# 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 (known as `tz`, `tzdb`, or `zoneinfo`) is used by several implementations, including the GNU C Library (used in GNU/Linux), Android, FreeBSD, NetBSD, OpenBSD, ChromiumOS, Cygwin, MariaDB, musl libc, MySQL, webOS, AIX, iOS, macOS, Microsoft Windows, OpenVMS, Oracle Database, Oracle Solaris, and QNX.

Each main entry in the database represents a timezone for a set of civil-time clocks that have all agreed since 1970. Timezones are typically identified by continent or ocean and then by the name of the largest city within the region containing the clocks. 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 a POSIX.1-2024 implementation set the `TZ` environment variable to the location’s full name, e.g., `TZ="America/New_York"`.

Associated with each timezone is a history of offsets from Universal Time (UT), which is Greenwich Mean Time (GMT) with days beginning at midnight; for timestamps 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.

## Downloading the `tz` database

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 2022a, each release has been distributed in POSIX ustar interchange format, compressed as described above; older releases use a nearly compatible format. Since version 2016h, each release has contained a text file named “`version`” whose first (and currently only) line is the version. Older archived releases are available via HTTPS, rsync, and FTP.

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. The binary files are in a special format specified by The Time Zone Information Format (TZif) (Internet RFC 9636). The code also lets you read a TZif file and interpret timestamps for that location.

## Changes to the `tz` database

The `tz` code and data are by no means authoritative. If you find errors, please email changes to `tz@iana.org`, the time zone mailing list. The mailing list and its archives are public, so please do not send confidential information. See the mailing list’s main page to subscribe or to browse its archive of old messages. Metadata for mailing list discussions and corresponding data changes can be generated automatically.

Changes to the `tz` code and data are often propagated to clients via operating system updates, so client `tz` data can often be corrected by applying these updates. With GNU/Linux and similar systems, if your maintenance provider has not yet adopted the latest `tz` data, you can often short-circuit the process by tailoring the generic instructions in the `tz README` file and installing the latest data yourself. System-specific instructions for installing the latest `tz` data have also been published for AIX, Android, ICU, IBM JDK, Joda-Time, MySQL, Noda Time, and OpenJDK/Oracle JDK.

Since version 2013a, sources for the `tz` database have been UTF-8 text files with lines terminated by LF, which can be modified by common text editors such as GNU Emacs, gedit, and vim. Specialized source-file editing can be done via the Sublime zoneinfo package for Sublime Text and the VSCode zoneinfo extension for Visual Studio Code.

For further information about updates, please see Procedures for Maintaining the Time Zone Database (Internet RFC 6557). More detail can be found in Theory and pragmatics of the `tz` code and data. A0 TimeZone Migration displays changes between recent `tzdb` versions.

## Coordinating with governments and distributors

As discussed in “How Time Zones Are Coordinated”, the time zone database relies on collaboration among governments, the time zone database volunteer community, and data distributors downstream.

If your government plans to change its time zone boundaries or daylight saving rules, please send email as described in “Changes to the `tz` database”. Do this well in advance, as this will lessen confusion and will coordinate updates to many cell phones, computers, and other devices around the world. In your email, please cite the legislation or regulation that specifies the change, so that it can be checked for details such as the exact times when clock transitions occur. It is OK if a rule change is planned to affect clocks far into the future, as a long-planned change can easily be reverted or otherwise altered with a year’s notice before the change would have affected clocks.

There is no fixed schedule for `tzdb` releases. However, typically a release occurs every few months. Many downstream timezone data distributors wait for a `tzdb` release before they produce an update to time zone behavior in consumer devices and software products. After a release, various parties must integrate, test, and roll out an update before end users see changes. These updates can be expensive, for both the quality assurance process and the overall cost of shipping and installing updates to each device’s copy of `tzdb`. Updates may be batched with other updates and may take substantial time to reach end users after a release. Older devices may no longer be supported and thus may never be updated, which means they will continue to use out-of-date rules.

