RFC 9636 | TZif | October 2024
Olson, et al. | Standards Track | [Page]

Stream:
  Internet Engineering Task Force (IETF)
RFC:
  9636
Obsoletes:
  8536
Category:
  Standards Track
Published:
  October 2024
ISSN:
  2070-1721
Authors:
  A. Olson
  P. Eggert
  UCLA
  K. Murchison
  Fastmail

# RFC 9636

# 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.¶

This document replaces and obsoletes RFC 8536.¶

## 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/rfc9636.¶

## Copyright Notice

Copyright (c) 2024 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

## 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].¶

This document obsoletes [RFC8536], providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of [RFC8536]. Additionally, a new version of the format is defined. The changes from [RFC8536] are summarized in Appendix C.¶

## 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 "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.¶
Leap Second:
  A one-second adjustment to keep UTC close to mean solar time at the prime meridian (see [ITU-R-TF.460]). Each inserted or deleted leap second occurs at the end of a UTC month, that is, a month using the Gregorian calendar and the UTC timescale.¶
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.¶
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 higher 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.¶

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+
    +-------------+   +-------------+
    |  Version 1  |   |  Version 1  |
    |   Header    |   |   Header    |
    +-------------+   +-------------+
    |  Version 1  |   |  Version 1  |
    |  Data Block |   |  Data Block |
    +-------------+   +-------------+
                      |  Version 2+ |
                      |   Header    |
                      +-------------+
                      |  Version 2+ |
                      |  Data Block |
                      +-------------+
                      |   Footer    |
                      +-------------+

Figure 1: 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) |
    +---------------+---------------+---------------+

Figure 2: 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.¶
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 non-empty, 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. The leap-second records MUST NOT be truncated at the start (Section 6.1) and MUST NOT contain an expiration time.¶

  - '3' (0x33) Version 3 - The file MUST conform to all version 2 requirements, except that any TZ string in the footer (Section 3.3) MAY use the TZ string extension described below (Section 3.3.2).¶

  - '4' (0x34) Version 4 - The file MUST conform to all version 3 requirements, except that the leap-second records MAY be truncated at the start and MAY contain an expiration time.¶

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 non-empty 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.¶

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 higher 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)                    |
    +---------------------------------------------------------+

Figure 3: 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 -259. (-259 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 -231 and SHOULD be in the range [-89999, 93599] (i.e., its value SHOULD be more than -25 hours and less than 26 hours). Avoiding -231 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". If the designation string is "-00", the time type is a placeholder indicating that local time is unspecified.¶
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. 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, also known as the leap-second table. 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 or at which the leap-second table expires. The first value, if present, MUST be non-negative, and each leap second MUST occur at the end of a UTC month.¶
  corr(ection):

    A four-octet signed integer specifying the value of LEAPCORR on or after the occurrence. If "leapcnt" is zero, LEAPCORR is zero for all timestamps. If "leapcnt" is nonzero, for timestamps before the first occurrence time, LEAPCORR is zero if the first correction is one (1) or minus one (-1) and is unspecified otherwise (which can happen only in files truncated at the start (Section 6.1)).¶

    The first leap second is a positive leap second if and only if its correction is positive. Each correction after the first MUST differ from the previous correction by either one (1) for a positive leap second or minus one (-1) for a negative leap second, except that in version 4 files with two or more leap-second records, the correction value of the last two records MAY be the same, with the occurrence of last record indicating the expiration time of the leap-second table.¶

    The leap-second table expiration time is the time at which the table no longer records the presence or absence of future leap-second corrections, and post-expiration timestamps cannot be accurately calculated. For example, a leap-second table published in January, which predicts the presence or absence of a leap second at June's end, might expire in mid-December because it is not known when the next leap second will occur.¶

    If leap seconds become permanently discontinued, as requested by the General Conference on Weights and Measures [CGPM-2022-R4], leap-second tables published after the discontinuation time SHOULD NOT expire, since they will not be updated in the foreseeable future.¶

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 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 non-empty; 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 non-empty; otherwise, it is specified by time type 0. A time type with a designation string of "-00" represents an unspecified local time.¶

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. 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-like 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.¶

