GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc8536

Internet Engineering Task Force (IETF) A. Olson Request for Comments: 8536 Category: Standards Track P. Eggert ISSN: 2070-1721 UCLA

                                                          K. Murchison
                                                              FastMail
                                                         February 2019
              The Time Zone Information Format (TZif)

Abstract

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

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8536.

Copyright Notice

 Copyright (c) 2019 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.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Olson, et al. Standards Track [Page 1] RFC 8536 TZif February 2019

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
 3.  The Time Zone Information Format (TZif) . . . . . . . . . . .   5
   3.1.  TZif Header . . . . . . . . . . . . . . . . . . . . . . .   6
   3.2.  TZif Data Block . . . . . . . . . . . . . . . . . . . . .   8
   3.3.  TZif Footer . . . . . . . . . . . . . . . . . . . . . . .  12
     3.3.1.  TZ String Extensions  . . . . . . . . . . . . . . . .  13
 4.  Interoperability Considerations . . . . . . . . . . . . . . .  13
 5.  Use with the Time Zone Data Distribution Service  . . . . . .  14
   5.1.  Truncating TZif Files . . . . . . . . . . . . . . . . . .  15
   5.2.  Example TZDIST Request for TZif Data  . . . . . . . . . .  15
 6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
 7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  17
 8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   8.1.  application/tzif  . . . . . . . . . . . . . . . . . . . .  17
   8.2.  application/tzif-leap . . . . . . . . . . . . . . . . . .  18
 9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
   9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
   9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
 Appendix A.  Common Interoperability Issues . . . . . . . . . . .  21
 Appendix B.  Example TZif Files . . . . . . . . . . . . . . . . .  23
   B.1.  Version 1 File Representing UTC (with Leap Seconds) . . .  24
   B.2.  Version 2 File Representing Pacific/Honolulu  . . . . . .  28
   B.3.  Truncated Version 3 File Representing Asia/Jerusalem  . .  33
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  34
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

Olson, et al. Standards Track [Page 2] RFC 8536 TZif February 2019

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 specifies the widely deployed Time Zone Information
 Format (TZif).  It is a binary format used by most UNIX systems to
 calculate local time.  This format was introduced in the 1980s and
 has evolved since then into multiple upward-compatible versions.
 There is a wide variety of interoperable software capable of
 generating and reading files in this format [tz-link].
 This specification does not define the source of the data assembled
 into a TZif file.  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 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.
 The following terms are used in this document (see "Sources for Time
 Zone and Daylight Saving Time Data" [tz-link] for more detailed
 information about civil timekeeping data and practice):
 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, when adjusted as necessary from standard time.  The
    adjustment may be positive or negative, and the amount of
    adjustment may vary depending on the date and time; the TZif
    format even allows the adjustment to be zero, although this is not
    common practice.
 International Atomic Time (TAI):  The time standard based on atomic
    clocks since 1972.  It is equal to UTC but without leap-second
    adjustments.

Olson, et al. Standards Track [Page 3] RFC 8536 TZif February 2019

 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:  Civil time for a particular location.  Its offset from
    universal time can depend on the date and time of day.
 POSIX Epoch:  1970-01-01 00:00:00 UTC, the basis for absolute
    timestamps in this document.
 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 daylight saving time is in effect
    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
    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.
 Transition Time:  The moment of occurrence of a time change that is
    not a leap second.  It is identified with a signed integer count
    of UNIX leap time seconds since the POSIX epoch.
 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.

Olson, et al. Standards Track [Page 4] RFC 8536 TZif February 2019

 UNIX Time:  The time as returned by the time() function provided by
    the C programming language (see Section 3 of the "System
    Interfaces" volume of [POSIX]).  This is an integer number of
    seconds since the POSIX epoch, 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,
    it accounts for the first leap second, so the UNIX leap time of
    1972-12-31 23:59:60 UTC would be 94694401, and the UNIX leap time
    of 1973-01-01 00:00:00 UTC would be 94694402.  If a TZif file
    specifies no leap-second records, UNIX leap time is equal to UNIX
    time.
 Wall Time:  Another name for local time; short for "wall-clock time".

3. The Time Zone Information Format (TZif)

 The Time Zone Information Format begins with a fixed 44-octet version
 1 header (Section 3.1) containing a field that specifies the version
 of the file's format.  Readers designed for version N can read
 version N+1 files without too much trouble; data specific to version
 N+1 either appears after version N data so that earlier-version
 readers can easily ignore later-version data they are not designed
 for, or it appears as a minor extension to version N that version N
 readers are likely to tolerate well.
 The version 1 header is followed by a variable-length version 1 data
 block (Section 3.2) containing four-octet (32-bit) transition times
 and leap-second occurrences.  These 32-bit values are limited to
 representing time changes from 1901-12-13 20:45:52 through 2038-01-19
 03:14:07 UT, and the version 1 header and data block are present only
 for backward compatibility with obsolescent readers, as discussed in
 Common Interoperability Issues (Appendix A).
 Version 1 files terminate after the version 1 data block.  Files from
 versions 2 and 3 extend the format by appending a second 44-octet
 version 2+ header, a variable-length version 2+ data block containing
 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.

