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41.1Schristos<title>Sources for time zone and daylight saving time data</title>
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101.1Schristos<body>
111.1Schristos<h1>Sources for time zone and daylight saving time data</h1>
121.1Schristos<p>
131.1Schristos<a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
141.1Schristos<a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
151.1Schristosrules are controlled by individual
161.1Schristosgovernments. They are sometimes changed with little notice, and their
171.1Schristoshistories and planned futures are often recorded only fitfully. Here
181.1Schristosis a summary of attempts to organize and record relevant data in this
191.1Schristosarea.
201.1Schristos</p>
211.6Schristos  <h3>Outline</h3>
221.6Schristos  <nav>
231.6Schristos    <ul>
241.6Schristos      <li>The <code><abbr>tz</abbr></code> database product and process
251.6Schristos	<ul>
261.6Schristos	  <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
271.6Schristos	  <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
281.6Schristos	  <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
291.6Schristos	  <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
301.6Schristos	</ul>
311.6Schristos      </li>
321.6Schristos      <li>Uses of the <code><abbr>tz</abbr></code> database
331.6Schristos	<ul>
341.6Schristos	  <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
351.6Schristos	  <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
361.6Schristos	  <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
371.6Schristos	  <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
381.6Schristos	  <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
391.6Schristos	</ul>
401.6Schristos      </li>
411.6Schristos      <li>Related data
421.6Schristos	<ul>
431.6Schristos	  <li><a href="#other-dbs">Other time zone databases</a></li>
441.6Schristos	  <li><a href="#maps">Maps</a></li>
451.6Schristos	  <li><a href="#boundaries">Time zone boundaries</a></li>
461.6Schristos	</ul>
471.6Schristos      </li>
481.6Schristos      <li>Timekeeping concepts
491.6Schristos	<ul>
501.6Schristos	  <li><a href="#civil">Civil time concepts and history</a></li>
511.6Schristos	  <li><a href="#national">National histories of legal time</a></li>
521.7Schristos	  <li><a href="#costs">Costs and benefits of time shifts</a></li>
531.6Schristos	  <li><a href="#precision">Precision timekeeping</a></li>
541.6Schristos	  <li><a href="#notation">Time notation</a></li>
551.6Schristos	  <li><a href="#see-also">See also</a></li>
561.6Schristos	</ul>
571.6Schristos      </li>
581.6Schristos    </ul>
591.6Schristos  </nav>
601.6Schristos
611.6Schristos<section>
621.1Schristos<h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
631.1Schristos<p>
641.1SchristosThe <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
651.1Schristostime zone database contains code and data
661.1Schristosthat represent the history of local time
671.1Schristosfor many representative locations around the globe.
681.1SchristosIt is updated periodically to reflect changes made by political bodies
691.1Schristosto time zone boundaries and daylight saving rules.
701.1SchristosThis database (known as <code><abbr>tz</abbr></code>,
711.1Schristos<code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
721.1Schristosis used by several implementations,
731.1Schristosincluding
741.1Schristos<a href="https://www.gnu.org/software/libc/">the
751.1Schristos<abbr title="GNU's Not Unix">GNU</abbr>
761.1SchristosC Library</a> (used in
771.1Schristos<a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
781.1Schristos<a href="https://www.android.com">Android</a>,
791.1Schristos<a href="https://www.freebsd.org">Free<abbr
801.1Schristostitle="Berkeley Software Distribution">BSD</abbr></a>,
811.1Schristos<a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
821.1Schristos<a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
831.1Schristos<a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
841.1Schristos<a href="https://cygwin.com">Cygwin</a>,
851.7Schristos<a href="https://mariadb.org">MariaDB</a>,
861.1Schristos<a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
871.1Schristos<a href="https://www.mysql.com">MySQL</a>,
881.1Schristos<a href="https://en.wikipedia.org/wiki/WebOS"><abbr
891.1Schristostitle="Web Operating System">webOS</abbr></a>,
901.7Schristos<a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
911.1Schristostitle="Advanced Interactive eXecutive">AIX</abbr></a>,
921.1Schristos<a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>,
931.6Schristos<a href="https://www.apple.com/ios"><abbr
941.1Schristostitle="iPhone OS">iOS</abbr></a>,
951.6Schristos<a href="https://www.apple.com/macos">macOS</a>,
961.1Schristos<a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
971.6Schristos<a href="https://www.vmssoftware.com">Open<abbr
981.1Schristostitle="Virtual Memory System">VMS</abbr></a>,
991.6Schristos<a href="https://www.oracle.com/database/">Oracle Database</a>, and
1001.1Schristos<a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
1011.1Schristos<p>
1021.2SchristosEach main entry in the database represents a <dfn>timezone</dfn>
1031.2Schristosfor a set of civil-time clocks that have all agreed since 1970.
1041.2SchristosTimezones are typically identified by continent or ocean and then by the
1051.2Schristosname of the largest city within the region containing the clocks.
1061.1SchristosFor example, <code>America/New_York</code>
1071.1Schristosrepresents most of the <abbr title="United States">US</abbr> eastern time zone;
1081.1Schristos<code>America/Phoenix</code> represents most of Arizona, which
1091.2Schristosuses mountain time without daylight saving time (<abbr>DST</abbr>);
1101.1Schristos<code>America/Detroit</code> represents most of Michigan, which uses
1111.1Schristoseastern time but with different <abbr>DST</abbr> rules in 1975;
1121.1Schristosand other entries represent smaller regions like Starke County,
1131.1SchristosIndiana, which switched from central to eastern time in 1991
1141.1Schristosand switched back in 2006.
1151.1SchristosTo use the database on an extended <a
1161.1Schristoshref="https://en.wikipedia.org/wiki/POSIX"><abbr
1171.1Schristostitle="Portable Operating System Interface">POSIX</abbr></a>
1181.1Schristosimplementation set the <code><abbr>TZ</abbr></code>
1191.1Schristosenvironment variable to the location's full name,
1201.1Schristose.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
1211.1Schristos<p>
1221.2SchristosAssociated with each timezone is a history of offsets from
1231.1Schristos<a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
1241.1SchristosTime</a> (<abbr>UT</abbr>), which is <a
1251.1Schristoshref="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
1261.1SchristosTime</a> (<abbr>GMT</abbr>) with days beginning at midnight;
1271.2Schristosfor timestamps after 1960 this is more precisely <a
1281.1Schristoshref="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
1291.1SchristosUniversal Time</a> (<abbr>UTC</abbr>).
