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Independent Submission                                          A. Olson
Internet-Draft
Intended status: Standards Track                               P. Eggert
Expires: March 16, 2019                                             UCLA
                                                            K. Murchison
                                                                FastMail
                                                      September 12, 2018


                The Time Zone Information Format (TZif)
                     draft-murchison-tzdist-tzif-14

Abstract

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

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-
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   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 March 16, 2019.

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.




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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 . . . . . . . . . . . . . . . . . . . . . . .  11
       3.3.1.  TZ String Extensions  . . . . . . . . . . . . . . . .  11
   4.  Interoperability Considerations . . . . . . . . . . . . . . .  12
   5.  Use with the Time Zone Data Distribution Service  . . . . . .  13
     5.1.  Truncating TZif Files . . . . . . . . . . . . . . . . . .  13
     5.2.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  16
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  application/tzif  . . . . . . . . . . . . . . . . . . . .  16
     8.2.  application/tzif-leap . . . . . . . . . . . . . . . . . .  17
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  18
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     10.2.  Informative References . . . . . . . . . . . . . . . . .  19
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Appendix A.  Common Interoperability Issues . . . . . . . . . . .  20
   Appendix B.  Change History (To be removed by RFC Editor before
                publication) . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

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 (TZif).  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.

   This specification does not define the source of leap second
   information, nor does it define the source the time zone data,
   metadata, identifiers, aliases, localized names, or versions as
   defined in Section 3 of [RFC7808].  One such source is the IANA-
   hosted time zone database [RFC6557].




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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 since 1972.  It is equal to UTC except without leap second
      adjustments.

   Leap Second Correction (LEAPCORR):  The value of TAI - UTC - 10 for
      timestamps after the first leap second, and zero for timestamps
      before that.  The expression "TAI - UTC - 10" comes from the fact
      that TAI - UTC was defined to be 10 just prior to the first leap
      second in 1972, so clocks with leap seconds have a zero LEAPCORR
      before the first leap second.

   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 LEAPCORR)

   Time Zone Data:  The Time Zone Data Distribution Service (TZDIST)
      [RFC7808] defines "Time zone data" as "data that defines a single



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      time zone, including an identifier, UTC offset values, DST rules,
      and other information such as time zone abbreviations."  The
      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 [2] 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.

   UNIX Leap Time:  UNIX time plus all preceding leap second
      corrections.  For example, if the first leap second record in a
      TZif file occurs at 1972-06-30 23:59:60 UTC, the UNIX leap time
      for the timestamp 1972-07-01 00:00:00 UTC would be 78796801, one
      greater than the UNIX time for the same timestamp.  Similarly, if
      the second leap second record occurs at 1972-12-31 23:59:60 UTC,
      its UNIX leap time would be 94694401; the second occurrence
      accounts for the first leap second.  If a TZif file specifies no
      leap second records, UNIX leap time is equivalent to UNIX time.

   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




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   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:

                        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

   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 [RFC0020] 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 the 32-bit header
         and data block, a 64-bit header and data block, and a footer.
         The TZ string in the footer (Section 3.3), if nonempty, MUST
         strictly adhere to the requirements for the TZ environment
         variable as defined in Section 8.3 of the "Base Definitions"
         Volume [3] of [POSIX], and MUST encode the POSIX portable
         character set as ASCII.

      '3' (0x33)  Version 3 - The file MUST contain the 32-bit header
         and data block, a 64-bit header and data block, and a footer.
         The TZ string in the footer (Section 3.3), if nonempty, MUST
         conform to POSIX requirements with ASCII encoding, except that
         it MAY use the TZ string extensions described below
         (Section 3.3.1).

   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'.




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

   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.  (Although time type 0 is not used in files that have
      nonempty TZ strings but no transitions, it is nevertheless
      required because many TZif readers reject files that lack time
      types.)

