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<!DOCTYPE html> <html lang="en"> <head> <title>Time zone and daylight saving time data</title> <meta charset="UTF-8"> <style> pre {margin-left: 2em; white-space: pre-wrap;} </style> </head> <body> <h1>Time zone and daylight saving time data</h1> <p> <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a> 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. </p> <h3>Outline</h3> <nav> <ul> <li>The <code><abbr>tz</abbr></code> database product and process <ul> <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li> <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li> <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li> <li><a href="#coordinating">Coordinating with governments and distributors</a></li> <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li> </ul> </li> <li>Uses of the <code><abbr>tz</abbr></code> database <ul> <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li> <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li> <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li> <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li> <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li> </ul> </li> <li>Related data <ul> <li><a href="#other-dbs">Other time zone databases</a></li> <li><a href="#maps">Maps</a></li> <li><a href="#boundaries">Time zone boundaries</a></li> </ul> </li> <li>Timekeeping concepts <ul> <li><a href="#civil">Civil time concepts and history</a></li> <li><a href="#national">National histories of legal time</a></li> <li><a href="#costs">Costs and benefits of time shifts</a></li> <li><a href="#precision">Precision timekeeping</a></li> <li><a href="#notation">Time notation</a></li> </ul> </li> <li><a href="#see-also">See also</a></li> </ul> </nav> <section> <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2> <p> The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a> 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 <code><abbr>tz</abbr></code>, <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>) is used by several implementations, including <a href="https://www.gnu.org/software/libc/">the <abbr title="GNU's Not Unix">GNU</abbr> C Library</a> (used in <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>), <a href="https://www.android.com">Android</a>, <a href="https://www.freebsd.org">Free<abbr title="Berkeley Software Distribution">BSD</abbr></a>, <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>, <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>, <a href="https://www.chromium.org/chromium-os/">Chromium OS</a>, <a href="https://cygwin.com">Cygwin</a>, <a href="https://mariadb.org">MariaDB</a>, <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>, <a href="https://www.mysql.com">MySQL</a>, <a href="https://en.wikipedia.org/wiki/WebOS"><abbr title="Web Operating System">webOS</abbr></a>, <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr title="Advanced Interactive eXecutive">AIX</abbr></a>, <a href="https://www.apple.com/ios"><abbr title="iPhone OS">iOS</abbr></a>, <a href="https://www.apple.com/macos">macOS</a>, <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>, <a href="https://www.vmssoftware.com">Open<abbr title="Virtual Memory System">VMS</abbr></a>, <a href="https://www.oracle.com/database/">Oracle Database</a>, and <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p> <p> Each main entry in the database represents a <dfn>timezone</dfn> 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, <code>America/New_York</code> represents most of the <abbr title="United States">US</abbr> eastern time zone; <code>America/Phoenix</code> represents most of Arizona, which uses mountain time without daylight saving time (<abbr>DST</abbr>); <code>America/Detroit</code> represents most of Michigan, which uses eastern time but with different <abbr>DST</abbr> 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 <a href="https://en.wikipedia.org/wiki/POSIX"><abbr title="Portable Operating System Interface">POSIX</abbr>.1-2017</a> implementation set the <code><abbr>TZ</abbr></code> environment variable to the location's full name, e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p> <p> Associated with each timezone is a history of offsets from <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal Time</a> (<abbr>UT</abbr>), which is <a href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean Time</a> (<abbr>GMT</abbr>) with days beginning at midnight; for timestamps after 1960 this is more precisely <a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated Universal Time</a> (<abbr>UTC</abbr>). The database also records when daylight saving time was in use, along with some time zone abbreviations such as <abbr>EST</abbr> for Eastern Standard Time in the <abbr>US</abbr>.</p> </section> <section> <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2> <p> The following <a href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download the latest release's two <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a> to a <abbr>GNU</abbr>/Linux or similar host.</p> <pre><code>mkdir tzdb cd tzdb <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz <a href="https://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="https://www.gnu.org/software/tar/">tar</a> -xf - gzip -dc tzdata-latest.tar.gz | tar -xf - </code></pre> <p>Alternatively, the following shell commands download the same release in a single-tarball format containing extra data useful for regression testing:</p> <pre><code>wget <a href="https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz">https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz</a> <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf - </code></pre> <p>These commands use convenience links to the latest release of the <code><abbr>tz</abbr></code> database hosted by the <a href="https://www.iana.org/time-zones">Time Zone Database website</a> of the <a href="https://www.iana.org">Internet Assigned Numbers Authority (IANA)</a>. Older releases are in files named <code>tzcode<var>V</var>.tar.gz</code>, <code>tzdata<var>V</var>.tar.gz</code>, and <code>tzdb-<var>V</var>.tar.lz</code>, where <code><var>V</var></code> is the version. Since 1996, each version has been a four-digit year followed by lower-case letter (<samp>a</samp> through <samp>z</samp>, then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp> through <samp>zzz</samp>, and so on). Since version 2022a, each release has been distributed in <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX ustar interchange format</a>, compressed as described above; older releases use a nearly compatible format. Since version 2016h, each release has contained a text file named "<code>version</code>" whose first (and currently only) line is the version. Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>, and are also available in an <a href="ftp://ftp.iana.org/tz/releases/"><abbr title="File Transfer Protocol">FTP</abbr> directory</a> via a less secure protocol.</p> <p>Alternatively, a development repository of code and data can be retrieved from <a href="https://github.com">GitHub</a> via the shell command:</p> <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a> </code></pre> <p> Since version 2012e, each release has been tagged in development repositories. Untagged commits are less well tested and probably contain more errors.</p> <p> After obtaining the code and data files, see the <code>README</code> file for what to do next. The code lets you compile the <code><abbr>tz</abbr></code> source files into machine-readable binary files, one for each location. The binary files are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>) specified by <a href="https://datatracker.ietf.org/doc/html/8536">Internet <abbr>RFC</abbr> 8536</a>. The code also lets you read a <abbr>TZif</abbr> file and interpret timestamps for that location.</p> </section> <section> <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2> <p> The <code><abbr>tz</abbr></code> code and data are by no means authoritative. If you find errors, please send changes to <a href="mailto:tz@iana.org"><code>tz@iana.org</code></a>, the time zone mailing list. You can also <a href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it and browse the <a href="https://mm.icann.org/pipermail/tz/">archive of old messages</a>. <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list discussions</a> and corresponding data changes can be generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>. </p> <p> Changes to the <code><abbr>tz</abbr></code> code and data are often propagated to clients via operating system updates, so client <code><abbr>tz</abbr></code> 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 <code><abbr>tz</abbr></code> data, you can often short-circuit the process by tailoring the generic instructions in the <code><abbr>tz</abbr> README</code> file and installing the latest data yourself. System-specific instructions for installing the latest <code><abbr>tz</abbr></code> data have also been published for <a href="https://developer.ibm.com/articles/au-aix-olson-time-zone/"><abbr>AIX</abbr></a>, <a href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>, <a href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr title="International Components for Unicode">ICU</abbr></a>, <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr> JDK</a>, <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>, <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>. </p> <p>Since version 2013a, sources for the <code><abbr>tz</abbr></code> database have been <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr title="Unicode Transformation Format 8-bit">UTF-8</abbr></a> <a href="https://en.wikipedia.org/wiki/Text_file">text files</a> with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr title="linefeed">LF</abbr></a>, which can be modified by common text editors such as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>, <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and <a href="https://www.vim.org">vim</a>. Specialized source-file editing can be done via the <a href="https://packagecontrol.io/packages/zoneinfo">Sublime zoneinfo</a> package for <a href="https://www.sublimetext.com">Sublime Text</a> and the <a href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual Studio Code</a>. </p> <p> For further information about updates, please see <a href="https://datatracker.ietf.org/doc/html/rfc6557">Procedures for Maintaining the Time Zone Database</a> (Internet <abbr title="Request For Comments">RFC</abbr> 6557). More detail can be found in <a href="theory.html">Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</a>. <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a> displays changes between recent <code><abbr>tzdb</abbr></code> versions. </p> </section> <section> <h2 id="coordinating">Coordinating with governments and distributors</h2> <p> As discussed in "<a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How Time Zones Are Coordinated</a>", the time zone database relies on collaboration among governments, the time zone database volunteer community, and data distributors downstream. <p> If your government plans to change its time zone boundaries or daylight saving rules, please send email to <a href="mailto:tz@iana.org"><code>tz@iana.org</code></a> 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.</p> <p> There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases. However, typically a release occurs every few months. Many downstream timezone data distributors wait for a <code><abbr>tzdb</abbr></code> 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 <a href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes. These updates can be expensive, for both the <a href="https://en.wikipedia.org/wiki/Quality_assurance">quality assurance</a> process and the overall cost of shipping and installing updates to each device's copy of <code><abbr>tzdb</abbr></code>. 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.</p> <p> 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 "<a href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On the Timing of Time Zone Changes</a>" for examples. </p> </section> <section> <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2> <ul> <li>The article <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is an encyclopedic summary.</li> <li><a href="tz-how-to.html">How to Read the tz Database Source Files</a> explains the <code><abbr>tz</abbr></code> database format.</li> <li><a href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the database's style.</li> <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it: managing time in the internet</a> analyzes the database longitudinally.