1301.1SchristosThe database also records when daylight saving time was in use,
1311.1Schristosalong with some time zone abbreviations such as <abbr>EST</abbr>
1321.1Schristosfor Eastern Standard Time in the <abbr>US</abbr>.</p>
1331.6Schristos</section>
1341.6Schristos
1351.6Schristos<section>
1361.1Schristos<h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
1371.1Schristos<p>
1381.1SchristosThe following <a
1391.1Schristoshref="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
1401.1Schristosthe latest release's two
1411.1Schristos<a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
1421.1Schristosto a <abbr>GNU</abbr>/Linux or similar host.</p>
1431.2Schristos<pre><code>mkdir tzdb
1441.1Schristoscd tzdb
1451.1Schristos<a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
1461.1Schristoswget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
1471.1Schristos<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 -
1481.1Schristosgzip -dc tzdata-latest.tar.gz | tar -xf -
1491.1Schristos</code></pre>
1501.1Schristos<p>Alternatively, the following shell commands download the same
1511.1Schristosrelease in a single-tarball format containing extra data
1521.1Schristosuseful for regression testing:</p>
1531.2Schristos<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>
1541.1Schristos<a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
1551.1Schristos</code></pre>
1561.1Schristos<p>These commands use convenience links to the latest release
1571.1Schristosof the <code><abbr>tz</abbr></code> database hosted by the
1581.1Schristos<a href="https://www.iana.org/time-zones">Time Zone Database website</a>
1591.1Schristosof the <a href="https://www.iana.org">Internet Assigned Numbers
1601.1SchristosAuthority (IANA)</a>.
1611.1SchristosOlder releases are in files named
1621.1Schristos<code>tzcode<var>V</var>.tar.gz</code>,
1631.1Schristos<code>tzdata<var>V</var>.tar.gz</code>, and
1641.1Schristos<code>tzdb-<var>V</var>.tar.lz</code>,
1651.1Schristoswhere <code><var>V</var></code> is the version.
1661.1SchristosSince 1996, each version has been a four-digit year followed by
1671.1Schristoslower-case letter (<samp>a</samp> through <samp>z</samp>,
1681.1Schristosthen <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
1691.1Schristosthrough <samp>zzz</samp>, and so on).
1701.10SchristosSince version 1999g, each release has been distributed in
1711.10Schristos<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
1721.10Schristosustar interchange format</a>, compressed as described above;
1731.10Schristosolder releases use a nearly-compatible format.
1741.1SchristosSince version 2016h, each release has contained a text file named
1751.1Schristos"<samp>version</samp>" whose first (and currently only) line is the version.
1761.7SchristosOlder releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
1771.7Schristosand are also available in an
1781.1Schristos<a href="ftp://ftp.iana.org/tz/releases/"><abbr
1791.1Schristostitle="File Transfer Protocol">FTP</abbr> directory</a> via a
1801.1Schristosless-secure protocol.</p>
1811.1Schristos<p>Alternatively, a development repository of code and data can be
1821.1Schristosretrieved from <a href="https://github.com">GitHub</a> via the shell
1831.1Schristoscommand:</p>
1841.2Schristos<pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
1851.1Schristos</code></pre>
1861.1Schristos<p>
1871.1SchristosSince version 2012e, each release has been tagged in development repositories.
1881.1SchristosUntagged commits are less well tested and probably contain
1891.1Schristosmore errors.</p>
1901.1Schristos<p>
1911.1SchristosAfter obtaining the code and data files, see the
1921.1Schristos<code>README</code> file for what to do next.
1931.1SchristosThe code lets you compile the <code><abbr>tz</abbr></code> source files into
1941.2Schristosmachine-readable binary files, one for each location. The binary files
1951.5Schristosare in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
1961.5Schristosspecified by <a href="https://tools.ietf.org/html/8536">Internet
1971.5Schristos<abbr>RFC</abbr> 8536</a>.
1981.2SchristosThe code also lets
1991.2Schristosyou read a <abbr>TZif</abbr> file and interpret timestamps for that
2001.1Schristoslocation.</p>
2011.6Schristos</section>
2021.6Schristos
2031.6Schristos<section>
2041.1Schristos<h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
2051.1Schristos<p>
2061.1SchristosThe <code><abbr>tz</abbr></code> code and data
2071.1Schristosare by no means authoritative. If you find errors, please
2081.1Schristossend changes to <a href="mailto:tz@iana.org"><code>tz@iana.org</code></a>,
2091.1Schristosthe time zone mailing list. You can also <a
2101.1Schristoshref="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
2111.1Schristosand browse the <a
2121.1Schristoshref="https://mm.icann.org/pipermail/tz/">archive of old
2131.5Schristosmessages</a>.
2141.5Schristos<a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
2151.5Schristosdiscussions</a> and corresponding data changes can be
2161.5Schristosgenerated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
2171.5Schristos</p>
2181.1Schristos<p>
2191.1SchristosIf your government plans to change its time zone boundaries or
2201.1Schristosdaylight saving rules, inform <code>tz@iana.org</code> well in
2211.1Schristosadvance, as this will coordinate updates to many cell phones,
2221.8Schristoscomputers, and other devices around the world.
2231.8SchristosThe change should be officially announced at least a year before it affects
2241.8Schristoshow clocks operate; otherwise, there is a good chance that some
2251.8Schristosclocks will operate incorrectly after the change, due
2261.1Schristosto delays in propagating updates to software and data. The shorter
2271.1Schristosthe notice, the more likely clock problems will arise; see "<a
2281.1Schristoshref="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
2291.1Schristosthe Timing of Time Zone Changes</a>" for examples.
2301.8SchristosThe <code><abbr>tz</abbr></code> data can represent planned changes
2311.8Schristosfar into the future, and a long-planned change can easily be reverted
2321.8Schristosor otherwise altered with a year's notice before the change would have
2331.8Schristosaffected clocks.
2341.1Schristos</p>
2351.1Schristos<p>
2361.1SchristosChanges to the <code><abbr>tz</abbr></code> code and data are often
2371.1Schristospropagated to clients via operating system updates, so
2381.1Schristosclient <code><abbr>tz</abbr></code> data can often be corrected by
2391.1Schristosapplying these updates. With GNU/Linux and similar systems, if your
2401.1Schristosmaintenance provider has not yet adopted the
2411.1Schristoslatest <code><abbr>tz</abbr></code> data, you can often short-circuit
2421.1Schristosthe process by tailoring the generic instructions in
2431.1Schristosthe <code><abbr>tz</abbr> README</code> file and installing the latest
2441.1Schristosdata yourself. System-specific instructions for installing the
2451.1Schristoslatest <code><abbr>tz</abbr></code> data have also been published
2461.1Schristosfor <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
2471.1Schristos<a
2481.10Schristoshref="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
2491.1Schristos<a
2501.1Schristoshref="http://userguide.icu-project.org/datetime/timezone"><abbr
2511.1Schristostitle="International Components for Unicode">ICU</abbr></a>,
2521.9Schristos<a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr>
2531.9SchristosJDK</a>,
2541.9Schristos<a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
2551.1Schristoshref="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
2561.9Schristos<a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
2571.9Schristoshref="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html#usage">OpenJDK/Oracle JDK</a>.
