# Sources for time zone and daylight saving time data

Time zone and daylight saving rules are controlled by individual governments. They are sometimes changed with little notice, and their histories and planned futures are often recorded only fitfully. Here is a summary of attempts to organize and record relevant data in this area.

## 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 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), Android, B2G OS, FreeBSD, NetBSD, OpenBSD, Chromium OS, Cygwin, DJGPP, MINIX, webOS, AIX, BlackBerry 10, iOS, macOS, Microsoft Windows, OpenVMS, Oracle Database, and Oracle Solaris.

Each location in the database represents a 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"`.

Associated with each region is a history of offsets from Universal Time (UT), which is Greenwich Mean Time (GMT) with days beginning at midnight; for time stamps after 1960 this is more precisely Coordinated Universal Time (UTC). The database also records when daylight saving time was in use, along with some time zone abbreviations such as EST for Eastern Standard Time in the US.

The following shell commands download the latest release's two tarballs to a GNU/Linux or similar host.

    mkdir tzdb
    cd tzdb
    wget https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
    wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
    gzip -dc tzcode-latest.tar.gz | tar -xf -
    gzip -dc tzdata-latest.tar.gz | tar -xf -

Alternatively, the following shell commands download the same release in a single-tarball format containing extra data useful for regression testing:

    wget https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz
    lzip -dc tzdb-latest.tar.lz | tar -xf -

These commands use convenience links to the latest release of the `tz` database hosted by the Time Zone Database website of the Internet Assigned Numbers Authority (IANA). Older releases are in files named `tzcodeV.tar.gz`, `tzdataV.tar.gz`, and `tzdb-V.tar.lz`, where `V` is the version. Since 1996, each version has been a four-digit year followed by lower-case letter (`a` through `z`, then `za` through `zz`, then `zza` through `zzz`, and so on). Since version 2016h, each release has contained a text file named "`version`" whose first (and currently only) line is the version. The releases are also available in an FTP directory via a less-secure protocol .

Alternatively, a development repository of code and data can be retrieved from GitHub via the shell command:

    git clone https://github.com/eggert/tz

Since version 2012e, each release has been tagged in development repositories. Untagged commits are less well tested and probably contain more errors.

After obtaining the code and data files, see the `README` file for what to do next. 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.

## Changes to the `tz` database

The `tz` code and data are by no means authoritative. If you find errors, please send changes to the time zone mailing list. You can also subscribe to it and browse the archive of old messages.

If your government plans to change its time zone boundaries or daylight saving rules, let the mailing list know well in advance. With less than a year's notice there is a good chance that some computer-based clocks will operate incorrectly after the change, due to delays in propagating updates to software and data. The shorter the notice, the more likely clock problems will arise.

Sources for the `tz` database are UTF-8 text files with lines terminated by LF, which can be modified by common text editors such as GNU Emacs, gedit, and vim. One editor has a package to simplify editing further:

- Sublime zoneinfo is a Sublime Text package for syntax highlighting `tz` source files.

For further information about updates, please see Procedures for Maintaining the Time Zone Database (Internet RFC 6557).

## Commentary on the `tz` database

- The article tz database is an encyclopedic summary.
- How to Read the tz Database Source Files explains the `tz` database format.
- On the Timing of Time Zone Changes gives examples of problems caused by inadequate notice by governments of time zone and daylight saving rule changes.
- A literary appreciation of the Olson/Zoneinfo/tz database comments on the database's style.

## Web sites using recent versions of the `tz` database

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

- Time.is shows locations' time and zones in a simple format.
- TimeJones.com is a simple time zone converter.
- 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 – Worldwide lets you sort zone names and convert times.
- Time Difference calculates the current time difference between locations.
- Weather Now lists the weather too.
- The Time Now also lists weather.

## Network protocols for `tz` data

- The Internet Engineering Task Force's Time Zone Data Distribution Service (tzdist) working group defined TZDIST (Internet RFC 7808), a time zone data distribution service, along with CalDAV (Internet RFC 7809), a calendar access protocol for transferring time zone data by reference. The draft TZDIST Geolocate Extension lets a client determine its time zone region from its geographic location using a 'geo' URI.
- The Internet Calendaring and Scheduling Core Object Specification (iCalendar) (Internet RFC 5445) covers time zone data; see its VTIMEZONE calendar component. The iCalendar format requires specialized parsers and generators; a variant xCal (Internet RFC 6321) uses XML format, and a variant jCal (Internet RFC 7265) uses JSON format.

