvent recurs by applying the recurrence rule to the start date-time. "start" date-
   time.

   A JSTask recurs by applying the recurrence rule to the start date-time, "start" date-
   time, if defined, otherwise it recurs by the due "due" date-time, if
   defined.  If the task
   neither defines neither a start or due "start" nor "due" date-time,
   its "recurrenceRule" property value MUST be "null". null.

   A Recurrence object is a JSON object mapping of a RECUR value type in
   iCalendar [RFC5545] [RFC7529] and has the same semantics.  It has the
   following properties:

   o  frequency: String (mandatory). "String" (mandatory)

      The time span covered by each iteration of this recurrence rule
      (see Section 4.3.2.1 for full semantics).  This MUST be one of the
      following values:

      *  "yearly"

      *  "monthly"

      *  "weekly"

      *  "daily"

      *  "hourly"

      *  "minutely"

      *  "secondly"

      To convert from iCalendar, simply lower-case

      This is the FREQ part. part from iCalendar, converted to lowercase.

   o  interval: Number "UnsignedInt" (optional, default: "1"). 1)

      The INTERVAL part from
      iCalendar. interval of iteration periods at which the recurrence repeats.
      If included, it MUST be an integer "x >= 1". 1.

      This is the INTERVAL part from iCalendar.

   o  rscale: String "String" (optional, default: ""gregorian""). "gregorian")

      The calendar system in which this recurrence rule operates, in
      lowercase.  This MUST be either a CLDR-registered calendar system
      name, or a non-standard, experimental calendar system name
      prefixed with the characters "x-".

      This is the RSCALE part from iCalendar RSCALE [RFC7529], converted
      to lower-case. lowercase.

   o  skip: String "String" (optional, default: ""omit""). "omit")

      The behaviour to use when the expansion of the recurrence produces
      invalid dates.  This MUST be one of the following values:

      *  "omit"

      *  "backward"

      *  "forward"

      This is the SKIP part from iCalendar RSCALE [RFC7529], converted
      to lower-case. lowercase.

   o  firstDayOfWeek: String "String" (optional, default: ""mo""). "mo")
      The WKST part
      from iCalendar, day on which the week is considered to start, represented as a lower-case
      lowercase abbreviated two-letter English day of the week.  If
      included, it MUST be one of the following values: ""mo"|"tu"|"we"|"th"|"fr"|"sa"|"su"".

      *  "mo"

      *  "tu"

      *  "we"

      *  "th"

      *  "fr"

      *  "sa"

      *  "su"

      This is the WKST part from iCalendar.

   o  byDay: NDay[] (optional). "NDay[]" (optional)

      Days of the week on which to repeat.  An *NDay* object has the
      following properties:

      *  day: String.  The "String" (mandatory)

         A day of the week on which to repeat; the allowed values are
         the same as for the "firstDayOfWeek" Recurrence property.

         This is the day-of-the-week part of the BYDAY value part in iCalendar, lower-cased.  MUST be one of the following values:
         ""mo"|"tu"|"we"|"th"|"fr"|"sa"|"su"".
         converted to lowercase.

      *  nthOfPeriod: Number (optional).  The ordinal part of the BYDAY
         value in iCalendar (e.g. ""+1"" or ""-3""). "Int" (optional)

         If present, rather than representing every occurrence of the
         weekday defined in the "day" property, it represents only a
         specific instance within the recurrence period.  The value can
         be positive or negative, but MUST NOT be zero.  A negative
         integer means nth-
         last nth-last of period.

         This is the ordinal part of the BYDAY value in iCalendar (e.g.
         1 or -3).

   o  byMonthDay: Number[] (optional).  The BYMONTHDAY part "Int[]" (optional)
      Days of the month on which to repeat.  Valid values are 1 to 31 or
      -31 to -1.  Negative values offset from
      iCalendar. the end of the month.  The
      array MUST have at least one entry if included.

      This is the BYMONTHDAY part in iCalendar.

   o  byMonth: String[] (optional). "String[]" (optional)

      The BYMONTH part from iCalendar. months in which to repeat.  Each entry is a string
      representation of a number, starting from "1" for the first month
      in the calendar (e.g. ""1" " "1" means
      ""January"" January with the Gregorian
      calendar), with an optional ""L"" "L" suffix (see [RFC7529]) for leap
      months (this MUST be upper-case, uppercase, e.g. ""3L""). "3L").  The array MUST have
      at least one entry if included.

      This is the BYMONTH part from iCalendar.

   o  byYearDay: Number[] (optional). "Int[]" (optional)

      The BYYEARDAY part days of the year on which to repeat.  Valid values are 1 to
      366 or -366 to -1.  Negative values offset from
      iCalendar. the end of the
      year.  The array MUST have at least one entry if included.

      This is the BYYEARDAY part from iCalendar.

   o  byWeekNo: Number[] (optional). "Int[]" (optional)

      Weeks of the year in which to repeat.  Valid values are 1 to 53 or
      -53 to -1.  The array MUST have at least one entry if included.

      This is the BYWEEKNO part from iCalendar.

   o  byHour: "UnsignedInt[]" (optional)

      The hours of the day in which to repeat.  Valid values are 0 to
      23.  The array MUST have at least one entry if included.

   o  byHour: Number[] (optional).  The  This is
      the BYHOUR part from iCalendar.

   o  byMinute: "UnsignedInt[]" (optional)

      The minutes of the hour in which to repeat.  Valid values are 0 to
      59.  The array MUST have at least one entry if included.

   o  byMinute: Number[] (optional).  The

      This is the BYMINUTE part from iCalendar.

   o  bySecond: "UnsignedInt[]" (optional)
      The seconds of the minute in which to repeat.  Valid values are 0
      to 60.  The array MUST have at least one entry if included.

   o  bySecond: Number[] (optional).  The

      This is the BYSECOND part from iCalendar.
      The array MUST have at least one entry if included.

   o  bySetPosition: Number[] (optional). "Int[]" (optional)

      The BYSETPOS part occurrences within the recurrence interval to include in the
      final results.  Negative values offset from
      iCalendar. the end of the list of
      occurrences.  The array MUST have at least one entry if included.

   o  count: Number (optional).  The COUNT
      This is the BYSETPOS part from iCalendar.

   o  count: "UnsignedInt" (optional)

      The number of occurrences at which to range-bound the recurrence.
      This MUST NOT be included if an "until" property is specified.

   o  until: LocalDateTime (optional).  The UNTIL

      This is the COUNT part from iCalendar.

   o  until: "LocalDateTime" (optional)

      The date-time at which to finish recurring.  The last occurrence
      is on or before this date-time.  This MUST NOT be included if a
      "count" property is specified.  Note: if not specified otherwise
      for a specific JSCalendar object, this date is presumed to be interpreted
      in the time zone specified in
      "timeZone".  As in iCalendar, the until value bounds JSCalendar object's "timeZone"
      property.

      This is the UNTIL part from iCalendar.

4.3.2.1.  Interpreting recurrence rule inclusively. rules

   A recurrence rule specifies a set of set of date-times for recurring
   calendar objects.  A recurrence rule has the following semantics.
   Note, wherever "year", "month" or "day of month" is used, this is
   within the calendar system given by the "rscale" property, which
   defaults to gregorian if omitted.