## 4. Interoperability Considerations

The following practices help ensure the interoperability of TZif applications.¶

- Version 1 files are considered a legacy format and SHOULD NOT be generated, as they do not support transition times after the year 2038.¶
- Readers that understand only version 1 MUST ignore any data that extends beyond the calculated end of the version 1 data block.¶
- Other than version 1, writers SHOULD generate the lowest version number needed by a file's data. This helps interoperability with older readers. For example, a writer SHOULD generate a version 4 file only if its leap-second table either expires or is truncated at the start. Likewise, a writer not generating a version 4 file SHOULD generate a version 3 file only if the TZ string extension is necessary to accurately model transition times.¶
- To save space, writers of version 2+ files MAY output a placeholder version 1 data block with all counts zero except that "typecnt" and "charcnt" are both one (1). If this is done, obsolescent version-1-only readers MUST interpret these files as lacking time changes and time zone abbreviations.¶
- Unless the version 1 data block is a placeholder, the sequence of timestamps defined by the version 1 header and data block SHOULD be a contiguous sub-sequence of the timestamps defined by the version 2+ header and data block and by the footer. This guideline helps obsolescent version 1 readers agree with current readers about timestamps within the contiguous sub-sequence.¶
- When a TZif file contains a leap-second table expiration time, TZif readers SHOULD either refuse to process post-expiration timestamps or process them as if the expiration time did not exist (possibly with an error indication). This lessens disagreement among implementations when processing far-future timestamps that cannot yet be handled exactly.¶
- Time zone designations MUST consist of at least three (3) and no more than six (6) ASCII characters from the set of alphanumerics, "-", and "+". This is compatible with POSIX requirements for time zone abbreviations.¶
- A reader that encounters a time zone designation containing bytes other than ASCII alphanumerics, "-", and "+" SHOULD act if the designation instead contained a signed numeric string derived from the UT offset, for example, "-10" and "+0530" to indicate 10 hours west and 5.5 hours east of Greenwich, respectively.¶
- When reading a version 2 or higher file, readers SHOULD ignore the version 1 header and data block except for the purpose of skipping over them. This improves compatibility among readers of non-conforming files where version 2+ data is not upward compatible with version 1.¶
- Readers 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.¶

- 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 9.2) 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 9.1) to indicate that leap-second records are not included in the TZif data; "leapcnt" in the header(s) MUST be zero (0).¶

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

## 5. Internationalization Considerations

TZif time zone designations contain only ASCII alphanumerics, "-", and "+". Commonly used designations include numeric strings like "-10" and "+0530" for UT offsets and English language abbreviations like "CEST" for Central European Summer Time and "GMT" for Greenwich Mean Time. It is the TZif reader's responsibility to substitute different abbreviations when needed for internationalization, such as substituting "HNC" (l'heure normale du Centre) for "CST" (Central Standard Time) in French-speaking regions. This substitution can be problematic, as abbreviations can be ambiguous; for example, "CST" commonly stands for China Standard Time and Cuba Standard Time as well as Central Standard Time. One approach for addressing this issue can be found in the time zone charts of the Unicode Common Locale Data Repository (CLDR) Project [CLDR].¶

Although the original TZif design allowed for any nonzero octets in time zone designations, and it was common practice until the mid-1990s for designations to contain ASCII spaces, designations are now limited to ASCII alphanumerics, "-", and "+" to avoid confusion and to encourage portability to a wide variety of locales.¶

## 6. 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 9.1) in its "capabilities" response (Section 5.1 of [RFC7808]). A TZDIST service MAY also include the media type "application/tzif-leap" (Section 9.2) 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".¶

TZDIST clients MUST use the HTTP "Accept" header field ([RFC9110], Section 12.5.1) to indicate their preference to receive data in the "application/tzif" and/or "application/tzif-leap" formats.¶

### 6.1. 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. Time type 0 MUST be a placeholder indicating that local time is unspecified, so that the reader is unambiguously informed of truncation at the start.¶

When truncating the start of a TZif file containing leap-second records, the service MUST keep all leap-second records governing timestamps within the truncation range, even if the first such record precedes the start point of the truncation range. If the truncated leap-second table is non-empty, its first record MUST have a positive correction if and only if it represents a positive leap second.¶

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. To this end, the time type of the last transition MUST be a placeholder indicating that local time is unspecified.¶

