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, Open UNIX, Solaris, and Tru64.

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, 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, 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.
• 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
• 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 tz compilers

• Olson -> VTIMEZONE Converter describes a program Vzic that compiles tz source into VTIMEZONE text as specified by the iCalendar specification published by the IETF Calendaring and Scheduling Working Group. Vzic is freely available under the GNU General Public License (GPL).
• Possible XML schemata for the tz data include one sketched out by Tim Berners-Lee and another used by the iPlanet Calendar Server.

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.

Other tz-based time zone conversion software

• HyperCard time zones calculator is a HyperCard stack.
• Time Zone Converter is a Microsoft Windows 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 Server is another time zone database.
• 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.
• Airlines, Airplanes and Airports lists current standard times for thousands of airports around the world. This seems to be derived from 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.
• 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.

Time zone boundaries

• Manifold.net's Free Maps and GIS Data includes a Manifold-format map of world time zone boundaries distributed under the 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 National Weather Service publishes outlines of US counties and their time zones 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 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.
• A few countries have well-documented histories of legal time.

  Australia

  Australia's Daylight Saving Times contains pointers to government records and to histories of daylight saving.

  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 and Daylight Saving Time.

  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).

  Netherlands

  Legal time in the Netherlands (in Dutch) covers the history of local time in the Netherlands from ancient times.

  United Kingdom

  History of legal time in Britain is an excellent discussion for 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 an excellent and thorough introduction to the theory and practice of precision timekeeping.
• Time Synchronization 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.
• The IAU's Standards Of Fundamental Astronomy (SOFA) initiative publishes Fortran code for converting among time scales like TAI, TDB, TT and UTC.
• Basics of Space Flight - Reference Systems - Time Conventions briefly explains interplanetary space flight timekeeping.
• 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. 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 (which has been superseded by ISO 8601:2000).
• Section 3.3 of Internet RFC 2822 specifies the time notation used in email and HTTP headers.
• Date and Time on the Internet: Timestamps proposes an ISO 8601 profile for use in new Internet protocols.
• 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 compatibility with POSIX the tz database contains English abbreviations for all time stamps but in many cases these are merely inventions of the database maintainers.

Related indexes

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