   1.  A set of candidates is generated.  This is every second within a
       period defined by the frequency property value:

       *  "yearly": every second from midnight on the 1st day of a year
          (inclusive) to midnight the 1st day of the following year
          (exclusive).

          If skip is not "omit", the calendar system has leap months and
          there is a byMonth property, generate candidates for the leap
          months even if they don't occur in this year.

          If skip is not "omit" and there is a byMonthDay property,
          presume each month has the maximum number of days any month
          may have in this calendar system when generating candidates,
          even if it's more than this month actually has.

       *  "monthly": every second from midnight on the 1st day of a
          month (inclusive) to midnight on the 1st of the following
          month (exclusive).

          If skip is not "omit" and there is a byMonthDay property,
          presume the month has the maximum number of days any month may
          have in this calendar system when generating candidates, even
          if it's more than this month actually has.

       *  "weekly": every second from midnight (inclusive) on the first
          day of the week (as defined by the firstDayOfWeek property, or
          Monday if omitted), to midnight 7 days later (exclusive).

       *  "daily": every second from midnight at the start of the day
          (inclusive) to midnight at the end of the day (exclusive).

       *  "hourly": every second from the beginning of the hour
          (inclusive) to the beginning of the next hour (exclusive).

       *  "minutely": every second from the beginning of the minute
          (inclusive) to the beginning of the next minute (exclusive).

       *  "secondly": the second itself, only.

   2.  Each date-time candidate is compared against all of the byX
       properties of the rule except bySetPosition.  If any property in
       the rule does not match the date-time, it is eliminated.  Each
       byX property is an array; the date-time matches the property if
       it matches any of the values in the array.  The properties have
       the following semantics:

       *  byMonth: the date-time is in the given month.

       *  byWeekNo: the date-time is in the nth week of the year.
          Negative numbers mean the nth last week of the year.  This
          corresponds to weeks according to week numbering as defined in
          ISO.8601.2004, with a week defined as a seven day period,
          starting on the firstDayOfWeek property value or Monday if
          omitted.  Week number one of the calendar year is the first
          week that contains at least four days in that calendar year.

          If the date-time is not valid (this may happen when generating
          candidates with a skip property in effect), it is always
          eliminated by this property.

       *  byYearDay: the date-time is on the nth day of year.  Negative
          numbers mean the nth last day of the year.

          If the date-time is not valid (this may happen when generating
          candidates with a skip property in effect), it is always
          eliminated by this property.

       *  byMonthDay: the date-time is on the given day of the month.
          Negative numbers mean the nth last day of the month.

       *  byDay: the date-time is on the given day of the week.  If the
          day is prefixed by a number, it is the nth occurrence of that
          day of the week within the month (if frequency is monthly) or
          year (if frequency is yearly).  Negative numbers means nth
          last occurrence within that period.

       *  byHour: the date-time has the given hour value.

       *  byMinute: the date-time has the given minute value.

       *  bySecond: the date-time has the given second value.

       If a skip property is defined and is not "omit", there may be
       candidates that do not correspond to valid dates (e.g. 31st
       February in the gregorian calendar).  In this case, the
       properties MUST be considered in the order above and:

       1.  After applying the byMonth filter, if the candidate's month
           is invalid for the given year increment it (if skip is
           "forward") or decrement it (if skip is "backward") until a
           valid month is found, incrementing/decrementing the year as
           well if you pass through the beginning/end of the year.  This
           only applies to calendar systems with leap months.

       2.  After applying the byMonthDay filter, if the day of the month
           is invalid for the given month and year, change the date to
           the first day of the next month (if skip == "forward") or the
           last day of the current month (if skip == "backward").

       3.  If any valid date produced after applying the skip is already
           a candidate, eliminate the duplicate.  (For example after
           adjusting, 30th February and 31st February would both become
           the same "real" date, so one is eliminated as a duplicate.)

   3.  If a bySetPosition property is included, this is now applied to
       the ordered list of remaining dates (this dates.  This property specifies the
       indexes of date-times to keep; all others should be eliminated.
       Negative numbers are indexes from the end of the list, with -1
       being the last item). item.

   4.  Any date-times before the start date of the event are eliminated
       (see below for why this might be needed).

   5.  If a skip property is included and is not "omit", eliminate any
       date-times that have already been produced by previous iterations
       of the algorithm.  (This is not possible if skip == "omit".)

   6.  If further dates are required (we have not reached the until
       date, or count limit) skip the next (interval - 1) sets of
       candidates, then continue from step 1.

   When determining the set of occurrence dates for an event or task,
   the following extra rules must be applied:

   1.  The start initial date-time to which the rule is applied (the "start"
       date-time for events; the "start" or "due" date-time for tasks)
       is always the first occurrence in the expansion (and is counted
       if the recurrence is limited by a "count" property), even if it
       would normally not match the rule.

   2.  The first set of candidates to consider is that which would
       contain the start initial date-time.  This means the first set may
       include candidates before the start; initial date-time; such candidates
       are eliminated from the results in step (4) as outlined before.

   3.  The following properties MUST be implicitly added to the rule
       under the given conditions:

       *  If frequency > is not "secondly" and no bySecond property: Add a
          bySecond property with the sole value being the seconds value
          of the start initial date-time.

       *  If frequency > "minutely" is not "secondly" or "minutely", and no byMinute
          property: Add a byMinute property with the sole value being
          the minutes value of the start initial date-time.

       *  If frequency > is not "secondly", "minutely" or "hourly" and no
          byHour property: Add a byHour property with the sole value
          being the hours value of the
          start initial date-time.

       *  If frequency is "weekly" and no byDay property: Add a byDay
          property with the sole value being the day-of-the-week of the
          start
          initial date-time.

       *  If frequency is "monthly" and no byDay property and no
          byMonthDay property: Add a byMonthDay property with the sole
          value being the day-of-the-month of the start initial date-time.

       *  If frequency is "yearly" and no byYearDay property:

          +  if  If there are no byMonth or byWeekNo properties, and either
             there is a byMonthDay property or there is no byDay
             property: Add a byMonth property with the sole value being
             the month of the start initial date-time.

          +  if  If there is no byMonthDay, byWeekNo or byDay properties:
             Add a byMonthDay property with the sole value being the
             day-of-the-month of the start initial date-time.

          +  if  If there is a byWeekNo property and no byMonthDay or byDay
             properties: Add a byDay property with the sole value being
             the day-of-the-week of the start initial date-time.

4.3.2.

4.3.3.  recurrenceOverrides

   Type: LocalDateTime[PatchObject] "LocalDateTime[PatchObject]" (optional).

   A map of the recurrence-ids recurrence ids (the date-time of the start of produced by the
   occurrence) recurrence
   rule) to an object of patches to apply to the generated occurrence
   object.

   If the recurrence-id recurrence id does not match an expanded start date from a date-time from the recurrence rule,
   rule (or no rule is specified), it is to be treated as an additional
   occurrence (like an RDATE from iCalendar).  The patch object may
   often be empty in this case.

   If the patch object defines the "excluded" property value to be
   "true", true,
   then the recurring calendar object does not occur at the
   recurrence-id recurrence
   id date-time (like an EXDATE from iCalendar).  Such a patch object
   MUST NOT patch any other property.