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.¶

### 6.2. 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.¶

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
    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

¶

## 7. 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.¶

As discussed in Section 8 of [RFC7808], transmission of time zone data over an insecure communication channel could result in tampered data, harming calendaring and scheduling operations. As such, TZif data transmitted over a public communications channel MUST be protected with a security layer such as that provided by Transport Layer Security (TLS) [RFC8446].¶

## 8. 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].¶

## 9. IANA Considerations

IANA has updated the "Media Types" registry as follows.¶

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

### 9.1. application/tzif

Type name:
  application¶
Subtype name:
  tzif¶
Required parameters:
  N/A¶
Optional parameters:
  N/A¶
Encoding considerations:
  binary¶
Security considerations:
  See Section 7 of RFC 9636.¶
Interoperability considerations:
  See Section 4 of RFC 9636.¶
Published specification:
  RFC 9636.¶
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 relative to UNIX time, 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 9636.¶
Change controller:
  IETF¶

### 9.2. application/tzif-leap

Type name:
  application¶
Subtype name:
  tzif-leap¶
Required parameters:
  none¶
Optional parameters:
  none¶
Encoding considerations:
  binary¶
Security considerations:
  See Section 7 of RFC 9636.¶
Interoperability considerations:
  See Section 4 of RFC 9636.¶
Published specification:
  RFC 9636.¶
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 relative to UNIX leap time, 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 9636.¶
Change controller:
  IETF¶

## 10. References

### 10.1. Normative References

[GNU-C]
  Free Software Foundation, "The GNU C Library", <https://www.gnu.org/software/libc/>.
[ITU-R-TF.460]
  International Telecommunication Union, "Standard-frequency and time-signal emissions", ITU-R Recommendation TF.460, February 2002, <https://www.itu.int/rec/R-REC-TF.460/en>.
[POSIX]
  IEEE, "IEEE Standard for Information Technology--Portable Operating System Interface (POSIX(TM)) Base Specifications, Issue 7", POSIX.1-2017, IEEE Std 1003.1-2017, DOI 10.1109/IEEESTD.2018.8277153, January 2018, <https://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>.
[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>.
[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>.
[RFC9110]
  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, <https://www.rfc-editor.org/info/rfc9110>.
[ZIC]
  Kerrisk, M., "zic(8) - Linux manual page", <http://man7.org/linux/man-pages/man8/zic.8.html>.

### 10.2. Informative References

[CGPM-2022-R4]
  General Conference on Weights and Measures, "Resolution 4 of the 27th CGPM (2022)", DOI 10.59161/CGPM2022RES4E, November 2022, <https://www.bipm.org/en/cgpm-2022/resolution-4>.
[CLDR]
  Unicode, Inc., "Unicode CLDR Project", <https://cldr.unicode.org/>.
[EGGERT-TZ]
  "History for tz", commit b5318b5, June 2024, <https://github.com/eggert/tz/commits/main/tzfile.5>.
[Err6426]
  RFC Errata, "Erratum ID 6426", RFC 8536, <https://www.rfc-editor.org/errata/eid6426>.
[Err6435]
  RFC Errata, "Erratum ID 6435", RFC 8536, <https://www.rfc-editor.org/errata/eid6435>.
[Err6757]
  RFC Errata, "Erratum ID 6757", RFC 8536, <https://www.rfc-editor.org/errata/eid6757>.
[Err7681]
  RFC Errata, "Erratum ID 7681", RFC 8536, <https://www.rfc-editor.org/errata/eid7681>.
[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>.
[RFC8536]
  Olson, A., Eggert, P., and K. Murchison, "The Time Zone Information Format (TZif)", RFC 8536, DOI 10.17487/RFC8536, February 2019, <https://www.rfc-editor.org/info/rfc8536>.
[tz-link]
  Eggert, P. and A. Olson, "Time zone and daylight saving time data", <https://www.iana.org/time-zones/repository/tz-link.html>.