2581.1Schristos</p>
2591.1Schristos<p>Sources for the <code><abbr>tz</abbr></code> database are
2601.1Schristos<a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
2611.1Schristostitle="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
2621.1Schristos<a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
2631.1Schristoswith lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
2641.1Schristostitle="linefeed">LF</abbr></a>,
2651.1Schristoswhich can be modified by common text editors such
2661.1Schristosas <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
2671.1Schristos<a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
2681.1Schristos<a href="https://www.vim.org">vim</a>.
2691.1SchristosSpecialized source-file editing can be done via the
2701.1Schristos<a href="https://packagecontrol.io/packages/zoneinfo">Sublime
2711.1Schristoszoneinfo</a> package for <a
2721.1Schristoshref="https://www.sublimetext.com">Sublime Text</a> and the <a
2731.1Schristoshref="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
2741.1Schristoszoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
2751.1SchristosStudio Code</a>.
2761.1Schristos</p>
2771.1Schristos<p>
2781.1SchristosFor further information about updates, please see
2791.1Schristos<a href="https://tools.ietf.org/html/rfc6557">Procedures for
2801.1SchristosMaintaining the Time Zone Database</a> (Internet <abbr
2811.1Schristostitle="Request For Comments">RFC</abbr> 6557). More detail can be
2821.7Schristosfound in <a href="theory.html">Theory and pragmatics of the
2831.7Schristos<code><abbr>tz</abbr></code> code and data</a>.
2841.2Schristos<a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
2851.2Schristosdisplays changes between recent <code><abbr>tzdb</abbr></code> versions.
2861.1Schristos</p>
2871.6Schristos</section>
2881.6Schristos
2891.6Schristos<section>
2901.1Schristos<h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
2911.1Schristos<ul>
2921.1Schristos<li>The article
2931.1Schristos<a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
2941.1Schristosan encyclopedic summary.</li>
2951.1Schristos<li><a href="tz-how-to.html">How to Read the
2961.1Schristostz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
2971.1Schristosdatabase format.</li>
2981.1Schristos<li><a
2991.1Schristoshref="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
3001.1Schristosliterary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
3011.1Schristosdatabase's style.</li>
3021.10Schristos<li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
3031.10Schristosmanaging time in the internet</a> analyzes the database longitudinally.</li>
3041.1Schristos</ul>
3051.6Schristos</section>
3061.6Schristos
3071.6Schristos<section>
3081.1Schristos<h2 id="web">Web sites using recent versions of the
3091.1Schristos<code><abbr>tz</abbr></code> database</h2>
3101.1Schristos<p>
3111.1SchristosThese are listed roughly in ascending order of complexity and fanciness.
3121.1Schristos</p>
3131.1Schristos<ul>
3141.1Schristos<li><a href="https://time.is">Time.is</a> shows locations'
3151.1Schristostime and zones.</li>
3161.1Schristos<li><a href="https://www.timejones.com">TimeJones.com</a>,
3171.1Schristos<a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
3181.7Schristos<a href="https://www.worldclock.com">The World Clock</a>
3191.1Schristosare time zone converters.</li>
3201.8Schristos<li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
3211.8Schristospublishes <code><abbr>tzdb</abbr></code>-derived data in
3221.8Schristos<a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
3231.8Schristostitle="comma-separated values">CSV</abbr></a> and
3241.8Schristosin <a href="https://en.wikipedia.org/wiki/SQL"><abbr
3251.8Schristostitle="Structured Query Language">SQL</abbr></a> form.</li>
3261.1Schristos<li><a
3271.6Schristoshref="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
3281.1Schristoslets you see the <code><abbr>TZ</abbr></code> values directly.</li>
3291.1Schristos<li><a
3301.1Schristoshref="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
3311.1SchristosTime in 1000 Places</a> uses descriptions of the values.</li>
3321.6Schristos<li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
3331.1Schristosuses a pulldown menu.</li>
3341.7Schristos<li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
3351.10Schristostimezone information for all countries</a>
3361.10Schristosdisplays tables of <abbr>DST</abbr> rules.
3371.1Schristos<li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
3381.1SchristosWorldwide</a> lets you sort zone names and convert times.</li>
3391.1Schristos<li><a href="https://24timezones.com">24TimeZones</a> has a world
3401.1Schristostime map and a time converter.</li>
3411.1Schristos<li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
3421.1Schristoscalculates the current time difference between locations.</li>
3431.1Schristos<li><a href="http://www.wx-now.com">Weather Now</a> and
3441.1Schristos<a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
3451.1Schristos</ul>
3461.6Schristos</section>
3471.6Schristos
3481.6Schristos<section>
3491.1Schristos<h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
3501.1Schristos<ul>
3511.1Schristos<li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
3521.1Schristos<a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
3531.1SchristosDistribution Service (tzdist) working group</a> defined <a
3541.1Schristoshref="https://tools.ietf.org/html/rfc7808">TZDIST</a>
3551.1Schristos(Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
3561.1Schristosalong with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
3571.1Schristos(Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
3581.2Schristostransferring time zone data by reference.
3591.8Schristos<a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
3601.8Schristosimplementations</a> are available.
3611.2SchristosThe <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
3621.9Schristosmailing list</a> discusses possible extensions.</li>
3631.1Schristos<li>The <a href="https://tools.ietf.org/html/rfc5545">
3641.1SchristosInternet Calendaring and Scheduling Core Object Specification
3651.1Schristos(iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
3661.1Schristoscovers time zone
3671.1Schristosdata; see its VTIMEZONE calendar component.