Olson, et al. Standards Track [Page 5] RFC 8536 TZif February 2019

 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.
 A TZif file is structured as follows:
                      Version 1       Versions 2 & 3
                   +-------------+   +-------------+
                   |  Version 1  |   |  Version 1  |
                   |   Header    |   |   Header    |
                   +-------------+   +-------------+
                   |  Version 1  |   |  Version 1  |
                   |  Data Block |   |  Data Block |
                   +-------------+   +-------------+
                                     |  Version 2+ |
                                     |   Header    |
                                     +-------------+
                                     |  Version 2+ |
                                     |  Data Block |
                                     +-------------+
                                     |   Footer    |
                                     +-------------+
                     General Format of TZif Files

3.1. TZif Header

 A TZif header is structured as follows (the lengths of multi-octet
 fields are shown in parentheses):
     +---------------+---+
     |  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 [RFC20] sequence "TZif" (0x54 0x5A 0x69
    0x66), which identifies the file as utilizing the Time Zone
    Information Format.

Olson, et al. Standards Track [Page 6] RFC 8536 TZif February 2019

 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 version 1
       header and data block.  Version 1 files MUST NOT contain a
       version 2+ header, data block, or footer.
    '2' (0x32)  Version 2 - The file MUST contain the version 1 header
       and data block, a version 2+ 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 of [POSIX] and MUST encode the POSIX
       portable character set as ASCII.
    '3' (0x33)  Version 3 - The file MUST contain the version 1 header
       and data block, a version 2+ 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".
 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 local time type records convey no useful
    information in files that have nonempty TZ strings but no
    transitions, at least one such record is nevertheless required
    because many TZif readers reject files that have zero 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 - MUST NOT be zero.  The count includes the
    trailing NUL (0x00) octet at the end of the last time zone
    designation.

Olson, et al. Standards Track [Page 7] RFC 8536 TZif February 2019

 Although the version 1 and 2+ headers have the same format, magic
 number, and version fields, their count fields may differ, because
 the version 1 data can be a subset of the version 2+ data.

3.2. TZif Data Block

 A TZif data block consists of seven variable-length elements, each of
 which is a series of 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 version 1 data block can calculate its
 total length and skip directly to the header of the version 2+ data
 block.
 In the version 1 data block, time values are 32 bits (TIME_SIZE = 4
 octets).  In the version 2+ data block, present only in version 2 and
 3 files, time values are 64 bits (TIME_SIZE = 8 octets).
 The data block is structured as follows (the lengths of multi-octet
 fields are shown in parentheses):
      +---------------------------------------------------------+
      |  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.

Olson, et al. Standards Track [Page 8] RFC 8536 TZif February 2019

    (-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 outlandishly 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 zero-based 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" - 1].
 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 (the lengths of multi-octet fields are shown in
    parentheses):
    +---------------+---+---+
    |  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
       [-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 POSIX-required range [-24:59:59, 25:59:59].
    (is)dst:  A one-octet value indicating whether local time should
       be considered Daylight Saving Time (DST).  The value MUST be 0
       or 1.  A value of one (1) indicates that this type of time is
       DST.  A value of zero (0) indicates that this time type is
       standard time.
    (desig)idx:  A one-octet unsigned integer specifying a zero-based
       index into the series of time zone designation octets, thereby
       selecting a particular designation string.  Each index MUST be
       in the range [0, "charcnt" - 1]; it designates the
       NUL-terminated string of octets starting at position "idx" in
       the time zone designations.  (This string MAY be empty.)  A NUL
       octet MUST exist in the time zone designations at or after
       position "idx".

Olson, et al. Standards Track [Page 9] RFC 8536 TZif February 2019

 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.  The character encoding of time zone designation
    strings is not specified; however, see Section 4 of this document.
 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 (the lengths of multi-octet fields are shown
    in parentheses):
    Version 1 Data Block:
    +---------------+---------------+
    |  occur (4)    |  corr (4)     |
    +---------------+---------------+
    version 2+ 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
       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.  The value
    MUST be set to one (1) if the corresponding UT/local indicator is

Olson, et al. Standards Track [Page 10] RFC 8536 TZif February 2019

    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; otherwise, it is unspecified.  If there are
 no transitions, local time for all timestamps is specified by the TZ
 string in the footer if present and nonempty; otherwise, it is
 specified by time type 0.
 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 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.

Olson, et al. Standards Track [Page 11] RFC 8536 TZif February 2019

3.3. TZif Footer

 The TZif footer is structured as follows (the lengths of multi-octet
 fields are shown in parentheses):
                    +---+--------------------+---+
                    | 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 version 2+ 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 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 version 2+ data, the string MUST be
    consistent with the last version 2+ transition.  In other words,
    evaluating the TZ string at the time of the last transition should
    yield the same time type as was specified in the last transition.
    The string MUST NOT contain NUL octets or be NUL-terminated, and
    it SHOULD NOT begin with the ':' (colon) character.
 The TZif footer is present only in version 2 and 3 files, as the
 obsolescent version 1 format was designed before the need for a
 footer was apparent.