## 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 to help:¶

1. TZif writers output files that avoid common pitfalls in older or buggy TZif readers,¶
2. TZif readers avoid common pitfalls when reading files generated by future TZif writers, and¶
3. 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 newer-version data useful even for older-version readers. This section attempts to document these compatibility issues and workarounds as well as other common bugs in readers.¶

Interoperability problems with TZif include the following:¶

- 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 timestamps have only 32 bits.¶
- Some readers designed for version 2 might mishandle timestamps after a version 3 or higher 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.¶
- Some readers designed for version 2 do not support permanent daylight saving time with transitions after 24:00 -- e.g., a TZ string "EST5EDT,0/0,J365/25" denoting permanent Eastern Daylight Time (-04). As a workaround, a writer can substitute standard time for two time zones east, e.g., "XXX3EDT4,0/0,J365/23" for a time zone with a never-used standard time (XXX, -03) and negative daylight saving time (EDT, -04) all year. Alternatively, 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).¶
- Some readers designed for version 2 or 3 and that require strict conformance to [RFC8536] reject version 4 files whose leap-second tables are truncated at the start or end in expiration times.¶
- 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.¶
- 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 placeholder (no-op) first transition at an early time.¶
- Some readers mishandle timestamps before the first transition that has a timestamp that is not less than -231. 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 placeholder transition at timestamp -231.¶
- Some readers mishandle a transition if its timestamp has the minimum possible signed 64-bit value. Timestamps less than -259 are not recommended.¶
- 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.¶
- Many readers mishandle time zone abbreviations that contain non-ASCII characters. These characters are not recommended.¶
- 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.¶
- This specification does not dictate how readers should deal with timestamps when local time is unspecified. Common practice is for readers to report UT with designation string "-00". A reader could return an error indication instead.¶
- 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 readers generate ambiguous timestamps for positive leap seconds that occur when the UTC offset is not a multiple of 60 seconds. For example, with UTC offset +01:23:45 and a positive leap second 78796801 (1972-06-30 23:59:60 UTC), some readers will map both 78796800 and 78796801 to 01:23:45 local time the next day instead of mapping the latter to 01:23:46, and they will map 78796815 to 01:23:59 instead of to 01:23:60. This has not yet been a practical problem, since no civil authority has observed such UTC offsets since leap seconds were introduced in 1972.¶

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

- 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.¶
- Some readers mishandle timestamps before the first transition that has a non-negative timestamp. Readers that do not support negative timestamps are likely to be more prone to this problem.¶
- Some readers mishandle time zone abbreviations like "-08" that contain "+", "-", or digits.¶
- Some readers mishandle UT offsets that are out of the conventional range of -12 through +12 hours and so do not support locations like Kiritimati that are outside this range.¶
- 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".¶
- 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.¶

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.¶

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

Table 1
File Offset | Hexadecimal Octets | Record Name / Field Name | Field Value
--- | --- | --- | ---
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 | isutcnt | 1
024 | 00 00 00 01 | isstdcnt | 1
028 | 00 00 00 1b | leapcnt | 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 | utoff | 0 (+00:00)
048 | 00 | isdst | 0 (no)
049 | 00 | desigidx | 0
050 | 55 54 43 00 | designations[0] | "UTC\0"
| 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
| 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)
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
| 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 | standard/wall[0] | 0 (wall)
271 | 00 | UT/local[0] | 0 (local)

