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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 8536

Independent Submission                                          A. Olson
Internet-Draft
Updates: 7808 (if approved)                                    P. Eggert
Intended status: Standards Track                                    UCLA
Expires: December 7, 2018                                   K. Murchison
                                                                FastMail
                                                            June 5, 2018


                The Time Zone Information Format (TZif)
                     draft-murchison-tzdist-tzif-05

Abstract

   This document defines the Time Zone Information Format (TZif) for
   representing and exchanging time zone information, independent of any
   particular service or protocol.  A MIME media type for this format is
   also defined.

Open Issues

   o  Should we require TZDIST servers to always return a particular
      format (2 or 3) with/without leap seconds?  Or should we allow the
      client to choose what they want (via parameters on the MIME type)?
      E.g. application/tzif+leap; version=3

   o  Is the MIME registration complete and correct (e.g. any security
      or privacy considerations)?

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   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 7, 2018.






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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  The Time Zone Information Format (TZif) . . . . . . . . . . .   4
     3.1.  TZif Header . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  TZif Data Block . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  TZif Footer . . . . . . . . . . . . . . . . . . . . . . .  10
       3.3.1.  TZ String Extensions  . . . . . . . . . . . . . . . .  11
   4.  Use with the Time Zone Data Distribution Service  . . . . . .  11
     4.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  13
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  15
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   Appendix A.  Change History (To be removed by RFC Editor before
                publication) . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   Time zone data typically consists of offsets from Universal Time
   (UT), daylight saving transition rules, one or more local time
   designations (acronyms or abbreviations), and optional leap second
   adjustments.  One such format for conveying this information is
   iCalendar [RFC5545].  It is a text-based format used by calendaring
   and scheduling systems.

   This document defines the Time Zone Information Format.  It is a
   binary format used by most UNIX systems to calculate local time.
   There is a wide variety of interoperable software [tz-link] capable
   of generating and reading files in this format.



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   This specification does not define the source of the time zone data
   or leap second information.  One such source is the IANA-hosted time
   zone database [RFC6557].

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [1] [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The following terms are used in this document:

   Coordinated Universal Time (UTC):  The basis for civil time since
      1960.  It is approximately equal to mean solar time at the prime
      meridian (0 degrees longitude).

   Daylight Saving Time (DST):  The time according to a location's law
      or practice, adjusted as necessary from standard time.  The
      adjustment may be positive, negative, or zero.

   International Atomic Time (TAI):  The time standard based on atomic
      clocks.  It is equal to UTC except without leap second
      adjustments.

   Local Time:  The time according to a location's current time zone
      offset from Universal Time.

   Standard Time:  The time according to a location's law or practice,
      unadjusted for Daylight Saving Time.

   Time Change:  A change to civil timekeeping practice.  It occurs when
      one or more of the following happen simultaneously:

      1.  a change in UT offset

      2.  a change in whether standard or daylight saving time is in use

      3.  a change in time zone abbreviation

      4.  a leap second (i.e., a change in UTC - TAI)

   Time Zone Data:  The Time Zone Data Distribution Service (TZDIST)
      [RFC7808] defines "Time zone data" as "data that defines a single
      time zone, including an identifier, UT offset values, DST rules,
      and other information such as time zone abbreviations."  The




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      interchange format defined in this document is one such form of
      time zone data.

   Universal Time (UT):  The basis of civil time.  This is the principal
      form of the mean solar time at the prime meridian (0 degrees
      longitude) for timestamps before UTC was introduced in 1960, and
      is UTC for timestamps thereafter.  Although UT is sometimes called
      "UTC" or "GMT" in other sources, this specification uses the term
      "UT" to avoid confusion with UTC or with GMT.

   UNIX Time:  The time as returned by the C time() function (see
      Section 3 of the "System Interfaces" Volume of [POSIX]).  This is
      an integer number of seconds since the POSIX Epoch (1970-01-01
      00:00:00 UTC) not counting leap seconds.  As an extension to
      POSIX, negative values represent times before the POSIX Epoch,
      using UT.

   Wall Time:  The time as shown on a clock set according to a
      location's law or practice.

3.  The Time Zone Information Format (TZif)

   The time zone information format begins with a fixed 44-octet header
   (Section 3.1) followed by a variable-length data block (Section 3.2)
   using four-octet (32-bit) transition times and leap second
   occurrences.  These 32-bit values are limited to representing times
   from 1901-12-13 20:45:52 through 2038-01-19 03:14:07 UT.

   The TZif header contains a field which specifies the version of the
   file's format.  Version 1 files terminate after the 32-bit data
   block.