For these reasons any rule change should be promulgated at least a year before it affects how clocks operate; otherwise, there is a good chance that many clocks will be wrong due to delays in propagating updates, and that residents will be confused or even actively resist the change. The shorter the notice, the more likely clock problems will arise; see “On the Timing of Time Zone Changes” for examples.

## 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.
- A literary appreciation of the Olson/Zoneinfo/tz database comments on the database’s style.
- What time is it: managing time in the internet analyzes the database longitudinally.

## 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.
- TimeJones.com, Time Zone Converter and The World Clock are time zone converters.
- TimeZoneDB Database publishes `tzdb`-derived data in CSV and in SQL form.
- Date and Time Gateway lets you see the `TZ` values directly.
- Current Time in 1000 Places uses descriptions of the values.
- The World Clock – Worldwide lets you sort zone names and convert times.
- 24TimeZones has a world time map and a time converter.
- Time Difference calculates the current time difference between locations.
- Weather Now and The Time Now list the weather too.

## Network protocols for `tz` data

- Time Zone Data Distribution Service (TZDIST, Internet RFC 7808) is associated with CalDAV (Internet RFC 7809), a calendar access protocol for transferring time zone data by reference. TZDIST implementations are available.
- The iCalendar format (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

Although some of these do not fully support `tz` data, in recent `tzdb` distributions you can generally work around compatibility problems by running the command `make rearguard_tarballs` and compiling from the resulting tarballs instead.

- 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).
- 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 with a `std::chrono` API that is a standard part of C++. It is freely available under the MIT 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 OpenJDK and Oracle JDK. Although its source code is proprietary, its executable is available under the Java SE Timezone Updater License Agreement.
  - The Java SE 8 Date and 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.
  - IANA Updater and ZIUpdater are alternatives to TZUpdater. IANA Updater’s license is unclear; ZIUpdater is licensed under the GPL.
  - Time4A: Advanced date and time library for Android and Time4J: Advanced date, time and interval library for Java compile `tz` source into a binary format. Time4A is available under the Apache License and 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 is like Joda-Time and Time4J, but for the .NET framework instead of Java. It is freely available under the Apache License.
- Many modern JavaScript runtimes support `tz` natively via the `timeZone` option of `Intl.DateTimeFormat`. This can be used as-is or with most of the following libraries, many of which also support runtimes lacking the `timeZone` option.
  - The `Intl.DateTimeFormat` timezone polyfill is freely available under a BSD-style license.
  - The date-fns library manipulates timezone-aware timestamps in browsers and in Node.js. It is freely available under the MIT license.
  - Day.js is a minimalist replacement for the date and time API of the now-legacy Moment.js date manipulation library. It is freely available under the MIT license.
  - Luxon improves timezone support for the `Intl` API. It is freely available under the MIT license.
  - Moment Timezone is a Moment.js plugin. It is freely available under the MIT license.
  - Timezone is a JavaScript library that supports date arithmetic that is time zone aware. It is freely available under the MIT license.
  - @tubular/time supports live `tzdb` updates, astronomical and atomic time, a command-line interface, and full TypeScript. Its companion @tubular/time-tzdb can generate TZif and other files, and a companion website Timezone Database Explorer lets you convert timestamps, view transition histories, and download code and data. It is freely available under the MIT license.
  The proposed `Temporal` objects let programs access an abstract view of `tzdb` data, and are designed to replace JavaScript’s problematic `Date` objects when working with dates and times.
- JuliaTime contains a compiler from `tz` source into Julia. It is freely available under the MIT license.
- TZDB – IANA Time Zone Database for Delphi/FPC compiles from `tz` source into Object Pascal as compiled by Delphi and FPC. It is freely available under a BSD-style license.
- pytz – World Timezone Definitions for Python compiles `tz` source into Python. It is freely available under a BSD-style license. In code that can assume Python 3.6 or later it is largely superseded; see pytz: The Fastest Footgun in the West.
- 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 TZif readers

