Internet-Draft TZif June 2022
Olson, et al. Expires 15 December 2022 [Page]
Workgroup:
Internet Engineering Task Force (IETF)
Internet-Draft:
draft-murchison-rfc8536bis-05
Obsoletes:
8536 (if approved)
Published:
Intended Status:
Standards Track
Expires:
Authors:
A.D. Olson
P. Eggert
UCLA
K. Murchison
Fastmail

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 Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 15 December 2022.

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Table of Contents

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 RFC 8536, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 8536. Additionally, a new version of the format is defined. The changes from RFC 8536 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 "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.
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

3.2. 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 -2**59. (-2**59 is the greatest negated power of 2 that predates the Big Bang, and avoiding earlier timestamps works around known TZif reader bugs relating to 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". 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. 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 or at which the leap second table expires. The first value, if present, MUST be nonnegative, 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; otherwise, 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 5.1)).

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 if there are two or more leap-second records the correction value of the last record MAY be the same as the second-to-last record, with the last record indicating the expiration time of the leap-second table.

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 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 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. 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-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.

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".

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. Time type 0 SHOULD be a placeholder indicating that local time is unspecified.

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 nonempty, 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 SHOULD 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.

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.

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
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

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 XXXX.
Interoperability considerations:
See Section 4 of RFC XXXX.
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 XXXX.
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 XXXX.
Interoperability considerations:
See Section 4 of RFC XXXX.
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 XXXX.
Change controller:
IETF

9. References

9.1. Normative References

[GNU-C]
"The GNU C Library (glibc)", <https://www.gnu.org/software/libc/>.
[ITU-R-TF.460]
International Telecommunications Union, "Standard-frequency and time-signal emissions", ITU-R Recommendation TF.460, , <https://www.itu.int/rec/R-REC-TF.460/en>.
[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, , <https://pubs.opengroup.org/onlinepubs/9699919799/>.
[RFC20]
Cerf, V., "ASCII format for network interchange", STD 80, RFC 20, DOI 10.17487/RFC0020, , <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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc7231>.
[RFC7808]
Douglass, M. and C. Daboo, "Time Zone Data Distribution Service", RFC 7808, DOI 10.17487/RFC7808, , <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, , <https://www.rfc-editor.org/info/rfc8174>.
[ZIC]
Kerrisk, M., "ZIC(8)", man-pages release 4.16, , <http://man7.org/linux/man-pages/man8/zic.8.html>.

9.2. Informative References

[EGGERT-TZ]
"History for tz", , <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>.
[RFC5545]
Desruisseaux, B., Ed., "Internet Calendaring and Scheduling Core Object Specification (iCalendar)", RFC 5545, DOI 10.17487/RFC5545, , <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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc8536>.
Eggert, P. and A.D. Olson, "Sources for 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:

  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:

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

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.

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"
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 (-10:21:26)
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"
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 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 (-10:21:26)
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"
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 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 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 can not 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).
  • The TZ string value has been line-wrapped for presentation purposes only.
Table 3
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] ""
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 01 typecnt 1
091 00 00 00 04 charcnt 4
095 00 00 00 00 7f e8 17 80 trans time[0] 2145916800 (2038-01-01T00:00:00Z)
103 00 trans type[0] 0
localtimetype[0]
104 00 00 1c 20 utoff 7200 (+02:00)
108 00 isdst 0 (no)
109 00 desigidx 0
110 49 53 54 00 designations[0] "IST"
114 0a NL '\n'
115 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"
141 0a NL '\n'

B.4. Truncated Version 4 File Representing America/New_York

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).
  • 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 4
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] ""
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 01 typecnt 1
091 00 00 00 04 charcnt 4
095 00 00 00 00 61 cf 99 9b trans time[0] 1640995227 (2022-01-01T00:00:00Z)
103 00 trans type[0] 0
localtimetype[0]
104 ff ff b9 b0 utoff -18000 (-05:00)
108 00 isdst 0 (no)
109 00 desigidx 0
110 45 53 54 00 designations[0] "EST"
leapsecond[0]
114 00 00 00 00 58 68 46 9a occurrence 1483228826 (2016-12-31T23:59:60Z)
122 00 00 00 1b correction 27
leapsecond[1]
126 00 00 00 00 62 ba 44 9b occurrence 1656374427 (2022-06-28T00:00:00Z)
134 00 00 00 1b correction 27
138 0a NL '\n'
139 45 53 54 35 45 44 54 2c 4d 33 2e 32 2e 30 2c 4d 31 31 2e 31 2e 30 TZ string "EST5EDT,
M3.2.0,M11.1.0"
161 0a NL '\n'

Appendix C. Changes from RFC 8536

Appendix D. Change Log

This section is to be removed by RFC Editor before publication.

D.1. Since rfc8536bis-04

D.2. Since rfc8536bis-03

D.3. Since rfc8536bis-02

D.4. Since rfc8536bis-01

D.5. Since rfc8536bis-00

D.6. Since RFC 8536

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, Alexey Melnikov, and Tim Parenti.

Authors' Addresses

Arthur David Olson
Paul Eggert
University of California, Los Angeles
Kenneth Murchison
Fastmail US LLC