   By default, an occurrence inherits all properties from the main
   object except the start (or due) date-time, which is shifted to match
   the
   new start time of the LocalDateTime key. recurrence id LocalDateTime.  However, individual properties of
   the occurrence can be modified by a patch, or multiple patches.  It
   is valid to patch the start "start" property value, and this patch takes
   precedence over the LocalDateTime key. value generated from the recurrence id.  Both the
   LocalDateTime key
   recurrence id as well as the patched start "start" date-time may occur
   before the original JSCalendar object's start "start" or due "due" date.

   A pointer in the PatchObject MUST be ignored if it starts with one of
   the following prefixes:

   o  @type

   o  uid

   o  relatedTo  method

   o  prodId

   o  method  recurrenceId

   o  recurrenceOverrides

   o  recurrenceRule

   o  recurrenceOverrides  relatedTo

   o  replyTo

4.3.3.

   o  uid

4.3.4.  excluded

   Type: Boolean "Boolean" (optional, default: "false"). false).

   Defines if this object is an overridden, excluded instance of a
   recurring JSCalendar object (also see (see Section 4.3.2). 4.3.3).  If this property
   value is "true", true, this calendar object instance MUST be removed from the
   occurrence expansion.  The absence of this property or its default
   value "false" false indicates that this instance MUST be
   added to included in the
   occurrence expansion.

4.4.  Sharing and scheduling properties Scheduling Properties

4.4.1.  priority

   Type: Number "Int" (optional, default: "0"). 0).

   Specifies a priority for the calendar object.  This may be used as
   part of scheduling systems to help resolve conflicts for a time
   period.

   The priority is specified as an integer in the range 0 to 9.  A value
   of 0 specifies an undefined priority.  A value of 1 is the highest
   priority.  A value of 2 is the second highest priority.  Subsequent
   numbers specify a decreasing ordinal priority.  A value of 9 is the
   lowest priority.  Other integer values are reserved for future use.

4.4.2.  freeBusyStatus

   Type: String "String" (optional, default: "busy").

   Specifies how this property should be treated when calculating free-
   busy state.  The value MUST be one of:

   o  ""free"":  "free": The object should be ignored when calculating whether the
      user is busy.

   o  ""busy"":  "busy": The object should be included when calculating whether the
      user is busy.

4.4.3.  privacy

   Type: String "String" (optional, default: "public").

   Calendar objects are normally collected together and may be shared
   with other users.  The privacy property allows the object owner to
   indicate that it should not be shared, or should only have the time
   information shared but the details withheld.  Enforcement of the
   restrictions indicated by this property are up to the
   implementations. API via which
   this object is accessed.

   This property MUST NOT affect the information sent to scheduled
   participants; it is only interpreted when the object is shared as
   part of a shared calendar.

   The value MUST be either one of the following values, registered in a
   future RFC, or a vendor-specific value.  Vendor specific values MUST
   be prefixed with a domain name controlled by the vendor, e.g.
   "example.com/topsecret".  Any value the client or server doesn't
   understand should be preserved but treated as equivalent to
   "private".

   o  "public": The full details of the object are visible to those whom
      the object's calendar is shared with.

   o  "private": The details of the object are hidden; only the basic
      time and metadata is shared.  The following properties MAY be
      shared, any other properties MUST NOT be shared:

      *  @type

      *  created
      *  due

      *  duration

      *  estimatedDuration

      *  freeBusyStatus

      *  privacy

      *  recurrenceOverrides.  Only patches whose keys are prefixed with
         one of the above properties which apply to another
         permissible property are allowed to be shared.

      *  sequence

      *  showWithoutTime

      *  start

      *  timeZone

      *  timeZones

      *  uid

      *  updated

   o  "secret": The object is hidden completely (as though it did not
      exist) when the calendar this object is in is shared.

4.4.4.  replyTo

   Type: String[String] "String[String]" (optional).

   Represents methods by which participants may submit their RSVP
   response to the organizer of the calendar object.  The keys in the
   property value are the available methods and MUST only contain ASCII
   alphanumeric characters (A-Za-z0-9).  The value is a URI to use that
   method.  Future methods may be defined in future specifications; a
   calendar client MUST ignore any method it does not understand, but
   MUST preserve the method key and URI.  This property MUST be omitted
   if no method is defined (rather than an empty object).  If this
   property is set, the "participants" property of this calendar object
   MUST contain at least one participant.

   The following methods are defined:

   o  "imip": The organizer accepts an iMIP [RFC6047] response at this
      email address.  The value MUST be a "mailto:" URI.

   o  "web": Opening this URI in a web browser will provide the user
      with a page where they can submit a reply to the organizer.

   o  "other": The organizer is identified by this URI but the method
      how to submit the RSVP is undefined.

4.4.5.  participants

   Type: String[Participant] "Id[Participant]" (optional).

   A map of participant identifiers ids to participants, describing their
   participation in the calendar object.

   If this property is set, then the "replyTo" property of this calendar
   object MUST define at least one reply method.

   A Participant object has the following properties:

   o  name: String (optional). "String" (optional)

      The display name of the participant (e.g.  "Joe Bloggs").

   o  email: String (optional). "String" (optional)

      The email address for the participant.

   o  sendTo: String[String]. "String[String]" (mandatory)

      Represents methods by which the participant may receive the
      invitation and updates to the calendar object.

      The keys in the property value are the available methods and MUST
      only contain ASCII alphanumeric characters (A-Za-z0-9).  The value
      is a URI to use that method.  Future methods may be defined in
      future specifications; a calendar client MUST ignore any method it
      does not understand, but MUST preserve the method key and URI.
      This property MUST be omitted if no method is defined (rather than
      an empty object).

      The following methods are defined:

      *  "imip": The participant accepts an iMIP [RFC6047] request at
         this email address.  The value MUST be a "mailto:" URI.  It MAY
         be different from the value of the participant's "email"
         property.

      *  "other": The participant is identified by this URI but the
         method how to submit the invitation or update is undefined.

   o  kind: String (optional). "String" (optional)

      What kind of entity this participant is, if known.

      This MUST be either one of the following values, registered in a
      future RFC, or a vendor-specific value.  Any value the client or
      server doesn't understand should be treated the same as if this
      property is omitted.

      *  "individual": a single person

      *  "group": a collection of people invited as a whole

      *  "resource": a non-human resource, e.g. a projector

      *  "location": a physical location involved in the calendar object
         that needs to be scheduled, e.g. a conference room.

   o  roles: String[Boolean]. "String[Boolean]" (mandatory)

      A set of roles that this participant fulfills.

      At least one role MUST be specified for the participant.  The keys
      in the set MUST be either one of the following values, registered
      in a future RFC, or a vendor-specific value:

      *  "owner": The participant is an owner of the object.  This
         signifies they have permission to make changes to it that
         affect the other participants.  Non-owner participants may only
         change properties that just affect themself (for example
         setting their own alerts).

      *  "attendee": The participant is an attendee of the calendar
         object.

      *  "chair": The participant is in charge of the calendar object
         when it occurs.

      The value for each key in the set MUST be "true". true.  Roles that are
      unknown to the implementation MUST be preserved and MAY be
      ignored. preserved.

   o  locationId: String (optional). "String" (optional)

      The location at which this participant is expected to be
      attending.

