Internet Engineering Task Force (IETF) A. Olson

Internet Engineering Task Force (IETF) A. Olson
Request for Comments: 8536
Category: Standards Track P. Eggert
ISSN: 2070-1721 UCLA
K. Murchison
FastMail
February 2019

The Time Zone Information Format (TZif)

Abstract

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

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8536.

Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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Table of Contents

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

Olson, et al. Standards Track [Page 2]

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1. Introduction

Time zone data typically consists of offsets from universal time
(UT), daylight saving transition rules, one or more local time
designations (acronyms or abbreviations), and optional leap-second
adjustments. One such format for conveying this information is
iCalendar [RFC5545]. It is a text-based format used by calendaring
and scheduling systems.

This document specifies the widely deployed Time Zone Information
Format (TZif). It is a binary format used by most UNIX systems to
calculate local time. This format was introduced in the 1980s and
has evolved since then into multiple upward-compatible versions.
There is a wide variety of interoperable software capable of
generating and reading files in this format [tz-link].

This specification does not define the source of the data assembled
into a TZif file. One such source is the IANA-hosted time zone
database [RFC6557].

2. Conventions Used in This Document

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

The following terms are used in this document (see "Sources for Time
Zone and Daylight Saving Time Data" [tz-link] for more detailed
information about civil timekeeping data and practice):

Coordinated Universal Time (UTC): The basis for civil time since
1960. It is approximately equal to mean solar time at the prime
meridian (0 degrees longitude).

Daylight Saving Time (DST): The time according to a location's law
or practice, when adjusted as necessary from standard time. The
adjustment may be positive or negative, and the amount of
adjustment may vary depending on the date and time; the TZif
format even allows the adjustment to be zero, although this is not
common practice.

International Atomic Time (TAI): The time standard based on atomic
clocks since 1972. It is equal to UTC but without leap-second
adjustments.

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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.

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UNIX Time: The time as returned by the time() function provided by
the C programming language (see Section 3 of the "System
Interfaces" volume of [POSIX]). This is an integer number of
seconds since the POSIX epoch, not counting leap seconds. As an
extension to POSIX, negative values represent times before the
POSIX epoch, using UT.

UNIX Leap Time: UNIX time plus all preceding leap-second
corrections. For example, if the first leap-second record in a
TZif file occurs at 1972-06-30 23:59:60 UTC, the UNIX leap time
for the timestamp 1972-07-01 00:00:00 UTC would be 78796801, one
greater than the UNIX time for the same timestamp. Similarly, if
the second leap-second record occurs at 1972-12-31 23:59:60 UTC,
it accounts for the first leap second, so the UNIX leap time of
1972-12-31 23:59:60 UTC would be 94694401, and the UNIX leap time
of 1973-01-01 00:00:00 UTC would be 94694402. If a TZif file
specifies no leap-second records, UNIX leap time is equal to UNIX
time.

Wall Time: Another name for local time; short for "wall-clock time".

3. The Time Zone Information Format (TZif)

The Time Zone Information Format begins with a fixed 44-octet version
1 header (Section 3.1) containing a field that specifies the version
of the file's format. Readers designed for version N can read
version N+1 files without too much trouble; data specific to version
N+1 either appears after version N data so that earlier-version
readers can easily ignore later-version data they are not designed
for, or it appears as a minor extension to version N that version N
readers are likely to tolerate well.

The version 1 header is followed by a variable-length version 1 data
block (Section 3.2) containing four-octet (32-bit) transition times
and leap-second occurrences. These 32-bit values are limited to
representing time changes from 1901-12-13 20:45:52 through 2038-01-19
03:14:07 UT, and the version 1 header and data block are present only
for backward compatibility with obsolescent readers, as discussed in
Common Interoperability Issues (Appendix A).

Version 1 files terminate after the version 1 data block. Files from
versions 2 and 3 extend the format by appending a second 44-octet
version 2+ header, a variable-length version 2+ data block containing
eight-octet (64-bit) transition times and leap-second occurrences,
and a variable-length footer (Section 3.3). These 64-bit values can
represent times approximately 292 billion years into the past or
future.