- The GNU C Library has an independent, thread-safe implementation of a TZif file reader. This library is freely available under the LGPL and is widely used in GNU/Linux systems.
- GNOME’s GLib has a TZif 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 TZif file reader. It is freely available under the Apache License.
- CCTZ is a simple C++ library that translates between UT and civil time and can read TZif files. It is freely available under the Apache License.
- The Go programming language has a TZif file reader `LoadLocationFromTZData`.
- The `posix_tz_db` package contains Python code to generate CSV and JSON tables that map `tz` settings to proleptic TZ approximations. For example, it maps `"Africa/Cairo"` to `"EET-2EEST,M4.5.5/0,M10.5.4/24"`, an approximation valid for Cairo timestamps from 2023 on. This can help porting to platforms that support only proleptic TZ. The package is freely available under the MIT license.
- Timelib is a C library that reads TZif files and converts timestamps from one time zone or format to another. It is used by PHP, HHVM, and MongoDB. It is freely available under the MIT license.
- Tcl, mentioned above, also contains a TZif file reader.
- DateTime::TimeZone::Tzfile is a TZif file reader written in Perl. It is freely available under the same terms as Perl (dual GPL and Artistic license).
- Python has a `zoneinfo.ZoneInfo` class that reads TZif data and creates objects that represent `tzdb` timezones. Python is freely available under the Python Software Foundation License. A companion PyPI module `tzdata` supplies TZif data if the underlying system data cannot be found; it is freely available under the Apache License.
- The public-domain tz.js library contains a Python tool that converts TZif 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 TZif data. It is freely available under a BSD-style license.

## Other `tz`-based time zone software

- FoxClocks is an extension for Google Chrome, 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.
- Microsoft Windows 8.1 and later has `tz` data and CLDR data (mentioned below) used by the Windows Runtime / Universal Windows Platform classes `DateTimeFormatter` and `Calendar`. 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. These mappings can be performed programmatically via the TimeZoneConverter .NET library, or the ICU Java and C++ libraries mentioned above.
- Oracle Java contains a copy of a subset of a recent `tz` database in a Java-specific format.

## Other time zone databases

- Time-zone Atlas is Astrodienst’s Web version of Shanks and Pottenger’s out-of-print time zone history atlases for the US and for the world. Although these extensive atlases were sources for much of the older `tz` data, they are 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.
- The Standard Schedules Information Manual of the International Air Transport Association gives current time zone rules for airports served by commercial aviation.

## Maps

- World Time Zone Map with current time has several fancy time zone maps; it covers Russia particularly well.
- 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.
- The Perry–Castañeda Library Map Collection of the University of Texas at Austin has copies of old maps taken from The World Factbook, formerly published by the US Central Intelligence Agency. Although the maps’ pictorial quality is good, the maps do not indicate daylight saving time.

## Time zone boundaries

Geographical boundaries between timezones are available from several Internet geolocation services and other sources.

- Timezone Boundary Builder extracts Open Street Map data to build boundaries of `tzdb` timezones. 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 borders appear to be quite accurate. Its main web page lists more than twenty libraries for looking up a timezone name from a GPS coordinate.
- 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 TimeZoneDB API. Commercial network API access is provided by AskGeo and GeoGarage.
- “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.
- Manifold Software – GIS and Database Tools includes a Manifold-format map of world time zone boundaries circa 2007, distributed under the GPL.
- 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 UT−12 and UT+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.
- The history of daylight saving time is surveyed in About Daylight Saving Time – History, rationale, laws & dates and summarized in A Brief History of Daylight Saving Time.
- Time Lords discusses how authoritarians manipulate civil time.
- Working with Time and Time Zones contains guidelines and best practices for software applications that deal with civil time.
- 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 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 and French).
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 and daylight saving time.
Chile
  The Hydrographic and Oceanographic Service of the Chilean Navy publishes a history of Chile’s official time (in Spanish).
China
  The Hong Kong Observatory maintains a history of summer time in Hong Kong, and Macau’s Meteorological and Geophysical Bureau maintains a similar history for Macau. Unfortunately the latter is incomplete and has errors.
Czech Republic
  When daylight saving time starts and ends (in Czech) summarizes and cites historical DST regulations.
Germany
  The National Institute for Science and Technology maintains the Realisation of Legal Time in Germany.
Israel
  Israel Timezone Files lists official time-change announcements and laws since 1940, almost all in Hebrew.
Malaysia
  See Singapore below.
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.
New Zealand
  The Department of Internal Affairs maintains a brief History of Daylight Saving in NZ.
Palestine
  The Ministry of Telecom and Digital Economy publishes a history of clock changes (in Arabic).
Portugal
  The Lisbon Astronomical Observatory publishes a history of legal time (in Portuguese).
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.
United States
  The Department of Transportation’s Recent Time Zone Proceedings lists changes to official written time zone boundaries, and its Time Zones dataset maps current boundaries. These boundaries are only for standard time, so the current map puts all of Arizona in one time zone even though part of Arizona observes DST and part does not.
Uruguay
  The Oceanography, Hydrography, and Meteorology Service of the Uruguayan Navy (SOHMA) publishes an annual almanac (in Spanish).

