Sources for Time Zone and Daylight Saving Time Data

@(#)tz-link.htm 7.48

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The `tz` database

The public-domain time zone database contains code and data that represent the history of local time for many representative locations around the globe. It is updated periodically to reflect changes made by political bodies to UTC offsets and daylight-saving rules. This database (often called tz or zoneinfo) is used by several implementations, including the GNU C Library used in GNU/Linux, FreeBSD, NetBSD, OpenBSD, Cygwin, DJGPP, HP-UX, IRIX, Mac OS X, OpenVMS, Solaris, Tru64, and UnixWare.

Each location in the database represents a national region where all clocks keeping local time have agreed since 1970. Locations are identified by continent or ocean and then by the name of the location, which is typically the largest city within the region. For example, America/New_York represents most of the US eastern time zone; America/Indianapolis represents most of Indiana, which uses eastern time without daylight saving time (DST); America/Detroit represents most of Michigan, which uses eastern time but with different DST rules in 1975; and other entries represent smaller regions like Starke County, Kentucky, which switched from central to eastern time in 1991. To use the database on an extended POSIX implementation set the TZ environment variable to the location's full name, e.g., TZ="America/New_York".

In the tz database's FTP distribution the code is in the file tzcodeC.tar.gz, where C is the code's version; similarly, the data are in tzdataD.tar.gz, where D is the data's version. The following shell commands download these files to a GNU/Linux or similar host; see the downloaded README file for what to do next.

wget 'ftp://elsie.nci.nih.gov/pub/tz*.tar.gz'
gzip -dc tzcode*.tar.gz | tar -xf -
gzip -dc tzdata*.tar.gz | tar -xf -

The code lets you compile the tz source files into machine-readable binary files, one for each location. It also lets you read a tz binary file and interpret time stamps for that location.

The data are by no means authoritative. If you find errors, please send changes to the time zone mailing list. You can also subscribe to the mailing list, retrieve the archive of old messages (in gzip compressed format), or retrieve archived older versions of code and data; there is also a smaller HTTP mirror.

The Web has several other sources for time zone and daylight saving time data. Here are some recent links that may be of interest.

Web pages using recent versions of the `tz` database

Date and Time Gateway is a text-based point-and-click interface to tables of current time throughout the world.
Fancier web interfaces, roughly in ascending order of complexity, include:
- Current Time in 1000 Places
- Time Zone Converter
The World Clock - Time Zones is a web interface to a time zone database derived from tz's.

Other time zone database formats

The Internet Calendaring and Scheduling Core Object Specification (iCalendar) specification published by the (now-concluded) IETF Calendaring and Scheduling Working Group (calsch) covers time zone data; see its VTIMEZONE calendar component. The Calendaring and Scheduling Consortium is promoting further work in this area.
The www-rdf-calendar list discusses RDF-based calendar and group scheduling systems, and has a workspace on time zone data converted from tz. An earlier schema was sketched out by Tim Berners-Lee.
XCal was a draft XML document type definition that corresponded to iCalendar.

Other `tz` compilers

Vzic iCalendar Timezone Converter describes a program Vzic that compiles tz source into iCalendar-compatible VTIMEZONE files. Vzic is freely available under the GNU General Public License (GPL).
DateTime::TimeZone contains a script parse_olson that compiles tz source into Perl modules. It is part of the Perl DateTime Project, which is freely available under both the GPL and the Perl Artistic License. DateTime::TimeZone also contains a script tests_from_zdump that generates test cases for each clock transition in the tz database.
ICU contains a C/C++ library for internationalization that has a compiler from tz source into an ICU-specific format. ICU is freely available under a BSD-style license.
Joda Time - Java date and time API contains a class org.joda.time.tz.ZoneInfoCompiler that compiles tz source into a Joda-specific binary format. Joda Time is freely available under a BSD-style license.
PyTZ - Python Time Zone Library compiles tz source into Python. It is freely available under a BSD-style license.

Other `tz` binary file readers

The GNU C Library has an independent, thread-safe implementation of a tz binary file reader. This library is freely available under the GNU Lesser General Public License (LGPL), and is widely used in GNU/Linux systems.
ZoneInfo.java is a tz binary file reader written in Java. It is freely available under the GNU LGPL.
Python time zones is a tz binary file reader written in Python. It is freely available under a BSD-style license.

Other `tz`-based time zone conversion software

Sun Java releases since 1.4 contain a copy of a subset of a recent tz database in a Java-specific format.
HyperCard time zones calculator is a HyperCard stack.
VelaTerra is a Mac OS X program. Its developers offer free licenses to tz contributors.
World Time Explorer is a Microsoft Windows program.

Other time zone databases

Atlas Query is Astrodienst's Web version of Shanks's excellent time zone history atlases published in both computer and book form by Astro Communications Services.
WORLDTIME: interactive atlas, time info, public holidays contains information on local time, sunrise and sunset, and public holidays in several hundred cities around the world.
World Time Server is another time zone database.
World Time Zones contains data from the Time Service Department of the US Naval Observatory, used as the source for the usno* files in the tz distribution.
The Standard Schedules Information Manual of the the International Air Transport Association, which gives current time zone rules for all the airports served by commercial aviation.

Maps

The United States Central Intelligence Agency (CIA) publishes a time zone map; the Perry-Castañeda Library Map Collection of the University of Texas at Austin has copies of recent editions. The pictorial quality is good, but the maps do not indicate summer time, and parts of the data are a few years out of date.
Current time around the world and standard time zones map of the world has several fancy time zone maps; it covers Russia particularly well. The maps' pictorial quality is not quite as good as the CIA's but the maps are more up to date.