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NOTE: All multi-octet integer values MUST be stored in network octet
order format (high-order octet first, otherwise known as big-endian),
with all bits significant. Signed integer values MUST be represented
using two's complement.

A TZif file is structured as follows:

Version 1 Versions 2 & 3
+-------------+ +-------------+
| Version 1 | | Version 1 |
| Header | | Header |
+-------------+ +-------------+
| Version 1 | | Version 1 |
| Data Block | | Data Block |
+-------------+ +-------------+
| Version 2+ |
| Header |
+-------------+
| Version 2+ |
| Data Block |
+-------------+
| Footer |
+-------------+

General Format of TZif Files

3.1. TZif Header

A TZif header is structured as follows (the lengths of multi-octet
fields are shown in parentheses):

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

TZif Header

The fields of the header are defined as follows:

magic: The four-octet ASCII [RFC20] sequence "TZif" (0x54 0x5A 0x69
0x66), which identifies the file as utilizing the Time Zone
Information Format.

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

NUL (0x00) Version 1 - The file contains only the version 1
header and data block. Version 1 files MUST NOT contain a
version 2+ header, data block, or footer.

'2' (0x32) Version 2 - The file MUST contain the version 1 header
and data block, a version 2+ header and data block, and a
footer. The TZ string in the footer (Section 3.3), if
nonempty, MUST strictly adhere to the requirements for the TZ
environment variable as defined in Section 8.3 of the "Base
Definitions" volume of [POSIX] and MUST encode the POSIX
portable character set as ASCII.

'3' (0x33) Version 3 - The file MUST contain the version 1 header
and data block, a version 2+ header and data block, and a
footer. The TZ string in the footer (Section 3.3), if
nonempty, MUST conform to POSIX requirements with ASCII
encoding, except that it MAY use the TZ string extensions
described below (Section 3.3.1).

isutcnt: A four-octet unsigned integer specifying the number of UT/
local indicators contained in the data block -- MUST either be
zero or equal to "typecnt".

isstdcnt: A four-octet unsigned integer specifying the number of
standard/wall indicators contained in the data block -- MUST
either be zero or equal to "typecnt".

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

timecnt: A four-octet unsigned integer specifying the number of
transition times contained in the data block.

typecnt: A four-octet unsigned integer specifying the number of
local time type records contained in the data block -- MUST NOT be
zero. (Although local time type records convey no useful
information in files that have nonempty TZ strings but no
transitions, at least one such record is nevertheless required
because many TZif readers reject files that have zero time types.)

charcnt: A four-octet unsigned integer specifying the total number
of octets used by the set of time zone designations contained in
the data block - MUST NOT be zero. The count includes the
trailing NUL (0x00) octet at the end of the last time zone
designation.

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

3.2. TZif Data Block

A TZif data block consists of seven variable-length elements, each of
which is a series of items. The number of items in each series is
determined by the corresponding count field in the header. The total
length of each element is calculated by multiplying the number of
items by the size of each item. Therefore, implementations that do
not wish to parse or use the version 1 data block can calculate its
total length and skip directly to the header of the version 2+ data
block.

In the version 1 data block, time values are 32 bits (TIME_SIZE = 4
octets). In the version 2+ data block, present only in version 2 and
3 files, time values are 64 bits (TIME_SIZE = 8 octets).

The data block is structured as follows (the lengths of multi-octet
fields are shown in parentheses):

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

TZif Data Block

The elements of the data block are defined as follows:

transition times: A series of four- or eight-octet UNIX leap-time
values sorted in strictly ascending order. Each value is used as
a transition time at which the rules for computing local time may
change. The number of time values is specified by the "timecnt"
field in the header. Each time value SHOULD be at least -2**59.

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(-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".

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time zone designations: A series of octets constituting an array of
NUL-terminated (0x00) time zone designation strings. The total
number of octets is specified by the "charcnt" field in the
header. Note that two designations MAY overlap if one is a suffix
of the other. The character encoding of time zone designation
strings is not specified; however, see Section 4 of this document.

leap-second records: A series of eight- or twelve-octet records
specifying the corrections that need to be applied to UTC in order
to determine TAI. The records are sorted by the occurrence time
in strictly ascending order. The number of records is specified
by the "leapcnt" field in the header. Each record has one of the
following structures (the lengths of multi-octet fields are shown
in parentheses):

Version 1 Data Block:

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

version 2+ Data Block:

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

occur(rence): A four- or eight-octet UNIX leap time value
specifying the time at which a leap-second correction occurs.
The first value, if present, MUST be nonnegative, and each
later value MUST be at least 2419199 greater than the previous
value. (This is 28 days' worth of seconds, minus a potential
negative leap second.)