      If the value does not correspond to any location id in the
      "locations" property of the instance, JSCalendar object, this MUST be
      treated the same as if the participant's locationId were omitted.

   o  participationStatus: String "String" (optional, default: "needs-action"). "needs-action")

      The participation status, if any, of this participant.

      The value MUST be either one of the following values, registered
      in a future RFC, or a vendor-specific value:

      *  "needs-action": No status yet set by the participant.

      *  "accepted": The invited participant will participate.

      *  "declined": The invited participant will not participate.

      *  "tentative": The invited participant may participate.

   o  participationComment: "String" (optional)

      A note from the participant to explain their participation status.

   o  attendance: String "String" (optional, default: "required"). "required")

      The required attendance of this participant.

      The value MUST be either one of the following values, registered
      in a future RFC, or a vendor-specific value.  Any value the client
      or server doesn't understand should be treated the same as
      "required".

      *  "none": Indicates a participant who is copied for information
         purposes only.

      *  "optional": Indicates a participant whose attendance is
         optional.

      *  "required": Indicates a participant whose attendance is
         required.

   o  expectReply: Boolean "Boolean" (optional, default: "false"). false)

      If true, the organizer is expecting the participant to notify them
      of their participation status.

   o  scheduleSequence: Number "UnsignedInt" (optional, default: "0"). 0)
      The sequence number of the last response from the participant.  If
      defined, this MUST be a non-negative integer.

      This can be used to determine whether the participant has sent a
      new RSVP following significant changes to the calendar object, and
      to determine if future responses are responding to a current or
      older view of the data.

   o  scheduleUpdated: UTCDateTime (optional). "UTCDateTime" (optional)

      The "updated" property of the last iMIP iTIP response from the
      participant.

      This can be compared to the "updated" property timestamp in future
      iMIP
      iTIP responses to determine if the response is older or newer than
      the current data.

   o  invitedBy: String (optional). "String" (optional)

      The participant id of the participant who invited this one, if
      known.

   o  delegatedTo: String[Boolean] (optional). "String[Boolean]" (optional)

      A set of participant ids that this participant has delegated their
      participation to.  Each key in the set MUST be the identifier id of a
      participant.  The value for each key in the set MUST be "true". true.
      This MUST be omitted if none (rather than an empty set).

   o  delegatedFrom: String[Boolean] (optional). "String[Boolean]" (optional)

      A set of participant ids that this participant is acting as a
      delegate for.  Each key in the set MUST be the identifier id of a
      participant.  The value for each key in the set MUST be "true". true.
      This MUST be omitted if none (rather than an empty set).

   o  memberOf: String[Boolean] (optional). "String[Boolean]" (optional)

      A set of group participants that were invited to this calendar
      object, which caused this participant to be invited due to their
      membership of the group(s).  Each key in the set MUST be the identifier id of
      a participant.  The value for each key in the set MUST be "true". true.
      This MUST be omitted if none (rather than an empty set).

   o  linkIds: String[Boolean] (optional). "String[Boolean]" (optional)

      A set of links to more information about this participant, for
      example in vCard format.  The keys in the set MUST be the identifier id of a
      Link object in the calendar object's "links" property.  The value
      for each key in the set MUST be "true". true.  This MUST be omitted if
      none (rather than an empty set).

4.5.  Alerts properties Properties

4.5.1.  useDefaultAlerts

   Type: Boolean "Boolean" (optional, default: "false"). false).

   If "true", true, use the user's default alerts and ignore the value of the
   "alerts" property.  Fetching user defaults is dependent on the API
   from which this JSCalendar object is being fetched, and is not
   defined in this specification.  If an implementation cannot determine
   the user's default alerts, or none are set, it MUST process the
   alerts property as if useDefaultAlerts "useDefaultAlerts" is set to "false". false.

4.5.2.  alerts

   Type: String[Alert] "Id[Alert]" (optional).

   A map of alert identifiers ids to Alert objects, representing alerts/
   reminders alerts/reminders to
   display or send to the user for this calendar object.

   An Alert Object has the following properties:

   o  trigger: OffsetTrigger|UnknownTrigger. "OffsetTrigger|AbsoluteTrigger|UnknownTrigger"

      Defines when to trigger the alert.  New types may be defined in
      future RFCs.

      An *OffsetTrigger* object has the following properties:

      *  type: String (mandatory). "String" (mandatory)

         The value of this property MUST be "offset".

      *  offset: SignedDuration "SignedDuration" (mandatory).

         Defines the offset at which to trigger the alert relative to
         the time property defined in the "relativeTo"
         property. property of the
         alert.  If the calendar object does not define a time zone, the
         user's default time zone SHOULD be used when determining the
         offset, if known.  Otherwise, the time zone to use is
         implementation specific.

      *  relativeTo: String "String" (optional, default: "start"). "start")
         Specifies the time property which that the alert offset is relative
         to.  The value MUST be one of:

         +  "start": triggers the alert relative to the start of the
            calendar object

         +  "end": triggers the alert relative to the end/due time of
            the calendar object

      An *AbsoluteTrigger* object has the following properties:

      *  type: "String" (mandatory)

         The value of this property MUST be "absolute".

      *  when: "UTCDateTime" (mandatory).

         Defines a specific UTC date-time when the alert is triggered.

      An *UnknownTrigger* object is an object that contains a *type*
      property whose value is not "offset", recognized (i.e., not "offset" or
      "absolute"), plus zero or more other properties.  This is for
      compatibility with client extensions and future RFCs.
      Implementations SHOULD NOT trigger for trigger types they do not
      understand, but MUST preserve them.

   o  acknowledged: UTCDateTime (optional).

      When "UTCDateTime" (optional)

      This records when an alert was last acknowledged.  This is set
      when the user has permanently dismissed the alert the client MUST
      set this to the current time in UTC.  Other alert; other clients which that sync
      this property can then SHOULD automatically dismiss or suppress duplicate
      alerts (alerts with the same alert id that triggered on or before
      this date-time).

      For a recurring calendar object, the "acknowledged" property of
      the parent object MUST be updated, unless the alert is already
      overridden in the "recurrenceOverrides" property.

   o  snoozed: UTCDateTime (optional).

      If the user temporarily dismisses the alert, this is the UTC date-
      time after which it should trigger again.  Setting this property
      on an instance of a recurring calendar object MUST update object, the
      alarm on "acknowledged" property of
      the top-level object, parent object MUST be updated, unless the respective instance
      already alert is defined already
      overridden in "recurrenceOverrides".  It MUST NOT generate
      an override for the sole use "recurrenceOverrides" property.

      Certain kinds of snoozing an alarm. alert action may not provide feedback as to when
      the user sees them, for example email based alerts.  For those
      kinds of alerts, this property SHOULD be set immediately when the
      alert is triggered and the action successfully carried out.

   o  action: String "String" (optional, default: "display"). "display")

      Describes how to alert the user.

      The value MUST be at most one of the following values, registered
      in a future RFC, or a vendor-specific value:

      *  "display": The alert should be displayed as appropriate for the
         current device and user context.

      *  "email": The alert should trigger an email sent out to the
         user, notifying about the alert.  This action is typically only
         appropriate for server implementations.

4.6.  Multilingual properties Properties

4.6.1.  localizations

   Type: String[PatchObject] "String[PatchObject]" (optional).