</li> </ul> </section> <section> <h2 id="web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</h2> <p> These are listed roughly in ascending order of complexity and fanciness. </p> <ul> <li><a href="https://time.is">Time.is</a> shows locations' time and zones.</li> <li><a href="https://www.timejones.com">TimeJones.com</a>, <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and <a href="https://www.worldclock.com">The World Clock</a> are time zone converters.</li> <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a> publishes <code><abbr>tzdb</abbr></code>-derived data in <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr title="comma-separated values">CSV</abbr></a> and in <a href="https://en.wikipedia.org/wiki/SQL"><abbr title="Structured Query Language">SQL</abbr></a> form.</li> <li><a href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a> lets you see the <code><abbr>TZ</abbr></code> values directly.</li> <li><a href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current Time in 1000 Places</a> uses descriptions of the values.</li> <li><a href="https://www.timeanddate.com/worldclock/">The World Clock – Worldwide</a> lets you sort zone names and convert times.</li> <li><a href="https://24timezones.com">24TimeZones</a> has a world time map and a time converter.</li> <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a> calculates the current time difference between locations.</li> <li><a href="https://www.wx-now.com">Weather Now</a> and <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li> </ul> </section> <section> <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2> <ul> <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data Distribution Service (tzdist) working group</a> defined <a href="https://datatracker.ietf.org/doc/html/rfc7808">TZDIST</a> (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service, along with <a href="https://datatracker.ietf.org/doc/html/rfc7809">CalDAV</a> (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for transferring time zone data by reference. <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST implementations</a> are available. The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis mailing list</a> discusses possible extensions.</li> <li>The <a href="https://datatracker.ietf.org/doc/html/rfc5545"> Internet Calendaring and Scheduling Core Object Specification (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445) covers time zone data; see its VTIMEZONE calendar component. The iCalendar format requires specialized parsers and generators; a variant <a href="https://datatracker.ietf.org/doc/html/rfc6321">xCal</a> (Internet <abbr>RFC</abbr> 6321) uses <a href="https://www.w3.org/XML/"><abbr title="Extensible Markup Language">XML</abbr></a> format, and a variant <a href="https://datatracker.ietf.org/doc/html/rfc7265">jCal</a> (Internet <abbr>RFC</abbr> 7265) uses <a href="https://www.json.org"><abbr title="JavaScript Object Notation">JSON</abbr></a> format.</li> </ul> </section> <section> <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2> <p>Although some of these do not fully support <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code> distributions you can generally work around compatibility problems by running the command <code>make rearguard_tarballs</code> and compiling from the resulting tarballs instead.</p> <ul> <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a> program that compiles <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files. Vzic is freely available under the <a href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr> General Public License (<abbr title="General Public License">GPL</abbr>)</a>.</li> <li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz database conversion utility</a> is like Vzic, except for the <a href="https://dotnet.microsoft.com">.NET framework</a> and with a <abbr>BSD</abbr>-style license.</li> <li><a href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a> contains a script <code>parse_olson</code> that compiles <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a> modules. It is part of the Perl <a href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>, which is freely available under both the <abbr>GPL</abbr> and the Perl Artistic License. DateTime::TimeZone also contains a script <code>tests_from_zdump</code> that generates test cases for each clock transition in the <code><abbr>tz</abbr></code> database.</li> <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone Database Parser</a> is a <a href="https://en.wikipedia.org/wiki/C++">C++</a> parser and runtime library with <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a> adopted by <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>, the current iteration of the C++ standard. It is freely available under the <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li> <li><a id="ICU" href="https://icu.unicode.org">International Components for Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a> libraries for internationalization that has a compiler from <code><abbr>tz</abbr></code> source and from <abbr title="Common Locale Data Repository">CLDR</abbr> data (mentioned <a href="#CLDR">below</a>) into an <abbr>ICU</abbr>-specific format. <abbr>ICU</abbr> is freely available under a <abbr>BSD</abbr>-style license.</li> <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for the <a href="https://elixir-lang.org">Elixir</a> language downloads and compiles <code><abbr>tz</abbr></code> source and exposes <abbr title="Application Program Interface">API</abbr>s for use. It is freely available under the <abbr>MIT</abbr> license.</li> <li>Java-based compilers and libraries include: <ul> <li>The <a href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by <a href="https://openjdk.java.net/">OpenJDK</a> and <a href="https://jdk.java.net/">Oracle JDK</a>. Although its source code is proprietary, its executable is available under the <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE Timezone Updater License Agreement</a>.</li> <li>The <a href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>, which is freely available under a <abbr>BSD</abbr>-style license.