Time zone boundaries

Administrative Divisions of Countries ("Statoids") contains detailed lists of tz-related zone subdivision data.
Time zone boundaries for multizone countries summarizes legal boundaries between time zones within countries.
Manifold.net's Free Maps and GIS Data includes a Manifold-format map of world time zone boundaries distributed under the GPL.
The US Geological Survey's National Atlas of the United States publishes the Time Zones of the United States in the public domain.
The GeoCommunity lists several commercial sources for International Time Zones and Time Zone Data.

Civil time concepts and history

A Walk through Time surveys the evolution of timekeeping.
About Daylight Saving Time - History, rationale, laws & dates is an overall history of DST.
Who Knew? A Brief History of Daylight Saving Time summarizes some of the contentious history of DST.
The Time of Internet describes time zones and daylight saving time, with diagrams. The time zone map is out of date, however.
A History of the International Date Line tells the story of the most important time zone boundary.
Basic Time Zone Concepts discusses terminological issues behind time zones.

National histories of legal time

Australia: The Community Relations Division of the New South Wales (NSW) Attorney General's Department maintains a history of daylight saving in NSW.
Austria: The Federal Office of Metrology and Surveying publishes a table of daylight saving time in Austria (in German).
Belgium: The Royal Observatory of Belgium maintains a table of time in Belgium (in Dutch).
Brazil: The Time Service Department of the National Observatory records Brazil's daylight saving time decrees (in Portuguese).
Canada: The Institute for National Measurement Standards publishes current and some older information about Time Zones & Daylight Saving Time.
Chile: WebExhibits publishes a history of official time (in Spanish) originally written by the Chilean Hydrographic and Oceanographic Service.
Germany: The National Institute for Science and Technology maintains the Realisation of Legal Time in Germany.
Israel: The Interior Ministry periodically issues announcements (in Hebrew).
Mexico: The Investigation and Analysis Service of the Mexican Library of Congress has published a history of Mexican local time (in Spanish).
Malaysia: See Singapore below.
Netherlands: Legal time in the Netherlands (in Dutch) covers the history of local time in the Netherlands from ancient times.
New Zealand: The Department of Internal Affairs maintains a brief history about daylight saving. The privately-maintained Time Changes in New Zealand has more details.
Singapore: Why is Singapore in the "Wrong" Time Zone? details the history of legal time in Singapore and Malaysia.
United Kingdom: History of legal time in Britain discusses in detail the country with perhaps the best-documented history of clock adjustments. The National Physical Laboratory also maintains an archive of summer time dates.

Precision timekeeping

The Science of Timekeeping is a thorough introduction to the theory and practice of precision timekeeping.
NTP: The Network Time Protocol discusses how to synchronize clocks of Internet hosts.
A Few Facts Concerning GMT, UT, and the RGO answers questions like "What is the difference between GMT and UTC?"
Astronomical Times explains more abstruse astronomical time scales like TDT, TCG, and TDB.
The IAU's SOFA initiative publishes Fortran code for converting among time scales like TAI, TDB, TDT and UTC.
Basics of Space Flight - Reference Systems - Time Conventions briefly explains interplanetary space flight timekeeping.
Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock briefly describes Mars Coordinated Time (MTC) and the diverse local time scales used by each landed mission on Mars.
Bulletins maintained by the IERS EOP (PC) contains official publications of the Earth Orientation Parameters Product Center of the International Earth Rotation Service, the committee that decides when leap seconds occur.
The Leap Second Discussion List covers McCarthy and Klepczynski's proposal to discontinue leap seconds, published in GPS World 10, 11 (1999-11), 50–57 and discussed further in R. A. Nelson et al., The leap second: its history and possible future, Metrologia 38 (2001), 509–529. The Future of Leap Seconds catalogs information about this contentious issue.

Time notation

A Summary of the International Standard Date and Time Notation is a good summary of ISO 8601:2004 -- Data elements and interchange formats -- Information interchange -- Representation of dates and times.
Section 3.3 of Internet RFC 2822 specifies the time notation used in email and HTTP headers.
Internet RFC 3339 specifies an ISO 8601 profile for use in new Internet protocols.
The Best of Dates, the Worst of Dates covers many problems encountered by software developers when handling dates and time stamps.
ICU contains a mechanism for localizing time zone labels and abbreviations; for example, one can use it to specify Russian translations for "Eastern European Summer Time", "EEST", and Europe/Bucharest. This mechanism is part of the Unicode CLDR Project; for example, the By-Type Chart for //ldml/dates/timeZoneNames/… shows values for time zone names in many locales.
Alphabetic time zone abbreviations should not be used as unique identifiers for UTC offsets as they are ambiguous in practice. For example, "EST" denotes 5 hours behind UTC in English-speaking North America, but it denotes 10 or 11 hours ahead of UTC in Australia; and French-speaking North Americans prefer "HNE" to "EST". For POSIX the tz database contains English abbreviations for all time stamps but in many cases these are merely inventions of the database maintainers.
Numeric time zone abbreviations typically count hours east of UTC, e.g., +09 for Japan and -10 for Hawaii. However, the POSIX TZ environment variable uses the opposite convention. For example, one might use TZ="JST-9" and TZ="HST10" for Japan and Hawaii, respectively. If the tz database is available, it is usually better to use settings like TZ="Asia/Tokyo" and TZ="Pacific/Honolulu" instead, as this should avoid confusion, handle old timestamps better, and insulate you better from any future changes to the rules. One should never set POSIX TZ to a value like "GMT-9", though, since this would falsely claim that local time is nine hours ahead of UTC and the time zone is called "GMT".