Calendaring extensions                                        N. Jenkins
Internet-Draft                                               R. Stepanek
Intended status: Standards Track                                FastMail
Expires: December 19, 26, 2019                                 June 17, 24, 2019

           JSCalendar: A JSON representation of calendar data
                    draft-ietf-calext-jscalendar-15
                    draft-ietf-calext-jscalendar-16

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 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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   This Internet-Draft will expire on December 19, 26, 2019.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Relation to the iCalendar format  . . . . . . . . . . . .   4
     1.2.  Relation to the jCal format . . . . . . . . . . . . . . .   5
     1.3.  Notational Conventions  . . . . . . . . . . . . . . . . .   5
   2.  JSCalendar objects  . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  JSEvent . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  JSTask  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  JSGroup . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Structure of JSCalendar objects . . . . . . . . . . . . . . .   6
     3.1.  Type signatures . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Data Types  . . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  UTCDate . . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.2.  LocalDate . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.3.  Duration  . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.4.  PatchObject . . . . . . . . . . . . . . . . . . . . .   8
       3.2.5.  Identifiers . . . . . . . . . . . . . . . . . . . . .   9
       3.2.6.  Time Zones  . . . . . . . . . . . . . . . . . . . . .   9
       3.2.7.  Normalization and equivalence . . . . . . . . . . . .   9
     3.3.  Custom property extensions and values . . . . . . . . . .  10
   4.  Common JSCalendar properties  . . . . . . . . . . . . . . . .  10
     4.1.  Metadata properties . . . . . . . . . . . . . . . . . . .  10
       4.1.1.  @type . . . . . . . . . . . . . . . . . . . . . . . .  10
       4.1.2.  uid . . . . . . . . . . . . . . . . . . . . . . . . .  11
       4.1.3.  relatedTo . . . . . . . . . . . . . . . . . . . . . .  11
       4.1.4.  prodId  . . . . . . . . . . . . . . . . . . . . . . .  12
       4.1.5.  created . . . . . . . . . . . . . . . . . . . . . . .  12
       4.1.6.  updated . . . . . . . . . . . . . . . . . . . . . . .  12
       4.1.7.  sequence  . . . . . . . . . . . . . . . . . . . . . .  12
       4.1.8.  method  . . . . . . . . . . . . . . . . . . . . . . .  13
     4.2.  What and where properties . . . . . . . . . . . . . . . .  13
       4.2.1.  title . . . . . . . . . . . . . . . . . . . . . . . .  13
       4.2.2.  description . . . . . . . . . . . . . . . . . . . . .  13
       4.2.3.  descriptionContentType  . . . . . . . . . . . . . . .  13
       4.2.4.  locations  showWithoutTime . . . . . . . . . . . . . . . . . . .  13
       4.2.5.  locations . . . .  13
       4.2.5.  virtualLocations . . . . . . . . . . . . . . . . . .  14
       4.2.6.  virtualLocations  . . . . . . . . . . . . . . . . . .  15
       4.2.7.  links . . . . . . . . . . . . . . . . . . . . . . . .  15
       4.2.7.
       4.2.8.  locale  . . . . . . . . . . . . . . . . . . . . . . .  16
       4.2.8.
       4.2.9.  keywords  . . . . . . . . . . . . . . . . . . . . . .  16
       4.2.9.  17
       4.2.10. categories  . . . . . . . . . . . . . . . . . . . . .  16
       4.2.10.  17
       4.2.11. color . . . . . . . . . . . . . . . . . . . . . . . .  17
     4.3.  Recurrence properties . . . . . . . . . . . . . . . . . .  17
       4.3.1.  recurrenceRule  . . . . . . . . . . . . . . . . . . .  17
       4.3.2.  recurrenceOverrides . . . . . . . . . . . . . . . . .  23
       4.3.3.  excluded  . . . . . . . . . . . . . . . . . . . . . .  24