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

standard/wall indicators: A series of one-octet values indicating
whether the transition times associated with local time types were
specified as standard time or wall-clock time. Each value MUST be
0 or 1. A value of one (1) indicates standard time. The value
MUST be set to one (1) if the corresponding UT/local indicator is

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set to one (1). A value of zero (0) indicates wall time. The
number of values is specified by the "isstdcnt" field in the
header. If "isstdcnt" is zero (0), all transition times
associated with local time types are assumed to be specified as
wall time.

UT/local indicators: A series of one-octet values indicating whether
the transition times associated with local time types were
specified as UT or local time. Each value MUST be 0 or 1. A
value of one (1) indicates UT, and the corresponding standard/wall
indicator MUST also be set to one (1). A value of zero (0)
indicates local time. The number of values is specified by the
"isutcnt" field in the header. If "isutcnt" is zero (0), all
transition times associated with local time types are assumed to
be specified as local time.

The type corresponding to a transition time specifies local time for
timestamps starting at the given transition time and continuing up
to, but not including, the next transition time. Local time for
timestamps before the first transition is specified by the first time
type (time type 0). Local time for timestamps on or after the last
transition is specified by the TZ string in the footer (Section 3.3)
if present and nonempty; otherwise, it is unspecified. If there are
no transitions, local time for all timestamps is specified by the TZ
string in the footer if present and nonempty; otherwise, it is
specified by time type 0.

A given pair of standard/wall and UT/local indicators is used to
designate whether the corresponding transition time was specified as
UT, standard time, or wall-clock time. Note that there are only
three combinations of the two indicators, given that the standard/
wall value MUST be one (1) if the UT/local value is one (1). This
information can be useful if the transition times in a TZif file need
to be transformed into transitions appropriate for another time zone
(e.g. when calculating transition times for a simple POSIX TZ string
such as "AKST9AKDT").

In order to eliminate unused space in a TZif file, every nonzero
local time type index SHOULD appear at least once in the transition
type array. Likewise, every octet in the time zone designations
array SHOULD be used by at least one time type record.

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3.3. TZif Footer

The TZif footer is structured as follows (the lengths of multi-octet
fields are shown in parentheses):

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

TZif Footer

The elements of the footer are defined as follows:

NL: An ASCII new line character (0x0A).

TZ string: A rule for computing local time changes after the last
transition time stored in the version 2+ data block. The string
is either empty or uses the expanded format of the "TZ"
environment variable as defined in Section 8.3 of the "Base
Definitions" volume of [POSIX] with ASCII encoding, possibly
utilizing extensions described below (Section 3.3.1) in version 3
files. If the string is empty, the corresponding information is
not available. If the string is nonempty and one or more
transitions appear in the version 2+ data, the string MUST be
consistent with the last version 2+ transition. In other words,
evaluating the TZ string at the time of the last transition should
yield the same time type as was specified in the last transition.
The string MUST NOT contain NUL octets or be NUL-terminated, and
it SHOULD NOT begin with the ':' (colon) character.

The TZif footer is present only in version 2 and 3 files, as the
obsolescent version 1 format was designed before the need for a
footer was apparent.

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3.3.1. TZ String Extensions

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

o The hours part of the transition times may be signed and range
from -167 through 167 (-167 <= hh <= 167) instead of the POSIX-
required unsigned values from 0 through 24.

Example: <-03>3<-02>,M3.5.0/-2,M10.5.0/-1
This represents a time zone that observes daylight saving time
from 22:00 on the day before March's last Sunday until 23:00 on
the day before October's last Sunday. Standard time is 3 hours
west of UT and is abbreviated "-03"; daylight saving time is 2
hours west of UT and is abbreviated "-02".

o DST is considered to be in effect all year if it starts January 1
at 00:00 and ends December 31 at 24:00 plus the difference between
daylight saving and standard time, leaving no room for standard
time in the calendar.