3681.1SchristosThe iCalendar format requires specialized parsers and generators; a
3691.1Schristosvariant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
3701.1Schristos(Internet <abbr>RFC</abbr> 6321) uses
3711.1Schristos<a href="https://www.w3.org/XML/"><abbr
3721.1Schristostitle="Extensible Markup Language">XML</abbr></a> format, and a variant
3731.1Schristos<a href="https://tools.ietf.org/html/rfc7265">jCal</a>
3741.1Schristos(Internet <abbr>RFC</abbr> 7265)
3751.1Schristosuses <a href="https://www.json.org"><abbr
3761.1Schristostitle="JavaScript Object Notation">JSON</abbr></a> format.</li>
3771.1Schristos</ul>
3781.6Schristos</section>
3791.6Schristos
3801.6Schristos<section>
3811.1Schristos<h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
3821.7Schristos<p>Although some of these do not fully support
3831.7Schristos<code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
3841.7Schristosdistributions you can generally work around compatibility problems by
3851.7Schristosrunning the command <code>make rearguard_tarballs</code> and compiling
3861.7Schristosfrom the resulting tarballs instead.</p>
3871.1Schristos<ul>
3881.1Schristos<li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
3891.1Schristoshref="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
3901.1Schristosprogram that compiles
3911.1Schristos<code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
3921.1SchristosVzic is freely
3931.1Schristosavailable under the <a
3941.1Schristoshref="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
3951.1SchristosGeneral Public License (<abbr
3961.1Schristostitle="General Public License">GPL</abbr>)</a>.</li>
3971.1Schristos<li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
3981.1Schristosdatabase conversion utility</a> is like Vzic, except for the <a
3991.6Schristoshref="https://dotnet.microsoft.com">.NET framework</a>
4001.1Schristosand with a <abbr>BSD</abbr>-style license.</li>
4011.1Schristos<li><a
4021.4Schristoshref="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
4031.1Schristoscontains a script <code>parse_olson</code> that compiles
4041.1Schristos<code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
4051.1Schristosmodules. It is part of the Perl <a
4061.4Schristoshref="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
4071.4Schristoswhich is freely
4081.1Schristosavailable under both the <abbr>GPL</abbr> and the Perl Artistic
4091.1SchristosLicense. DateTime::TimeZone also contains a script
4101.1Schristos<code>tests_from_zdump</code> that generates test cases for each clock
4111.1Schristostransition in the <code><abbr>tz</abbr></code> database.</li>
4121.1Schristos<li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
4131.1SchristosDatabase Parser</a> is a
4141.1Schristos<a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
4151.9Schristosruntime library with <a
4161.9Schristoshref="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
4171.9Schristosadopted by
4181.6Schristos<a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
4191.9Schristosthe current iteration of the C++ standard.
4201.1SchristosIt is freely available under the
4211.1Schristos<abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
4221.1Schristos<li><a id="ICU" href="http://site.icu-project.org">International Components for
4231.1SchristosUnicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
4241.1Schristoshref="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
4251.1Schristoslibraries for internationalization that
4261.1Schristoshas a compiler from <code><abbr>tz</abbr></code> source
4271.1Schristosand from <abbr title="Common Locale Data Repository">CLDR</abbr> data
4281.1Schristos(mentioned <a href="#CLDR">below</a>)
4291.1Schristosinto an <abbr>ICU</abbr>-specific format.
4301.1Schristos<abbr>ICU</abbr> is freely available under a
4311.1Schristos<abbr>BSD</abbr>-style license.</li>
4321.1Schristos<li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
4331.1Schristosthe <a href="https://elixir-lang.org">Elixir</a> language downloads
4341.7Schristosand compiles <code><abbr>tz</abbr></code> source and exposes <abbr
4351.1Schristostitle="Application Program Interface">API</abbr>s for use. It is
4361.1Schristosfreely available under the <abbr>MIT</abbr> license.</li>
4371.1Schristos<li>Java-based compilers and libraries include:
4381.1Schristos<ul>
4391.1Schristos<li>The <a
4401.9Schristoshref="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
4411.1Schristostool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
4421.9Schristos<a href="https://openjdk.java.net/">OpenJDK</a> and
4431.9Schristos<a href="https://jdk.java.net/">Oracle JDK</a>.
4441.9SchristosAlthough its source code is proprietary, its executable is available under the
4451.9Schristos<a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
4461.9SchristosTimezone Updater License Agreement</a>.</li>
4471.1Schristos<li>The <a
4481.4Schristoshref="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
4491.6SchristosSE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
4501.4Schristoshref="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
4511.1Schristoswhich is freely available under a <abbr>BSD</abbr>-style license.</li>
4521.4Schristos<li><a href="https://www.joda.org/joda-time/">Joda-Time &ndash; Java date
4531.1Schristosand time <abbr>API</abbr></a> contains a class
4541.1Schristos<code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
4551.1Schristos<code><abbr>tz</abbr></code> source into a binary format. It inspired
4561.1SchristosJava 8 <code>java.time</code>, which its users should migrate to once
4571.1Schristosthey can assume Java 8 or later. It is available under the <a
4581.1Schristoshref="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
4591.9Schristos<li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
4601.7Schristoshref="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
4611.9Schristosare alternatives to TZUpdater. IANA Updater's license is unclear;
4621.9SchristosZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
4631.4Schristos<li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
4641.4Schristostime library for Android</a> and
4651.4Schristos<a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
4661.4Schristostime and interval library for Java</a> compile
4671.4Schristos<code><abbr>tz</abbr></code> source into a binary format.
4681.4SchristosTime4A is available under the Apache License and Time4J is
4691.1Schristosavailable under the <a
4701.1Schristoshref="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
4711.1SchristosGeneral Public License (<abbr title="Lesser General Public
4721.1SchristosLicense">LGPL</abbr>)</a>.</li>
4731.1Schristos<li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
4741.1SchristosJava-based libraries.</li>
4751.1Schristos</ul>
4761.1Schristos<li><a href="https://nodatime.org">Noda Time &ndash; Date and
4771.1Schristostime <abbr>API</abbr> for .NET</a>
4781.4Schristosis like Joda-Time and Time4J, but for the .NET framework instead of Java.
4791.4SchristosIt is freely available under the Apache License.</li>
4801.8Schristos<li>Many modern
4811.8Schristos<a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
4821.8Schristosruntimes support <code><abbr>tz</abbr></code> natively via the
4831.8Schristos<samp>timeZone</samp> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><samp>Intl.DateTimeFormat</samp></a>.
4841.10SchristosThis can be used as-is or with most of the following libraries,
4851.10Schristosmany of which also support runtimes lacking the <samp>timeZone</samp> option.
4861.1Schristos<ul>
4871.8Schristos<li>The <a
4881.8Schristoshref="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
4891.8Schristostimezone polyfill</a>
4901.8Schristosis freely available under a <abbr>BSD</abbr>-style license.</li>
4911.1Schristos<li><a
4921.1Schristoshref="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
4931.1Schristoscompiles time zone data into a compact form designed for
4941.1SchristosJavaScript. It is freely available under a combination of
4951.1Schristosthe <abbr>MIT</abbr> license and the Apache License.</li>
4961.10Schristos<li>The <a href="https://date-fns.org/">date-fns</a>
4971.10Schristoslibrary manipulates timezone-aware timestamps in browsers and
4981.10Schristosin <a href="https://nodejs.org/en/">Node.js</a>.
4991.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
5001.10Schristos<li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
5011.10Schristosminimalist replacement for the date and time API of
5021.10Schristosthe <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
5031.10Schristosmanipulation library.
5041.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
5051.10Schristos<li><a href="https://moment.github.io/luxon/">Luxon</a> improves
5061.10Schristostimezone support for the <samp>Intl</samp> API.
5071.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
5081.1Schristos<li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
5091.10SchristosMoment.js plugin.