   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):

















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        +---------------------------------------------------------+
        |  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:

   transition times:  A series of four- or eight-octet UNIX leap 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.  Each time value SHOULD be at least -2**59.
      (-2**59 is the greatest negated power of 2 that predates the Big
      Bang, and avoiding earlier timestamps works around known TZif
      reader bugs relating to outlandlishly negative timestamps.)

   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.
         The value MUST NOT be -2**31, and SHOULD be in the range



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         [-89999, 93599] (i.e., its value SHOULD be more than -25 hours
         and less than 26 hours).  (Avoiding -2**31 allows 32-bit
         clients to negate the value without overflow.  Restricting it
         to [-89999, 93599] allows easy support by implementations that
         already support the the POSIX-required range [-24:59:59,
         25:59:59].)

      (is)dst:  A one-octet value indicating whether local time should
         be considered Daylight Savings Time (DST).  The value MUST be 0
         or 1.  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 octets, thereby selecting a
         particular designation string.  Each index MUST be in the range
         [0, 'charcnt'), and MUST index a NUL-terminated (0x00) sequence
         of octets.

   time zone designations:  A series of octets constituting an array of
      NUL-terminated (0x00) time zone designation strings.  The total
      number of octets 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:

      32-bit Data Block:

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

      64-bit Data Block:

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

      occur(rence):  A four- or eight-octet UNIX leap time value
         specifying the time at which a leap second correction occurs.
         The first value, if present, MUST be nonnegative, and each
         later value MUST be at least 2419199 greater than the previous




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         value.  (This is 28 days' worth of seconds, minus a potential
         negative leap second.)

      corr(ection):  A four-octet signed integer specifying the value of
         LEAPCORR on or after the occurrence.  The correction value in
         the first leap second record, if present, MUST be either one
         (1) or minus one (-1).  The correction values in adjacent leap
         second records MUST differ by exactly one (1).  The value of
         LEAPCORR is zero for timestamps that occur before the
         occurrence time in the first leap second record (or for all
         timestamps if there are no leap second records).

   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.  Each value MUST be
      0 or 1.  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.  Each value MUST be 0 or 1.  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, but 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
   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



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   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 nonzero
   local time type index SHOULD appear at least once in the transition
   type array.  Likewise, every octet 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 8.3 of the "Base Definitions"
      Volume [4] of [POSIX] with ASCII encoding, possibly utilizing
      extensions described below (Section 3.3.1) in version 3 files.  If
      the string is empty, the corresponding information is not
      available.  If the string is nonempty and one or more transitions
      appear in the 64-bit data, the 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.  The string MUST NOT
      contain NUL octets or be NUL-terminated, and SHOULD NOT begin with
      the ':' (colon) character.

3.3.1.  TZ String Extensions

   The TZ string in a Version 3 TZif file MAY use the following
   extensions to POSIX TZ strings.  These extensions are described using
   the terminology of Section 8.3 of the "Base Definitions" Volume [5]
   of [POSIX].





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   o  The hours part of the transition times may be signed and range
      from -167 through 167 (-167 <= hh <= 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.  Interoperability Considerations

   These recommended practices should be followed in order to assure
   interoperability of TZif applications.

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

   o  Implementations SHOULD generate a version 3 file if TZ string
      extensions are necessary to accurately model transition times.
      Otherwise, version 2 files SHOULD be generated.

   o  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, and by the footer.
      This guideline helps obsolescent version 1 readers agree with
      current readers about timestamps within the contiguous
      subsequence.  It also lets writers not catering to obsolescent
      readers use a 'timecnt' of zero in the 32-bit data to save space.

   o  Time zone designations SHOULD consist of at least three (3) and no
      more than six (6) ASCII characters from the set of alphanumerics,
      '-', and '+'.  This is for compatibility with POSIX requirements
      for time zone abbreviations.






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   o  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.

   o  Implementations SHOULD calculate the total lengths of the 32- and
      64-bit headers and data blocks and compare them against the actual
      file size, as part of a validity check for the file.

   o  When a TZif file is used in a MIME message entity it SHOULD be
      indicated by one of the following media types:

      *  "application/tzif-leap" (Section 8.2) to indicate that leap
         second records are included in the TZif data.