Example: EST5EDT,0/0,J365/25
This represents a time zone that observes daylight saving time
all year. It is 4 hours west of UT and is abbreviated "EDT".

4. Interoperability Considerations

The following practices help ensure the interoperability of TZif
applications.

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

o Implementations that only understand version 1 MUST ignore any
data that extends beyond the calculated end of the version 1 data
block.

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

o The sequence of time changes defined by the version 1 header and
data block SHOULD be a contiguous sub-sequence of the time changes
defined by the version 2+ header and data block, and by the
footer. This guideline helps obsolescent version 1 readers agree

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with current readers about timestamps within the contiguous sub-
sequence. It also lets writers not supporting obsolescent readers
use a "timecnt" of zero in the version 1 data block to save space.

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

o When reading a version 2 or 3 file, implementations SHOULD ignore
the version 1 header and data block except for the purpose of
skipping over them.

o Implementations SHOULD calculate the total lengths of the headers
and data blocks and check that they all fit within the actual file
size, as part of a validity check for the file.

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

* "application/tzif-leap" (Section 8.2) to indicate that leap-
second records are included in the TZif data as necessary (none
are necessary if the file is truncated to a range that precedes
the first leap second).

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

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

5. Use with the Time Zone Data Distribution Service

The Time Zone Data Distribution Service (TZDIST) [RFC7808] is a
service that allows reliable, secure, and fast delivery of time zone
data and leap-second rules to client systems such as calendaring and
scheduling applications or operating systems.

A TZDIST service MAY supply time zone data to clients in the Time
Zone Information Format. Such a service MUST indicate that it
supports this format by including the media type "application/tzif"
(Section 8.1) in its "capabilities" response (see Section 5.1 of
[RFC7808]). A TZDIST service MAY also include the media type
"application/tzif-leap" (Section 8.2) in its "capabilities" response
if it is able to generate TZif files containing leap-second records.
A TZDIST service MUST NOT advertise the "application/tzif-leap" media
type without also advertising "application/tzif".

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

5.1. Truncating TZif Files

As described in Section 3.9 of [RFC7808], a TZDIST service MAY
truncate time zone transition data. A truncated TZif file is valid
from its first and up to, but not including, its last version 2+
transition time, if present.

When truncating the start of a TZif file, the service MUST supply in
the version 2+ data a first transition time that is the start point
of the truncation range. As with untruncated TZif files, time type 0
indicates local time immediately before the start point, and the time
type of the first transition indicates local time thereafter.

When truncating the end of a TZif file, the service MUST supply in
the version 2+ data a last transition time that is the end point of
the truncation range and MUST supply an empty TZ string. As with
untruncated TZif files with empty TZ strings, a truncated TZif file
does not indicate local time after the last transition.

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

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

5.2. Example TZDIST Request for TZif Data

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

Olson, et al. Standards Track [Page 15]

RFC 8536 TZif February 2019

Note that this example presumes that the time zone context path has
been discovered (see [RFC7808], Section 4.2.1) to be "/tzdist".

>> Request <<

GET /tzdist/capabilities HTTP/1.1
Host: tz.example.com

>> Response <<

HTTP/1.1 200 OK
Date: Fri, 01 Jun 2018 14:52:23 GMT
Content-Type: application/json; charset="utf-8"
Content-Length: xxxx

{
"version": 1,

"info": {
"primary-source": "IANA:2018e",
"formats": [
"text/calendar",
"application/tzif",
"application/tzif-leap"
],
...
},
...
}

>> Request <<

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

>> Response <<

HTTP/1.1 200 OK
Date: Fri, 01 Jun 2018 14:52:24 GMT
Content-Type: application/tzif
Content-Length: xxxx
ETag: "123456789-000-111"

TZif2...[binary data without leap-second records]...
EST5EDT,M3.2.0,M11.1.0

Olson, et al. Standards Track [Page 16]

RFC 8536 TZif February 2019

6. Security Considerations

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

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

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

7. Privacy Considerations

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

As discussed in Section 9 of [RFC7808], transmission of time zone
data over an insecure communications channel could leak the past,
current, or future location of a device or user. As such, TZif data
transmitted over a public communications channel MUST be protected
with a confidentiality layer such as that provided by Transport Layer
Security (TLS) [RFC8446].