5101.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
5111.8Schristos<li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
5121.8SchristosJavaScript library that supports date arithmetic that is time zone
5131.8Schristosaware. It is freely available under the <abbr>MIT</abbr> license.</li>
5141.10Schristos<li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
5151.10Schristossupports live <code><abbr>tzdb</abbr></code> updates,
5161.10Schristosastronomical and atomic time, a command-line interface,
5171.10Schristosand full <a
5181.10Schristoshref="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
5191.10SchristosIts companion <a
5201.10Schristoshref="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
5211.10Schristoscan generate <abbr>TZif</abbr> and other files, and a companion website
5221.10Schristos<a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
5231.10Schristosconvert timestamps, view transition histories, and download code and data.
5241.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
5251.1Schristos</ul>
5261.10SchristosThe proposed <a
5271.10Schristoshref="https://github.com/tc39/proposal-temporal"><samp>Temporal</samp>
5281.10Schristosobjects</a> let programs access an abstract view of
5291.10Schristos<code><abbr>tzdb</abbr></code> data, and are designed to replace <a
5301.10Schristoshref="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
5311.10Schristosproblematic <samp>Date</samp> objects</a> when working with dates and times.
5321.1Schristos<li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
5331.1Schristoscompiler from <code><abbr>tz</abbr></code> source into
5341.1Schristos<a href="https://julialang.org/">Julia</a>. It is freely available
5351.1Schristosunder the <abbr>MIT</abbr> license.</li>
5361.6Schristos<li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> &ndash;
5371.6Schristos<abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
5381.6Schristostitle="Free Pascal Compiler">FPC</abbr></a>
5391.2Schristoscompiles from <code><abbr>tz</abbr></code> source into
5401.2Schristos<a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
5411.2Schristosas compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
5421.2Schristosand <a
5431.2Schristoshref="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
5441.2SchristosIt is freely available under a <abbr>BSD</abbr>-style license.</li>
5451.1Schristos<li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
5461.1SchristosDefinitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
5471.1Schristos<a href="https://www.python.org">Python</a>.
5481.9SchristosIt is freely available under a <abbr>BSD</abbr>-style license.
5491.9SchristosIn code that can assume Python 3.9 or later it is superseded by
5501.9Schristos<a href="#python-zoneinfo"><code>zoneinfo.ZoneInfo</code></a>
5511.9Schristosand the <a href="#pypi-tzdata"><code>tzdata</code> Python package</a>.</li>
5521.1Schristos<li><a href="https://tzinfo.github.io">TZInfo &ndash;
5531.1SchristosRuby Timezone Library</a>
5541.1Schristoscompiles <code><abbr>tz</abbr></code> source into
5551.1Schristos<a href="https://www.ruby-lang.org/en/">Ruby</a>.
5561.1SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
5571.1Schristos<li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
5581.1SchristosLibrary</a> is
5591.1Schristosa <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
5601.1Schristoslibrary that compiles <code><abbr>tz</abbr></code> source into a time
5611.1Schristoszone repository whose format
5621.1Schristosis either proprietary or an <abbr>XML</abbr>-encoded
5631.1Schristosrepresentation.</li>
5641.1Schristos<li><a id="Tcl" href="https://tcl.tk">Tcl</a>
5651.1Schristoscontains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
5661.1Schristossource into text files, along with a runtime that can read those
5671.1Schristosfiles. Tcl is freely available under a <abbr>BSD</abbr>-style
5681.1Schristoslicense.</li>
5691.1Schristos</ul>
5701.6Schristos</section>
5711.6Schristos
5721.6Schristos<section>
5731.2Schristos<h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
5741.1Schristos<ul>
5751.1Schristos<li>The <a
5761.1Schristoshref="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
5771.1SchristosLibrary</a>
5781.1Schristoshas an independent, thread-safe implementation of
5791.2Schristosa <abbr>TZif</abbr> file reader.
5801.1SchristosThis library is freely available under the LGPL
5811.1Schristosand is widely used in <abbr>GNU</abbr>/Linux systems.</li>
5821.1Schristos<li><a href="https://www.gnome.org">GNOME</a>'s
5831.1Schristos<a href="https://developer.gnome.org/glib/">GLib</a> has
5841.2Schristosa <abbr>TZif</abbr> file reader written in C that
5851.1Schristoscreates a <code>GTimeZone</code> object representing sets
5861.1Schristosof <abbr>UT</abbr> offsets.
5871.1SchristosIt is freely available under the <abbr>LGPL</abbr>.</li>
5881.1Schristos<li>The
5891.1Schristos<a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
5901.1Schristos<code>baltzo::TimeZoneUtil</code> component contains a C++
5911.2Schristosimplementation of a <abbr>TZif</abbr> file reader. It is freely available under
5921.1Schristosthe Apache License.</li>
5931.1Schristos<li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
5941.1Schristoslibrary that translates between <abbr>UT</abbr> and civil time and
5951.2Schristoscan read <abbr>TZif</abbr> files. It is freely available under the Apache
5961.1SchristosLicense.</li>
5971.1Schristos<li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
5981.2Schristoslibrary that reads <abbr>TZif</abbr> files and converts
5991.2Schristostimestamps from one time zone or format to another.
6001.1SchristosIt is used by <a href="https://secure.php.net"><abbr
6011.1Schristostitle="PHP: Hypertext Preprocessor">PHP</abbr></a>,
6021.1Schristos<a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
6031.1Schristosand <a href="https://www.mongodb.com">MongoDB</a>.
6041.1SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
6051.1Schristos<li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
6061.2Schristos<abbr>TZif</abbr> file reader.</li>
6071.4Schristos<li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
6081.1SchristosDateTime::TimeZone::Tzfile</a>
6091.2Schristosis a <abbr>TZif</abbr> file reader written in Perl.
6101.1SchristosIt is freely available under the same terms as Perl
6111.1Schristos(dual <abbr>GPL</abbr> and Artistic license).</li>
6121.9Schristos<li>Python has a <a id="python-zoneinfo"
6131.8Schristoshref="https://docs.python.org/3.9/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
6141.8Schristosclass</a> that reads <abbr>TZif</abbr> data and creates objects
6151.8Schristosthat represent <code><abbr>tzdb</abbr></code> timezones.
6161.8SchristosPython is freely available under the
6171.8Schristos<a href="https://docs.python.org/3.9/license.html">Python Software Foundation
6181.8SchristosLicense</a>.