Olson, et al. Standards Track [Page 12] RFC 8536 TZif February 2019

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 of
 [POSIX].
 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

 The following practices help ensure the 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 that only understand version 1 MUST ignore any
    data that extends beyond the calculated end of the version 1 data
    block.
 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 version 1 header and
    data block SHOULD be a contiguous sub-sequence of the time changes
    defined by the version 2+ header and data block, and by the
    footer.  This guideline helps obsolescent version 1 readers agree

Olson, et al. Standards Track [Page 13] RFC 8536 TZif February 2019

    with current readers about timestamps within the contiguous sub-
    sequence.  It also lets writers not supporting obsolescent readers
    use a "timecnt" of zero in the version 1 data block 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.
 o  When reading a version 2 or 3 file, implementations SHOULD ignore
    the version 1 header and data block except for the purpose of
    skipping over them.
 o  Implementations SHOULD calculate the total lengths of the headers
    and data blocks and check that they all fit within 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 as necessary (none

       are necessary if the file is truncated to a range that precedes
       the first leap second).
  • "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 media type "application/tzif"
 (Section 8.1) in its "capabilities" response (see Section 5.1 of
 [RFC7808]).  A TZDIST service MAY also include the 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" media
 type without also advertising "application/tzif".

Olson, et al. Standards Track [Page 14] RFC 8536 TZif February 2019

 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 version 2+
 transition time, if present.
 When truncating the start of a TZif file, the service MUST supply in
 the version 2+ 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 version 2+ 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 TZDIST Request for TZif Data

 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.

Olson, et al. Standards Track [Page 15] RFC 8536 TZif February 2019

 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

Olson, et al. Standards Track [Page 16] RFC 8536 TZif February 2019

6. Security Considerations

 The Time Zone Information Format contains no executable code, and it
 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 it does not define any extensible areas that could be used to
 store private data.
 As discussed in Section 9 of [RFC7808], transmission of time zone
 data over an insecure communications channel could leak the past,
 current, or future location of a device or user.  As such, TZif data
 transmitted over a public communications channel MUST be protected
 with a confidentiality layer such as that provided by Transport Layer
 Security (TLS) [RFC8446].

8. IANA Considerations

 This document defines two media types [RFC6838] 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 RFC 8536.

Olson, et al. Standards Track [Page 17] RFC 8536 TZif February 2019

 Interoperability considerations:  See Section 4 of RFC 8536.
 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) [ZIC] and the GNU C Library [GNU-C].  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 "Authors' Addresses" section of RFC 8536.
 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 RFC 8536.
 Interoperability considerations:  See Section 4 of RFC 8536.
 Published specification:  This specification.

Olson, et al. Standards Track [Page 18] RFC 8536 TZif February 2019

 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) [ZIC] and the GNU C Library [GNU-C].  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 "Authors' Addresses" section of RFC 8536.
 Change controller:  IETF

9. References

9.1. Normative References

 [GNU-C]     "The GNU C Library (glibc)",
             <https://www.gnu.org/software/libc/>.
 [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/>.
 [RFC20]     Cerf, V., "ASCII format for network interchange", STD 80,
             RFC 20, DOI 10.17487/RFC0020, October 1969,
             <https://www.rfc-editor.org/info/rfc20>.
 [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>.

Olson, et al. Standards Track [Page 19] RFC 8536 TZif February 2019

 [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>.
 [ZIC]       Kerrisk, M., "ZIC(8)", man-pages release 4.16, February
             2010, <http://man7.org/linux/man-pages/man8/zic.8.html>.

9.2. Informative References

 [EGGERT-TZ] "History for tz", October 2018,
             <https://github.com/eggert/tz/commits/master/tzfile.5>.
 [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>.
 [RFC8446]   Rescorla, E., "The Transport Layer Security (TLS)
             Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446,
             August 2018, <https://www.rfc-editor.org/info/rfc8446>.
 [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>.

Olson, et al. Standards Track [Page 20] RFC 8536 TZif February 2019

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
 [EGGERT-TZ].  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 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
 documenting other common bugs in readers.
 Interoperability problems with TZif include the following:
 o  Some readers examine only version 1 data.  As a partial
    workaround, a writer can output as much version 1 data as
    possible.  However, a reader should ignore version 1 data and use
    version 2+ data, even if the reader's native timestamps have only
    32 bits.
 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).

Olson, et al. Standards Track [Page 21] RFC 8536 TZif February 2019

 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.
 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.

Olson, et al. Standards Track [Page 22] RFC 8536 TZif February 2019

 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, 15 minutes, or 1 minute.

Appendix B. Example TZif Files

 The following sections contain annotated hexadecimal dumps of example
 TZif files.
 Note that these examples should only be considered informative.
 Although the example data entries are current as of the publication
 date of this document, the data will likely change in the future as
 leap seconds are added and changes are made to civil time.

Olson, et al. Standards Track [Page 23] RFC 8536 TZif February 2019

B.1. Version 1 File Representing UTC (with Leap Seconds)