     4.4.  Sharing and scheduling properties . . . . . . . . . . . .  24
       4.4.1.  priority  . . . . . . . . . . . . . . . . . . . . . .  24
       4.4.2.  freeBusyStatus  . . . . . . . . . . . . . . . . . . .  24  25
       4.4.3.  privacy . . . . . . . . . . . . . . . . . . . . . . .  25
       4.4.4.  replyTo . . . . . . . . . . . . . . . . . . . . . . .  26
       4.4.5.  participants  . . . . . . . . . . . . . . . . . . . .  26  27
     4.5.  Alerts properties . . . . . . . . . . . . . . . . . . . .  30
       4.5.1.  useDefaultAlerts  . . . . . . . . . . . . . . . . . .  30
       4.5.2.  alerts  . . . . . . . . . . . . . . . . . . . . . . .  30
     4.6.  Multilingual properties . . . . . . . . . . . . . . . . .  32
       4.6.1.  localizations . . . . . . . . . . . . . . . . . . . .  32
     4.7.  Time zone properties  . . . . . . . . . . . . . . . . . .  33
       4.7.1.  timeZones . . . . . . . . . . . . . . . . . . . . . .  33
   5.  Type-specific JSCalendar properties . . . . . . . . . . . . .  34  35
     5.1.  JSEvent properties  . . . . . . . . . . . . . . . . . . .  34  35
       5.1.1.  start . . . . . . . . . . . . . . . . . . . . . . . .  35
       5.1.2.  timeZone  . . . . . . . . . . . . . . . . . . . . . .  35
       5.1.3.  duration  . . . . . . . . . . . . . . . . . . . . . .  35
       5.1.4.  isAllDay  . . . . . . . . . . . . . . . . . . . . . .  35
       5.1.5.  status  . . . . . . . . . . . . . . . . . . . . . . .  36
     5.2.  JSTask properties . . . . . . . . . . . . . . . . . . . .  36
       5.2.1.  due . . . . . . . . . . . . . . . . . . . . . . . . .  36
       5.2.2.  start . . . . . . . . . . . . . . . . . . . . . . . .  36
       5.2.3.  timeZone  . . . . . . . . . . . . . . . . . . . . . .  36
       5.2.4.  estimatedDuration . . . . . . . . . . . . . . . . . .  36
       5.2.5.  statusUpdatedAt . . . . . . . . . . . . . . . . . . .  37
       5.2.6.  progress  . . . . . . . . . . . . . . . . . . . . . .  37
       5.2.7.  status  . . . . . . . . . . . . . . . . . . . . . . .  38
     5.3.  JSGroup properties  . . . . . . . . . . . . . . . . . . .  38
       5.3.1.  entries . . . . . . . . . . . . . . . . . . . . . . .  39
       5.3.2.  source  . . . . . . . . . . . . . . . . . . . . . . .  39
   6.  JSCalendar object examples  . . . . . . . . . . . . . . . . .  39
     6.1.  Simple event  . . . . . . . . . . . . . . . . . . . . . .  39
     6.2.  Simple task . . . . . . . . . . . . . . . . . . . . . . .  40
     6.3.  Simple group  . . . . . . . . . . . . . . . . . . . . . .  40
     6.4.  All-day event . . . . . . . . . . . . . . . . . . . . . .  41
     6.5.  Task with a due date  . . . . . . . . . . . . . . . . . .  41
     6.6.  Event with end time-zone  . . . . . . . . . . . . . . . .  42
     6.7.  Floating-time event (with recurrence) . . . . . . . . . .  42
     6.8.  Event with multiple locations and localization  . . . . .  43
     6.9.  Recurring event with overrides  . . . . . . . . . . . . .  44
     6.10. Recurring event with participants . . . . . . . . . . . .  45
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  47
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  47
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  48
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  48
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  48
     10.2.  Informative References . . . . . . . . . . . . . . . . .  50  51
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  51
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  51

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, reducing complexity of its
      representation.

   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 should be taken from the
      iCalendar data format ([RFC5545] [RFC5545] and [RFC7986], also see
      Section 1.1), [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 not fully preserve semantic meaning.

   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.