   Version 2 and 3 files extend the format by appending a second
   44-octet header, another variable-length data block using eight-octet
   (64-bit) transition times and leap second occurrences, and a variable
   length footer (Section 3.3).  These 64-bit values can represent times
   approximately 292 billion years into the past or future.

   A TZif file is structured as follows:












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                        Version 1       Versions 2 & 3
                     +-------------+   +-------------+
                     | Header for  |   | Header for  |
                     |   32-bit    |   |   32-bit    |
                     | Transitions |   | Transitions |
                     +-------------+   +-------------+
                     |  Data with  |   |  Data with  |
                     |   32-bit    |   |   32-bit    |
                     | Transitions |   | Transitions |
                     +-------------+   +-------------+
                                       | Header for  |
                                       |   64-bit    |
                                       | Transitions |
                                       +-------------+
                                       |  Data with  |
                                       |   64-bit    |
                                       | Transitions |
                                       +-------------+
                                       |   Footer    |
                                       +-------------+

                       General Format of TZif Files

   Version 1 files are considered a legacy format and SHOULD NOT be
   generated, as they do not support transition times after the year
   2038.

   Implementations SHOULD generate version 2 or 3 files.  The sequence
   of time changes defined by the 32-bit header and data block SHOULD be
   a contiguous subsequence of the time changes defined by the 64-bit
   header and data block.  When reading a version 2 or 3 file,
   implementations SHOULD ignore the 32-bit header and data block except
   for the purpose of skipping over them.

   NOTE: All multi-octet integer values MUST be stored in network octet
   order format (high-order octet first, otherwise known as big-endian),
   with all bits significant.  Signed integer values MUST be represented
   using two's complement.

3.1.  TZif Header

   The TZif header is structured as follows (the number of octets
   occupied by a field is shown in parenthesis):








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       +---------------+---+
       |  magic    (4) |ver|
       +---------------+---+---------------------------------------+
       |           [unused - reserved for future use] (15)         |
       +---------------+---------------+---------------+-----------+
       |  isutcnt  (4) |  isstdcnt (4) |  leapcnt  (4) |
       +---------------+---------------+---------------+
       |  timecnt  (4) |  typecnt  (4) |  charcnt  (4) |
       +---------------+---------------+---------------+

                                TZif Header

   The fields of the header are defined as follows:

   magic:  The four-octet ASCII sequence "TZif" (0x54 0x5A 0x69 0x66)
      which identifies the file as utilizing the Time Zone Information
      Format.

   ver(sion):  An octet identifying the version of the file's format.
      The value MUST be one of the following:

      NUL (0x00)  Version 1 - The file contains only the 32-bit header
         and data block.  Version 1 files MUST NOT contain a 64-bit
         header, data block, or footer.

      '2' (0x32)  Version 2 - The file MUST contain both the 32-bit
         header and data block and a 64-bit header, data block, and
         footer.  The TZ string in the footer MUST strictly adhere to
         the expanded format of the "TZ" environment variable as defined
         in Section 8 of the "Base Definitions" Volume of [POSIX].

      '3' (0x33)  Version 3 - The file MUST contain both the 32-bit
         header and data block and a 64-bit header, data block, and
         footer.  The TZ string in the footer string MAY use extensions
         (Section 3.3.1) to the expanded format of the "TZ" environment
         variable as defined in Section 8 of the "Base Definitions"
         Volume of [POSIX].

   isutcnt:  A four-octet unsigned integer specifying the number of UT/
      local indicators contained in the data block - MUST either be zero
      or equal to 'typecnt'.

   isstdcnt:  A four-octet unsigned integer specifying the number of
      standard/wall indicators contained in the data block - MUST either
      be zero or equal to 'typecnt'.

   leapcnt:  A four-octet unsigned integer specifying the number of leap
      second records contained in the data block.



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   timecnt:  A four-octet unsigned integer specifying the number of
      transition times contained in the data block.

   typecnt:  A four-octet unsigned integer specifying the number of
      local time type records contained in the data block - MUST NOT be
      zero.

   charcnt:  A four-octet unsigned integer specifying the total number
      of octets used by the set of time zone designations contained in
      the data block.

3.2.  TZif Data Block

   The TZif data block consists of seven variable-length elements, each
   of which is series of zero or more items.  The number of items in
   each series is determined by the corresponding count field in the
   header.  The total length of each element is calculated by
   multiplying the number of items by the size of each item.  Therefore,
   implementations that do not wish to parse or use the 32-bit data
   block can calculate its total length and skip directly to the header
   of the 64-bit data block.