 +-------+---------------+------------------+------------------------+
 | File  | Data Octets   | Record Name /    | Field Value            |
 | Offset| (hexadecimal) | Field Name       |                        |
 +-------+---------------+------------------+------------------------+
 | 000   | 54 5a 69 66   | magic            | "TZif"                 |
 | 004   | 00            | version          | 0 (1)                  |
 | 005   | 00 00 00 00   |                  |                        |
 |       | 00 00 00 00   |                  |                        |
 |       | 00 00 00 00   |                  |                        |
 |       | 00 00 00      |                  |                        |
 | 020   | 00 00 00 01   | isutccnt         | 1                      |
 | 024   | 00 00 00 01   | isstdcnt         | 1                      |
 | 028   | 00 00 00 1b   | isleapcnt        | 27                     |
 | 032   | 00 00 00 00   | timecnt          | 0                      |
 | 036   | 00 00 00 01   | typecnt          | 1                      |
 | 040   | 00 00 00 04   | charcnt          | 4                      |
 |       |               |                  |                        |
 |       |               | localtimetype[0] |                        |
 | 044   | 00 00 00 00   | utcoff           | 00:00                  |
 | 048   | 00            | isdst            | 0 (no)                 |
 | 049   | 00            | desigidx         | 0                      |
 |       |               |                  |                        |
 | 050   | 55 54 43 00   | designations[0]  | "UTC"                  |
 |       |               |                  |                        |
 |       |               | leapsecond[0]    |                        |
 | 054   | 04 b2 58 00   | occurrence       | 78796800               |
 |       |               |                  | (1972-06-30T23:59:60Z) |
 | 058   | 00 00 00 01   | correction       | 1                      |
 |       |               |                  |                        |
 |       |               | leapsecond[1]    |                        |
 | 062   | 05 a4 ec 01   | occurrence       | 94694401               |
 |       |               |                  | (1972-12-31T23:59:60Z) |
 | 066   | 00 00 00 02   | correction       | 2                      |
 |       |               |                  |                        |
 |       |               | leapsecond[2]    |                        |
 | 070   | 07 86 1f 82   | occurrence       | 126230402              |
 |       |               |                  | (1973-12-31T23:59:60Z) |
 | 074   | 00 00 00 03   | correction       | 3                      |
 |       |               |                  |                        |
 |       |               | leapsecond[3]    |                        |
 | 078   | 09 67 53 03   | occurrence       | 157766403              |
 |       |               |                  | (1974-12-31T23:59:60Z) |
 | 082   | 00 00 00 04   | correction       | 4                      |
 |       |               |                  |                        |

Olson, et al. Standards Track [Page 24] RFC 8536 TZif February 2019

 |       |               | leapsecond[4]    |                        |
 | 086   | 0b 48 86 84   | occurrence       | 189302404              |
 |       |               |                  | (1975-12-31T23:59:60Z) |
 | 090   | 00 00 00 05   | correction       | 5                      |
 |       |               |                  |                        |
 |       |               | leapsecond[5]    |                        |
 | 094   | 0d 2b 0b 85   | occurrence       | 220924805              |
 |       |               |                  | (1976-12-31T23:59:60Z) |
 | 098   | 00 00 00 06   | correction       | 6                      |
 |       |               |                  |                        |
 |       |               | leapsecond[6]    |                        |
 | 102   | 0f 0c 3f 06   | occurrence       | 252460806              |
 |       |               |                  | (1977-12-31T23:59:60Z) |
 | 106   | 00 00 00 07   | correction       | 7                      |
 |       |               |                  |                        |
 |       |               | leapsecond[7]    |                        |
 | 110   | 10 ed 72 87   | occurrence       | 283996807              |
 |       |               |                  | (1978-12-31T23:59:60Z) |
 | 114   | 00 00 00 08   | correction       | 8                      |
 |       |               |                  |                        |
 |       |               | leapsecond[8]    |                        |
 | 118   | 12 ce a6 08   | occurrence       | 315532808              |
 |       |               |                  | (1979-12-31T23:59:60Z) |
 | 122   | 00 00 00 09   | correction       | 9                      |
 |       |               |                  |                        |
 |       |               | leapsecond[9]    |                        |
 | 126   | 15 9f ca 89   | occurrence       | 362793609              |
 |       |               |                  | (1981-06-30T23:59:60Z) |
 | 130   | 00 00 00 0a   | correction       | 10                     |
 |       |               |                  |                        |
 |       |               | leapsecond[10]   |                        |
 | 134   | 17 80 fe 0a   | occurrence       | 394329610              |
 |       |               |                  | (1982-06-30T23:59:60Z) |
 | 138   | 00 00 00 0b   | correction       | 11                     |
 |       |               |                  |                        |
 |       |               | leapsecond[11]   |                        |
 | 142   | 19 62 31 8b   | occurrence       | 425865611              |
 |       |               |                  | (1983-06-30T23:59:60Z) |
 | 146   | 00 00 00 0c   | correction       | 12                     |
 |       |               |                  |                        |
 |       |               | leapsecond[12]   |                        |
 | 150   | 1d 25 ea 0c   | occurrence       | 489024012              |
 |       |               |                  | (1985-06-30T23:59:60Z) |
 | 154   | 00 00 00 0d   | correction       | 13                     |
 |       |               |                  |                        |
 |       |               | leapsecond[13]   |                        |
 | 158   | 21 da e5 0d   | occurrence       | 567993613              |
 |       |               |                  | (1987-12-31T23:59:60Z) |