   A map of [RFC5646] language tags to patch objects, which localize the
   calendar object into the locale of the respective language tag.

   See the description of PatchObject (Section 3.2.4) 1.4.8) for the structure
   of the PatchObject.  The patches are applied to the top-level
   calendar object.  In addition addition, the "locale" property of the patched
   object is set to all the restrictions on language tag.  All pointers for patches specified there, the
   pointer also MUST NOT start end
   with one of the following prefixes; suffixes; any patch with a such a key that does not follow
   this MUST be ignored:

   o  @type

   o  due

   o  duration

   o  freeBusyStatus

   o  localization

   o  method

   o  participants

   o  prodId

   o  progress

   o  relatedTo
   o  sequence

   o  start

   o  status ignored unless otherwise specified in a future RFC:

   o  timeZone  title

   o  uid  description

   o  useDefaultAlerts  name

   For example, a patch to "recurrenceOverrides/2018-01-
   05T14:00:00/locations/abcd1234/title" is permissible, but a patch to
   "uid" is not.

   Note that this specification does not define how to maintain validity
   of localized content.  For example, a client application changing a
   JSCalendar object's title property might also need to update any
   localizations of this property.  Client implementations SHOULD
   provide the means to manage localizations, but how to achieve this is
   specific to the application's workflow and requirements.

4.7.  Time zone properties Zone Properties
4.7.1.  timeZone

   Type: "String|null" (optional, default: null).

   Identifies the time zone the object is scheduled in, or null for
   floating time.  This is either a name from the IANA Time Zone
   Database [4] or the id of a custom time zone from the "timeZones"
   property (see Section 1.4.9).  If omitted, this MUST be presumed to
   be null (i.e., floating time).

4.7.2.  timeZones

   Type: String[TimeZone] "String[TimeZone]" (optional).

   Maps identifiers of custom time zones to their time zone definition.
   The following restrictions apply for each key in the map:

   o  It MUST start with the "/" character (ASCII decimal 47; also see
      sections
      Sections 3.2.19 of [RFC5545] and 3.6. of [RFC7808] for discussion
      of the forward slash character in time zone identifiers).

   o  It MUST be a valid "paramtext" value as specified in section Section 3.1.
      of [RFC5545].

   o  At least one other property in the same JSCalendar object MUST
      reference a time zone using this identifier (i.e. orphaned time
      zones are not allowed).

   An identifier need only be unique to this JSCalendar object.

   A TimeZone object maps a VTIMEZONE component from iCalendar
   ([RFC5545]). [RFC5545]
   and the semantics are as defined there.  A valid time zone MUST
   define at least one transition rule in the "standard" or "daylight"
   property.  Its properties are:

   o  tzId: String "String" (mandatory).

      The TZID property from iCalendar.

   o  lastModified: UTCDateTime (optional). "UTCDateTime" (optional)

      The LAST-MODIFIED property from iCalendar.

   o  url: String (optional). "String" (optional)

      The TZURL property from iCalendar.

   o  validUntil: UTCDateTime (optional). "UTCDateTime" (optional)
      The TZUNTIL property from iCalendar specified in [RFC7808].

   o  aliases: String[Boolean] (optional). "String[Boolean]" (optional)

      Maps the TZID-ALIAS-OF properties from iCalendar specified in
      [RFC7808] to a JSON set of aliases.  The set is represented as an
      object, with the keys being the aliases.  The value for each key
      in the set MUST be "true". true.

   o  standard: TimeZoneRule[] (optional). "TimeZoneRule[]" (optional)

      The STANDARD sub-components from iCalendar.  The order MUST be
      preserved during conversion.

   o  daylight: TimeZoneRule[] "TimeZoneRule[]" (optional).

      The DAYLIGHT sub-components from iCalendar.  The order MUST be
      preserved during conversion.

   A TimeZoneRule object maps a STANDARD or DAYLIGHT sub-component from
   iCalendar, with the restriction that at most one recurrence rule is
   allowed per rule.  It has the following properties:

   o  start: LocalDateTime (mandatory). "LocalDateTime" (mandatory)

      The DTSTART property from iCalendar.

   o  offsetTo: String (mandatory). "String" (mandatory)

      The TZOFFSETTO property from iCalendar.

   o  offsetFrom: String (mandatory). "String" (mandatory)

      The TZOFFSETFROM property from iCalendar.

   o  recurrenceRule: RecurrenceRule (optional). "RecurrenceRule" (optional)

      The RRULE property mapped as specified in Section 4.3.1. 4.3.2.  During
      recurrence rule evaluation, the "until" property value MUST be
      interpreted as a local time in the UTC time zone.

   o  recurrenceDates: LocalDateTime[Boolean] (optional). "LocalDateTime[Boolean]" (optional)

      Maps the RDATE properties from iCalendar to a JSON set.  The set
      is represented as an object, with the keys being the recurrence
      dates.  The value for each key in the set MUST be "true". true.

   o  names: String[Boolean] (optional). "String[Boolean]" (optional)
      Maps the TZNAME properties from iCalendar to a JSON set.  The set
      is represented as an object, with the keys being the names.  The
      value for each key in the set MUST be "true". true.

   o  comments: String[] "String[]" (optional).  Maps the COMMENT properties from
      iCalendar.  The order MUST be preserved during conversion.

5.  Type-specific JSCalendar properties Properties

5.1.  JSEvent properties Properties

   In addition to the common JSCalendar object properties (Section 4) a
   JSEvent has the following properties:

5.1.1.  start

   Type: LocalDateTime (mandatory).

   The date/time the event would start in the event's time zone.

5.1.2.  timeZone

   Type: String|null (optional, default: "null").

   Identifies "LocalDateTime" (mandatory).

   The date/time the event starts in the event's time zone (as specified
   in the event is scheduled in, or "null" for
   floating time.  If omitted, this MUST be presumed to be "null" (i.e.
   floating time).  Also "timeZone" property, see Section 3.2.6.

5.1.3. 4.7.1).

5.1.2.  duration

   Type: Duration "Duration" (optional, default: "PT0S").

   The zero or positive duration of the event in the event's start time
   zone.  The same rules as for the iCalendar DURATION value type
   ([RFC5545]) apply: The duration of

   Note that a week duration specified using weeks or a day in hours/minutes/
   seconds days does not always
   correspond to an exact multiple of 24 hours.  The number of
   hours/minutes/seconds may vary if it overlaps a period of
   discontinuity in the event's time zone, for example a change from
   standard time to daylight-savings time.  Leap seconds MUST NOT be
   considered when computing an exact duration.  When computing an exact
   duration, the greatest order time components MUST be added first,
   that is, the number of days MUST be added first, followed by the
   number of hours, number of minutes, and number of seconds.
   Fractional seconds MUST be added last.  These semantics match the
   iCalendar DURATION value type ([RFC5545], Section 3.3.6).

   A JSEvent MAY involve start and end locations that are in different
   time zones (e.g. a trans-continental flight).  This can be expressed
   using the "relativeTo" and "timeZone" properties of the JSEvent's
   "location" objects.

5.1.4.
   Location objects (see Section 4.2.5).

5.1.3.  status

   Type: String "String" (optional, default: "confirmed").