   In the initial data block, time values are 32-bit (TIME_SIZE = 4
   octets).  In the second data block, present only in version 2 and 3
   files, time values are 64-bit (TIME_SIZE = 8 octets).

   The data block is structured as follows (the number of octets
   occupied by a field is shown in parenthesis):

        +---------------------------------------------------------+
        |  transition times          (timecnt x TIME_SIZE)        |
        +---------------------------------------------------------+
        |  transition types          (timecnt)                    |
        +---------------------------------------------------------+
        |  local time type records   (typecnt x 6)                |
        +---------------------------------------------------------+
        |  time zone designations    (charcnt)                    |
        +---------------------------------------------------------+
        |  leap second records       (leapcnt x (TIME_SIZE + 4))  |
        +---------------------------------------------------------+
        |  standard/wall indicators  (isstdcnt)                   |
        +---------------------------------------------------------+
        |  UT/local indicators       (isutcnt)                    |
        +---------------------------------------------------------+

                              TZif Data Block

   The elements of the data block are defined as follows:



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   transition times:  A series of four- or eight-octet UNIX time values
      sorted in strictly ascending order.  Each value is used as a
      transition time at which the rules for computing local time may
      change.  The number of time values is specified by the 'timecnt'
      field in the header.

   transition types:  A series of one-octet unsigned integers specifying
      the type of local time of the corresponding transition time.
      These values serve as indices into the array of local time type
      records.  The number of type indices is specified by the 'timecnt'
      field in the header.  Each type index MUST be in the range [0,
      'typecnt').

   local time type records:  A series of six-octet records specifying a
      local time type.  The number of records is specified by the
      'typecnt' field in the header.  Each record has the following
      format:

      +---------------+-+-+---+
      |  utoff (4)    |dst|idx|
      +---------------+---+---+

      utoff:  A four-octet signed integer specifying the number of
         seconds to be added to UT in order to determine local time.

      (is)dst:  A one-octet value indicating whether local time should
         be considered Daylight Savings Time (DST).  A value of one (1)
         indicates that DST is in effect.  A value of zero (0) indicates
         that standard time in effect.

      (desig)idx:  A one-octet unsigned integer specifying an index into
         the series of time zone designation characters, thereby
         selecting a particular designation string.  Each index MUST be
         in the range [0, 'charcnt').

   time zone designations:  A series of ASCII characters constituting an
      array of NUL-terminated (0x00) time zone designation strings.  The
      total number of characters is specified by the 'charcnt' field in
      the header.  Note that two designations MAY overlap if one is a
      suffix of the other.

   leap second records:  A series of eight- or twelve-octet records
      specifying the corrections that need to be applied to UTC in order
      to determine TAI.  The records are sorted by the occurrence time
      in strictly ascending order.  The number of records is specified
      by the 'leapcnt' field in the header.  Each record has one of the
      following structures:




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      32-bit Data Block:

      +---------------+---------------+
      |  occur (4)    |  corr (4)     |
      +---------------+---------------+

      64-bit Data Block:

      +---------------+---------------+---------------+
      |  occur (8)                    |  corr (4)     |
      +---------------+---------------+---------------+

      occur(rence):  A four- or eight-octet UNIX time value specifying
         the time at which a leap second correction occurs.

      corr(ection):  A four-octet signed integer specifying the total
         number of leap seconds to be applied to UTC on or after the
         occurrence.  The correction values in adjacent leap second
         records MUST differ by exactly one (1).

   standard/wall indicators:  A series of one-octet values indicating
      whether the transition times associated with local time types were
      specified as standard time or wall clock time.  A value of one (1)
      indicates standard time, and MUST be set to one (1) if the
      corresponding UT/local indicator is set to one (1).  A value of
      zero (0) indicates wall time.  The number of values is specified
      by the 'isstdcnt' field in the header.  If 'isstdcnt' is zero (0),
      all transition times associated with local time types are assumed
      to be specified as wall time.

   UT/local indicators:  A series of one-octet values indicating whether
      the transition times associated with local time types were
      specified as UT or local time.  A value of one (1) indicates UT,
      and the corresponding standard/wall indicator MUST also be set to
      one (1).  A value of zero (0) indicates local time.  The number of
      values is specified by the 'isutcnt' field in the header.  If
      'isutcnt' is zero (0), all transition times associated with local
      time types are assumed to be specified as local time.

   The type corresponding to a transition time specifies local time for
   timestamps starting at the given transition time and continuing up to
   and not including the next transition time.  Local time for
   timestamps before the first transition is specified by the first time
   type (time type 0).  Local time for timestamps on or after the last
   transition is specified by the TZ string in the footer (Section 3.3)
   if present and nonempty, and is unspecified otherwise.  If there are
   no transitions, local time for all timestamps is specified by the TZ




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   string in the footer if present and nonempty, and is specified by
   time type 0 otherwise.