      *  "application/tzif" (Section 8.1) to indicate that leap second
         records are not included in the TZif data; 'leapcnt' in the
         header(s) MUST be zero (0).

   o  Common interoperability issues and possible workarounds are
      described in Appendix A.

5.  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 8.1) in its "capabilities" response (see Section 5.1
   of [RFC7808]).  A TZDIST service MAY also include the MIME media type
   "application/tzif-leap" (Section 8.2) in its "capabilities" response
   if it is able to generate TZif files containing leap second records.
   A TZDIST service MUST NOT advertise the "application/tzif-leap" MIME
   media type without also advertising "application/tzif".

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

5.1.  Truncating TZif Files

   As described in Section 3.9 of [RFC7808], a TZDIST service MAY
   truncate time zone transition data.  A truncated TZif file is valid
   from its first and up to, but not including, its last 64-bit
   transition time, if present.



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   When truncating the start of a TZif file, the service MUST supply in
   the 64-bit data a first transition time that is the start point of
   the truncation range.  As with untruncated TZif files, time type 0
   indicates local time immediately before the start point, and the time
   type of the first transition indicates local time thereafter.

   When truncating the end of a TZif file, the service MUST supply in
   the 64-bit data a last transition time that is the end point of the
   truncation range, and MUST supply an empty TZ string.  As with
   untruncated TZif files with empty TZ strings, a truncated TZif file
   does not indicate local time after the last transition.

   All represented information that falls inside the truncation range
   MUST be the same as that represented by a corresponding untruncated
   TZif file.

   TZDIST clients SHOULD NOT use a truncated TZif file (as described
   above) to interpret timestamps outside the truncation time range.

5.2.  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 identifier 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",
         "application/tzif-leap"
       ],
   ...
     },
   ...
   }


   >> 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 without leap second records]...
   EST5EDT,M3.2.0,M11.1.0




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6.  Security Considerations

   The Time Zone Information Format contains no executable code and the
   format does not define any extensible areas that could be used to
   store such code.

   TZif contains counted arrays of data elements.  All counts should be
   checked when processing TZif objects to guard against references past
   the end of the object.

   TZif provides no confidentiality or integrity protection.  Time zone
   information is normally public and does not call for confidentiality
   protection.  Since time zone information is used in many critical
   applications, integrity protection may be required, and must be
   provided externally.

7.  Privacy Considerations

   The Time Zone Information Format contains publicly available data and
   the format does not define any extensible areas that could be used to
   store private data.

8.  IANA Considerations

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

8.1.  application/tzif

   Type name:  application

   Subtype name:  tzif

   Required parameters:  none

   Optional parameters:  none

   Encoding considerations:  binary

   Security considerations:  See Section 6 of this document.

   Interoperability considerations:  See Section 4 of this document.

   Published specification:  This specification.

   Applications that use this media type:  This media type is designed
      for widespread use by applications that need to use or exchange
      time zone information, such as the Time Zone Information Compiler



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      (zic) [6] and the GNU C Library [7].  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>

   Intended usage:  COMMON

   Restrictions on usage:  N/A

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

   Change controller:  IETF

8.2.  application/tzif-leap

   Type name:  application

   Subtype name:  tzif-leap

   Required parameters:  none

   Optional parameters:  none

   Encoding considerations:  binary

   Security considerations:  See Section 6 of this document.

   Interoperability considerations:  See Section 4 of this document.

   Published specification:  This specification.

   Applications that use this media type:  This media type is designed
      for widespread use by applications that need to use or exchange
      time zone information, such as the Time Zone Information Compiler
      (zic) [8] and the GNU C Library [9].  The Time Zone Distribution
      Service [RFC7808] can directly use this media type.



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   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>

   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 following individuals for
   contributing their ideas and support for writing this specification:
   Michael Douglass, Ned Freed, Guy Harris, Eliot Lear, and Alexey
   Melnikov.