   The scheduling status (Section 4.4) of a JSEvent.  If set, it MUST be
   one of:

   o  "confirmed": Indicates the event is definite. definitely happening.

   o  "cancelled": Indicates the event is has been cancelled.

   o  "tentative": Indicates the event is tentative. may happen.

5.2.  JSTask properties Properties

   In addition to the common JSCalendar object properties (Section 4) a
   JSTask has the following properties:

5.2.1.  due

   Type: LocalDateTime "LocalDateTime" (optional).

   The date/time the task is due in the task's time zone.

5.2.2.  start

   Type: LocalDateTime "LocalDateTime" (optional).

   The date/time the task should start in the task's time zone.

5.2.3.  timeZone

   Type: String|null (optional, default: "null").

   Identifies the time zone the task is scheduled in, or "null" for
   floating time.  If omitted, this MUST be presumed to be "null" (i.e.
   floating time).  Also see Section 3.2.6.

5.2.4.  estimatedDuration

   Type: Duration "Duration" (optional).

   Specifies the estimated positive duration of time the task takes to
   complete.

5.2.5.

5.2.4.  statusUpdatedAt

   Type: UTCDateTime "UTCDateTime" (optional).

   Specifies the date/time the "status" property of either the task status properties
   overall (Section 5.2.6) or a specific participant (Section 5.2.5) was
   last updated.

   If the task is recurring and has future instances, a client may want
   to keep track of the last status update timestamp of a specific task
   recurrence, but leave other instances unchanged.  One way to achieve
   this is by overriding the statusUpdatedAt property in the task
   "recurrenceOverrides" property.  However, this could produce a long
   list of timestamps for regularly recurring tasks.  An alternative
   approach is to split the JSTask into a current, single instance of
   JSTask with this instance status update time and a future recurring
   instance.  Also see  See also Section 4.1.3 on splitting.

5.2.6.

5.2.5.  progress

   In addition to the common properties of a Participant object
   (Section 4.4.5), a Participant within a JSTask supports the following
   property:

   o  progress: ParticipantProgress (optional).  The progress of the
      participant for this task, if known.  This property MUST NOT be
      set if the "participationStatus" of this participant is any other
      value but "accepted".

   A ParticipantProgress object has the following properties:

   o  status: String (mandatory). "String" (mandatory)

      Describes the completion status of the participant's progress.

      The value MUST be at most one of the following values, registered
      in a future RFC, or a vendor-specific value:

      *  "completed": The participant completed their this task.

      *  "in-process": The participant has started this task.

      *  "failed": The participant failed to complete their this task.

   o  timestamp: UTCDateTime (mandatory).  Describes "UTCDateTime" (mandatory)

      Represents the last time when the participant progress got was updated.

5.2.7.

5.2.6.  status

   Type: String "String" (optional).

   Defines the overall status of this task.  If omitted, the default
   status (Section 4.4) of a JSTask is defined as follows (in order of
   evaluation):

   o  "completed": if the "status" property value of all participant
      progresses is "completed".

   o  "failed": if at least one "status" property value of the
      participant progresses is "failed".

   o  "in-process": if at least one "status" property value of the
      participant progresses is "in-process".

   o  "needs-action": If none of the other criteria match.

   If set, it MUST be one of:

   o  "needs-action": Indicates the task needs action.

   o  "completed": Indicates the task is completed.

   o  "in-process": Indicates the task is in process.

   o  "cancelled": Indicates the task is cancelled.

   o  "pending": Indicates the task has been created and accepted for
      processing, but not yet started.

   o  "failed": Indicates the task failed.

5.3.  JSGroup properties Properties

   JSGroup supports the following common JSCalendar properties
   (Section 4):

   o  @type

   o  uid

   o  created

   o  updated

   o  categories

   o  keywords

   o  name  title

   o  description

   o  color

   o  links

   as well as
   In addition, the following JSGroup-specific properties: properties are supported:

5.3.1.  entries

   Type: String[JSTask|JSEvent] "String[JSTask|JSEvent]" (mandatory).

   A collection of group members.  This is represented as a map of the
   "uid" property value to the JSCalendar object member having that uid.
   Implementations MUST ignore entries of unknown type.

5.3.2.  source

   Type: String "String" (optional).

   The source from which updated versions of this group may be retrieved
   from.  The value MUST be a URI.

6.  JSCalendar object examples  Examples

   The following examples illustrate several aspects of the JSCalendar
   data model and format.  The examples may omit mandatory or additional
   properties, which is indicated by a placeholder property with key
   "...".  While most of the examples use calendar event objects, they
   are also illustrative for tasks.

6.1.  Simple event Event

   This example illustrates a simple one-time event.  It specifies a
   one-time event that begins on January 15, 2018 at 1pm New York local
   time and ends after 1 hour. at 2pm.

   {
     "@type": "jsevent",
     "uid": "2a358cee-6489-4f14-a57f-c104db4dc2f1",
     "updated": "2018-01-15T18:00:00Z", "2018-01-01T12:00:00Z",
     "title": "Some event",
     "start": "2018-01-15T13:00:00",
     "timeZone": "America/New_York",
     "duration": "PT1H"
   }

6.2.  Simple task Task

   This example illustrates a simple task for a plain to-do item.

   {
     "@type": "jstask",
     "uid": "2a358cee-6489-4f14-a57f-c104db4dc2f2",
     "updated": "2018-01-15T18:00:00Z", "2018-01-19T18:00:00Z",
     "title": "Do something"
   }

6.3.  Simple group Group

   This example illustrates a simple calendar object group that contains
   an event and a task.

   {
     "@type": "jsgroup",
     "uid": "2a358cee-6489-4f14-a57f-c104db4dc343",
     "updated": "2018-01-15T18:00:00Z",
     "name":
     "title": "A simple group",
     "entries": [
       {
         "@type": "jsevent",
         "uid": "2a358cee-6489-4f14-a57f-c104db4dc2f1",
         "updated": "2018-01-15T18:00:00Z",
         "title": "Some event",
         "start": "2018-01-15T13:00:00",
         "timeZone": "America/New_York",
         "duration": "PT1H"
       },
       {
         "@type": "jstask",
         "uid": "2a358cee-6489-4f14-a57f-c104db4dc2f2",
         "updated": "2018-01-15T18:00:00Z",
         "title": "Do something"
       }
     ]
   }

6.4.  All-day event Event

   This example illustrates an event for an international holiday.  It
   specifies an all-day event on April 1 that occurs every year since
   the year 1900.

   {
     "...": "",
     "title": "April Fool's Day",
     "showWithoutTime": true,
     "start": "1900-04-01T00:00:00",
     "duration": "P1D",
     "recurrenceRule": {
       "frequency": "yearly"
     }
   }

6.5.  Task with a due date Due Date

   This example illustrates a task with a due date.  It is a reminder to
   buy groceries before 6pm Vienna local time on January 19, 2018.  The
   calendar user expects to need 1 hour for shopping.

   {
     "...": "",
     "title": "Buy groceries",
     "due": "2018-01-19T18:00:00",
     "timeZone": "Europe/Vienna",
     "estimatedDuration": "PT1H"
   }

6.6.  Event with end time-zone End Time Zone

   This example illustrates the use of end time-zones by use of an
   international flight.  The flight starts on April 1, 2018 at 9am in
   Berlin local time.  The duration of the flight is scheduled at 10
   hours 30 minutes.  The time at the flights destination is in the same
   time-zone as Tokyo.  Calendar clients could use the end time-zone to
   display the arrival time in Tokyo local time and highlight the time-
   zone difference of the flight.  The location names can serve as input
   for navigation systems.