Olson, et al. Standards Track [Page 25] RFC 8536 TZif February 2019

 | 162   | 00 00 00 0e   | correction       | 14                     |
 |       |               |                  |                        |
 |       |               | leapsecond[14]   |                        |
 | 166   | 25 9e 9d 8e   | occurrence       | 631152014              |
 |       |               |                  | (1989-12-31T23:59:60Z) |
 | 170   | 00 00 00 0f   | correction       | 15                     |
 |       |               |                  |                        |
 |       |               | leapsecond[15]   |                        |
 | 174   | 27 7f d1 0f   | occurrence       | 662688015              |
 |       |               |                  | (1990-12-31T23:59:60Z) |
 | 178   | 00 00 00 10   | correction       | 16                     |
 |       |               |                  |                        |
 |       |               | leapsecond[16]   |                        |
 | 182   | 2a 50 f5 90   | occurrence       | 709948816              |
 |       |               |                  | (1992-06-30T23:59:60Z) |
 | 186   | 00 00 00 11   | correction       | 17                     |
 |       |               |                  |                        |
 |       |               | leapsecond[17]   |                        |
 | 190   | 2c 32 29 11   | occurrence       | 741484817              |
 |       |               |                  | (1993-06-30T23:59:60Z) |
 | 194   | 00 00 00 12   | correction       | 18                     |
 |       |               |                  |                        |
 |       |               | leapsecond[18]   |                        |
 | 198   | 2e 13 5c 92   | occurrence       | 773020818              |
 |       |               |                  | (1994-06-30T23:59:60Z) |
 | 202   | 00 00 00 13   | correction       | 19                     |
 |       |               |                  |                        |
 |       |               | leapsecond[19]   |                        |
 | 206   | 30 e7 24 13   | occurrence       | 820454419              |
 |       |               |                  | (1995-12-31T23:59:60Z) |
 | 210   | 00 00 00 14   | correction       | 20                     |
 |       |               |                  |                        |
 |       |               | leapsecond[20]   |                        |
 | 214   | 33 b8 48 94   | occurrence       | 867715220              |
 |       |               |                  | (1997-06-30T23:59:60Z) |
 | 218   | 00 00 00 15   | correction       | 21                     |
 |       |               |                  |                        |
 |       |               | leapsecond[21]   |                        |
 | 222   | 36 8c 10 15   | occurrence       | 915148821              |
 |       |               |                  | (1998-12-31T23:59:60Z) |
 | 226   | 00 00 00 16   | correction       | 22                     |
 |       |               |                  |                        |
 |       |               | leapsecond[22]   |                        |
 | 230   | 43 b7 1b 96   | occurrence       | 1136073622             |
 |       |               |                  | (2005-12-31T23:59:60Z) |
 | 234   | 00 00 00 17   | correction       | 23                     |
 |       |               |                  |                        |

Olson, et al. Standards Track [Page 26] RFC 8536 TZif February 2019

 |       |               | leapsecond[23]   |                        |
 | 238   | 49 5c 07 97   | occurrence       | 1230768023             |
 |       |               |                  | (2008-12-31T23:59:60Z) |
 | 242   | 00 00 00 18   | correction       | 24                     |
 |       |               |                  |                        |
 |       |               | leapsecond[24]   |                        |
 | 246   | 4f ef 93 18   | occurrence       | 1341100824             |
 |       |               |                  | (2012-06-30T23:59:60Z) |
 | 250   | 00 00 00 19   | correction       | 25                     |
 |       |               |                  |                        |
 |       |               | leapsecond[25]   |                        |
 | 254   | 55 93 2d 99   | occurrence       | 1435708825             |
 |       |               |                  | (2015-06-30T23:59:60Z) |
 | 258   | 00 00 00 1a   | correction       | 26                     |
 |       |               |                  |                        |
 |       |               | leapsecond[26]   |                        |
 | 262   | 58 68 46 9a   | occurrence       | 1483228826             |
 |       |               |                  | (2016-12-31T23:59:60Z) |
 | 266   | 00 00 00 1b   | correction       | 27                     |
 |       |               |                  |                        |
 | 270   | 00            | UT/local[0]      | 0 (local)              |
 |       |               |                  |                        |
 | 271   | 00            | standard/wall[0] | 0 (wall)               |
 +-------+---------------+------------------+------------------------+
 To determine TAI corresponding to 2000-01-01T00:00:00Z
 (UNIX time = 946684800), the following procedure would be followed:
 1.  Find the latest leap-second occurrence prior to the time of
     interest (leapsecond[21]) and note the correction value
     (LEAPCORR = 22).
 2.  Add LEAPCORR + 10 to the time of interest to yield TAI of
     2000-01-01T00:00:32.