6191.9SchristosA companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
6201.8Schristos<a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
6211.8Schristossupplies TZif data if the underlying system data cannot be found;
6221.8Schristosit is freely available under the Apache License.</li>
6231.1Schristos<li>The
6241.1Schristospublic-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
6251.1Schristoslibrary contains a Python tool that
6261.2Schristosconverts <abbr>TZif</abbr> data into
6271.1Schristos<abbr>JSON</abbr>-format data suitable for use
6281.1Schristosin its JavaScript library for time zone conversion. Dates before 1970
6291.1Schristosare not supported.</li>
6301.1Schristos<li>The <a
6311.1Schristoshref="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
6321.1Schristospackage contains <a href="https://www.haskell.org">Haskell</a> code that
6331.2Schristosparses and uses <abbr>TZif</abbr> data. It is freely
6341.1Schristosavailable under a <abbr>BSD</abbr>-style license.</li>
6351.1Schristos</ul>
6361.6Schristos</section>
6371.6Schristos
6381.6Schristos<section>
6391.1Schristos<h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
6401.1Schristos<ul>
6411.1Schristos<li><a href="https://foxclocks.org">FoxClocks</a>
6421.1Schristosis an extension for <a href="https://www.google.com/chrome/">Google
6431.1SchristosChrome</a> and for <a
6441.1Schristoshref="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
6451.1SchristosToolkit</a> applications like <a
6461.1Schristoshref="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
6471.1Schristoshref="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
6481.1SchristosIt displays multiple clocks in the application window, and has a mapping
6491.1Schristosinterface to <a href="https://www.google.com/earth/">Google Earth</a>.
6501.1SchristosIt is freely available under the <abbr>GPL</abbr>.</li>
6511.1Schristos<li><a href="https://golang.org">Go programming language</a>
6521.1Schristosimplementations contain a copy of a 32-bit subset of a recent
6531.1Schristos<code><abbr>tz</abbr></code> database in a
6541.1SchristosGo-specific format.</li>
6551.1Schristos<li>Microsoft Windows 8.1
6561.1Schristosand later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
6571.3Schristosdata (mentioned <a href="#CLDR">below</a>) used by the
6581.3Schristos<a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
6591.3Schristos<a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
6601.3Schristos<a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
6611.3Schristos<a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
6621.1Schristos<a id="System.TimeZoneInfo"
6631.1Schristoshref="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
6641.1SchristosWindows Time Zones with <code>System.TimeZoneInfo</code></a> describes
6651.1Schristosthe older, proprietary method of Microsoft Windows 2000 and later,
6661.1Schristoswhich stores time zone data in the
6671.1Schristos<a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
6681.1Schristos<a
6691.1Schristoshref="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
6701.1SchristosTzid table</a> or <a
6711.6Schristoshref="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
6721.1Schristosfile</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
6731.1Schristosto <code><abbr>tz</abbr></code> names.
6741.3SchristosThese mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
6751.3Schristosor the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
6761.1Schristos<li><a
6771.1Schristoshref="https://www.oracle.com/java/index.html">Oracle
6781.1SchristosJava</a> contains a copy of a subset of a recent
6791.1Schristos<code><abbr>tz</abbr></code> database in a
6801.1SchristosJava-specific format.</li>
6811.1Schristos</ul>
6821.6Schristos</section>
6831.6Schristos
6841.6Schristos<section>
6851.1Schristos<h2 id="other-dbs">Other time zone databases</h2>
6861.1Schristos<ul>
6871.1Schristos<li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
6881.4Schristosis Astrodienst's Web version of Shanks and Pottenger's out-of-print
6891.4Schristostime zone history atlases
6901.4Schristos<a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
6911.9Schristos<a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
6921.4SchristosAlthough these extensive atlases
6931.4Schristos<a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
6941.4Schristossources for much of the older <code><abbr>tz</abbr></code> data</a>,
6951.4Schristosthey are unreliable as Shanks appears to have
6961.1Schristosguessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
6971.1Schristossources and do not indicate which entries are guesswork.</li>
6981.1Schristos<li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
6991.1Schristosits own <code>tztab</code>(4) format.</li>
7001.1Schristos<li>Microsoft Windows has proprietary data mentioned
7011.1Schristos<a href="#System.TimeZoneInfo">above</a>.</li>
7021.1Schristos<li><a href="https://www.worldtimeserver.com">World Time Server</a>
7031.1Schristosis another time zone database.</li>
7041.1Schristos<li>The <a
7051.1Schristoshref="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
7061.1SchristosSchedules Information Manual</a> of the
7071.1SchristosInternational Air Transport Association
7081.1Schristosgives current time zone rules for airports served by commercial aviation.</li>
7091.1Schristos</ul>
7101.6Schristos</section>
7111.6Schristos
7121.6Schristos<section>
7131.1Schristos<h2 id="maps">Maps</h2>
7141.1Schristos<ul>
7151.9Schristos<li>The <a
7161.9Schristoshref="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
7171.9Schristosand Regional Maps section</a> of <em>The World Factbook</em>, published by the
7181.9Schristos<a href="https://www.cia.gov">US Central Intelligence
7191.9SchristosAgency (<abbr
7201.9Schristostitle="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
7211.9Schristoszone map; the
7221.1Schristos<a
7231.6Schristoshref="https://legacy.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
7241.1SchristosLibrary Map Collection</a>
7251.1Schristosof the University of Texas at Austin has copies of
7261.1Schristosrecent editions.
7271.1SchristosThe pictorial quality is good,
7281.1Schristosbut the maps do not indicate daylight saving time,
7291.1Schristosand parts of the data are a few years out of date.</li>
7301.6Schristos<li><a href="https://www.worldtimezone.com">World Time Zone Map
7311.6Schristoswith current time</a>
7321.1Schristoshas several fancy time zone maps; it covers Russia particularly well.
7331.1SchristosThe maps' pictorial quality is not quite as good as the
7341.1Schristos<abbr>CIA</abbr>'s
7351.1Schristosbut the maps are more up to date.</li>
7361.1Schristos<li><a
7371.1Schristoshref="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
7381.1Schristosmuch is time wrong around the world?</a> maps the difference between
7391.1Schristosmean solar and standard time, highlighting areas such as western China
7401.1Schristoswhere the two differ greatly. It's a bit out of date, unfortunately.</li>
7411.1Schristos</ul>
7421.6Schristos</section>
7431.6Schristos
7441.6Schristos<section>
7451.1Schristos<h2 id="boundaries">Time zone boundaries</h2>
7461.2Schristos<p>Geographical boundaries between timezones are available
7471.9Schristosfrom several <a
7481.9Schristoshref="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
7491.9Schristosgeolocation</a>
7501.1Schristosservices and other sources.</p>
7511.1Schristos<ul>
7521.1Schristos<li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
7531.1SchristosBoundary Builder</a> extracts
7541.1Schristos<a href="https://www.openstreetmap.org">Open Street Map</a> data to build
7551.2Schristosboundaries of <code><abbr>tzdb</abbr></code> timezones.