   {
     "...": "",
     "title": "Flight XY51 to Tokyo",
     "start": "2018-04-01T09:00:00",
     "timeZone": "Europe/Berlin",
     "duration": "PT10H30M",
     "locations": {
       "2a358cee-6489-4f14-a57f-c104db4dc2f1": {
         "rel": "start",
         "name": "Frankfurt Airport (FRA)"
       },
       "c2c7ac67-dc13-411e-a7d4-0780fb61fb08": {
         "rel": "end",
         "name": "Narita International Airport (NRT)",
         "timeZone": "Asia/Tokyo"
       }
     }
   }

6.7.  Floating-time event Event (with recurrence) Recurrence)

   This example illustrates the use of floating-time.  Since January 1,
   2018, a calendar user blocks 30 minutes every day to practice Yoga at
   7am local time, in whatever time-zone the user is located on that
   date.

   {
     "...": "",
     "title": "Yoga",
     "start": "2018-01-01T07:00:00",
     "duration": "PT30M",
     "recurrenceRule": {
       "frequency": "daily"
     }
   }

6.8.  Event with multiple locations Multiple Locations and localization Localization

   This example illustrates an event that happens at both a physical and
   a virtual location.  Fans can see a live convert on premises or
   online.  The event title and descriptions are localized.

   {
     "...": "",
     "title": "Live from Music Bowl: The Band",
     "description": "Go see the biggest music event ever!",
     "locale": "en",
     "start": "2018-07-04T17:00:00",
     "timeZone": "America/New_York",
     "duration": "PT3H",
     "locations": {
       "c0503d30-8c50-4372-87b5-7657e8e0fedd": {
         "name": "The Music Bowl",
         "description": "Music Bowl, Central Park, New York",
         "coordinates": "geo:40.7829,73.9654"
       }
     },
     "virtualLocations": {
       "6f3696c6-1e07-47d0-9ce1-f50014b0041a": {
         "name": "Free live Stream from Music Bowl",
         "uri": "https://stream.example.com/the_band_2018"
       }
     },
     "localizations": {
       "de": {
         "title": "Live von der Music Bowl: The Band!",
         "description": "Schau dir das groesste Musikereignis an!",
         "virtualLocations/6f3696c6-1e07-47d0-9ce1-f50014b0041a/name":
                                 "Gratis Live-Stream aus der Music Bowl"
       }
     }
   }

6.9.  Recurring event Event with overrides Overrides

   This example illustrates the use of recurrence overrides.  A math
   course at a University is held for the first time on January 8, 2018
   at 9am London time and occurs every week until June 25, 2018. 2018
   (inclusive).  Each lecture lasts for one hour and 30 thirty minutes and
   is located at the Mathematics department.  This event has exceptional
   occurrences: at the last occurrence of the course is an exam, which
   lasts for 2 hours and starts at 10am.  Also, the location of the exam
   differs from the usual location.  On April 2 no course is held.  On
   January 5 at 2pm is an optional introduction course, that occurs
   before the first regular lecture.

{
  "...": "",
  "title": "Calculus I",
  "start": "2018-01-08T09:00:00",
  "timeZone": "Europe/London",
  "duration": "PT1H30M",
  "locations": {
    "2a358cee-6489-4f14-a57f-c104db4dc2f1": {
      "title": "Math lab room 1",
      "description": "Math Lab I, Department of Mathematics" Mathematics\n1 University Drive"
    }
  },
  "recurrenceRule": {
    "frequency": "weekly",
    "until": "2018-06-25T09:00:00"
  },
  "recurrenceOverrides": {
    "2018-01-05T14:00:00": {
      "title": "Introduction to Calculus I (optional)"
    },
    "2018-04-02T09:00:00": {
      "excluded": "true" true
    },
    "2018-06-25T09:00:00": {
      "title": "Calculus I Exam",
      "start": "2018-06-25T10:00:00",
      "duration": "PT2H",
      "locations": {
        "2a358cee-6489-4f14-a57f-c104db4dc2f1": {
          "title": "Big Auditorium",
          "description": "Big Auditorium, Other Road"
        }
      }
    }
  }
}

6.10.  Recurring event Event with participants Participants

   This example illustrates scheduled events.  A team meeting occurs
   every week since January 8, 2018 at 9am Johannesburg time.  The event
   owner also chairs the event.  Participants meet in a virtual meeting
   room.  An attendee has accepted the invitation, but on March 8, 2018
   he is unavailable and declined participation for this occurrence. occurrence,
   leaving a short explanation for the organizer.

  {
    "...": "",
    "title": "FooBar team meeting",
    "start": "2018-01-08T09:00:00",
    "timeZone": "Africa/Johannesburg",
    "duration": "PT1H",
    "virtualLocations": {
      "2a358cee-6489-4f14-a57f-c104db4dc2f1": {
        "name": "ChatMe meeting room",
        "uri": "https://chatme.example.com?id=1234567"
      }
    },
    "recurrenceRule": {
      "frequency": "weekly"
    },
    "replyTo": {
      "imip": "mailto:6489-4f14-a57f-c1@schedule.example.com"
    },
    "participants": {
      "dG9tQGZvb2Jhci5leGFtcGxlLmNvbQ": {
        "name": "Tom Tool",
        "email": "tom@foobar.example.com",
        "sendTo": {
          "imip": "mailto:6489-4f14-a57f-c1@calendar.example.com" "mailto:41f3-8b10-516a4d0f42bc@calendar.example.com"
        },
        "participationStatus": "accepted",
        "roles": {
          "attendee": true
        }
      },
      "em9lQGZvb2Jhci5leGFtcGxlLmNvbQ": {
        "name": "Zoe Zelda",
        "email": "zoe@foobar.example.com",
        "sendTo": {
          "imip": "mailto:zoe@foobar.example.com"
        },
        "participationStatus": "accepted",
        "roles": {
          "owner": true,
          "attendee": true,
          "chair": true
        }
      },
      "...": ""
    },
    "recurrenceOverrides": {
     "2018-03-08T09:00:00": {
    "participants/dG9tQGZvb2Jhci5leGFtcGxlLmNvbQ/participationStatus":
                                                              "declined"
                                                             "declined",
    "participants/dG9tQGZvb2Jhci5leGFtcGxlLmNvbQ/participationComment":

                        "Sorry, kid's recital this week, can't make it."
      }
    }
  }

7.  Security Considerations

   The

   Calendaring and scheduling information is very privacy-sensitive.
   The transmission of such information must be careful to protect it
   from possible threats, such as eavesdropping, replay, message
   insertion, deletion, modification, and man-in-the-middle attacks.
   This document just defines the data format; such considerations are
   primarily the concern of the API or method of storage and
   transmission of such files.

7.1.  Expanding Recurrences

   A recurrence rule may produce infinite occurrences of an event.
   Implementations MUST handle expansions carefully to prevent
   accidental or deliberate resource exhaustion.

   Conversely, a recurrence rule may be specified that does not expand
   to anything.  It is not always possible to tell this through static
   analysis of the rule, so implementations MUST be careful to avoid
   getting stuck in an infinite loop, or otherwise exhausting resources,
   searching for the next occurrence.