1.1.  Relation to the iCalendar format

   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.  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 have a way to express a concise difference between two
   calendar components, which results in verbose exchanges during
   scheduling.

1.2.  Relation to the jCal format

   The JSON format for iCalendar data, jCal [RFC7265], is a direct
   mapping between iCalendar and JSON.  It does not attempt to extend or
   update iCalendar semantics, and consequently does not address the
   issues outlined in Section 1.1.

   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, which often are
   incompatible with each other.  JSCalendar is intended to meet this
   demand for JSON formatted calendar data, and to provide a standard
   representation as an alternative to new proprietary formats.

1.3.  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 in the event at multiple locations.

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

2.2.  JSTask

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

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

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

   A JSTask 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.
   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 this document.
   The following conventions are used:

   o  "Boolean|String": The value is either a JSON "Boolean" value, or a
      JSON "String" value.

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

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

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

3.2.  Data Types

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

3.2.1.  UTCDate

   This is a string in [RFC3339] "date-time" format, with the further
   restrictions that any letters MUST be in upper-case, the time
   component MUST be included and the time MUST be in UTC.  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.  LocalDate

   This is a date-time string _with no time zone/offset information_.
   It is otherwise in the same format as UTCDate, 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 LocalDate with comes from an associated property, or if no time
   zone is associated it defines _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.  Duration

   A Duration object is 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.

   A SignedDuration object is represented as a duration, optionally
   preceeded
   preceded by a sign character.  It typically is used to express the
   offset of a point in time relative to an associated time.  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.  PatchObject

   A PatchObject is of type "String[*|null]", 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, 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": 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 property on the
      patch object with (if present) or add to the property (if not
      present).

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

3.2.5.  Identifiers

   If not stated otherwise in the respective property definition,
   properties and object keys that define identifiers MUST be string
   values, MUST be at least 1 character and maximum 256 characters in
   size, and MUST only contain characters from the "URL and Filename
   safe" Base 64 Alphabet, as defined in section 5 of [RFC4648].  This
   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 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, a UUID typically is a good choice.

3.2.6.  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 (see Section 4.7.1).

3.2.7.  Normalization and 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 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.  Custom property extensions and 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.

4.  Common JSCalendar 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

4.1.1.  @type

   Type: 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 (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] (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:

   o  relation: String[Boolean] (optional).  Describes how the linked
      object is related to this object as a set of relation types.  If
      not null, the set MUST NOT be empty.

      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".

   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 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 (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 3.3.11 in [RFC5545]).

   This property SHOULD NOT be used to alter the interpretation of an
   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: UTCDate (optional).

   The date and time this object was initially created.

4.1.6.  updated

   Type: UTCDate (mandatory).

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

4.1.7.  sequence

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

   Initially zero, this MUST be a non-negative integer that is
   monotonically incremented each time a change is made to the object.

4.1.8.  method

   Type: String (optional).

   The iTIP ([RFC5546]) method, in lower-case.  Used for scheduling.

4.2.  What and where properties

4.2.1.  title

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

   A short summary of the object.

4.2.2.  description

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

   Describes the media type ([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]).  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]) to reference links in the calendar object by use of
   the "cid" property of the Link object.

4.2.4.  showWithoutTime

   Type: Boolean (optional, default: "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.

4.2.5.  locations

   Type: String[Location] (optional).

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

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

   o  name: String (optional, default: empty String).  The human-
      readable name of the location.

   o  description: 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).  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).  A time zone for this location.  Also
      see Section 3.2.6.

   o  coordinates: String (optional).  An [RFC5870] "geo:" URI for the
      location.

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

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

4.2.5.

4.2.6.  virtualLocations

   Type: String[VirtualLocation] (optional).

   A map of identifiers 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 (optional, default: empty String).  The human-
      readable name of the virtual location.

   o  description: 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).  A URI that represents how to connect to
      this virtual location.

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

4.2.6.

4.2.7.  links

   Type: String[Link] (optional).

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

   A Link object has the following properties:

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

   o  type: String (optional).  The content-type [RFC6838] of the
      resource, if known.

