The Time Zone Information Format (TZif)
draft-murchison-tzdist-tzif-01

Abstract

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

Open Issues

• Need to review definitions of terms.
• Should we require TZDIST servers to always return a particular format (2 or 3) with/without leap seconds? Or should we allow the client to choose what they want (via parameters on the MIME type)?
• Is the MIME registration complete and correct (e.g. any security or privacy considerations)?

Status of This Memo

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

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This Internet-Draft will expire on November 24, 2018.

Copyright Notice

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

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

Table of Contents

• 1. Introduction
• 2. Conventions Used in This Document
• 3. The Time Zone Information Format (TZif)
• 3.1. TZif Header
• 3.2. TZif Data Block
• 3.3. TZif Footer
• 4. Use with the Time Zone Data Distribution Service
• 4.1. Example
• 5. Security Considerations
• 6. Privacy Considerations
• 7. IANA Considerations
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Appendix A. Change History (To be removed by RFC Editor before publication)
• Authors' Addresses

1. Introduction

Time zone data typically consists of an offset from Universal Coordinated Time (UTC), 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. It is a text-based format used by calendaring and scheduling systems.

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

This specification does not define the source of the time zone data or leap second information. It is assumed that a reliable and accurate source is available. One such source is the IANA-hosted time zone database [RFC6557].

2. Conventions Used in This Document

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

The following terms are used in this document:

Coordinated Universal Time (UTC):

The time at the prime meridian (0 degrees longitude) as determined by atomic clocks and the Earth's rotational time.

Daylight Saving Time (DST):

The time according to a location's law or practice, adjusted forward from standard time during summer months to gain more evening daylight.

International Atomic Time (TAI):

The time at the prime meridian (0 degrees longitude) based on a continuous counting of atomic clock seconds.

Local Time:

The time according to a locations's current time zone offset from Coordinated Universal Time.

POSIX Time:

The time as returned by the C time() function (see Section 3 of the "System Interfaces" Volume of [POSIX]).

Standard Time:

The time according to a location's law or practice, unadjusted for changes such as Daylight Saving Time.

Wall Time:

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

3. The Time Zone Information Format (TZif)

The time zone information format begins with a fixed 44-octet header followed by a variable-length data block using four-octet (32-bit) transition times and leap second occurrences. These 32-bit values are limited to representing times no later than 19 January, 2038 03:14:07 UTC.

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

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

A TZif file is structured as follows:

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

General Format of TZif Files

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

Implementations SHOULD generate version 2 or 3 files, and SHOULD strive to make the pre-2038 transition times in the 32-bit and 64-bit data blocks consistent with one another.

NOTE: All multi-octet integer values MUST be stored in network byte order format (high-order octet first, otherwise known as big-endian).

3.1. TZif Header

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

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

TZif Header

The fields of the header are defined as follows:

magic:

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

ver(sion):

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

NUL (0x00)

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

'2' (0x32)

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

'3' (0x33)

Version 3 - The file MUST contain both the 32-bit header and data block and a 64-bit header, data block, and footer. The POSIX TZ string in the footer string MAY use the following extensions:

• The hours part of the transition times may be signed and range from -167 through 167 instead of the POSIX-required unsigned values from 0 through 24.
• DST is considered to be in effect all year if it starts January 1 at 00:00 and ends December 31 at 24:00 plus the difference between daylight saving and standard time, leaving no room for standard time in the calendar.

isutccnt:

A four-octet unsigned integer specifying the number of UTC/local indicators contained in the data block.

isstdcnt:

A four-octet unsigned integer specifying the number of standard/wall indicators contained in the data block.

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.

charcnt:

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

3.2. TZif Data Block

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

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

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

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

TZif Data Block

The elements of the data block are defined as follows:

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

   +---------------+---------------+
   |  epoch (4)    |  corr (4)     |               
   +---------------+---------------+

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

transition times:

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

transition time index:

A series of one-octet unsigned integers specifying the type of local time of the same-indexed transition time. These values serve as indices into the array of local time type records. The number of time indices is specified by the 'timecnt' field in the header.

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:

utcoff:

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

(is)dst:

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

(desig)idx:

A one-octet unsigned integer specifying an index into the series of time zone designation characters, thereby selecting a particular designation string.

time zone designations:

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

leap second records:

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

32-bit Data Block:

64-bit Data Block:

epoch:

A four- or eight-octet POSIX time value specifying the time at which a leap second occurs.

corr:

A four-octet signed integer specifying the total number of leap seconds to be applied to UTC on or after the epoch.

standard/wall indicators:

A series of one-octet values indicating whether the transition times associated with local time types were specified as standard time or wall clock time. A value of one (1) indicates standard time. A value of zero (0) indicates wall time. The number of values is specified by the 'isstdcnt' field in the header.

UTC/local indicators:

A series of one-octet values indicating whether the transition times associated with local time types were specified as UTC or local time. A value of one (1) indicates UTC. A value of zero (0) indicates local time. The number of values is specified by the 'isutccnt' field in the header.

3.3. TZif Footer

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

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

TZif Footer

The elements of the footer are defined as follows:

NL:

An ASCII new line character (0x0A).

POSIX TZ string:

A rule for computing local time changes after the last transition time stored in the 64-bit data block. The string uses the expanded format of the "TZ" environment variable as defined in Section 8 of the "Base Definitions" Volume of [POSIX]. Note that the string MUST NOT be NUL-terminated, and MAY be empty.

4. Use with the Time Zone Data Distribution Service

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

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

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

4.1. Example

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

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: Wed, 22 Nov 2017 09:32:12 GMT
Content-Type: application/json; charset="utf-8"
Content-Length: xxxx

{
  "version": 1,

  "info": {
    "primary-source": "IANA:2017c",
    "formats": [
      "text/calendar",
      "application/tzif",
    ],
...
  },    
...
}


>> Request <<

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

>> Response <<

HTTP/1.1 200 OK
Date: Wed, 22 Nov 2017 09:32:12 GMT
Content-Type: application/tzif
Content-Length: xxxx
ETag: "123456789-000-111"

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

5. Security Considerations

None.

6. Privacy Considerations

None.

7. IANA Considerations

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

Type name:

application

Subtype name:

tzif

Required parameters:

N/A

Optional parameters:

N/A

Encoding considerations:

Binary

Security considerations:

N/A

Interoperability considerations:

N/A

Published specification:

This specification.

Applications that use this media type:

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

Fragment identifier considerations:

N/A

Additional information:

Magic number(s):

The first 4 bytes are 0x54, 0x5A, 0x69, 0x66

File extensions(s):

N/A

Macintosh file type code(s):

N/A

Person & email address to contact for further information:

Time Zone Database mailing list <tz@iana.org>

Intended usage:

COMMON

Restrictions on usage:

N/A

Author:

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

Change controller:

IETF

8. References

8.1. Normative References

8.2. Informative References

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

Changes since -00:

• Split TZif format description into a general overview and 3 subsections.
• Updated Keywords boilerplate.
• Updated POSIX reference.
• Editorial changes from Eliot Lear.

Authors' Addresses