7.2.  JSON Parsing

   The Security Considerations of [RFC8259] apply to the use of JSON as a format does have its own inherent security risks
   as discussed in Section 12
   the data interchange format.

   As for any serialization format, parsers need to thoroughly check the
   syntax of [RFC8259].  Even though the supplied data.  JSON uses opening and closing tags for
   several types and structures, and it is
   considered a safe subset possible that the end of JavaScript, it should the
   supplied data will be kept in mind
   that reached when scanning for a flaw in matching closing
   tag; this is an error condition, and implementations need to stop
   scanning at the parser processing end of the supplied data.

   JSON could still impose also uses a
   threat, which doesn't arise string encoding with conventional iCalendar data.

   With this in mind, some escape sequences to encode
   special characters within a parser for JSON data aware of string.  Care is needed when processing
   these escape sequences to ensure that they are fully formed before
   the security
   implications should be used for special processing is triggered, with special care taken when the format described
   escape sequences appear adjacent to other (non-escaped) special
   characters or adjacent to the end of data (as in this
   document.  For example, the previous
   paragraph).

   If parsing JSON into a non-textual structured data format,
   implementations may need to allocate storage to hold JSON string
   elements.  Since JSON does not use explicit string lengths, the risk
   of JavaScript's "eval()" function is
   considered an unacceptable security risk, as described in Section 12
   of[RFC8259].  A native parser with full awareness denial of service due to resource exhaustion is small, but
   implementations may still wish to place limits on the JSON format
   should be preferred. size of
   allocations they are willing to make in any given context, to avoid
   untrusted data causing excessive memory allocation.

7.3.  URI Values

   Several JSCalendar properties contain URIs as values, and processing
   these properties requires extra care.  Section 7 of [RFC3986]
   discusses security risk related to URIs.

8.  IANA Considerations

   This document defines a MIME media type for use with JSCalendar data
   formatted in JSON.

   Type name:  application

   Subtype name:  jscalendar+json

   Required parameters:  type

      The "type" parameter conveys the type of the JSCalendar data in
      the body part, with the value being one of "jsevent", "jstask", or
      "jsgroup".  The parameter MUST NOT occur more than once.  It MUST
      match the value of the "@type" property of the JSON-formatted
      JSCalendar object in the body.

   Optional parameters:  none

   Encoding considerations:  Same as encoding considerations of
      application/json as specified in RFC8529, Section 11 [RFC8259].

   Security considerations:  See Section 7 of this document.

   Interoperability considerations:  This media type provides an
      alternative to iCalendar, jCal and proprietary JSON-based
      calendaring data formats.

   Published specification:  This specification.

   Applications that use this media type:  Applications that currently
      make use of the text/calendar and application/calendar+json media
      types can use this as an alternative.  Similarly, applications
      that use the application/json media type to transfer calendaring
      data can use this to further specify the content.

   Fragment identifier considerations:  N/A

   Additional information:

      Magic number(s):  N/A

      File extensions(s):  N/A

      Macintosh file type code(s):  N/A

   Person & email address to contact for further information:
      calext@ietf.org

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  See the "Author's Address" section of this document.

   Change controller:  IETF

9.  Acknowledgments

   The authors would like to thank the members of CalConnect for their
   valuable contributions.  This specification originated from the work
   of the API technical committee of CalConnect, the Calendaring and
   Scheduling Consortium.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2392]  Levinson, E., "Content-ID and Message-ID Uniform Resource
              Locators", RFC 2392, DOI 10.17487/RFC2392, August 1998,
              <https://www.rfc-editor.org/info/rfc2392>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <https://www.rfc-editor.org/info/rfc4122>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC4791]  Daboo, C., Desruisseaux, B., and L. Dusseault,
              "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
              DOI 10.17487/RFC4791, March 2007,
              <https://www.rfc-editor.org/info/rfc4791>.

   [RFC5545]  Desruisseaux, B., Ed., "Internet Calendaring and
              Scheduling Core Object Specification (iCalendar)",
              RFC 5545, DOI 10.17487/RFC5545, September 2009,
              <https://www.rfc-editor.org/info/rfc5545>.

   [RFC5546]  Daboo, C., Ed., "iCalendar Transport-Independent
              Interoperability Protocol (iTIP)", RFC 5546,
              DOI 10.17487/RFC5546, December 2009,
              <https://www.rfc-editor.org/info/rfc5546>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.

   [RFC5870]  Mayrhofer, A. and C. Spanring, "A Uniform Resource
              Identifier for Geographic Locations ('geo' URI)",
              RFC 5870, DOI 10.17487/RFC5870, June 2010,
              <https://www.rfc-editor.org/info/rfc5870>.

   [RFC6047]  Melnikov, A., Ed., "iCalendar Message-Based
              Interoperability Protocol (iMIP)", RFC 6047,
              DOI 10.17487/RFC6047, December 2010,
              <https://www.rfc-editor.org/info/rfc6047>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC6901]  Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
              "JavaScript Object Notation (JSON) Pointer", RFC 6901,
              DOI 10.17487/RFC6901, April 2013,
              <https://www.rfc-editor.org/info/rfc6901>.

   [RFC7265]  Kewisch, P., Daboo, C., and M. Douglass, "jCal: The JSON
              Format for iCalendar", RFC 7265, DOI 10.17487/RFC7265, May
              2014, <https://www.rfc-editor.org/info/rfc7265>.

   [RFC7493]  Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
              DOI 10.17487/RFC7493, March 2015,
              <https://www.rfc-editor.org/info/rfc7493>.

   [RFC7529]  Daboo, C. and G. Yakushev, "Non-Gregorian Recurrence Rules
              in the Internet Calendaring and Scheduling Core Object
              Specification (iCalendar)", RFC 7529,
              DOI 10.17487/RFC7529, May 2015,
              <https://www.rfc-editor.org/info/rfc7529>.

   [RFC7808]  Douglass, M. and C. Daboo, "Time Zone Data Distribution
              Service", RFC 7808, DOI 10.17487/RFC7808, March 2016,
              <https://www.rfc-editor.org/info/rfc7808>.

   [RFC7986]  Daboo, C., "New Properties for iCalendar", RFC 7986,
              DOI 10.17487/RFC7986, October 2016,
              <https://www.rfc-editor.org/info/rfc7986>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

10.2.  Informative References

   [MIME]     "IANA Media Types", <https://www.iana.org/assignments/
              media-types/media-types.xhtml>.

10.3.  URIs

   [1] https://www.iana.org/time-zones

   [2] https://www.iana.org/assignments/link-relations/link-
       relations.xhtml

   [3] https://www.w3.org/TR/2011/REC-css3-color-20110607/#svg-color

   [4] https://www.iana.org/time-zones

Authors' Addresses

   Neil Jenkins
   FastMail
   Fastmail
   PO Box 234
   Collins St West
   Melbourne  VIC 8007
   Australia

   Email: neilj@fastmailteam.com
   URI:   https://www.fastmail.com

   Robert Stepanek
   FastMail
   Fastmail
   PO Box 234
   Collins St West
   Melbourne  VIC 8007
   Australia

   Email: rsto@fastmailteam.com
   URI:   https://www.fastmail.com