## Other `tz` compilers

- Vzic is 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 and with a BSD-style license.
- 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.
- The Time Zone Database Parser is a C++ parser and runtime library. It is freely available under the Creative Commons Attribution 4.0 International Public License.
- International Components for Unicode (ICU) contains C/C++ and Java libraries for internationalization that has a compiler from `tz` source and from CLDR data (mentioned below) into an ICU-specific format. ICU is freely available under a BSD-style license.
- The Tzdata package for the Elixir language downloads and compiles tz source and exposes APIs for use. It is freely available under the MIT license.
- Java-based compilers and libraries include:
  - The TZUpdater tool compiles `tz` source into the format used by Oracle Java.
  - The Java 8 `java.time` API can be supplemented by ThreeTen-Extra, which 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 binary format. It inspired Java 8 `java.time`, which its users should migrate to once they can assume Java 8 or later. It is available under the Apache License.
  - Time4J – Advanced date, time and interval library for Java contains a class `net.time4j.tool.TimezoneRepositoryCompiler` that compiles `tz` source into a binary format. Time4J is available under the GNU Lesser General Public License (LGPL).
  - ICU (mentioned above) contains compilers and Java-based libraries.
- Noda Time – Date and time API for .NET and TZ4Net are similar to Joda-Time and Time4J, but for the .NET framework instead of Java. They are freely available under the Apache License and a BSD-style license, respectively.
- JavaScript-based compilers and libraries include:
  - Moment Timezone is a plugin for the Moment.js date manipulation library. It is freely available under the MIT license.
  - TimezoneJS.Date's API is upward compatible with standard JavaScript Dates. It is freely available under the Apache License.
  - Walltime-js translates UT to local time. It is freely available under the MIT license.
- JuliaTime contains a compiler from `tz` source into Julia. It is freely available under the MIT license.
- pytz – World Timezone Definitions for Python 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.
- 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 LGPL and is widely used in GNU/Linux systems.
- GNOME's GLib has a `tz` binary file reader written in C that creates a `GTimeZone` object representing sets of UT offsets. It is freely available under the LGPL.
- The BDE Standard Library's `baltzo::TimeZoneUtil` component contains a C++ implementation of a binary file reader. It is freely available under the Apache License.
- CCTZ is a simple C++ library that translates between UTC and civil time and can read binary files. It is freely available under the Apache License.
- ZoneInfo.java is a `tz` binary file reader written in Java. It is freely available under the LGPL.
- Timezone is a JavaScript library that supports date arithmetic that is time zone aware. It is freely available under the MIT license.
- Tcl, mentioned above, also contains a `tz` binary file reader.
- DateTime::TimeZone::Tzfile is a `tz` binary file reader written in Perl. It is freely available under the same terms as Perl (dual GPL and Artistic license).
- The public-domain tz.js library contains a Python tool that converts `tz` binary data into JSON-format data suitable for use in its JavaScript library for time zone conversion. Dates before 1970 are not supported.
- The timezone-olson package contains Haskell code that parses and uses `tz` binary data. It is freely available under a BSD-style license.

## Other `tz`-based time zone software

- FoxClocks is an extension for Google Chrome and for Mozilla Toolkit applications like Firefox and Thunderbird. It displays multiple clocks in the application window, and has a mapping interface to Google Earth. It is freely available under the GPL.
- Go programming language implementations contain a copy of a 32-bit subset of a recent `tz` database in a Go-specific format.
- International clock (intclock) is a clock that displays multiple time zones on GNU/Linux and similar systems. It is freely available under the GPL.
- Microsoft Windows 8.1 and later has `tz` data and CLDR data (mentioned below) used by Windows Runtime classes such as `DateTimeFormatter`. Exploring Windows Time Zones with `System.TimeZoneInfo` describes the older, proprietary method of Microsoft Windows 2000 and later, which stores time zone data in the Windows Registry. The Zone → Tzid table or XML file of the CLDR data maps proprietary zone IDs to `tz` names.
- Oracle Java contains 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.
- Time Zone Master is a Microsoft Windows clock program that can automatically download, compile and use `tz` releases. The Basic version is free.
- VelaTerra is a macOS program. Its developers offer free licenses to `tz` contributors.

## Other time zone databases

- Time-zone Atlas is Astrodienst's Web version of Shanks and Pottenger's time zone history atlases also published in software form by ACS-Starcrafts. These atlases are extensive but unreliable, as Shanks appears to have guessed many UT offsets and transitions. The atlases cite no sources and do not indicate which entries are guesswork.
- HP-UX has a database in its own `tztab`(4) format.
- Microsoft Windows has proprietary data mentioned above.
- World Time Server is another time zone database.
- World Time Zones contains data from the Time Service Department of the US Naval Observatory.
- The Standard Schedules Information Manual of the International Air Transport Association gives current time zone rules for 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.
- 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.
- How much is time wrong around the world? maps the difference between mean solar and standard time, highlighting areas such as western China where the two differ greatly. It's a bit out of date, unfortunately.