8. IANA Considerations

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

8.1. application/tzif

Type name: application

Subtype name: tzif

Required parameters: none

Optional parameters: none

Encoding considerations: binary

Security considerations: See Section 6 of RFC 8536.

Olson, et al. Standards Track [Page 17]

RFC 8536 TZif February 2019

Interoperability considerations: See Section 4 of RFC 8536.

Published specification: This specification.

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

Fragment identifier considerations: N/A

Additional information:

Magic number(s): The first 4 octets are 0x54, 0x5A, 0x69, 0x66

File extensions(s): N/A

Macintosh file type code(s): N/A

Person & email address to contact for further information:
Time Zone Database mailing list <[email protected]>

Intended usage: COMMON

Restrictions on usage: N/A

Author: See the "Authors' Addresses" section of RFC 8536.

Change controller: IETF

8.2. application/tzif-leap

Type name: application

Subtype name: tzif-leap

Required parameters: none

Optional parameters: none

Encoding considerations: binary

Security considerations: See Section 6 of RFC 8536.

Interoperability considerations: See Section 4 of RFC 8536.

Published specification: This specification.

Olson, et al. Standards Track [Page 18]

RFC 8536 TZif February 2019

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

Fragment identifier considerations: N/A

Additional information:

Magic number(s): The first 4 octets are 0x54, 0x5A, 0x69, 0x66

File extensions(s): N/A

Macintosh file type code(s): N/A

Person & email address to contact for further information:
Time Zone Database mailing list <[email protected]>

Intended usage: COMMON

Restrictions on usage: N/A

Author: See the "Authors' Addresses" section of RFC 8536.

Change controller: IETF

9. References

9.1. Normative References

[GNU-C] "The GNU C Library (glibc)",
<https://www.gnu.org/software/libc/>.

[POSIX] IEEE, "Standard for Information Technology--Portable
Operating System Interface (POSIX(R)) Base
Specifications, Issue 7", IEEE 1003.1-2017,
DOI 10.1109/IEEESTD.2018.8277153, January 2018,
<http://pubs.opengroup.org/onlinepubs/9699919799/>.

[RFC20] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

Olson, et al. Standards Track [Page 19]

RFC 8536 TZif February 2019

[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.

[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Semantics and Content",
RFC 7231, DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.

[RFC7808] Douglass, M. and C. Daboo, "Time Zone Data Distribution
Service", RFC 7808, DOI 10.17487/RFC7808, March 2016,
<https://www.rfc-editor.org/info/rfc7808>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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

9.2. Informative References

[EGGERT-TZ] "History for tz", October 2018,
<https://github.com/eggert/tz/commits/master/tzfile.5>.

[RFC5545] Desruisseaux, B., Ed., "Internet Calendaring and
Scheduling Core Object Specification (iCalendar)",
RFC 5545, DOI 10.17487/RFC5545, September 2009,
<https://www.rfc-editor.org/info/rfc5545>.

[RFC6557] Lear, E. and P. Eggert, "Procedures for Maintaining the
Time Zone Database", BCP 175, RFC 6557,
DOI 10.17487/RFC6557, February 2012,
<https://www.rfc-editor.org/info/rfc6557>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS)
Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446,
August 2018, <https://www.rfc-editor.org/info/rfc8446>.

[tz-link] Eggert, P. and A. Olson, "Sources for Time Zone and
Daylight Saving Time Data", 2018,
<https://www.iana.org/time-zones/repository/
tz-link.html>.

Olson, et al. Standards Track [Page 20]

RFC 8536 TZif February 2019

Appendix A. Common Interoperability Issues

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

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

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

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

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

Interoperability problems with TZif include the following:

o Some readers examine only version 1 data. As a partial
workaround, a writer can output as much version 1 data as
possible. However, a reader should ignore version 1 data and use
version 2+ data, even if the reader's native timestamps have only
32 bits.

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

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

Olson, et al. Standards Track [Page 21]

RFC 8536 TZif February 2019

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

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

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

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

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

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

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

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

Olson, et al. Standards Track [Page 22]

RFC 8536 TZif February 2019

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

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

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

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

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

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

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

Appendix B. Example TZif Files

The following sections contain annotated hexadecimal dumps of example
TZif files.