7561.1SchristosIts code is freely available under the <abbr>MIT</abbr> license, and
7571.1Schristosits data entries are freely available under the
7581.1Schristos<a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
7591.1SchristosOpen Database License</a>. The maps' borders appear to be quite accurate.</li>
7601.1Schristos<li>Programmatic interfaces that map geographical coordinates via tz_world to
7611.2Schristos<code><abbr>tzdb</abbr></code> timezones include:
7621.1Schristos<ul>
7631.1Schristos<li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
7641.1Schristoswritten in <a
7651.1Schristoshref="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
7661.1Schristosand is freely available under the <abbr>MIT</abbr> license.</li>
7671.1Schristos<li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
7681.1Schristosis written in Go and is freely available under the Apache License.</li>
7691.1Schristos<li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
7701.1Schristosin both Java and
7711.1Schristos<a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
7721.1Schristosform, is freely available under the MIT license.</li>
7731.10Schristos<li>For Node.js,
7741.1Schristosthe <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
7751.1Schristosis freely available under the MIT license, and
7761.1Schristosthe <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
7771.1Schristosis in the public domain.</li>
7781.1Schristos<li>The <a
7791.1Schristoshref="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
7801.1Schristoslibrary for Python is freely available under the MIT license.
7811.1Schristos<li>The <a
7821.1Schristoshref="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
7831.1Schristoslibrary for Ruby is freely available under the MIT license.</li>
7841.1Schristos</ul></li>
7851.1Schristos<li>Free access via a network API, if you register a key, is provided by
7861.4Schristosthe <a
7871.4Schristoshref="https://www.geonames.org/export/web-services.html#timezone">GeoNames
7881.4SchristosTimezone web service</a>, the <a
7891.4Schristoshref="https://developers.google.com/maps/documentation/timezone/intro">Google
7901.4SchristosMaps Time Zone API</a>, and
7911.8Schristosthe <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
7921.1SchristosCommercial network API access is provided
7931.1Schristosby <a href="https://askgeo.com">AskGeo</a>
7941.1Schristosand <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
7951.1Schristos</li>
7961.1Schristos<li>"<a
7971.1Schristoshref="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
7981.1Schristosto get a time zone from a location using latitude and longitude
7991.1Schristoscoordinates?</a>" discusses other geolocation possibilities.</li>
8001.1Schristos<li><a href="http://statoids.com/statoids.html">Administrative
8011.1SchristosDivisions of Countries ("Statoids")</a> lists
8021.1Schristospolitical subdivision data related to time zones.</li>
8031.7Schristos<li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
8041.1Schristoszone boundaries for multizone countries</a> summarizes legal
8051.1Schristosboundaries between time zones within countries.</li>
8061.1Schristos<li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
8071.1Schristos&ndash; GIS and Database Tools</a> includes a Manifold-format map of
8081.1Schristosworld time zone boundaries distributed under the
8091.1Schristos<abbr>GPL</abbr>.</li>
8101.1Schristos<li>A ship within the <a
8111.1Schristoshref="https://en.wikipedia.org/wiki/Territorial_waters">territorial
8121.1Schristoswaters</a> of any nation uses that nation's time. In international
8131.1Schristoswaters, time zone boundaries are meridians 15&deg; apart, except that
8141.1Schristos<abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
8151.1Schristoswide and are separated by
8161.1Schristosthe 180&deg; meridian (not by the International Date Line, which is
8171.1Schristosfor land and territorial waters only). A captain can change ship's
8181.1Schristosclocks any time after entering a new time zone; midnight changes are
8191.1Schristoscommon.</li>
8201.1Schristos</ul>
8211.6Schristos</section>
8221.6Schristos
8231.6Schristos<section>
8241.1Schristos<h2 id="civil">Civil time concepts and history</h2>
8251.1Schristos<ul>
8261.1Schristos<li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
8271.1SchristosWalk through Time</a>
8281.1Schristossurveys the evolution of timekeeping.</li>
8291.4Schristos<li>The history of daylight saving time is surveyed in <a
8301.4Schristoshref="http://www.webexhibits.org/daylightsaving/">About Daylight
8311.4SchristosSaving Time &ndash; History, rationale, laws &amp; dates</a> and summarized in
8321.4Schristos<a href="http://seizethedaylight.com/dst/">A Brief
8331.4SchristosHistory of Daylight Saving Time</a>.</li>
8341.4Schristos<li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
8351.4SchristosLords</a> discusses how authoritarians manipulate civil time.</li>
8361.1Schristos<li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
8371.1Schristoscontains guidelines and best practices for software applications that
8381.1Schristosdeal with civil time.</li>
8391.1Schristos<li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
8401.1Schristosthe International Date Line</a> tells the story of the most important
8411.1Schristostime zone boundary.</li>
8421.1Schristos<li><a href="http://statoids.com/tconcept.html">Basic Time
8431.1SchristosZone Concepts</a> discusses terminological issues behind time zones.</li>
8441.1Schristos</ul>
8451.6Schristos</section>
8461.6Schristos
8471.6Schristos<section>
8481.1Schristos<h2 id="national">National histories of legal time</h2>
8491.1Schristos<dl>
8501.1Schristos<dt>Australia</dt>
8511.6Schristos<dd>The Parliamentary Library commissioned a <a
8521.1Schristoshref="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
8531.1Schristospaper on daylight saving time in Australia</a>.
8541.1SchristosThe Bureau of Meteorology publishes a list of <a
8551.1Schristoshref="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
8561.1SchristosDates of Daylight Savings Time within Australia</a>.</dd>
8571.1Schristos<dt>Belgium</dt>
8581.7Schristos<dd>The Royal Observatory of Belgium maintains a table of time in
8591.7SchristosBelgium (in
8601.7Schristos<a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
8611.7Schristoshreflang="nl">Dutch</a> and <a
8621.7Schristoshref="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
8631.7Schristoshreflang="fr">French</a>).</dd>
8641.1Schristos<dt>Brazil</dt>
8651.1Schristos<dd>The Time Service Department of the National Observatory
8661.1Schristosrecords <a href="http://pcdsh01.on.br/DecHV.html"
8671.1Schristoshreflang="pt-BR">Brazil's daylight saving time decrees (in
8681.1SchristosPortuguese)</a>.</dd>
8691.1Schristos<dt>Canada</dt>
8701.1Schristos<dd>National Research Council Canada publishes current
8711.1Schristosand some older information about <a
8721.6Schristoshref="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
8731.6Schristoszones and daylight saving time</a>.</dd>
8741.1Schristos<dt>Chile</dt>
8751.1Schristos<dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
8761.4Schristos<a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
8771.1SchristosChile's official time (in Spanish)</a>.</dd>
8781.3Schristos<dt>China</dt>
8791.3Schristos<dd>The Hong Kong Observatory maintains a
8801.7Schristos<a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
8811.3Schristos summer time in Hong Kong</a>,
8821.3Schristosand Macau's Meteorological and Geophysical Bureau maintains a <a
8831.7Schristoshref="https://www.smg.gov.mo/en/subpage/224/page/174">similar
8841.3Schristoshistory for Macau</a>.