10.  References

10.1.  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,
              <http://pubs.opengroup.org/onlinepubs/9699919799/>.

              This standard is also
              <https://ieeexplore.ieee.org/servlet/
              opac?punumber=8277151> published by ieeexplore.ieee.org.

   [RFC0020]  Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/info/rfc20>.





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   [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>.

   [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>.

10.2.  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>.

10.3.  URIs

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

   [2] http://pubs.opengroup.org/onlinepubs/9699919799/functions/
       time.html

   [3] http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
       V1_chap08.html#tag_08_03



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   [4] http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
       V1_chap08.html#tag_08_03

   [5] http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
       V1_chap08.html#tag_08_03

   [6] http://man7.org/linux/man-pages/man8/zic.8.html

   [7] https://www.gnu.org/software/libc/

   [8] http://man7.org/linux/man-pages/man8/zic.8.html

   [9] https://www.gnu.org/software/libc/

Appendix A.  Common Interoperability Issues

   This section documents common problems in implementing this
   specification.  Most of these are problems in generating TZif files
   for use by readers conforming to predecessors of this specification.
   The goals of this section are:

   1.  to help TZif writers output files that avoid common pitfalls in
       older or buggy TZif readers,

   2.  to help TZif readers avoid common pitfalls when reading files
       generated by future TZif writers, and

   3.  to help any future specification authors see what sort of
       problems arise when the TZif format is changed.

   When new versions of the TZif format have been defined, a design goal
   has been that a reader can successfully use a TZif file even if the
   file is of a later TZif version than what the reader was designed
   for.  When complete compatibility was not achieved, an attempt was
   made to limit glitches to rarely-used timestamps, and to allow simple
   partial workarounds in writers designed to generate new-version data
   useful even for older-version readers.  This section attempts to
   document these compatibility issues and workarounds, as well as to
   document other common bugs in readers.

   Interoperability problems with TZif include the following:

   o  Some readers examine only 32-bit data.  As a partial workaround, a
      writer can output as much 32-bit data as possible.  However, a
      reader should ignore 32-bit data, and should use 64-bit data even
      if the reader's native timestamps have only 32 bits.





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   o  Some readers designed for version 2 might mishandle timestamps
      after a version 3 file's last transition, because they cannot
      parse extensions to POSIX in the TZ-like string.  As a partial
      workaround, a writer can output more transitions than necessary,
      so that only far-future timestamps are mishandled by version 2
      readers.

   o  Some readers designed for version 2 do not support permanent
      daylight saving time, e.g., a TZ string "EST5EDT,0/0,J365/25"
      denoting permanent Eastern Daylight Time (-04).  As a partial
      workaround, a writer can substitute standard time for the next
      time zone east, e.g., "AST4" for permanent Atlantic Standard Time
      (-04).

   o  Some readers ignore the footer, and instead predict future
      timestamps from the time type of the last transition.  As a
      partial workaround, a writer can output more transitions than
      necessary.

   o  Some readers do not use time type 0 for timestamps before the
      first transition, in that they infer a time type using a heuristic
      that does not always select time type 0.  As a partial workaround,
      a writer can output a dummy (no-op) first transition at an early
      time.

   o  Some readers mishandle timestamps before the first transition that
      has a timestamp not less than -2**31.  Readers that support only
      32-bit timestamps are likely to be more prone to this problem, for
      example, when they process 64-bit transitions only some of which
      are representable in 32 bits.  As a partial workaround, a writer
      can output a dummy transition at timestamp -2**31.

   o  Some readers mishandle a transition if its timestamp has the
      minimum possible signed 64-bit value.  Timestamps less than -2**59
      are not recommended.

   o  Some readers mishandle POSIX-style TZ strings that contain '<' or
      '>'.  As a partial workaround, a writer can avoid using '<' or '>'
      for time zone abbreviations containing only alphabetic characters.

   o  Many readers mishandle time zone abbreviations that contain non-
      ASCII characters.  These characters are not recommended.

   o  Some readers may mishandle time zone abbreviations that contain
      fewer than 3 or more than 6 characters, or that contain ASCII
      characters other than alphanumerics, '-', and '+'.  These
      abbreviations are not recommended.




