Sources for Time Zone and Daylight Saving Time Data

This file is in the public domain, so clarified as of 2009-05-17 by Arthur David Olson.

Please send corrections to this web page to the time zone mailing list.

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 time zone boundaries, UTC offsets, and daylight-saving rules. This database (often called zoneinfo or tz) is used by several implementations, including the GNU C Library used in GNU/Linux, FreeBSD, NetBSD, OpenBSD, Cygwin, DJGPP, AIX, Mac OS X, OpenVMS, Oracle Database, 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/Phoenix represents most of Arizona, which uses mountain 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, Indiana, which switched from central to eastern time in 1991 and switched back in 2006. 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 releases 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. Each version is a four-digit year followed by lower-case letters (a through z, then za through zz, then zza through zzz, and so on). Convenience links to the latest code and latest data revisions are also available. The following shell commands download these files to a GNU/Linux or similar host; see the downloaded README file for what to do next.

mkdir tz
cd tz
wget --retr-symlinks '*-latest.tar.gz'
gzip -dc tzcode-latest.tar.gz | tar -xf -
gzip -dc tzdata-latest.tar.gz | tar -xf -

The code and data files can also be obtained from the IANA timezone web page.

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 browse recent messages sent to the mailing list, subscribe to it. browse the archive of old messages (message by message or in gzip compressed format), or retrieve archived older versions of code and data.

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

These are listed roughly in ascending order of complexity and fanciness.

Other time zone database formats

Other tz compilers

Other tz binary file readers

Other tz-based time zone software

Other time zone databases


Time zone boundaries

Civil time concepts and history

National histories of legal time

The Parliamentary Library has commissioned research note on daylight saving time in Australia. The Bureau of Meteorology publishes a list of Implementation Dates of Daylight Savings Time within Australia.
The Royal Observatory of Belgium maintains a table of time in Belgium (in Dutch).
The Time Service Department of the National Observatory records Brazil's daylight saving time decrees (in Portuguese).
The Institute for National Measurement Standards publishes current and some older information about Time Zones & Daylight Saving Time.
The Chilean Hydrographic and Oceanographic Service publishes a history of official time (in Spanish).
The National Institute for Science and Technology maintains the Realisation of Legal Time in Germany.
The Interior Ministry periodically issues announcements (in Hebrew).
The Investigation and Analysis Service of the Mexican Library of Congress has published a history of Mexican local time (in Spanish).
See Singapore below.
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 History of New Zealand time has more details.
The Norwegian Meteorological Institute lists Summer time in Norway (in Norwegian), citing the Institute of Theoretical Astrophysics, Oslo.
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

Time notation

Related indexes