   A given pair of standard/wall and UT/local indicators is used to
   designate whether the corresponding transition time was specified as
   UT, standard time, or wall clock time.  Note that there are only
   three combinations of the two indicators given that the standard/wall
   value MUST be one (1) if the UT/local value is one (1).  This
   information can be useful if the transition times in a TZif file need
   to be transformed into transitions appropriate for another time zone
   (e.g. when calculating transition times for a simple POSIX TZ string
   such as "AKST9AKDT").

   In order to eliminate unused space in a TZif file, every local time
   type record SHOULD be used by at least one transition (except that
   time type 0 need not be used if there is at least one transition).
   Likewise, every character in the time zone designations array SHOULD
   be used by at least one time type record.

3.3.  TZif Footer

   The TZif footer is structured as follows (the number of octets
   occupied by a field is shown in parenthesis):

                      +---+--------------------+---+
                      | NL|  TZ string (0...)  |NL |
                      +---+--------------------+---+

                                TZif Footer

   The elements of the footer are defined as follows:

   NL:  An ASCII new line character (0x0A).

   TZ string:  A rule for computing local time changes after the last
      transition time stored in the 64-bit data block.  The string is
      either empty or uses the expanded format of the "TZ" environment
      variable as defined in Section 8 of the "Base Definitions" Volume
      of [POSIX].  If empty, the corresponding information is not
      available.  If the string is nonempty, at least one transition
      MUST be present in the 64-bit data, and the TZ string MUST be
      consistent with the last 64-bit transition - i.e., evaluating the
      TZ string at the time of the last transition should yield the same
      time type as the time type specified in the last transition.  Note
      that the string MUST NOT be NUL-terminated and SHOULD NOT begin
      with the ':' (colon) character.





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3.3.1.  TZ String Extensions

   Version 3 TZif files MAY use the following extensions in the TZ
   string:

   o  The hours part of the transition times may be signed and range
      from -167 through 167 instead of the POSIX-required unsigned
      values from 0 through 24.

      Example: <-03>3<-02>,M3.5.0/-2,M10.5.0/-1
         This represents a time zone that observes daylight saving time
         from 22:00 on the day before March's last Sunday until 23:00 on
         the day before October's last Sunday.  Standard time is 3 hours
         west of UT and is abbreviated "-03"; daylight saving time is 2
         hours west of UT and is abbreviated "-02".

   o  DST is considered to be in effect all year if it starts January 1
      at 00:00 and ends December 31 at 24:00 plus the difference between
      daylight saving and standard time, leaving no room for standard
      time in the calendar.

      Example: EST5EDT,0/0,J365/25
         This represents a time zone that observes daylight saving time
         all year.  It is 4 hours west of UT and is abbreviated "EDT".

4.  Use with the Time Zone Data Distribution Service

   The Time Zone Data Distribution Service (TZDIST) [RFC7808] is a
   service that allows reliable, secure, and fast delivery of time zone
   data and leap second rules to client systems such as calendaring and
   scheduling applications or operating systems.

   A TZDIST service MAY supply time zone data to clients in the Time
   Zone Information Format.  Such a service MUST indicate that it
   supports this format by including the MIME media type "application/
   tzif" (Section 7) in its "capabilities" response (see Section 5.1 of
   [RFC7808].

   TZDIST clients MUST use the HTTP "Accept" [RFC7231] header field to
   indicate their preference to receive data in the "application/tzif"
   format.

4.1.  Example

   In this example, the client checks the server for the available
   formats and then requests that the time zone with a specific time
   zone identifer be returned in Time Zone Information Format.




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   Note that this example presumes that the time zone context path has
   been discovered (see [RFC7808] Section 4.2.1) to be "/tzdist".

   >> Request <<

   GET /tzdist/capabilities HTTP/1.1
   Host: tz.example.com

   >> Response <<

   HTTP/1.1 200 OK
   Date: Fri, 01 Jun 2018 14:52:23 GMT
   Content-Type: application/json; charset="utf-8"
   Content-Length: xxxx

   {
     "version": 1,

     "info": {
       "primary-source": "IANA:2018e",
       "formats": [
         "text/calendar",
         "application/tzif",
       ],
   ...
     },
   ...
   }


   >> Request <<

   GET /tzdist/zones/America%2FNew_York HTTP/1.1
   Host: tz.example.com
   Accept: application/tzif

   >> Response <<

   HTTP/1.1 200 OK
   Date: Fri, 01 Jun 2018 14:52:24 GMT
   Content-Type: application/tzif
   Content-Length: xxxx
   ETag: "123456789-000-111"

   TZif2...[binary data]...
   EST5EDT,M3.2.0,M11.1.0





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5.  Security Considerations

   None.