</li> <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date and time <abbr>API</abbr></a> contains a class <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles <code><abbr>tz</abbr></code> source into a binary format. It inspired Java 8 <code>java.time</code>, which its users should migrate to once they can assume Java 8 or later. It is available under the <a href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li> <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a> are alternatives to TZUpdater. IANA Updater's license is unclear; ZIUpdater is licensed under the <abbr>GPL</abbr>.</li> <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and time library for Android</a> and <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date, time and interval library for Java</a> compile <code><abbr>tz</abbr></code> source into a binary format. Time4A is available under the Apache License and Time4J is available under the <a href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser General Public License (<abbr title="Lesser General Public License">LGPL</abbr>)</a>.</li> <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and Java-based libraries.</li> </ul> <li><a href="https://nodatime.org">Noda Time – Date and time <abbr>API</abbr> for .NET</a> is like Joda-Time and Time4J, but for the .NET framework instead of Java. It is freely available under the Apache License.</li> <li>Many modern <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a> runtimes support <code><abbr>tz</abbr></code> natively via the <code>timeZone</code> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><code>Intl.DateTimeFormat</code></a>. This can be used as-is or with most of the following libraries, many of which also support runtimes lacking the <code>timeZone</code> option. <ul> <li>The <a href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code> timezone polyfill</a> is freely available under a <abbr>BSD</abbr>-style license.</li> <li>The <a href="https://date-fns.org/">date-fns</a> library manipulates timezone-aware timestamps in browsers and in <a href="https://nodejs.org/en/">Node.js</a>. It is freely available under the <abbr>MIT</abbr> license.</li> <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a minimalist replacement for the date and time API of the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date manipulation library. It is freely available under the <abbr>MIT</abbr> license.</li> <li><a href="https://moment.github.io/luxon/">Luxon</a> improves timezone support for the <code>Intl</code> API. It is freely available under the <abbr>MIT</abbr> license.</li> <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a Moment.js plugin. It is freely available under the <abbr>MIT</abbr> license.</li> <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a JavaScript library that supports date arithmetic that is time zone aware. It is freely available under the <abbr>MIT</abbr> license.</li> <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a> supports live <code><abbr>tzdb</abbr></code> updates, astronomical and atomic time, a command-line interface, and full <a href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>. Its companion <a href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a> can generate <abbr>TZif</abbr> and other files, and a companion website <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you convert timestamps, view transition histories, and download code and data. It is freely available under the <abbr>MIT</abbr> license.</li> </ul> The proposed <a href="https://github.com/tc39/proposal-temporal"><code>Temporal</code> objects</a> let programs access an abstract view of <code><abbr>tzdb</abbr></code> data, and are designed to replace <a href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's problematic <code>Date</code> objects</a> when working with dates and times. <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a compiler from <code><abbr>tz</abbr></code> source into <a href="https://julialang.org/">Julia</a>. It is freely available under the <abbr>MIT</abbr> license.</li> <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> – <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr title="Free Pascal Compiler">FPC</abbr></a> compiles from <code><abbr>tz</abbr></code> source into <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a> as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a> and <a href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>. It is freely available under a <abbr>BSD</abbr>-style license.</li> <li><a href="http://pytz.sourceforge.net">pytz – World Timezone Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into <a href="https://www.python.org">Python</a>. It is freely available under a <abbr>BSD</abbr>-style license. In code that can assume Python 3.6 or later it is largely superseded; see <a href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz: The Fastest Footgun in the West</a>.</li> <li><a href="https://tzinfo.github.io">TZInfo – Ruby Timezone Library</a> compiles <code><abbr>tz</abbr></code> source into <a href="https://www.ruby-lang.org/en/">Ruby</a>. It is freely available under the <abbr>MIT</abbr> license.</li> <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time Library</a> is a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class library that compiles <code><abbr>tz</abbr></code> source into a time zone repository whose format is either proprietary or an <abbr>XML</abbr>-encoded representation.</li> <li><a id="Tcl" href="https://tcl.tk">Tcl</a> contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code> source into text files, along with a runtime that can read those files. Tcl is freely available under a <abbr>BSD</abbr>-style license.</li> </ul> </section> <section> <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2> <ul> <li>The <a href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C Library</a> has an independent, thread-safe implementation of a <abbr>TZif</abbr> file reader. This library is freely available under the LGPL and is widely used in <abbr>GNU</abbr>/Linux systems.</li> <li><a href="https://www.gnome.org">GNOME</a>'s <a href="https://developer.gnome.org/glib/">GLib</a> has a <abbr>TZif</abbr> file reader written in C that creates a <code>GTimeZone</code> object representing sets of <abbr>UT</abbr> offsets. It is freely available under the <abbr>LGPL</abbr>.</li> <li>The <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s <code>baltzo::TimeZoneUtil</code> component contains a C++ implementation of a <abbr>TZif</abbr> file reader. It is freely available under the Apache License.