Olson, et al. Standards Track [Page 27] RFC 8536 TZif February 2019

B.2. Version 2 File Representing Pacific/Honolulu

 +--------+--------------+------------------+------------------------+
 | File   | Hexadecimal  | Record Name /    | Field Value            |
 | Offset | Octets       | Field Name       |                        |
 +--------+--------------+------------------+------------------------+
 | 000    | 54 5a 69 66  | magic            | "TZif"                 |
 | 004    | 32           | version          | '2' (2)                |
 | 005    | 00 00 00 00  |                  |                        |
 |        | 00 00 00 00  |                  |                        |
 |        | 00 00 00 00  |                  |                        |
 |        | 00 00 00     |                  |                        |
 | 020    | 00 00 00 06  | isutccnt         | 6                      |
 | 024    | 00 00 00 06  | isstdcnt         | 6                      |
 | 028    | 00 00 00 00  | isleapcnt        | 0                      |
 | 032    | 00 00 00 07  | timecnt          | 7                      |
 | 036    | 00 00 00 06  | typecnt          | 6                      |
 | 040    | 00 00 00 14  | charcnt          | 20                     |
 |        |              |                  |                        |
 | 044    | 80 00 00 00  | trans time[0]    | -2147483648            |
 |        |              |                  | (1901-12-13T20:45:52Z) |
 | 048    | bb 05 43 48  | trans time[1]    | -1157283000            |
 |        |              |                  | (1933-04-30T12:30:00Z) |
 | 052    | bb 21 71 58  | trans time[2]    | -1155436200            |
 |        |              |                  | (1933-05-21T21:30:00Z) |
 | 056    | cb 89 3d c8  | trans time[3]    | -880198200             |
 |        |              |                  | (1942-02-09T12:30:00Z) |
 | 060    | d2 23 f4 70  | trans time[4]    | -769395600             |
 |        |              |                  | (1945-08-14T23:00:00Z) |
 | 064    | d2 61 49 38  | trans time[5]    | -765376200             |
 |        |              |                  | (1945-09-30T11:30:00Z) |
 | 068    | d5 8d 73 48  | trans time[6]    | -712150200             |
 |        |              |                  | (1947-06-08T12:30:00Z) |
 |        |              |                  |                        |
 | 072    | 01           | trans type[0]    | 1                      |
 | 073    | 02           | trans type[1]    | 2                      |
 | 074    | 01           | trans type[2]    | 1                      |
 | 075    | 03           | trans type[3]    | 3                      |
 | 076    | 04           | trans type[4]    | 4                      |
 | 077    | 01           | trans type[5]    | 1                      |
 | 078    | 05           | trans type[6]    | 5                      |
 |        |              |                  |                        |
 |        |              | localtimetype[0] |                        |
 | 079    | ff ff 6c 02  | utcoff           | -37886 (-10:21:26)     |
 | 083    | 00           | isdst            | 0 (no)                 |
 | 084    | 00           | desigidx         | 0                      |
 |        |              |                  |                        |

Olson, et al. Standards Track [Page 28] RFC 8536 TZif February 2019

 |        |              | localtimetype[1] |                        |
 | 085    | ff ff 6c 58  | utcoff           | -37800 (-10:30)        |
 | 089    | 00           | isdst            | 0 (no)                 |
 | 090    | 04           | desigidx         | 4                      |
 |        |              |                  |                        |
 |        |              | localtimetype[2] |                        |
 | 091    | ff ff 7a 68  | utcoff           | -34200 (-09:30)        |
 | 095    | 01           | isdst            | 1 (yes)                |
 | 096    | 08           | desigidx         | 8                      |
 |        |              |                  |                        |
 |        |              | localtimetype[3] |                        |
 | 097    | ff ff 7a 68  | utcoff           | -34200 (-09:30)        |
 | 101    | 01           | isdst            | 1 (yes)                |
 | 102    | 0c           | desigidx         | 12                     |
 |        |              |                  |                        |
 |        |              | localtimetype[4] |                        |
 | 103    | ff ff 7a 68  | utcoff           | -34200 (-09:30)        |
 | 107    | 01           | isdst            | 1 (yes)                |
 | 108    | 10           | desigidx         | 16                     |
 |        |              |                  |                        |
 |        |              | localtimetype[5] |                        |
 | 109    | ff ff 73 60  | utcoff           | -36000 (-10:00)        |
 | 113    | 00           | isdst            | 0 (no)                 |
 | 114    | 04           | desigidx         | 4                      |
 |        |              |                  |                        |
 | 115    | 4c 4d 54 00  | designations[0]  | "LMT"                  |
 | 119    | 48 53 54 00  | designations[4]  | "HST"                  |
 | 123    | 48 44 54 00  | designations[8]  | "HDT"                  |
 | 127    | 48 57 54 00  | designations[12] | "HWT"                  |
 | 131    | 48 50 54 00  | designations[16] | "HPT"                  |
 |        |              |                  |                        |
 | 135    | 00           | UT/local[0]      | 1 (UT)                 |
 | 136    | 00           | UT/local[1]      | 0 (local)              |
 | 137    | 00           | UT/local[2]      | 0 (local)              |
 | 138    | 00           | UT/local[3]      | 0 (local)              |
 | 139    | 01           | UT/local[4]      | 1 (UT)                 |
 | 140    | 00           | UT/local[5]      | 0 (local)              |
 |        |              |                  |                        |
 | 141    | 00           | standard/wall[0] | 1 (standard)           |
 | 142    | 00           | standard/wall[1] | 0 (wall)               |
 | 143    | 00           | standard/wall[2] | 0 (wall)               |
 | 144    | 00           | standard/wall[3] | 0 (wall)               |
 | 145    | 01           | standard/wall[4] | 1 (standard)           |
 | 146    | 00           | standard/wall[5] | 0 (wall)               |
 |        |              |                  |                        |
 | 147    | 54 5a 69 66  | magic            | "TZif"                 |
 | 151    | 32           | version          | '2' (2)                |