## Costs and benefits of time shifts

Various sources argue for and against daylight saving time and time zone shifts, and many scientific studies have been conducted. This section summarizes reviews and position statements based on scientific literature in the area.

- Carey RN, Sarma KM. Impact of daylight saving time on road traffic collision risk: a systematic review. BMJ Open. 2017;7(6):e014319. doi:10.1136/bmjopen-2016-014319. This reviews research literature and concludes that the evidence neither supports nor refutes road safety benefits from shifts in time zones.
- Havranek T, Herman D, Irsova D. Does daylight saving save electricity? A meta-analysis. Energy J. 2018;39(2):35–61. doi:10.5547/01956574.39.2.thav. This analyzes research literature and concludes, “Electricity savings are larger for countries farther away from the equator, while subtropical regions consume more electricity because of DST.”
- Neumann P, von Blanckenburg K. What time will it be? A comprehensive literature review on daylight saving time. Time Soc. 2025;34(4):684–745. doi:10.1177/0961463X241310562. This reviews DST’s effects on electricity, health, crime, road safety, and the economy, focusing on research since 2010, and concludes that year-round standard time is preferable overall.
- Romigi A, Franco V, Scoditti E et al. The effects of daylight saving time and clock time transitions on sleep and sleepiness: a systematic review. Sleep Med Rev. 2025;84:102161. doi:10.1016/j.smrv.2025.102161. This reviews DST and DST transitions, and concludes that they both harm sleep, health and behavior.

The following medical societies have taken positions on the advisability of clock shifts:

- In 2022 the American Medical Association issued a statement supporting permanent standard time on health grounds.
- Crawford MR, Winnebeck EC, von Schantz M et al. The British Sleep Society position statement on Daylight Saving Time in the UK. J Sleep Res. 2025;34(3):e14352. doi:10.1111/jsr.14352. This recommends that the UK abolish DST for health reasons.
- Malow BA. It is time to abolish the clock change and adopt permanent standard time in the United States: a Sleep Research Society position statement. Sleep. 2022;45(12):zsac236. doi:10.1093/sleep/zsac236. The Sleep Research Society advocates permanent standard time due to its health benefits.
- Rishi MA, Cheng JY, Strang AR et al. Permanent standard time is the optimal choice for health and safety: an American Academy of Sleep Medicine position statement. J Clin Sleep Med. 2024;20(1):121–125. doi:10.5664/jcsm.10898. The AASM argues for permanent standard time due to health and safety risks and economic costs of both DST transitions and permanent DST.
- Roenneberg T, Wirz-Justice A, Skene DJ et al. Why should we abolish Daylight Saving Time? J Biol Rhythms. 2019;34(3):227–230. doi:10.1177/0748730419854197. The Society for Research on Biological Rhythms opposes DST changes and permanent DST, and advocates that governments adopt “permanent Standard Time for the health and safety of their citizens”.