6.  Privacy Considerations

   None.

7.  IANA Considerations

   This document defines a MIME [RFC6838] media type for the exchange of
   data utilizing the Time Zone Information Format.

   Type name:  application

   Subtype name:  tzif

   Required parameters:  N/A

   Optional parameters:  N/A

   Encoding considerations:  Binary

   Security considerations:  N/A

   Interoperability considerations:  N/A

   Published specification:  This specification.

   Applications that use this media type:  This media type is designed
      for widespread use by applications that need to exchange time zone
      information.  The Time Zone Distribution Service [RFC7808] can
      directly use this media type.

   Fragment identifier considerations:  N/A

   Additional information:

      Magic number(s):  The first 4 octets are 0x54, 0x5A, 0x69, 0x66

      File extensions(s):  N/A

      Macintosh file type code(s):  N/A

   Person & email address to contact for further
   information:
      Time Zone Database mailing list <tz@iana.org>




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   Intended usage:  COMMON

   Restrictions on usage:  N/A

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

   Change controller:  IETF

8.  Acknowledgments

   The authors would like to thank the following individuals for
   contributing their ideas and support for writing this specification:
   Michael Douglass and Eliot Lear.

9.  References

9.1.  Normative References

   [POSIX]    IEEE, "Standard for Information Technology--Portable
              Operating System Interface (POSIX(R)) Base Specifications,
              Issue 7", IEEE 1003.1-2017,
              DOI 10.1109/IEEESTD.2018.8277153, January 2018,
              <https://ieeexplore.ieee.org/servlet/
              opac?punumber=8277151>.

              This is identical to The Open Group Base Specifications
              Issue 7, 2018 edition [2].

   [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>.

   [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>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [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>.





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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [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>.

   [RFC6557]  Lear, E. and P. Eggert, "Procedures for Maintaining the
              Time Zone Database", BCP 175, RFC 6557,
              DOI 10.17487/RFC6557, February 2012,
              <https://www.rfc-editor.org/info/rfc6557>.

   [tz-link]  Eggert, P. and A. Olson, "Sources for Time Zone and
              Daylight Saving Time Data", 2018,
              <https://www.iana.org/time-zones/repository/tz-link.html>.

9.3.  URIs

   [1] https://tools.ietf.org/html/bcp14

Appendix A.  Change History (To be removed by RFC Editor before
             publication)

   Changes since -04:

   o  Edited text discussing timestamps before first and after last
      transition.

   o  Specified legal range of time type indices and time zone
      designation indices.

   o  Notes that corrections in adjacent leap second records must differ
      by one.

   o  Added recommendations to eliminate unused space.

   o  Minor editorial changes.

   Changes since -03:

   o  Removed definition of GMT.

   o  Updated definitions of UTC, TAI, and UT




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   o  Switched to using UT rather than UTC.

   o  Added more text about the use of standard/wall and UT/local
      indicators.

   o  Added Acknowledgments.

   o  Minor editorial changes.

   Changes since -02:

   o  Updated definitions of Standard Time and DST.

   o  Added definitions of GMT and UT.

   o  Added a definition of Time Zone Data from RFC7808.

   o  Removed sentence stating that TZDB is accurate.

   o  Added more text for standard/wall and UTC/local indicators and
      counts.

   o  Added text discussing timestamps before first and after last
      transition.

   o  Added more guidance text regarding 32-bit and 64-bit data
      consistency.

   o  Minor editorial changes.

   Changes since -01:

   o  Renamed "POSIX Time" to "UNIX Time" and noted that values can be
      negative.

   o  Noted that signed values MUST be represented using two's
      complement.

   o  Renamed "POSIX TZ string" to "TZ string" and noted that it can be
      empty.

   o  Moved TZ string extensions into its own subsection with examples.

   o  Renamed leap second "epoch" to "occurrence".

   o  Editorial changes from Paul Eggert.

   Changes since -00:



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   o  Split TZif format description into a general overview and 3
      subsections.

   o  Updated Keywords boilerplate.

   o  Updated POSIX reference.

   o  Editorial changes from Eliot Lear.

Authors' Addresses

   Arthur David Olson

   Email: arthurdavidolson@gmail.com


   Paul Eggert
   University of California, Los Angeles

   Email: eggert@cs.ucla.edu


   Kenneth Murchison
   FastMail US LLC

   Email: murch@fastmailteam.com

























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