Olson, et al. Standards Track [Page 29] RFC 8536 TZif February 2019

 | 152    | 00 00 00 00  |                  |                        |
 |        | 00 00 00 00  |                  |                        |
 |        | 00 00 00 00  |                  |                        |
 |        | 00 00 00     |                  |                        |
 | 167    | 00 00 00 06  | isutccnt         | 6                      |
 | 171    | 00 00 00 06  | isstdcnt         | 6                      |
 | 175    | 00 00 00 00  | isleapcnt        | 0                      |
 | 179    | 00 00 00 07  | timecnt          | 7                      |
 | 183    | 00 00 00 06  | typecnt          | 6                      |
 | 187    | 00 00 00 14  | charcnt          | 20                     |
 |        |              |                  |                        |
 | 191    | ff ff ff ff  | trans time[0]    | -2334101314            |
 |        | 74 e0 70 be  |                  | (1896-01-13T22:31:26Z) |
 | 199    | ff ff ff ff  | trans time[1]    | -1157283000            |
 |        | bb 05 43 48  |                  | (1933-04-30T12:30:00Z) |
 | 207    | ff ff ff ff  | trans time[2]    | -1155436200            |
 |        | bb 21 71 58  |                  | (1933-05-21T21:30:00Z) |
 | 215    | ff ff ff ff  | trans time[3]    | -880198200             |
 |        | cb 89 3d c8  |                  | (1942-02-09T12:30:00Z) |
 | 223    | ff ff ff ff  | trans time[4]    | -769395600             |
 |        | d2 23 f4 70  |                  | (1945-08-14T23:00:00Z) |
 | 231    | ff ff ff ff  | trans time[5]    | -765376200             |
 |        | d2 61 49 38  |                  | (1945-09-30T11:30:00Z) |
 | 239    | ff ff ff ff  | trans time[6]    | -712150200             |
 |        | d5 8d 73 48  |                  | (1947-06-08T12:30:00Z) |
 |        |              |                  |                        |
 | 247    | 01           | trans type[0]    | 1                      |
 | 248    | 02           | trans type[1]    | 2                      |
 | 249    | 01           | trans type[2]    | 1                      |
 | 250    | 03           | trans type[3]    | 3                      |
 | 251    | 04           | trans type[4]    | 4                      |
 | 252    | 01           | trans type[5]    | 1                      |
 | 253    | 05           | trans type[6]    | 5                      |
 |        |              |                  |                        |
 |        |              | localtimetype[0] |                        |
 | 254    | ff ff 6c 02  | utcoff           | -37886 (-10:21:26)     |
 | 258    | 00           | isdst            | 0 (no)                 |
 | 259    | 00           | desigidx         | 0                      |
 |        |              |                  |                        |
 |        |              | localtimetype[1] |                        |
 | 260    | ff ff 6c 58  | utcoff           | -37800 (-10:30)        |
 | 264    | 00           | isdst            | 0 (no)                 |
 | 265    | 04           | desigidx         | 4                      |
 |        |              |                  |                        |
 |        |              | localtimetype[2] |                        |
 | 266    | ff ff 7a 68  | utcoff           | -34200 (-09:30)        |
 | 270    | 01           | isdst            | 1 (yes)                |
 | 271    | 08           | desigidx         | 8                      |

Olson, et al. Standards Track [Page 30] RFC 8536 TZif February 2019

 |        |              |                  |                        |
 |        |              | localtimetype[3] |                        |
 | 272    | ff ff 7a 68  | utcoff           | -34200 (-09:30)        |
 | 276    | 01           | isdst            | 1 (yes)                |
 | 277    | 0c           | desigidx         | 12                     |
 |        |              |                  |                        |
 |        |              | localtimetype[4] |                        |
 | 278    | ff ff 7a 68  | utcoff           | -34200 (-09:30)        |
 | 282    | 01           | isdst            | 1 (yes)                |
 | 283    | 10           | desigidx         | 16                     |
 |        |              |                  |                        |
 |        |              | localtimetype[5] |                        |
 | 284    | ff ff 73 60  | utcoff           | -36000 (-10:00)        |
 | 288    | 00           | isdst            | 0 (no)                 |
 | 289    | 04           | desigidx         | 4                      |
 |        |              |                  |                        |
 | 290    | 4c 4d 54 00  | designations[0]  | "LMT"                  |
 | 294    | 48 53 54 00  | designations[4]  | "HST"                  |
 | 298    | 48 44 54 00  | designations[8]  | "HDT"                  |
 | 302    | 48 57 54 00  | designations[12] | "HWT"                  |
 | 306    | 48 50 54 00  | designations[16] | "HPT"                  |
 |        |              |                  |                        |
 | 310    | 00           | UT/local[0]      | 0 (local)              |
 | 311    | 00           | UT/local[1]      | 0 (local)              |
 | 312    | 00           | UT/local[2]      | 0 (local)              |
 | 313    | 00           | UT/local[3]      | 0 (local)              |
 | 314    | 01           | UT/local[4]      | 1 (UT)                 |
 | 315    | 00           | UT/local[5]      | 0 (local)              |
 |        |              |                  |                        |
 | 316    | 00           | standard/wall[0] | 0 (wall)               |
 | 317    | 00           | standard/wall[1] | 0 (wall)               |
 | 318    | 00           | standard/wall[2] | 0 (wall)               |
 | 319    | 00           | standard/wall[3] | 0 (wall)               |
 | 320    | 01           | standard/wall[4] | 1 (standard)           |
 | 321    | 00           | standard/wall[5] | 0 (wall)               |
 |        |              |                  |                        |
 | 322    | 0a           | NL               | '\n'                   |
 | 323    | 48 53 54 31  | TZ string        | "HST10"                |
 |        | 30           |                  |                        |
 | 328    | 0a           | NL               | '\n'                   |
 +--------+--------------+------------------+------------------------+