## Precision timekeeping

- The Science of Timekeeping is a thorough introduction to the theory and practice of precision timekeeping.
- The Science of Time 2016 contains several freely readable papers.
- NTP: The Network Time Protocol (Internet RFC 5905) discusses how to synchronize clocks of Internet hosts.
- The Huygens family of software algorithms can achieve accuracy to a few tens of nanoseconds in scalable server farms without special hardware.
- The Precision Time Protocol (IEEE 1588) can achieve submicrosecond clock accuracy on a local area network with special-purpose hardware.
- 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.
- Time Scales describes astronomical time scales like TDT, TCG, and TDB.
- The IAU’s SOFA collection contains C and Fortran code for converting among time scales like TAI, TDB, TDT and UTC. It is freely available under the SOFA license.
- 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.
- The rules for leap seconds are specified in Annex 1 (Time scales) of Standard-frequency and time-signal emissions, International Telecommunication Union – Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).
- 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 where a computer’s internal `time_t` integer clock counts every TAI second, as opposed to the default `"posix"` configuration where the internal clock ignores leap seconds. The two configurations agree for timestamps starting with 1972-01-01 00:00:00 UTC (`time_t` 63 072 000) and diverge for timestamps starting with `time_t` 78 796 800, which corresponds to the first leap second 1972-06-30 23:59:60 UTC in the `"right"` configuration, and to 1972-07-01 00:00:00 UTC in the `"posix"` configuration. In practice the two configurations also agree for timestamps before 1972 even though the historical situation is messy, partly because neither UTC nor TAI is well-defined for sufficiently old timestamps.
- The NTP Leap Second File covers the text file `leap-seconds.list`, which lists the currently known leap seconds. The IERS maintains this file, and a copy is distributed by `tzdb` for use by NTP implementations like classic `ntpd` and NTPsec. The `tz` database also distributes leap second information in a differently-formatted `leapseconds` text file, as well as in the `"right"` configuration in binary form; for example, `right/UTC` can be used by `chrony`, another NTP implementation.
- 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 abovementioned NTP implementations, supports conversion between UTC and smeared POSIX timestamps, and is used by major cloud service providers. However, according to §3.7.1 of Network Time Protocol Best Current Practices (Internet RFC 8633), leap smearing is not suitable for applications requiring accurate UTC or civil time, and is intended for use only in single, well-controlled environments.
- The LEAPSECS 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. The General Conference on Weights and Measures (CGPM) decided in 2022 to discontinue the use of leap seconds by 2035, and requested that no discontinuous adjustments be made to UTC for at least a century. The World Radiocommunication Conference resolved in 2023 to cooperate with this process. A draft Resolution C to make continuous UTC effective on 2027-05-20, and thereby discontinue leap seconds, has been scheduled for the 28th CGPM starting 2026-10-13 in Paris. One proposal to implement this would replace leap seconds with seven 13-second leap smears occurring once per decade until 2100, with leap smears after that gradually increasing in size. See:
  - Levine J. A proposal to change the leap-second adjustments to coordinated universal time. Metrologia. 2024;61(5):055002. doi:10.1088/1681-7575/ad6266 with followups in doi:10.1088/1681-7575/ade314 and doi:10.1088/1681-7575/ade315.
  However, there is still no consensus on whether this is the best way to replace leap seconds.

## 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 covers ISO 8601-1:2019 – Date and time – Representations for information interchange – Part 1: Basic rules.
- 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. An extension, Date and Time on the Internet: Timestamps with Additional Information (Internet RFC 9557) extends this profile to let you specify the `tzdb` timezone of a timestamp via suffixes like `[Asia/Tokyo]`.
- Date & Time Formats on the Web surveys web- and Internet-oriented date and time formats.
- Alphabetic time zone abbreviations should not be used as unique identifiers for UT offsets as they are ambiguous in practice. For example, in English-speaking North America “CST” denotes 6 hours behind UT, but in China it denotes 8 hours ahead of UT, and French-speaking North Americans prefer “HNC” to “CST”. The `tz` database contains English abbreviations for many timestamps; unfortunately some of these abbreviations were merely the database maintainers’ inventions, and these have been removed when possible.
- Numeric time zone abbreviations typically count hours east of UT, e.g., +09 for Japan and −10 for Hawaii. However, POSIX proleptic `TZ` settings use 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 timestamps 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 UT and the time zone is called “GMT”.

## See also

- Theory and pragmatics of the `tz` code and data
- Time and the Arts