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.¶

### B.2. Version 2 File Representing Pacific/Honolulu

Table 2
File Offset | Hexadecimal Octets | Record Name / Field Name | Field Value
--- | --- | --- | ---
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 | isutcnt | 6
024 | 00 00 00 06 | isstdcnt | 6
028 | 00 00 00 00 | leapcnt | 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 | utoff | -37886 (-10:31:26)
083 | 00 | isdst | 0 (no)
084 | 00 | desigidx | 0
| localtimetype[1] |
085 | ff ff 6c 58 | utoff | -37800 (-10:30)
089 | 00 | isdst | 0 (no)
090 | 04 | desigidx | 4
| localtimetype[2] |
091 | ff ff 7a 68 | utoff | -34200 (-09:30)
095 | 01 | isdst | 1 (yes)
096 | 08 | desigidx | 8
| localtimetype[3] |
097 | ff ff 7a 68 | utoff | -34200 (-09:30)
101 | 01 | isdst | 1 (yes)
102 | 0c | desigidx | 12
| localtimetype[4] |
103 | ff ff 7a 68 | utoff | -34200 (-09:30)
107 | 01 | isdst | 1 (yes)
108 | 10 | desigidx | 16
| localtimetype[5] |
109 | ff ff 73 60 | utoff | -36000 (-10:00)
113 | 00 | isdst | 0 (no)
114 | 04 | desigidx | 4
115 | 4c 4d 54 00 | designations[0] | "LMT\0"
119 | 48 53 54 00 | designations[4] | "HST\0"
123 | 48 44 54 00 | designations[8] | "HDT\0"
127 | 48 57 54 00 | designations[12] | "HWT\0"
131 | 48 50 54 00 | designations[16] | "HPT\0"
135 | 00 | standard/wall[0] | 0 (wall)
136 | 00 | standard/wall[1] | 0 (wall)
137 | 00 | standard/wall[2] | 0 (wall)
138 | 00 | standard/wall[3] | 0 (wall)
139 | 01 | standard/wall[4] | 1 (standard)
140 | 00 | standard/wall[5] | 0 (wall)
141 | 00 | UT/local[0] | 0 (local)
142 | 00 | UT/local[1] | 0 (local)
143 | 00 | UT/local[2] | 0 (local)
144 | 00 | UT/local[3] | 0 (local)
145 | 01 | UT/local[4] | 1 (UT)
146 | 00 | UT/local[5] | 0 (local)
147 | 54 5a 69 66 | magic | "TZif"
151 | 32 | version | '2' (2)
152 | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |
167 | 00 00 00 06 | isutcnt | 6
171 | 00 00 00 06 | isstdcnt | 6
175 | 00 00 00 00 | leapcnt | 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 74 e0 70 be | trans time[0] | -2334101314 (1896-01-13T22:31:26Z)
199 | ff ff ff ff bb 05 43 48 | trans time[1] | -1157283000 (1933-04-30T12:30:00Z)
207 | ff ff ff ff bb 21 71 58 | trans time[2] | -1155436200 (1933-05-21T21:30:00Z)
215 | ff ff ff ff cb 89 3d c8 | trans time[3] | -880198200 (1942-02-09T12:30:00Z)
223 | ff ff ff ff d2 23 f4 70 | trans time[4] | -769395600 (1945-08-14T23:00:00Z)
231 | ff ff ff ff d2 61 49 38 | trans time[5] | -765376200 (1945-09-30T11:30:00Z)
239 | ff ff ff ff d5 8d 73 48 | trans time[6] | -712150200 (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 | utoff | -37886 (-10:31:26)
258 | 00 | isdst | 0 (no)
259 | 00 | desigidx | 0
| localtimetype[1] |
260 | ff ff 6c 58 | utoff | -37800 (-10:30)
264 | 00 | isdst | 0 (no)
265 | 04 | desigidx | 4
| localtimetype[2] |
266 | ff ff 7a 68 | utoff | -34200 (-09:30)
270 | 01 | isdst | 1 (yes)
271 | 08 | desigidx | 8
| localtimetype[3] |
272 | ff ff 7a 68 | utoff | -34200 (-09:30)
276 | 01 | isdst | 1 (yes)
277 | 0c | desigidx | 12
| localtimetype[4] |
278 | ff ff 7a 68 | utoff | -34200 (-09:30)
282 | 01 | isdst | 1 (yes)
283 | 10 | desigidx | 16
| localtimetype[5] |
284 | ff ff 73 60 | utoff | -36000 (-10:00)
288 | 00 | isdst | 0 (no)
289 | 04 | desigidx | 4
290 | 4c 4d 54 00 | designations[0] | "LMT\0"
294 | 48 53 54 00 | designations[4] | "HST\0"
298 | 48 44 54 00 | designations[8] | "HDT\0"
302 | 48 57 54 00 | designations[12] | "HWT\0"
306 | 48 50 54 00 | designations[16] | "HPT\0"
310 | 00 | standard/wall[0] | 0 (wall)
311 | 00 | standard/wall[1] | 0 (wall)
312 | 00 | standard/wall[2] | 0 (wall)
313 | 00 | standard/wall[3] | 0 (wall)
314 | 01 | standard/wall[4] | 1 (standard)
315 | 00 | standard/wall[5] | 0 (wall)
316 | 00 | UT/local[0] | 0 (local)
317 | 00 | UT/local[1] | 0 (local)
318 | 00 | UT/local[2] | 0 (local)
319 | 00 | UT/local[3] | 0 (local)
320 | 01 | UT/local[4] | 1 (UT)
321 | 00 | UT/local[5] | 0 (local)
322 | 0a | NL | '\n'
323 | 48 53 54 31 30 | TZ string | "HST10"
328 | 0a | NL | '\n'