</li> <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++ library that translates between <abbr>UT</abbr> and civil time and can read <abbr>TZif</abbr> files. It is freely available under the Apache License.</li> <li>The <a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code> package</a> contains Python code to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map <code><abbr>tz</abbr></code> settings to POSIX.1-2017-like approximations. For example, it maps <code>"Africa/Cairo"</code> to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>, an approximation valid for Cairo timestamps from 2023 on. This can help porting to platforms that support only POSIX.1-2017. The package is freely available under the MIT license.</li> <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C library that reads <abbr>TZif</abbr> files and converts timestamps from one time zone or format to another. It is used by <a href="https://secure.php.net"><abbr title="PHP: Hypertext Preprocessor">PHP</abbr></a>, <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>, and <a href="https://www.mongodb.com">MongoDB</a>. It is freely available under the <abbr>MIT</abbr> license.</li> <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a <abbr>TZif</abbr> file reader.</li> <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile"> DateTime::TimeZone::Tzfile</a> is a <abbr>TZif</abbr> file reader written in Perl. It is freely available under the same terms as Perl (dual <abbr>GPL</abbr> and Artistic license).</li> <li>Python has a <a id="python-zoneinfo" href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code> class</a> that reads <abbr>TZif</abbr> data and creates objects that represent <code><abbr>tzdb</abbr></code> timezones. Python is freely available under the <a href="https://docs.python.org/3/license.html">Python Software Foundation License</a>. A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a> supplies TZif data if the underlying system data cannot be found; it is freely available under the Apache License.</li> <li>The public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a> library contains a Python tool that converts <abbr>TZif</abbr> data into <abbr>JSON</abbr>-format data suitable for use in its JavaScript library for time zone conversion. Dates before 1970 are not supported.</li> <li>The <a href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a> package contains <a href="https://www.haskell.org">Haskell</a> code that parses and uses <abbr>TZif</abbr> data. It is freely available under a <abbr>BSD</abbr>-style license.</li> </ul> </section> <section> <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2> <ul> <li><a href="https://foxclocks.org">FoxClocks</a> is an extension for <a href="https://www.google.com/chrome/">Google Chrome</a>, <a href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>. It displays multiple clocks in the application window, and has a mapping interface to <a href="https://www.google.com/earth/">Google Earth</a>. It is freely available under the <abbr>GPL</abbr>.</li> <li><a href="https://golang.org">Go programming language</a> implementations contain a copy of a 32-bit subset of a recent <code><abbr>tz</abbr></code> database in a Go-specific format.</li> <li>Microsoft Windows 8.1 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr> data (mentioned <a href="#CLDR">below</a>) used by the <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> / <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>. <a id="System.TimeZoneInfo" href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes the older, proprietary method of Microsoft Windows 2000 and later, which stores time zone data in the <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The <a href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone → Tzid table</a> or <a href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr> file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs to <code><abbr>tz</abbr></code> names. These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library, or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>. <li><a href="https://www.oracle.com/java/index.html">Oracle Java</a> contains a copy of a subset of a recent <code><abbr>tz</abbr></code> database in a Java-specific format.</li> </ul> </section> <section> <h2 id="other-dbs">Other time zone databases</h2> <ul> <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a> is Astrodienst's Web version of Shanks and Pottenger's out-of-print time zone history atlases <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and <a href="https://www.worldcat.org/oclc/76950459">for the world</a>. Although these extensive atlases <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were sources for much of the older <code><abbr>tz</abbr></code> data</a>, they are unreliable as Shanks appears to have guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no sources and do not indicate which entries are guesswork.</li> <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in its own <code>tztab</code>(4) format.</li> <li>Microsoft Windows has proprietary data mentioned <a href="#System.TimeZoneInfo">above</a>.</li> <li><a href="https://www.worldtimeserver.com">World Time Server</a> is another time zone database.</li> <li>The <a href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard Schedules Information Manual</a> of the International Air Transport Association gives current time zone rules for airports served by commercial aviation.</li> </ul> </section> <section> <h2 id="maps">Maps</h2> <ul> <li>The <a href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World and Regional Maps section</a> of <em>The World Factbook</em>, published by the <a href="https://www.cia.gov">US Central Intelligence Agency (<abbr title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time zone map; the <a href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda Library Map Collection</a> of the University of Texas at Austin has copies of recent editions. The pictorial quality is good, but the maps do not indicate daylight saving time, and parts of the data are a few years out of date.</li> <li><a href="https://www.worldtimezone.com">World Time Zone Map with current time</a> has several fancy time zone maps; it covers Russia particularly well. The maps' pictorial quality is not quite as good as the <abbr>CIA</abbr>'s but the maps are more up to date.