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

Olson, et al. Standards Track [Page 23]

RFC 8536 TZif February 2019

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

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

Olson, et al. Standards Track [Page 24]

RFC 8536 TZif February 2019

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

Olson, et al. Standards Track [Page 25]

RFC 8536 TZif February 2019

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

Olson, et al. Standards Track [Page 26]

RFC 8536 TZif February 2019

| | | leapsecond[23] | |
| 238 | 49 5c 07 97 | occurrence | 1230768023 |
| | | | (2008-12-31T23:59:60Z) |
| 242 | 00 00 00 18 | correction | 24 |
| | | | |
| | | leapsecond[24] | |
| 246 | 4f ef 93 18 | occurrence | 1341100824 |
| | | | (2012-06-30T23:59:60Z) |
| 250 | 00 00 00 19 | correction | 25 |
| | | | |
| | | leapsecond[25] | |
| 254 | 55 93 2d 99 | occurrence | 1435708825 |
| | | | (2015-06-30T23:59:60Z) |
| 258 | 00 00 00 1a | correction | 26 |
| | | | |
| | | leapsecond[26] | |
| 262 | 58 68 46 9a | occurrence | 1483228826 |
| | | | (2016-12-31T23:59:60Z) |
| 266 | 00 00 00 1b | correction | 27 |
| | | | |
| 270 | 00 | UT/local[0] | 0 (local) |
| | | | |
| 271 | 00 | standard/wall[0] | 0 (wall) |
+-------+---------------+------------------+------------------------+

To determine TAI corresponding to 2000-01-01T00:00:00Z
(UNIX time = 946684800), the following procedure would be followed:

1. Find the latest leap-second occurrence prior to the time of
interest (leapsecond[21]) and note the correction value
(LEAPCORR = 22).

2. Add LEAPCORR + 10 to the time of interest to yield TAI of
2000-01-01T00:00:32.

Olson, et al. Standards Track [Page 27]

RFC 8536 TZif February 2019

B.2. Version 2 File Representing Pacific/Honolulu

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

Olson, et al. Standards Track [Page 28]

RFC 8536 TZif February 2019

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

Olson, et al. Standards Track [Page 29]

RFC 8536 TZif February 2019

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

Olson, et al. Standards Track [Page 30]

RFC 8536 TZif February 2019

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

Olson, et al. Standards Track [Page 31]

RFC 8536 TZif February 2019

To determine the local time in this time zone corresponding to
1933-05-04T12:00:00Z (UNIX time = -1156939200), the following
procedure would be followed:

1. Find the latest time transition prior to the time of interest
(trans time[1]).

2. Reference the corresponding transition type (trans type[1]) to
determine the local time type index (2).

3. Reference the corresponding local time type (localtimetype[2]) to
determine the offset from UTC (-09:30), the daylight saving
indicator (1 = yes), and the index into the time zone designation
strings (8).

4. Look up the corresponding time zone designation string
(designations[8] = "HDT").

5. Add the UTC offset to the time of interest to yield a local
daylight saving time of 1933-05-04T02:30:00-09:30 (HDT).

To determine the local time in this time zone corresponding to
2019-01-01T00:00:00Z (UNIX time = 1546300800), the following
procedure would be followed:

1. Find the latest time transition prior to the time of interest
(there is no such transition).

2. Look up the TZ string in the footer ("HST10"), which indicates
that the time zone designation is "HST" year-round, and the
offset to UTC is 10:00.

3. Subtract the UTC offset from the time of interest to yield a
standard local time of 2018-12-31T14:00:00-10:00 (HST).

Olson, et al. Standards Track [Page 32]

RFC 8536 TZif February 2019

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

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

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

Olson, et al. Standards Track [Page 33]

RFC 8536 TZif February 2019

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

Acknowledgments

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

Authors' Addresses

Arthur David Olson

Email: [email protected]

Paul Eggert
University of California, Los Angeles

Email: [email protected]

Kenneth Murchison
FastMail US LLC

Email: [email protected]

Olson, et al. Standards Track [Page 34]