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).¶

### B.3. Truncated Version 2 File Representing Pacific/Johnston

The following TZif file has been truncated to end on 2004-06-161T00:00:00Z (the atoll was abandoned sometime on 2004-06-15).¶

In this example:¶

- The version 1 header contains only the required minimum data, which will be ignored by readers.¶
- The version 2 header leverages the fact that, by specifying 'isutcnt' and 'isstdcnt' as zero, all transition times associated with local time types are assumed to be specified as local wall-clock time (see the definitions of UT/local indicators and standard/wall indicators in Section 3.2).¶
- The time type of the last transition has designation "-00", indicating that local time is unspecified.¶
- The TZ string is empty, indicating that there are no known future transitions.¶

Table 3
File Offset | Hexadecimal Octets | Record Name / Field Name | Field Value
--- | --- | --- | ---
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 00 | isutcnt | 0
024 | 00 00 00 00 | isstdcnt | 0
028 | 00 00 00 00 | leapcnt | 0
032 | 00 00 00 00 | timecnt | 0
036 | 00 00 00 01 | typecnt | 1
040 | 00 00 00 01 | charcnt | 1
| localtimetype[0] |
044 | 00 00 00 00 | utoff | 0 (+00:00)
048 | 00 | isdst | 0 (no)
049 | 00 | desigidx | 0
050 | 00 | designations[0] | "\0"
051 | 54 5a 69 66 | magic | "TZif"
055 | 32 | version | '2' (2)
056 | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |
071 | 00 00 00 00 | isutcnt | 0
075 | 00 00 00 00 | isstdcnt | 0
079 | 00 00 00 00 | leapcnt | 0
083 | 00 00 00 08 | timecnt | 8
087 | 00 00 00 07 | typecnt | 7
091 | 00 00 00 18 | charcnt | 24
095 | ff ff ff ff 74 e0 70 be | trans time[0] | -2334101314 (1896-01-13T22:31:26Z)
103 | ff ff ff ff bb 05 43 48 | trans time[1] | -1157283000 (1933-04-30T12:30:00Z)
111 | ff ff ff ff bb 21 71 58 | trans time[2] | -1155436200 (1933-05-21T21:30:00Z)
119 | ff ff ff ff cb 89 3d c8 | trans time[3] | -880198200 (1942-02-09T12:30:00Z)
127 | ff ff ff ff d2 23 f4 70 | trans time[4] | -769395600 (1945-08-14T23:00:00Z)
135 | ff ff ff ff d2 61 49 38 | trans time[5] | -765376200 (1945-09-30T11:30:00Z)
143 | ff ff ff ff d5 8d 73 48 | trans time[6] | -712150200 (1947-06-08T12:30:00Z)
151 | 00 00 00 00 40 cf 8d 80 | trans time[7] | 1087344000 (2004-06-16T00:00:00Z)
159 | 02 | trans type[0] | 2
160 | 03 | trans type[1] | 3
161 | 02 | trans type[2] | 2
162 | 04 | trans type[3] | 4
163 | 05 | trans type[4] | 5
164 | 02 | trans type[5] | 2
165 | 06 | trans type[6] | 6
166 | 01 | trans type[7] | 1
| localtimetype[0] |
167 | ff ff 6c 02 | utoff | -37886 (-10:31:26)
171 | 00 | isdst | 0 (no)
172 | 04 | desigidx | 4
| localtimetype[1] |
173 | 00 00 00 00 | utoff | 0 (+00:00)
177 | 00 | isdst | 0 (no)
178 | 00 | desigidx | 0
| localtimetype[2] |
179 | ff ff 6c 58 | utoff | -37800 (-10:30)
183 | 00 | isdst | 0 (no)
184 | 08 | desigidx | 8
| localtimetype[3] |
185 | ff ff 7a 68 | utoff | -34200 (-09:30)
189 | 01 | isdst | 1 (yes)
190 | 0c | desigidx | 12
| localtimetype[4] |
191 | ff ff 7a 68 | utoff | -34200 (-09:30)
195 | 01 | isdst | 1 (yes)
196 | 10 | desigidx | 16
| localtimetype[5] |
197 | ff ff 7a 68 | utoff | -34200 (-09:30)
201 | 01 | isdst | 1 (yes)
202 | 14 | desigidx | 20
| localtimetype[6] |
203 | ff ff 73 60 | utoff | -36000 (-10:00)
207 | 00 | isdst | 0 (no)
208 | 08 | desigidx | 8
209 | 2d 30 30 00 | designations[0] | "-00\0"
213 | 4c 4d 54 00 | designations[4] | "LMT\0"
217 | 48 53 54 00 | designations[8] | "HST\0"
221 | 48 44 54 00 | designations[12] | "HDT\0"
225 | 48 57 54 00 | designations[16] | "HWT\0"
229 | 48 50 54 00 | designations[20] | "HPT\0"
233 | 0a | NL | '\n'
234 |  | TZ string | ""
234 | 0a | NL | '\n'