</li> <li><a href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How much is time wrong around the world?</a> 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.</li> </ul> </section> <section> <h2 id="boundaries">Time zone boundaries</h2> <p>Geographical boundaries between timezones are available from several <a href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet geolocation</a> services and other sources.</p> <ul> <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone Boundary Builder</a> extracts <a href="https://www.openstreetmap.org">Open Street Map</a> data to build boundaries of <code><abbr>tzdb</abbr></code> timezones. Its code is freely available under the <abbr>MIT</abbr> license, and its data entries are freely available under the <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons Open Database License</a>. 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.</li> <li>Free access via a network API, if you register a key, is provided by the <a href="https://www.geonames.org/export/web-services.html#timezone">GeoNames Timezone web service</a>, the <a href="https://developers.google.com/maps/documentation/timezone/intro">Google Maps Time Zone API</a>, and the <a href="https://timezonedb.com/api">TimeZoneDB API</a>. Commercial network API access is provided by <a href="https://askgeo.com">AskGeo</a> and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>. </li> <li>"<a href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How to get a time zone from a location using latitude and longitude coordinates?</a>" discusses other geolocation possibilities.</li> <li><a href="http://statoids.com/statoids.html">Administrative Divisions of Countries ("Statoids")</a> lists political subdivision data related to time zones.</li> <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software – GIS and Database Tools</a> includes a Manifold-format map of world time zone boundaries circa 2007, distributed under the <abbr>GPL</abbr>.</li> <li>A ship within the <a href="https://en.wikipedia.org/wiki/Territorial_waters">territorial waters</a> of any nation uses that nation's time. In international waters, time zone boundaries are meridians 15° apart, except that <abbr>UT</abbr>−12 and <abbr>UT</abbr>+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.</li> </ul> </section> <section> <h2 id="civil">Civil time concepts and history</h2> <ul> <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A Walk through Time</a> surveys the evolution of timekeeping.</li> <li>The history of daylight saving time is surveyed in <a href="http://www.webexhibits.org/daylightsaving/">About Daylight Saving Time – History, rationale, laws & dates</a> and summarized in <a href="http://seizethedaylight.com/dst/">A Brief History of Daylight Saving Time</a>.</li> <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time Lords</a> discusses how authoritarians manipulate civil time.</li> <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a> contains guidelines and best practices for software applications that deal with civil time.</li> <li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of the International Date Line</a> tells the story of the most important time zone boundary.</li> <li><a href="http://statoids.com/tconcept.html">Basic Time Zone Concepts</a> discusses terminological issues behind time zones.</li> </ul> </section> <section> <h2 id="national">National histories of legal time</h2> <dl> <dt>Australia</dt> <dd>The Parliamentary Library commissioned a <a href="https://parlinfo.aph.gov.au/parlInfo/download/library/prspub/359V6/upload_binary/359v60.pdf">research paper on daylight saving time in Australia</a>. The Bureau of Meteorology publishes a list of <a href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation Dates of Daylight Savings Time within Australia</a>.</dd> <dt>Belgium</dt> <dd>The Royal Observatory of Belgium maintains a table of time in Belgium (in <a href="https://robinfo.oma.be/nl/astro-info/tijd/" hreflang="nl">Dutch</a> and <a href="https://robinfo.oma.be/fr/astro-info/heure/" hreflang="fr">French</a>).</dd> <dt>Brazil</dt> <dd>The Time Service Department of the National Observatory records <a href="http://pcdsh01.on.br/DecHV.html" hreflang="pt-BR">Brazil's daylight saving time decrees (in Portuguese)</a>.</dd> <dt>Canada</dt> <dd>National Research Council Canada publishes current and some older information about <a href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time zones and daylight saving time</a>.</dd> <dt>Chile</dt> <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of Chile's official time (in Spanish)</a>.</dd> <dt>China</dt> <dd>The Hong Kong Observatory maintains a <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of summer time in Hong Kong</a>, and Macau's Meteorological and Geophysical Bureau maintains a <a href="https://www.smg.gov.mo/en/subpage/224/page/174">similar history for Macau</a>. Unfortunately the latter is incomplete and has errors.</dd> <dt>Czech Republic</dt> <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas" hreflang="cs">When daylight saving time starts and ends (in Czech)</a> summarizes and cites historical <abbr>DST</abbr> regulations.</dd> <dt>Germany</dt> <dd>The National Institute for Science and Technology maintains the <a href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation of Legal Time in Germany</a>.</dd> <dt>Israel</dt> <dd>The Interior Ministry periodically issues <a href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements" hreflang="he">announcements (in Hebrew)</a>.</dd> <dt>Malaysia</dt> <dd>See Singapore <a href="#Singapore">below</a>.</dd> <dt>Mexico</dt> <dd>The Investigation and Analysis Service of the Mexican Library of Congress has published a <a href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm" hreflang="es">history of Mexican local time (in Spanish)</a>.</dd> <dt>Netherlands</dt> <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm" hreflang="nl">Legal time in the Netherlands (in Dutch)</a> covers the history of local time in the Netherlands from ancient times.</dd> <dt>New Zealand</dt> <dd>The Department of Internal Affairs maintains a brief <a href="https://www.dia.govt.nz/Daylight-Saving-History">History of Daylight Saving</a>.