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

   o  rel: 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).  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

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

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

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

   o  title: String (optional).  A human-readable plain-text description
      of the resource.

4.2.7.

4.2.8.  locale

   Type: String (optional).

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

4.2.8.

4.2.9.  keywords

   Type: 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".

4.2.9.

4.2.10.  categories

   Type: 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".

   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".

4.2.10.

4.2.11.  color

   Type: String (optional).

   Specifies 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 [3] or a CSS3 RGB color hex value.

4.3.  Recurrence properties

4.3.1.  recurrenceRule

   Type: 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.  A JSTask recurs by
   applying the recurrence rule to the start date-time, if defined,
   otherwise it recurs by the due date-time, if defined.  If the task
   neither defines a start or due date-time, its "recurrenceRule"
   property value MUST be "null".

   A Recurrence object has the following properties:

   o  frequency: String (mandatory).  This MUST be one of the following
      values:

      *  "yearly"

      *  "monthly"

      *  "weekly"

      *  "daily"

      *  "hourly"

      *  "minutely"

      *  "secondly"

      To convert from iCalendar, simply lower-case the FREQ part.

   o  interval: Number (optional, default: "1").  The INTERVAL part from
      iCalendar.  If included, it MUST be an integer "x >= 1".

   o  rscale: String (optional, default: ""gregorian"").  The RSCALE
      part from iCalendar RSCALE [RFC7529], converted to lower-case.

   o  skip: String (optional, default: ""omit"").  The SKIP part from
      iCalendar RSCALE [RFC7529], converted to lower-case.

   o  firstDayOfWeek: String (optional, default: ""mo"").  The WKST part
      from iCalendar, represented as a lower-case 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"".

   o  byDay: NDay[] (optional).  An *NDay* object has the following
      properties:

      *  day: String.  The day-of-the-week part of the BYDAY value in
         iCalendar, lower-cased.  MUST be one of the following values:
         ""mo"|"tu"|"we"|"th"|"fr"|"sa"|"su"".

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

   o  byMonthDay: Number[] (optional).  The BYMONTHDAY part from
      iCalendar.  The array MUST have at least one entry if included.

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

   o  byYearDay: Number[] (optional).  The BYYEARDAY part from
      iCalendar.  The array MUST have at least one entry if included.

   o  byWeekNo: Number[] (optional).  The BYWEEKNO part from iCalendar.
      The array MUST have at least one entry if included.

   o  byHour: Number[] (optional).  The BYHOUR part from iCalendar.  The
      array MUST have at least one entry if included.

   o  byMinute: Number[] (optional).  The BYMINUTE part from iCalendar.
      The array MUST have at least one entry if included.

   o  bySecond: Number[] (optional).  The BYSECOND part from iCalendar.
      The array MUST have at least one entry if included.

   o  bySetPosition: Number[] (optional).  The BYSETPOS part from
      iCalendar.  The array MUST have at least one entry if included.

   o  count: Number (optional).  The COUNT part from iCalendar.  This
      MUST NOT be included if an "until" property is specified.

   o  until: LocalDate (optional).  The UNTIL part from iCalendar.  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 in the time zone specified in "timeZone".
      As in iCalendar, the until value bounds the recurrence rule
      inclusively.

   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
       Februrary
       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 Februrary 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 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).

   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 date-time 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 date-time.  This means the first set may
       include candidates before the start; 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 > "secondly" and no bySecond property: Add a
          bySecond property with the sole value being the seconds value
          of the start date-time.

       *  If frequency > "minutely" and no byMinute property: Add a
          byMinute property with the sole value being the minutes value
          of the start date-time.

       *  If frequency > "hourly" and no byHour property: Add a byHour
          property with the sole value being the hours value of the
          start 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 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 date-time.

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

          +  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 date-time.

          +  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 date-time.

          +  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 date-time.

4.3.2.  recurrenceOverrides

   Type: LocalDate[PatchObject] (optional).

