# 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 time zone boundaries, 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, 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 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 browse recent messages sent to the mailing list, subscribe to it, retrieve the full 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

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

- Date and Time Gateway lets you see the `TZ` values directly.
- Current Time in 1000 Places uses descriptions of the values.
- Time Zone Converter uses a pulldown menu.
- Complete timezone information for all countries displays tables of DST rules.
- The World Clock - Time Zones lets you sort zone names and convert times.
- Graphical Display of Time Zones and Daylight Saving Times shows a graph of time difference versus time for any pair of locations.
- The World Time Engine also contains data about time zone boundaries; it supports queries via place names and shows location maps.

## Other time zone database formats

- The Internet Calendaring and Scheduling Core Object Specification (iCalendar), Internet RFC 2445, 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. iCalendar TIMEZONE Problems and Recommendations offers guidelines and recommendations for the use of VTIMEZONE and `tz`.
- Extended Daylight Saving Time Links, Advisories and Changes lists vendor material on recent daylight saving time changes.
- Timezone Registry and Service Recommendations discusses a strategy for defining and deploying a time zone registration process that would establish unique names for each version of each `tz` zone, along with a polygonal representation of the geographical area corresponding to the zone.
- 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.

## Other `tz` compilers

- Vzic iCalendar Timezone Converter describes a C program that compiles `tz` source into iCalendar-compatible VTIMEZONE files. Vzic is freely available under the GNU General Public License (GPL).
- tziCal - tz database conversion utility is like Vzic, except for the .NET framework.
- 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.
- International Components for Unicode (ICU) contains C/C++ and Java libraries 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.
- TZInfo - Ruby Timezone Library compiles `tz` source into Ruby. It is freely available under the MIT license.
- The Chronos Date/Time Library is a Smalltalk class library that compiles `tz` source into a time zone repository whose format is either proprietary or an XML-encoded representation.
- Starting with version 8.5, Tcl contains a developer-oriented parser that compiles `tz` source into text files, along with a runtime that can read those files. Tcl 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 LGPL.
- Tcl, mentioned above, also contains a `tz` binary file reader.

## Other `tz`-based time zone software

- FoxClocks is an extension for Mozilla Toolkit applications like Firefox, Thunderbird, and Sunbird. It displays multiple clocks in the application window, and has a mapping interface to Google Earth. It is freely available under the GPL.
- International clock (intclock) is a multi-timezone clock for GNU/Linux and similar systems. It is freely available under the GPL.
- PublicDomain 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 Java-specific format.
- Time Zone is a WordPress plugin. It is freely available under a BSD-style license.
- 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 (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 for the `usno*` files in the `tz` distribution.
- The Standard Schedules Information Manual of the International Air Transport Association 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 `TZ` values using the Windows → Tzid table maintained by the CLDR data mentioned below.

## 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

- A map of the TZ timezones in the US contains a shapefile of the `tz` regions in the US.
- 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.
- A ship within the territorial waters of any nation uses that nation's time. In international waters, time zone boundaries are meridians 15° apart, except that UTC−12 and UTC+12 are each 7.5° wide and are separated by the 180° meridian (not by the International Date Line, which is for land and territorial waters only). A captain can change ship's clocks any time after entering a new time zone; midnight changes are common.

## 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.
- Saving Time, Saving Energy discusses a primary justification for 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 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.
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
  The Chilean Hydrographic and Oceanographic Service publishes a history of official time (in Spanish).
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 History of New Zealand time has more details.
Norway
  The Norwegian Meteorological Institute lists Summer time in Norway (in Norwegian), citing the Institute of Theoretical Astrophysics, Oslo.
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.
- Timezone 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 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. Time Scales goes into more detail, particularly for historical variants.
- 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.
- 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.
- 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, discussed further in The leap second: its history and possible future. The AAS Leap Second Committee has solicited input on this proposal. The Future of Leap Seconds covers 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.
- XML Schema: Datatypes - dateTime specifies a format inspired by ISO 8601 that is in common use in XML data.
- Internet Message Format (Internet RFC 2822) §3.3 specifies the time notation used in email and HTTP headers.
- Date and Time on the Internet: Timestamps (Internet RFC 3339) specifies an ISO 8601 profile for use in new Internet protocols.
- Date & Time Formats on the Web surveys web- and Internet-oriented date and time formats.
- The 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 "Bucharest". By-Type 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 "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 time stamps 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".

## Related indexes

- Time and the Arts
- Open Directory - Reference: Time
- Google Directory - Reference > Time
- Yahoo! Directory > Science > Measurements and Units > Time