</dd> <dt>Palestine</dt> <dd>The Ministry of Telecom and IT publishes a <a href="https://mtit.pna.ps/home/TimeZone" hreflang="ar">history of clock changes (in Arabic)</a>.</dd> <dt>Portugal</dt> <dd>The Lisbon Astronomical Observatory publishes a <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of legal time (in Portuguese)</a>.</dd> <dt>Singapore</dt> <dd><a id="Singapore" href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why is Singapore in the "Wrong" Time Zone?</a> details the history of legal time in Singapore and Malaysia.</dd> <dt>United Kingdom</dt> <dd><a href="https://www.polyomino.org.uk/british-time/">History of legal time in Britain</a> discusses in detail the country with perhaps the best-documented history of clock adjustments.</dd> <dt>United States</dt> <dd>The Department of Transportation's <a href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent Time Zone Proceedings</a> lists changes to time zone boundaries.</dd> <dt>Uruguay</dt> <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan Navy (SOHMA) publishes an annual <a href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac (in Spanish)</a>.</dd> </dl> </section> <section> <h2 id="costs">Costs and benefits of time shifts</h2> <p>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.</p> <ul> <li>In 2022 the American Medical Association issued a <a href="https://www.ama-assn.org/press-center/press-releases/ama-calls-permanent-standard-time">statement supporting permanent standard time</a> on health grounds.</li> <li>Carey RN, Sarma KM. <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of daylight saving time on road traffic collision risk: a systematic review</a>. <em>BMJ Open.</em> 2017;7(6):e014319. doi:<a href="https://doi.org/10.1136/bmjopen-2016-014319">10.1136/bmjopen-2016-014319</a>. This reviews research literature and concludes that the evidence neither supports nor refutes road safety benefits from shifts in time zones.</li> <li>Havranek T, Herman D, Irsova D. <a href="https://www.iaee.org/en/publications/ejarticle.aspx?id=3051">Does daylight saving save electricity? A meta-analysis</a>. <em>Energy J.</em> 2018;39(2):35–61. doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>. 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 <abbr>DST</abbr>."</li> <li>Malow BA. <a href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time to abolish the clock change and adopt permanent standard time in the United States: a Sleep Research Society position statement</a>. <em>Sleep.</em> 2022;45(12):zsac236. doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>. After reviewing the scientific literature, the Sleep Research Society advocates permanent standard time due to its health benefits. <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>. <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time is the optimal choice for health and safety: an American Academy of Sleep Medicine position statement</a>. <em>J Clin Sleep Med.</em> 2024;20(1):121–125. doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>. The AASM argues for permanent standard time due to health and safety risks and economic costs of both <abbr>DST</abbr> transitions and permanent <abbr>DST</abbr>.</li> <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>. <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why should we abolish Daylight Saving Time?</a> <em>J Biol Rhythms.</em> 2019;34(3):227–230. doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>. 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".</li> </ul> </section> <section> <h2 id="precision">Precision timekeeping</h2> <ul> <li><a href="http://leapsecond.com/hpan/an1289.pdf">The Science of Timekeeping</a> is a thorough introduction to the theory and practice of precision timekeeping.</li> <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of Time 2016</a> contains several freely readable papers.</li> <li><a href="https://www.ntp.org"><abbr title="Network Time Protocol">NTP</abbr>: The Network Time Protocol</a> (Internet <abbr>RFC</abbr> 5905) discusses how to synchronize clocks of Internet hosts.</li> <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a> family of software algorithms can achieve accuracy to a few tens of nanoseconds in scalable server farms without special hardware.</li> <li>The <a href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision Time Protocol</a> (<abbr title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588) can achieve submicrosecond clock accuracy on a local area network with special-purpose hardware.</li> <li><a href="https://datatracker.ietf.org/doc/html/rfc4833">Timezone Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a> (Internet <abbr>RFC</abbr> 4833) specifies a <a href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a> option for a server to configure a client's time zone and daylight saving settings automatically.</li> <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time Scales</a> describes astronomical time scales like <abbr title="Terrestrial Dynamic Time">TDT</abbr>, <abbr title="Geocentric Coordinate Time">TCG</abbr>, and <abbr title="Barycentric Dynamic Time">TDB</abbr>. <li>The <a href="https://www.iau.org"><abbr title="International Astronomical Union">IAU</abbr></a>'s <a href="https://www.iausofa.org"><abbr title="Standards Of Fundamental Astronomy">SOFA</abbr></a> collection contains C and <a href="https://en.wikipedia.org/wiki/Fortran">Fortran</a> code for converting among time scales like <abbr title="International Atomic Time">TAI</abbr>, <abbr>TDB</abbr>, <abbr>TDT</abbr> and <abbr>UTC</abbr>. It is freely available under the <a href="https://www.iausofa.org/tandc.html">SOFA license</a>.</li> <li><a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the diverse local time scales used by each landed mission on Mars.</li> <li><a href="http://leapsecond.com">LeapSecond.com</a> 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.