Sources for Time Zone and Daylight Saving Time Data

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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 GNU/Linux, FreeBSD, NetBSD, OpenBSD, DJGPP, HP-UX, IRIX, SCO OpenServer, Solaris, and UnixWare. 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 code lets you compile the text data into time zone files, one for each location. It also lets you read a compiled time zone 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, or retrieve the archive of old messages (in gzip compressed format).

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.
• CGI/1.0 URL Timezone Converter is a similar interface, with source code written in Perl, using what Paul Stewart calls the "20-line hack no one will let me forget".
• Fancier web interfaces, roughly in ascending order of complexity, include:
  • Local Times Around the World
  • Current Time in 1000 Places
  • Time Zone Converter
• Tick2Tock time zone converter is a multilingual interface.
• The Worldwide Holiday & Festival Site lists DST-related clock changes along with holidays.
• The World Clock is a web interface to a time zone database derived from tz's.

Other time zone file readers

• The GNU C Library has an independent, thread-safe implementation of a time zone file reader that is compatible with tz. 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 time zone file reader written in Java. It is freely available under the GNU LGPL.

Other tz-based time zone conversion software

• HyperCard time zones calculator is a HyperCard stack.
• Time Zone Converter is a Microsoft Windows 95/98/NT4 program.

Other time zone databases

• Astrodienst - 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 Zones contains data from the Time Service Department of the US Naval Observatory (USNO), used as the source for the usno* files in the tz distribution.
• The International Air Transport Association publishes the Standard Schedules Information Manual, 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.
• World timezones map with current time 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.
• World Time Server has a separate little outline map for each region of each country that currently observes a different time.

Time zone boundaries

• Manifold.net's Free Maps and GIS Data includes a Manifold-format map of world time zone boundaries distributed under the GNU General Public License (GPL). The GeoCommunity's International Time Zones publishes the same data in other formats.
• 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.

Daylight saving time concepts and history

• About Daylight Saving Time - History, rationale, laws and dates is a good overall history of DST.
• The Time of Internet contains good descriptions of Time Zones and daylight saving time, with diagrams. The time zone map is out of date, however.
• A few countries have well-documented histories of legal time.

  Australia

  The Community Relations Division of the NSW Attorney General's Department maintains a history of daylight saving in New South Wales.

  Brazil

  The Time Service Department of the National Observatory records Brazil's daylight saving time decrees (in Portuguese).

  Germany

  The National Institute for Science and Technology maintains the Realisation of Legal Time in Germany.

  Israel

  The Ministry of Interior 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).

  United Kingdom

  History of legal time in Britain is an excellent discussion for the country with perhaps the best-documented history of clock adjustments.

Precision timekeeping

• The Science of Timekeeping is an excellent and thorough introduction to the theory and practice of precision timekeeping.
• Time Server discusses the Network Time Protocol (NTP), used 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 TT, TCG, and TDB.
• Basics of Space Flight - Reference Systems - Time Conventions briefly explains interplanetary space flight timekeeping.
• Bulletins of IERS contains official publications 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 remove leap seconds (which requires discussion list membership to read). This proposal is being studied by the URSI Commission J Working Group on the Leap Second, and the IAU has plans to report on this proposal by 2003.

Time notation

• A Summary of the International Standard Date and Time Notation is a good summary of ISO 8601:1988 - Data elements and interchange formats - Information interchange - Representation of dates and times.

Related indexes

• Open Directory - Reference: Time
• Time and the Arts
• Yahoo! - Science:Measurements and Units:Time:Time Zones