Independent Submission A. Olson
Internet-Draft
Updates: 7808 (if approved) P. Eggert
Intended status: Standards Track UCLA
Expires: December 2, 2018 K. Murchison
FastMail
May 31, 2018
The Time Zone Information Format (TZif)
draft-murchison-tzdist-tzif-04
Abstract
This document defines the Time Zone Information Format (TZif) 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
o 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)?
E.g. application/tzif+leap; version=3
o 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.
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 December 2, 2018.
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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
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. The Time Zone Information Format (TZif) . . . . . . . . . . . 4
3.1. TZif Header . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. TZif Data Block . . . . . . . . . . . . . . . . . . . . . 7
3.3. TZif Footer . . . . . . . . . . . . . . . . . . . . . . . 10
3.3.1. TZ String Extensions . . . . . . . . . . . . . . . . 11
4. Use with the Time Zone Data Distribution Service . . . . . . 11
4.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Normative References . . . . . . . . . . . . . . . . . . 14
9.2. Informative References . . . . . . . . . . . . . . . . . 15
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Appendix A. Change History (To be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
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 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 [tz-link] capable
of generating and reading files in this format.
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This specification does not define the source of the time zone data
or leap second information. 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 [1] [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 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, adjusted as necessary from standard time. The
adjustment may be positive, negative, or zero.
International Atomic Time (TAI): The time standard based on atomic
clocks. It is equal to UTC except without leap second
adjustments.
Local Time: The time according to a location's current time zone
offset from Universal Time.
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 standard or daylight saving time is in use
3. a change in time zone abbreviation
4. a leap second (i.e., a change in UTC - TAI)
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, UT offset values, DST rules,
and other information such as time zone abbreviations." The
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interchange format defined in this document is one such form of
time zone data.
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 C time() function (see
Section 3 of the "System Interfaces" Volume of [POSIX]). This is
an integer number of seconds since the POSIX Epoch (1970-01-01
00:00:00 UTC) not counting leap seconds. As an extension to
POSIX, negative values represent times before the POSIX Epoch,
using UT.
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
(Section 3.1) followed by a variable-length data block (Section 3.2)
using four-octet (32-bit) transition times and leap second
occurrences. These 32-bit values are limited to representing times
from 1901-12-13 20:45:52 through 2038-01-19 03:14:07 UT.
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 (Section 3.3). These 64-bit values can represent times
approximately 292 billion years into the past or future.
A TZif file is structured as follows:
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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. The sequence
of time changes defined by the 32-bit header and data block SHOULD be
a contiguous subsequence of the time changes defined by the 64-bit
header and data block. When reading a version 2 or 3 file,
implementations SHOULD ignore the 32-bit header and data block except
for the purpose of skipping over them.
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.
3.1. TZif Header
The TZif header is structured as follows (the number of octets
occupied by a field is shown in parenthesis):
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+---------------+---+
| 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 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 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 TZ string in the footer string MAY use extensions
(Section 3.3.1) to the expanded format of the "TZ" environment
variable as defined in Section 8 of the "Base Definitions"
Volume of [POSIX].
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.
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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 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, time 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 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:
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transition times: A series of four- or eight-octet UNIX time values
sorted in 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.
transition types: A series of one-octet unsigned integers specifying
the type of local time of the corresponding transition time.
These values serve as indices into the array of local time type
records. The number of type 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:
+---------------+-+-+---+
| 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.
(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:
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+---------------+---------------+
| occur (4) | corr (4) |
+---------------+---------------+
64-bit Data Block:
+---------------+---------------+---------------+
| occur (8) | corr (4) |
+---------------+---------------+---------------+
occur(rence): A four- or eight-octet UNIX time value specifying
the time at which a leap second correction occurs.
corr(ection): A four-octet signed integer specifying the total
number of leap seconds to be applied to UTC on or after the
occurrence.
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, and 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. 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
and not including the next transition time. 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, and
otherwise is specified by the last transition type. Local time for
timestamps before the first transition time (or for all timestamps if
no transitions are present) is specified by the time type computed
from the following rules:
1. Time type 0 (the first time type) if this time type is not used
in any transitions.