</li> <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency and time-signal emissions</a>, International Telecommunication Union – Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li> <li><a href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr title="International Earth Rotation and Reference Systems Service">IERS</abbr> Bulletins</a> contains official publications of the International Earth Rotation and Reference Systems Service, which decides when leap seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds via an optional "<code>right</code>" configuration where a computer's internal <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second, as opposed to the default "<code>posix</code>" configuration where the internal clock ignores leap seconds. The two configurations agree for timestamps starting with 1972-01-01 00:00:00 <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for timestamps starting with <code>time_t</code> 78 796 800, which corresponds to the first leap second 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration, and to 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration. In practice the two configurations also agree for timestamps before 1972 even though the historical situation is messy, partly because neither <abbr>UTC</abbr> nor <abbr>TAI</abbr> is well-defined for sufficiently old timestamps.</li> <li><a href="https://developers.google.com/time/smear">Leap Smear</a> discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a leap second so that they disagree with <abbr>UTC</abbr> by at most a half second, even though every <abbr>POSIX</abbr> minute has exactly sixty seconds. This approach works with the default <code><abbr>tz</abbr></code> "<code>posix</code>" configuration, is <a href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by the <abbr>NTP</abbr> reference implementation, <a href="https://github.com/google/unsmear">supports</a> conversion between <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major cloud service providers. However, according to <a href="https://datatracker.ietf.org/doc/html/rfc8633#section-3.7.1">§3.7.1 of Network Time Protocol Best Current Practices</a> (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for applications requiring accurate <abbr>UTC</abbr> or civil time, and is intended for use only in single, well-controlled environments.</li> <li>The <a href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap Second Discussion List</a> covers <a href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy and Klepczynski's 1999 proposal to discontinue leap seconds</a>, discussed further in <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The leap second: its history and possible future</a>. <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr> might be redefined without Leap Seconds</a> gives pointers on this contentious issue. The General Conference on Weights and Measures <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a> to discontinue the use of leap seconds by 2035, replacing them with an as-yet-undetermined scheme some time after the year 2135. The World Radiocommunication Conference <a href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved in 2023</a> to cooperate with this process. </li> </ul> </section> <section> <h2 id="notation">Time notation</h2> <ul> <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time zone names, abbreviations, identifiers, and formats. For example, it contains French translations for "Eastern European Summer Time", "<abbr title="Eastern European Summer Time">EEST</abbr>", and "Bucharest". Its <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type charts</a> show these values for many locales. Data values are available in both <abbr title="Locale Data Markup Language">LDML</abbr> (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>. <li> <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of the international standard date and time notation</a> covers <a href="https://www.iso.org/standard/70907.html"><em><abbr title="International Organization for Standardization">ISO</abbr> 8601-1:2019 – Date and time – Representations for information interchange – Part 1: Basic rules</em></a>.</li> <li> <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr> Schema: Datatypes – dateTime</a> specifies a format inspired by <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li> <li><a href="https://datatracker.ietf.org/doc/html/rfc5322#section-3.3">§3.3 of Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322) specifies the time notation used in email and <a href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a> headers.</li> <li> <a href="https://datatracker.ietf.org/doc/html/rfc3339">Date and Time on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339) specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.</li> <li> <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time Formats on the Web</a> surveys web- and Internet-oriented date and time formats.</li> <li>Alphabetic time zone abbreviations should not be used as unique identifiers for <abbr>UT</abbr> offsets as they are ambiguous in practice. For example, in English-speaking North America "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>, but in China it denotes 8 hours ahead of <abbr>UT</abbr>, and French-speaking North Americans prefer "<abbr title="Heure Normale du Centre">HNC</abbr>" to "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code> 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.</li> <li>Numeric time zone abbreviations typically count hours east of <abbr>UT</abbr>, e.g., +09 for Japan and −10 for Hawaii. However, the <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> environment variable uses the opposite convention. For example, one might use <code><abbr>TZ</abbr>="<abbr title="Japan Standard Time">JST</abbr>-9"</code> and <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code> for Japan and Hawaii, respectively. If the <code><abbr>tz</abbr></code> database is available, it is usually better to use settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> 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 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like <code>"GMT-9"</code>, though, since this would incorrectly imply that local time is nine hours ahead of <abbr>UT</abbr> and the time zone is called "<abbr>GMT</abbr>".</li> </ul> </section> <section> <h2 id="see-also">See also</h2> <ul> <li><a href="theory.html">Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</a></li> <li><a href="tz-art.html">Time and the Arts</a></li> </ul> </section> <footer> <hr> This web page is in the public domain, so clarified as of 2009-05-17 by Arthur David Olson. <br> Please send corrections to this web page to the <a href="mailto:tz@iana.org">time zone mailing list</a>. </footer> </body> </html>