Olson, et al. Standards Track [Page 31] RFC 8536 TZif February 2019

 To determine the local time in this time zone corresponding to
 1933-05-04T12:00:00Z (UNIX time = -1156939200), the following
 procedure would be followed:
 1.  Find the latest time transition prior to the time of interest
     (trans time[1]).
 2.  Reference the corresponding transition type (trans type[1]) to
     determine the local time type index (2).
 3.  Reference the corresponding local time type (localtimetype[2]) to
     determine the offset from UTC (-09:30), the daylight saving
     indicator (1 = yes), and the index into the time zone designation
     strings (8).
 4.  Look up the corresponding time zone designation string
     (designations[8] = "HDT").
 5.  Add the UTC offset to the time of interest to yield a local
     daylight saving time of 1933-05-04T02:30:00-09:30 (HDT).
 To determine the local time in this time zone corresponding to
 2019-01-01T00:00:00Z (UNIX time = 1546300800), the following
 procedure would be followed:
 1.  Find the latest time transition prior to the time of interest
     (there is no such transition).
 2.  Look up the TZ string in the footer ("HST10"), which indicates
     that the time zone designation is "HST" year-round, and the
     offset to UTC is 10:00.
 3.  Subtract the UTC offset from the time of interest to yield a
     standard local time of 2018-12-31T14:00:00-10:00 (HST).

Olson, et al. Standards Track [Page 32] RFC 8536 TZif February 2019

B.3. Truncated Version 3 File Representing Asia/Jerusalem

 The following TZif file has been truncated to start on
 2038-01-01T00:00:00Z.
 +--------+--------------+------------------+------------------------+
 | File   | Hexadecimal  | Record Name /    | Field Value            |
 | Offset | Octets       | Field Name       |                        |
 +--------+--------------+------------------+------------------------+
 | 000    | 54 5a 69 66  | magic            | "TZif"                 |
 | 004    | 33           | version          | '3' (3)                |
 | 005    | 00 00 00 00  |                  |                        |
 |        | 00 00 00 00  |                  |                        |
 |        | 00 00 00 00  |                  |                        |
 |        | 00 00 00     |                  |                        |
 | 020    | 00 00 00 00  | isutccnt         | 0                      |
 | 024    | 00 00 00 00  | isstdcnt         | 0                      |
 | 028    | 00 00 00 00  | isleapcnt        | 0                      |
 | 032    | 00 00 00 00  | timecnt          | 0                      |
 | 036    | 00 00 00 00  | typecnt          | 0                      |
 | 040    | 00 00 00 00  | charcnt          | 0                      |
 |        |              |                  |                        |
 | 044    | 54 5a 69 66  | magic            | "TZif"                 |
 | 048    | 33           | version          | '3' (3)                |
 | 049    | 00 00 00 00  |                  |                        |
 |        | 00 00 00 00  |                  |                        |
 |        | 00 00 00 00  |                  |                        |
 |        | 00 00 00     |                  |                        |
 | 064    | 00 00 00 03  | isutccnt         | 1                      |
 | 068    | 00 00 00 03  | isstdcnt         | 1                      |
 | 072    | 00 00 00 00  | isleapcnt        | 0                      |
 | 076    | 00 00 00 03  | timecnt          | 1                      |
 | 080    | 00 00 00 03  | typecnt          | 1                      |
 | 084    | 00 00 00 08  | charcnt          | 4                      |
 |        |              |                  |                        |
 | 088    | 00 00 00 00  | trans time[0]    | 2145916800             |
 |        | 7f e8 17 80  |                  | (2038-01-01T00:00:00Z) |
 |        |              |                  |                        |
 | 096    | 00           | trans type[0]    | 0                      |
 |        |              |                  |                        |
 |        |              | localtimetype[0] |                        |
 | 097    | 00 00 1c 20  | utcoff           | 7200 (+02:00)          |
 | 101    | 00           | isdst            | 0 (no)                 |
 | 102    | 00           | desigidx         | 0                      |
 |        |              |                  |                        |
 | 103    | 49 53 54 00  | designations[0]  | "IST"                  |
 |        |              |                  |                        |
 | 107    | 01           | UT/local[0]      | 1 (UT)                 |

Olson, et al. Standards Track [Page 33] RFC 8536 TZif February 2019

 |        |              |                  |                        |
 | 108    | 01           | standard/wall[0] | 1 (standard)           |
 |        |              |                  |                        |
 | 109    | 0a           | NL               | '\n'                   |
 | 110    | 49 53 54 2d  | TZ string        | "IST-2IDT,             |
 |        | 32 49 44 54  |                  | M3.4.4/26,M10.5.0"     |
 |        | 2c 4d 33 2e  |                  |                        |
 |        | 34 2e 34 2f  |                  |                        |
 |        | 32 36 2c 4d  |                  |                        |
 |        | 31 30 2e 35  |                  |                        |
 |        | 2e 30        |                  |                        |
 | 136    | 0a           | NL               | '\n'                   |
 +--------+--------------+------------------+------------------------+

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.

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

Olson, et al. Standards Track [Page 34]

/data/webs/external/dokuwiki/data/pages/rfc/rfc8536.txt · Last modified: 2019/02/12 20:45 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki