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.
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
This database (often called
is used by several implementations,
C Library used in
Mac OS X,
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.
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.,
the code is in the file
C is the code's version;
similarly, the data are in
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).
The following shell commands download
these files to a GNU/Linux or similar host;
see the downloaded
README file for what to do next.
gzip -dc tzcode*.tar.gz | tar -xf -
gzip -dc tzdata*.tar.gz | tar -xf -
The code and data files can also be obtained from the
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
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
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
These are listed roughly in ascending order of complexity and fanciness.
Other time zone database formats
tz binary file readers
- The GNU C
has an independent, thread-safe implementation of
tz binary file reader.
This library is freely available under the
GNU Lesser General Public License
and is widely used in GNU/Linux systems.
tz binary file reader written in Java.
It is freely available under the LGPL.
- Tcl, mentioned above, also contains a
tz binary file reader.
tz binary file reader written in Perl.
It is freely available under the same terms as Perl
(dual GPL and Artistic license).
tz-based time zone software
is an extension for Mozilla
Toolkit applications like Firefox, Thunderbird, and
It displays multiple clocks in the application window, and has a mapping
interface to Google Earth.
It is freely available under the GPL.
clock (intclock) is a multi-timezone clock for
GNU/Linux and similar systems. It is freely available
under the GPL.
has a copy of a recent
tz database, accessed via a C# library. As its
name suggests, it is in the public domain. Only current time stamps
are well supported; historical data are compiled into the runtime but
are not easily accessible.
- Sun Java releases since 1.4
contain a copy of a subset of a recent
tz database in a
- Time Zone is
a WordPress plugin. It is freely
available under a BSD-style license.
- VelaTerra is
a Mac OS X program. Its developers
- World Time Explorer is a
Microsoft Windows program.
WorldClock for Windows and Windows Mobile
lets users "see the time in up to 25 locations in the world at once."
(From Hans Nieuwenhuis, 2009-11-02.)
Time Zone Master Basic
"allows people to display multiple desktop clocks, and to
research current and historical time information, as well as times of
astronomical events (sunrise/transit/set, moonrise/transit/set, phases,
season starts) for user-selected dates in the past and future. It can
automatically download, compile and use the tzdata**.gz database files
as they are released to keep the data up to date. The software is
free." (Davie Patte)
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 (one volume
for the USA, and one for
other locations) 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
usno* files in the
- The Standard
Schedules Information Manual of the
International Air Transport
gives current time zone rules for airports served by commercial aviation.
- Some Microsoft Windows versions contain time zone information in
an undocumented format, with IDs that can be mapped to
values using the Windows
→ Tzid table maintained by the CLDR data mentioned
provides programming-language-specific representations of timezone
formatted data. The repository is updated as soon as the FTP
distribution is updated. All data can be downloaded as a zip and/or it
can be obtained/synced via anonymous SVN. Data is made available under
the MIT license. (From Rich Tibbett.)
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
- 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
time in Norway (in Norwegian), citing the
Institute of Theoretical Astrophysics, Oslo.
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.
Science of Timekeeping is a thorough introduction
to the theory and practice of precision timekeeping.
- NTP: The Network
discusses how to synchronize clocks of
Options for DHCP
(Internet RFC 4833)
specifies a DHCP option for a server to configure
a client's time zone and daylight saving settings automatically.
- A Few
Facts Concerning GMT, UT, and
answers questions like "What is the
difference between GMT and UTC?"
Times explains more abstruse astronomical time scales like
Scales goes into more detail, particularly for historical variants.
- The IAU's SOFA
initiative publishes Fortran
code for converting among time scales like
TDB, TDT and
- Basics of
Space Flight - Reference Systems - Time Conventions
briefly explains interplanetary space flight timekeeping.
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.
- LeapSecond.com is
dedicated not only to leap seconds but to precise time and frequency
in general. It covers the state of the art in amateur timekeeping, and
how the art has progressed over the past few decades.
maintained by the
(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,
discussed further in
leap second: its history and possible future.
The (now disbanded) AAS Leap Second
Committee has solicited input on this proposal.
Future of Leap Seconds covers this
A Summary of
the International Standard Date and Time Notation is a good
8601:2004 -- Data elements and interchange formats -- Information
interchange -- Representation of dates and times.
Schema: Datatypes - dateTime specifies a format inspired by
ISO 8601 that is in common use in XML data.
Message Format (Internet RFC 2822) §3.3
specifies the time notation used in email and HTTP
Date and Time
on the Internet: Timestamps (Internet RFC 3339)
specifies an ISO 8601
profile for use in new Internet
Date & Time
Formats on the Web surveys web- and Internet-oriented date and time
Best of Dates, the Worst of Dates covers many problems encountered
by software developers when handling dates and time stamps.
- The Unicode Common Locale Data Repository
(CLDR) Project has localizations for time zone names,
abbreviations, identifiers, and formats. For example, it contains
French translations for "Eastern European Summer Time", "EEST", and
Chart: names.metazone shows these values for many locales.
ICU contains a mechanism for using this data.
- 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
"EST". For POSIX the
database contains English abbreviations for all time stamps but in
many cases these are merely inventions of the database
- Numeric time zone abbreviations typically count hours east of
+09 for Japan and
-10 for Hawaii. However, the POSIX
TZ environment variable uses the opposite convention. For
example, one might use
TZ="HST10" for Japan and Hawaii, respectively. If the
tz database is available, it is usually better to use
TZ="Pacific/Honolulu" instead, as this should avoid
confusion, handle old time stamps better, and insulate you better from
any future changes to the rules. One should never set
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".