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

   If the recurrence-id does not match an expanded start date from a
   recurrence rule, 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", then the recurring calendar object does not occur at the
   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 the
   new start time of the LocalDate key.  However, individual properties
   of the occurrence can be modified by a patch, or multiple patches.
   It is valid to patch the start property value, and this patch takes
   precedence over the LocalDate key.  Both the LocalDate key as well as
   the patched start date-time may occur before the original JSCalendar
   object's start or due date.

   A pointer in the PatchObject MUST NOT start with one of the following
   prefixes; any patch with such a key MUST be ignored:

   o  @type

   o  uid

   o  relatedTo

   o  prodId

   o  method

   o  recurrenceRule

   o  recurrenceOverrides

   o  replyTo

4.3.3.  excluded

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

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

4.4.  Sharing and scheduling properties

4.4.1.  priority

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

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

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

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

4.4.3.  privacy

   Type: 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.

   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

      *  uid

      *  created

      *  updated

      *  sequence

      *  freeBusyStatus

      *  privacy

      *  start

      *  isAllDay

      *  timeZone
      *  timeZones  due
      *  duration

      *  estimatedDuration

      *  due  freeBusyStatus

      *  privacy

      *  recurrenceOverrides.  Only patches whose keys are prefixed with
         one of the above properties 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 object is shared.

4.4.4.  replyTo

   Type: 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] (optional).

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

   o  email: String (optional).  The email address for the participant.

   o  sendTo: String[String].  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).  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].  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.

      *  "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".  Roles that are
      unknown to the implementation MUST be preserved and MAY be
      ignored.

   o  locationId: 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, this MUST be treated the
      same as if the participant's locationId were omitted.

   o  participationStatus: String (optional, default: "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  attendance: String (optional, default: "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 (optional, default: "false").  If true, the
      organizer is expecting the participant to notify them of their
      status.

   o  scheduleSequence: Number (optional, default: "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: UTCDate (optional).  The "updated" property of
      the last iMIP response from the participant.

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

   o  invitedBy: String (optional).  The participant id of the
      participant who invited this one, if known.

   o  delegatedTo: 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 of a participant.  The value
      for each key in the set MUST be "true".  This MUST be omitted if
      none (rather than an empty set).

   o  delegatedFrom: 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 of a participant.  The value for
      each key in the set MUST be "true".  This MUST be omitted if none
      (rather than an empty set).

   o  memberOf: 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 of a participant.  The
      value for each key in the set MUST be "true".  This MUST be
      omitted if none (rather than an empty set).

   o  linkIds: 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 of a Link object in the
      calendar object's "links" property.  The value for each key in the
      set MUST be "true".  This MUST be omitted if none (rather than an
      empty set).

4.5.  Alerts properties

4.5.1.  useDefaultAlerts

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

   If "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 is set to "false".

4.5.2.  alerts

   Type: String[Alert] (optional).

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

   An Alert Object has the following properties:

   o  trigger: OffsetTrigger|UnknownTrigger.  Defines when to trigger
      the alert.

      An *OffsetTrigger* object has the following properties:

      *  type: String (mandatory).  The value of this property MUST be
         "offset".

      *  offset: SignedDuration (mandatory).  Defines to trigger the
         alert relative to the time property defined in the "relativeTo"
         property.  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 (optional, default: "start").  Specifies the
         time property which 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 *UnknownTrigger* object is an object that contains a *type*
      property whose value is not "offset", 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: UTCDate (optional).

      When the user has permanently dismissed the alert the client MUST
      set this to the current time in UTC.  Other clients which sync
      this property can then 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: UTCDate (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 the
      alarm on the top-level object, unless the respective instance
      already is defined in "recurrenceOverrides".  It MUST NOT generate
      an override for the sole use of snoozing an alarm.

   o  action: String (optional, default: "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

4.6.1.  localizations

   Type: 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) for the structure
   of the PatchObject.  The patches are applied to the top-level object.
   In addition to all the restrictions on patches specified there, the
   pointer also MUST NOT start with one of the following prefixes; any
   patch with a such a key 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

   o  timeZone

   o  uid

   o  useDefaultAlerts

   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

4.7.1.  timeZones

   Type: 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 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 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]).  A valid time zone MUST define at least one transition
   rule in the "standard" or "daylight" property.  Its properties are:

   o  tzId: String (mandatory).  The TZID property from iCalendar.

   o  lastModified: UTCDate (optional).  The LAST-MODIFIED property from
      iCalendar.

   o  url: String (optional).  The TZURL property from iCalendar.