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   o  Some readers mishandle TZif files that specify daylight-saving
      time UT offsets that are less than the UT offsets for the
      corresponding standard time.  These readers do not support
      locations like Ireland, which uses the equivalent of the POSIX TZ
      string "IST-1GMT0,M10.5.0,M3.5.0/1", observing standard time (IST,
      +01) in summer and daylight saving time (GMT, +00) in winter.  As
      a partial workaround, a writer can output data for the equivalent
      of the POSIX TZ string "GMT0IST,M3.5.0/1,M10.5.0", thus swapping
      standard and daylight saving time.  Although this workaround
      misidentifies which part of the year uses daylight saving time, it
      records UT offsets and time zone abbreviations correctly.

   Some interoperability problems are reader bugs that are listed here
   mostly as warnings to developers of readers.

   o  Some readers do not support negative timestamps.  Developers of
      distributed applications should keep this in mind if they need to
      deal with pre-1970 data.

   o  Some readers mishandle timestamps before the first transition that
      has a nonnegative timestamp.  Readers that do not support negative
      timestamps are likely to be more prone to this problem.

   o  Some readers mishandle time zone abbreviations like "-08" that
      contain '+', '-', or digits.

   o  Some readers mishandle UT offsets that are out of the traditional
      range of -12 through +12 hours, and so do not support locations
      like Kiritimati that are outside this range.

   o  Some readers mishandle UT offsets in the range [-3599, -1] seconds
      from UT, because they integer-divide the offset by 3600 to get 0
      and then display the hour part as "+00".

   o  Some readers mishandle UT offsets that are not a multiple of one
      hour, or of 15 minutes, or of 1 minute.

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

   Changes since -13:

   o  Added text to Privacy Considerations.

   Changes since -12:

   o  Added reference to RFC0020.




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   o  Fully fleshed out application/tzif-leap declaration rather than
      referencing application/tzif.

   o  Added definition for "Leap Second Correction".

   o  Added external references directly to the relevant sections of
      POSIX.

   Changes since -11:

   o  Removed text requiring leapcnt by non-zero for application/tzif-
      leap files.

   Changes since -10:

   o  Removed text mandating all TZDIST features be supported.

   o  Minor editorial changes.

   Changes since -09:

   o  Removed "Update 7808" from header as this spec doesn't introduce
      new TZDIST functionality.

   o  Added text regarding truncation of TZif via TZDIST.

   o  Expanded what this spec DOESN'T define.

   o  Added reasons for some of the recommended practices.

   o  Added common interoperability issues appendix.

   o  Minor editorial changes.

   Changes since -08:

   o  Clarifying text regarding MIME types.

   o  Consolidated/referenced duplicate security and interoperability
      text.

   o  Switched to 'octets' instead of 'characters' when describing time
      zone designations.

   o  Minor editorial changes.

   Changes since -07:




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   o  Clarifying text regarding TZ string.

   o  Added "application/tzif-leap" MIME media type.

   o  New reference for zic(8) man page.

   o  Minor editorial changes.

   Changes since -06:

   o  Added definition of UNIX Leap Time and used it to describe
      transition times and leap second occurrences.

   o  Moved TZif generation recommendations into discussion of version
      field.

   o  Repeated TZif generation recommendations in TZDIST section.

   o  Rewrote part of the TZ string text.

   o  Minor editorial changes.

   Changes since -05:

   o  Clarify TAI, leap seconds, some descriptions, and some field
      values/ranges with text from Paul Eggert.

   o  Refined MIME declaration based on feedback from Ned Freed.

   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.




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   o  Updated definitions of UTC, TAI, and UT

   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.



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   Changes since -00:

   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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