### B.4. Truncated Version 3 File Representing Asia/Jerusalem

The following TZif file has been truncated to start on 2038-01-01T00:00:00Z.¶

In this example:¶

- The start time value cannot be represented using 32 bits, so the version 1 header contains only the required minimum data, which will be ignored by readers.¶
- The version 3 header leverages the fact that, by specifying 'isutcnt' and 'isstdcnt' as zero, all transition times associated with local time types are assumed to be specified as local wall-clock time (see the definitions of UT/local indicators and standard/wall indicators in Section 3.2).¶
- Time type 0 has designation "-00", indicating that local time is unspecified prior to the truncation time.¶
- The TZ string value has been line-wrapped for presentation purposes only.¶

Table 4
File Offset | Hexadecimal Octets | Record Name / Field Name | Field Value
--- | --- | --- | ---
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 | isutcnt | 0
024 | 00 00 00 00 | isstdcnt | 0
028 | 00 00 00 00 | leapcnt | 0
032 | 00 00 00 00 | timecnt | 0
036 | 00 00 00 01 | typecnt | 1
040 | 00 00 00 01 | charcnt | 1
| localtimetype[0] |
044 | 00 00 00 00 | utoff | 0 (+00:00)
048 | 00 | isdst | 0 (no)
049 | 00 | desigidx | 0
050 | 00 | designations[0] | "\0"
051 | 54 5a 69 66 | magic | "TZif"
055 | 33 | version | '3' (3)
056 | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |
071 | 00 00 00 00 | isutcnt | 0
075 | 00 00 00 00 | isstdcnt | 0
079 | 00 00 00 00 | leapcnt | 0
083 | 00 00 00 01 | timecnt | 1
087 | 00 00 00 02 | typecnt | 2
091 | 00 00 00 08 | charcnt | 8
095 | 00 00 00 00 7f e8 17 80 | trans time[0] | 2145916800 (2038-01-01T00:00:00Z)
103 | 01 | trans type[0] | 1
| localtimetype[0] |
104 | 00 00 00 00 | utoff | 0 (+00:00)
108 | 00 | isdst | 0 (no)
109 | 00 | desigidx | 0
| localtimetype[1] |
110 | 00 00 1c 20 | utoff | 7200 (+02:00)
114 | 00 | isdst | 0 (no)
115 | 04 | desigidx | 4
116 | 2d 30 30 00 | designations[0] | "-00\0"
120 | 49 53 54 00 | designations[4] | "IST\0"
124 | 0a | NL | '\n'
125 | 49 53 54 2d 32 49 44 54 2c 4d 33 2e 34 2e 34 2f 32 36 2c 4d 31 30 2e 35 2e 30 | TZ string | "IST-2IDT,M3.4.4/26,M10.5.0"
151 | 0a | NL | '\n'