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

   o  aliases: 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".

   o  standard: TimeZoneRule[] (optional).  The STANDARD sub-components
      from iCalendar.  The order MUST be preserved during conversion.

   o  daylight: 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: LocalDate (mandatory).  The DTSTART property from
      iCalendar.

   o  offsetTo: String (mandatory).  The TZOFFSETTO property from
      iCalendar.

   o  offsetFrom: String (mandatory).  The TZOFFSETFROM property from
      iCalendar.

   o  recurrenceRule: RecurrenceRule (optional).  The RRULE property
      mapped as specified in Section 4.3.1.  During recurrence rule
      evaluation, the "until" property value MUST be interpreted as a
      local time in the UTC time zone.

   o  recurrenceDates: LocalDate[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".

   o  names: 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".

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

5.  Type-specific JSCalendar properties

5.1.  JSEvent properties

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

5.1.1.  start

   Type: LocalDate (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 the time zone the event 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.

5.1.3.  duration

   Type: 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 a week or a day in 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.

   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.  isAllDay

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

   Indicates whether this event is meant to represent an all-day event,
   and SHOULD be presented accordingly in a calendaring application.
   The value of this property is independent of the actual time-span
   covered by this event.

5.1.5.  status

   Type: 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.

   o  "cancelled": Indicates the event is cancelled.

   o  "tentative": Indicates the event is tentative.

5.2.  JSTask properties

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

5.2.1.  due

   Type: LocalDate (optional).

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

5.2.2.  start

   Type: LocalDate (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 (optional).

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

5.2.5.  statusUpdatedAt

   Type: UTCDate (optional).

   Specifies the date/time the task status properties 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 Section 4.1.3 on splitting.

5.2.6.  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).  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 task.

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

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

   o  timestamp: UTCDate (mandatory).  Describes the last time when the
      participant progress got updated.

5.2.7.  status

   Type: 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

   JSGroup supports the following JSCalendar properties (Section 4):

   o  @type

   o  uid

   o  created

   o  updated

   o  categories
   o  keywords

   o  name

   o  description

   o  color

   o  links

   as well as the following JSGroup-specific properties:

5.3.1.  entries

   Type: 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 (optional).

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

6.  JSCalendar object 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

   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.

   {
     "@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"
   }

6.2.  Simple 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",
     "title": "Do something"
   }

6.3.  Simple 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": "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

   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",
     "isAllDay":
     "showWithoutTime": true,
     "start": "1900-04-01T00:00:00",
     "duration": "P1D",
     "recurrenceRule": {
       "frequency": "yearly"
     }
   }

6.5.  Task with a 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

   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 (with 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 and 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 with 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.  Each
   lecture lasts for one hour and 30 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"
       }
     },
     "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"
       },
       "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 with 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.

   {
     "...": "",
     "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"
         },
         "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"
       }
     }

   }

7.  Security Considerations

   The use of JSON as a format does have its own inherent security risks
   as discussed in Section 12 of [RFC8259].  Even though JSON is
   considered a safe subset of JavaScript, it should be kept in mind
   that a flaw in the parser processing JSON could still impose a
   threat, which doesn't arise with conventional iCalendar data.

   With this in mind, a parser for JSON data aware of the security
   implications should be used for the format described in this
   document.  For example, the use of JavaScript's "eval()" function is
   considered an unacceptable security risk, as described in Section 12
   of[RFC8259].  A native parser with full awareness of the JSON format
   should be preferred.

   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.  Similarily,  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

Authors' Addresses

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

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

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

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