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2. If rule (1) does not apply, time type T if the first transition
is to a daylight saving time type V, T is a standard time type
that is less than V, and no standard time type U is in the range
T < U < V.
3. If rules (1) and (2) do not apply, the lowest-numbered standard
time type.
4. If rules (1) through (3) do not apply, 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").
3.3. TZif Footer
The TZif footer is structured as follows (the number of octets
occupied by a field is shown in parenthesis):
+---+--------------------+---+
| 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 64-bit data block. The string is
either empty or uses the expanded format of the "TZ" environment
variable as defined in Section 8 of the "Base Definitions" Volume
of [POSIX]. If empty, the corresponding information is not
available. If the string is nonempty, at least one transition
MUST be present in the 64-bit data, and the TZ string MUST be
consistent with the last 64-bit transition - i.e., evaluating the
TZ string at the time of the last transition should yield the same
time type as the time type specified in the last transition. Note
that the string MUST NOT be NUL-terminated and SHOULD NOT begin
with the ':' (colon) character.
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3.3.1. TZ String Extensions
Version 3 TZif files MAY use the following extensions in the TZ
string:
o 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.
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. 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 MIME media type "application/
tzif" (Section 7) in its "capabilities" response (see Section 5.1 of
[RFC7808].
TZDIST clients MUST use the HTTP "Accept" [RFC7231] 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.
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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",
],
...
},
...
}
>> 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]...
EST5EDT,M3.2.0,M11.1.0
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5. Security Considerations
None.
6. Privacy Considerations
None.
7. IANA Considerations
This document defines a MIME [RFC6838] 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 [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>
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Intended usage: COMMON
Restrictions on usage: N/A
Author: See the "Author's Address" section of this document.
Change controller: IETF
8. Acknowledgments
The authors would like to thank the following individuals for
contributing their ideas and support for writing this specification:
Michael Douglass and Eliot Lear.
9. References
9.1. Normative References
[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,
<https://ieeexplore.ieee.org/servlet/
opac?punumber=8277151>.
This is identical to The Open Group Base Specifications
Issue 7, 2018 edition [2].
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[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>.
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[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>.
9.2. Informative References
[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>.
[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>.
9.3. URIs
[1] https://tools.ietf.org/html/bcp14
Appendix A. Change History (To be removed by RFC Editor before
publication)
Changes since -03:
o Removed definition of GMT.
o Updated definitions of UTC, TAI, and UT
o Switched to using UT rather than UTC.
o Added more text about the use of standard/wall and UT/local
indicators.
o Added Acknowledgments.
o Minor editorial changes.
Changes since -02:
o Updated definitions of Standard Time and DST.
o Added definitions of GMT and UT.
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o Added a definition of Time Zone Data from RFC7808.
o Removed sentence stating that TZDB is accurate.
o Added more text for standard/wall and UTC/local indicators and
counts.
o Added text discussing timestamps before first and after last
transition.
o Added more guidance text regarding 32-bit and 64-bit data
consistency.
o Minor editorial changes.
Changes since -01:
o Renamed "POSIX Time" to "UNIX Time" and noted that values can be
negative.
o Noted that signed values MUST be represented using two's
complement.
o Renamed "POSIX TZ string" to "TZ string" and noted that it can be
empty.
o Moved TZ string extensions into its own subsection with examples.
o Renamed leap second "epoch" to "occurrence".
o Editorial changes from Paul Eggert.
Changes since -00:
o Split TZif format description into a general overview and 3
subsections.
o Updated Keywords boilerplate.
o Updated POSIX reference.
o Editorial changes from Eliot Lear.
Authors' Addresses
Arthur David Olson
Email: arthurdavidolson@gmail.com
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Paul Eggert
University of California, Los Angeles
Email: eggert@cs.ucla.edu
Kenneth Murchison
FastMail US LLC
Email: murch@fastmailteam.com
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