### B.5. Truncated Version 4 File Representing Europe/London

The following TZif file has been truncated to start on 2022-01-01T00:00:00Z.¶

In this example:¶

- The version 1 header contains only the required minimum data, which will be ignored by readers.¶
- The version 4 header leverages the fact that, by specifying 'isutcnt' and 'isstdcnt' as zero, all transition times associated with local time types are assumed to be specified as local wall-clock time (see the definitions of UT/local indicators and standard/wall indicators in Section 3.2).¶
- Time type 0 has designation "-00", indicating that local time is unspecified prior to the truncation time.¶
- The first leap-second occurrence is the most recent one prior to the truncation time.¶
- The last leap-second correction matches the second-to-last leap-second correction, indicating the expiration time of the leap-second table.¶
- The TZ string value has been line-wrapped for presentation purposes only.¶

Table 5
File Offset | Hexadecimal Octets | Record Name / Field Name | Field Value
--- | --- | --- | ---
000 | 54 5a 69 66 | magic | "TZif"
004 | 34 | version | '4' (4)
005 | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |
020 | 00 00 00 00 | isutcnt | 0
024 | 00 00 00 00 | isstdcnt | 0
028 | 00 00 00 00 | leapcnt | 0
032 | 00 00 00 00 | timecnt | 0
036 | 00 00 00 01 | typecnt | 1
040 | 00 00 00 01 | charcnt | 1
| localtimetype[0] |
044 | 00 00 00 00 | utoff | 0 (+00:00)
048 | 00 | isdst | 0 (no)
049 | 00 | desigidx | 0
050 | 00 | designations[0] | "\0"
051 | 54 5a 69 66 | magic | "TZif"
055 | 34 | version | '4' (4)
056 | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |
071 | 00 00 00 00 | isutcnt | 0
075 | 00 00 00 00 | isstdcnt | 0
079 | 00 00 00 02 | leapcnt | 2
083 | 00 00 00 01 | timecnt | 1
087 | 00 00 00 02 | typecnt | 2
091 | 00 00 00 08 | charcnt | 8
095 | 00 00 00 00 61 cf 99 9b | trans time[0] | 1640995227 (2022-01-01T00:00:27Z)
103 | 01 | trans type[0] | 1
| localtimetype[0] |
104 | 00 00 00 00 | utoff | 0 (+00:00)
108 | 00 | isdst | 0 (no)
109 | 00 | desigidx | 0
| localtimetype[1] |
110 | 00 00 00 00 | utoff | 0 (+00:00)
114 | 00 | isdst | 0 (no)
115 | 04 | desigidx | 4
116 | 2d 30 30 00 | designations[0] | "-00\0"
120 | 47 4d 54 00 | designations[4] | "GMT\0"
| leapsecond[0] |
124 | 00 00 00 00 58 68 46 9a | occurrence | 1483228826 (2016-12-31T23:59:60Z)
132 | 00 00 00 1b | correction | 27
| leapsecond[1] |
136 | 00 00 00 00 66 7d fd 1b | occurrence | 1719532827 (2024-06-28T00:00:01Z)
144 | 00 00 00 1b | correction | 27
148 | 0a | NL | '\n'
149 | 47 4d 54 30 42 53 54 2c 4d 33 2e 35 2e 30 2f 31 2c 4d 31 30 2e 35 2e 30 | TZ string | "GMT0BST,M3.5.0/1,M10.5.0"
173 | 0a | NL | '\n'

## Appendix C. Changes from RFC 8536

- Added definition of Leap Second.¶
- Added specification of the version 4 format and the optional leap-second table truncation and expiration, along with an example and relevant interoperability considerations.¶
- Documented the longstanding practice that UT with designation string "-00" denotes unspecified local time. Added recommendation that this designation string should be used for timestamps excluded by TZif file truncation.¶
- Required support in version 2 files for all-year daylight saving time, using POSIX TZ strings with negative DST, as this is not an extension to POSIX (Section 3.3.1).¶
- Applied erratum [Err6435].¶
- Addressed errata [Err6426], [Err6757], and [Err7681] as well as several other errors in the examples.¶
- Added additional interoperability considerations and common issues.¶
- Added an example of a TZif file truncated at the end (Appendix B.3).¶
- Added informational notes to Appendix B.4.¶
- Miscellaneous editorial changes.¶

## Acknowledgments

The authors thank the following individuals for contributing their ideas and support for writing this specification: Michael Douglass, Ned Freed, Guy Harris, Eliot Lear, Alexey Melnikov, and Tim Parenti.¶

## Authors' Addresses