## Time zone boundaries

Geographical boundaries between time zone regions are available from several geolocation services and other sources.

- Databases of time zone boundaries include:
  - Timezone Boundary Builder extracts Open Street Map data to build boundaries of `tz` regions. Its code is freely available under the MIT license, and its data entries are freely available under the Open Data Commons Open Database License. The maps' borders appear to be quite accurate.
  - TZ timezones maps contains shapefiles of sets of `tz` regions. This includes tz_world, a shapefile for all the world's regions
  - Whereonearth-timezone is in GeoJSON format (Internet RFC 7946), and combines the the tz_world shapefiles with the GeoPlanet dataset.
- Programmatic interfaces that map geographical coordinates via tz_world to `tz` regions include:
  - GeoTimeZone is written in C# and is freely available under the MIT license.
  - The latlong package is written in Go and is freely available under the Apache License.
  - LatLongToTimezone, in both Java and Swift form, is freely available under the MIT license.
  - For Node.js, the geo-tz module is freely available under the MIT license, and the tz-lookup module is in the public domain.
  - The timezonefinder library for Python is freely available under the MIT license.
  - The timezone_finder library for Ruby is freely available under the MIT license.
  - What Time is It Here? applies MongoDB geospatial query operators to shapefiles' data.
- Free access via a network API, if you register a key, is provided by the GeoNames Timezone web service, the Google Maps Time Zone API, and the Time Zone Database & API. Commercial network API access is provided by AskGeo and GeoGarage. Also, an experimental server is available for the TZDIST Geolocate Extension mentioned above.
- "How to get a time zone from a location using latitude and longitude coordinates?" discusses other geolocation possibilities.
- Administrative Divisions of Countries ("Statoids") lists political subdivision data related to time zones.
- Time zone boundaries for multizone countries summarizes legal boundaries between time zones within countries.
- Manifold.net's Free Stuff for Manifold System Users includes a Manifold-format map of world time zone boundaries distributed under the GPL.
- 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.
- Working with Time Zones contains guidelines and best practices for software applications that deal with civil time.
- A Brief History of Daylight Saving Time summarizes some of the contentious history of DST.
- 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 a research paper 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
  National Research Council Canada publishes current and some older information about time zones & daylight saving time.
Chile
  The Hydrographic and Oceanographic Service of the Chilean Navy publishes a history of Chile's 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).
Italy
  The National Institute of Metrological Research maintains a table of civil time (in Italian).
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 of Daylight Saving. The privately-maintained History of New Zealand time has more details.
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.
United States
  The Department of Transportation's Recent Time Zone Proceedings lists changes to time zone boundaries.

## Precision timekeeping

- The Science of Timekeeping is a thorough introduction to the theory and practice of precision timekeeping.
- NTP: The Network Time Protocol (Internet RFC 5905) discusses how to synchronize clocks of Internet hosts.
- The Precision Time Protocol (IEEE 1588) can achieve submicrosecond clock accuracy on a local area network.
- 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.
- 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 collection contains C and Fortran code for converting among time scales like TAI, TDB, TDT and UTC.
- Mars24 Sunclock – Time on Mars describes Airy Mean Time (AMT) 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.
- IERS Bulletins contains official publications of the International Earth Rotation and Reference Systems Service, which decides when leap seconds occur. The `tz` code and data support leap seconds via an optional "`right`" configuration, as opposed to the default "`posix`" configuration.
- Leap Smear discusses how to gradually adjust POSIX clocks near a leap second so that they disagree with UTC by at most a half second, even though every POSIX minute has exactly sixty seconds. This approach works with the default `tz` "`posix`" configuration, is supported by the NTP reference implementation, and is used by major cloud service providers.
- The Leap Second Discussion List covers McCarthy and Klepczynski's 1999 proposal to discontinue leap seconds, discussed further in The leap second: its history and possible future. UTC might be redefined without Leap Seconds gives pointers on this contentious issue, which was active until 2015 and could become active again.

## Time notation

- 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". Its by-type charts show these values for many locales. Data values are available in both LDML (an XML format) and JSON.
- 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.
- §3.3 of Internet Message Format (Internet RFC 5322) 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.
- Alphabetic time zone abbreviations should not be used as unique identifiers for UTC offsets as they are ambiguous in practice. For example, in English-speaking North America "CST" denotes 6 hours behind UTC, but in China it denotes 8 hours ahead of UTC, and French-speaking North Americans prefer "HNC" to "CST". The `tz` database contains English abbreviations for many time stamps; unfortunately some of these abbreviations were merely the database maintainers' inventions, and are gradually being removed.
- 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 incorrectly imply that local time is nine hours ahead of UTC and the time zone is called "GMT".

## See also

- Time and the Arts