8851.3SchristosUnfortunately the latter is incomplete and has errors.</dd>
8861.1Schristos<dt>Czech Republic</dt>
8871.1Schristos<dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
8881.1Schristoshreflang="cs">When daylight saving time starts and ends (in Czech)</a>
8891.10Schristossummarizes and cites historical <abbr>DST</abbr> regulations.</dd>
8901.1Schristos<dt>Germany</dt>
8911.1Schristos<dd>The National Institute for Science and Technology maintains the <a
8921.1Schristoshref="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
8931.1Schristosof Legal Time in Germany</a>.</dd>
8941.1Schristos<dt>Israel</dt>
8951.1Schristos<dd>The Interior Ministry periodically issues <a
8961.1Schristoshref="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
8971.1Schristoshreflang="he">announcements (in Hebrew)</a>.</dd>
8981.2Schristos<dt>Malaysia</dt>
8991.2Schristos<dd>See Singapore <a href="#Singapore">below</a>.</dd>
9001.1Schristos<dt>Mexico</dt>
9011.1Schristos<dd>The Investigation and Analysis Service of the Mexican Library of
9021.1SchristosCongress has published a <a
9031.1Schristoshref="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
9041.1Schristoshreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
9051.1Schristos<dt>Netherlands</dt>
9061.1Schristos<dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
9071.1Schristoshreflang="nl">Legal time in the Netherlands (in Dutch)</a>
9081.1Schristoscovers the history of local time in the Netherlands from ancient times.</dd>
9091.1Schristos<dt>New Zealand</dt>
9101.1Schristos<dd>The Department of Internal Affairs maintains a brief <a
9111.1Schristoshref="https://www.dia.govt.nz/Daylight-Saving-History">History of
9121.6SchristosDaylight Saving</a>.</dd>
9131.10Schristos<dt>Portugal</dt>
9141.10Schristos<dd>The Lisbon Astronomical Observatory publishes a
9151.10Schristos<a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
9161.10Schristoslegal time (in Portuguese)</a>.</dd>
9171.1Schristos<dt>Singapore</dt>
9181.1Schristos<dd><a id="Singapore"
9191.7Schristoshref="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
9201.1Schristosis Singapore in the "Wrong" Time Zone?</a> details the
9211.1Schristoshistory of legal time in Singapore and Malaysia.</dd>
9221.1Schristos<dt>United Kingdom</dt>
9231.1Schristos<dd><a
9241.1Schristoshref="https://www.polyomino.org.uk/british-time/">History of
9251.1Schristoslegal time in Britain</a> discusses in detail the country
9261.6Schristoswith perhaps the best-documented history of clock adjustments.</dd>
9271.1Schristos<dt>United States</dt>
9281.1Schristos<dd>The Department of Transportation's <a
9291.1Schristoshref="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
9301.1SchristosTime Zone Proceedings</a> lists changes to time zone boundaries.</dd>
9311.1Schristos<dt>Uruguay</dt>
9321.1Schristos<dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
9331.1SchristosNavy (SOHMA) publishes an annual <a
9341.7Schristoshref="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
9351.1Schristos(in Spanish)</a>.</dd>
9361.1Schristos</dl>
9371.6Schristos</section>
9381.6Schristos
9391.6Schristos<section>
9401.7Schristos<h2 id="costs">Costs and benefits of time shifts</h2>
9411.7Schristos<p>Various sources argue for and against daylight saving time and time
9421.7Schristoszone shifts, and many scientific studies have been conducted. This
9431.8Schristossection summarizes reviews and position statements based on
9441.8Schristosscientific literature in the area.</p>
9451.7Schristos<ul>
9461.7Schristos<li>Carey RN, Sarma KM.
9471.7Schristos<a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
9481.7Schristosdaylight saving time on road traffic collision risk: a systematic
9491.7Schristosreview</a>.
9501.7Schristos<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>.
9511.7SchristosThis reviews research literature and concludes that the evidence
9521.7Schristosneither supports nor refutes road safety benefits from
9531.7Schristosshifts in time zones.</li>
9541.7Schristos<li>Havranek T, Herman D, Irsova D.
9551.7Schristos<a href="https://www.econstor.eu/handle/10419/174191">Does daylight
9561.7Schristossaving save electricity? A meta-analysis</a>. <em>Energy J.</em>
9571.7Schristos2018;39(2).
9581.7Schristosdoi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
9591.7SchristosThis analyzes research literature and concludes, "Electricity savings
9601.7Schristosare larger for countries farther away from the equator, while
9611.10Schristossubtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
9621.8Schristos<li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
9631.8Schristos<a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
9641.8Schristosan American Academy of Sleep Medicine position statement</a>.
9651.8Schristos<em>J Clin Sleep Med.</em>
9661.8Schristos2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
9671.8SchristosThis argues for permanent standard time due to health risks of both
9681.10Schristos<abbr>DST</abbr> transitions and permanent <abbr>DST</abbr>.</li>
9691.7Schristos<li>Roenneberg T, Winnebeck EC, Klerman EB.
9701.7Schristos<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
9711.7Schristossaving time and artificial time zones &ndash; a battle between
9721.7Schristosbiological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
9731.7Schristosdoi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
9741.10SchristosThis reviews evidence about the health effects of <abbr>DST</abbr>
9751.10Schristosand concludes,
9761.7Schristos"In summary, the scientific literature strongly argues against the
9771.10Schristosswitching between <abbr>DST</abbr> and Standard Time and even more so against
9781.10Schristosadopting <abbr>DST</abbr> permanently."</li>
9791.7Schristos</ul>
9801.7Schristos</section>
9811.7Schristos
9821.7Schristos<section>
9831.1Schristos<h2 id="precision">Precision timekeeping</h2>
9841.1Schristos<ul>
9851.1Schristos<li><a
9861.1Schristoshref="http://leapsecond.com/hpan/an1289.pdf">The
9871.1SchristosScience of Timekeeping</a> is a thorough introduction
9881.1Schristosto the theory and practice of precision timekeeping.</li>
9891.1Schristos<li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
9901.1SchristosTime 2016</a> contains several freely-readable papers.</li>
9911.1Schristos<li><a href="http://www.ntp.org"><abbr
9921.1Schristostitle="Network Time Protocol">NTP</abbr>: The Network
9931.1SchristosTime Protocol</a> (Internet <abbr>RFC</abbr> 5905)
9941.1Schristosdiscusses how to synchronize clocks of
9951.1SchristosInternet hosts.</li>
9961.2Schristos<li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
9971.2Schristosfamily of software algorithms can achieve accuracy to a few tens of
9981.2Schristosnanoseconds in scalable server farms without special hardware.</li>
9991.1Schristos<li>The <a
10001.1Schristoshref="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
10011.1SchristosTime Protocol</a> (<abbr
10021.1Schristostitle="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
10031.2Schristoscan achieve submicrosecond clock accuracy on a local area network
10041.2Schristoswith special-purpose hardware.</li>
10051.1Schristos<li><a
10061.1Schristoshref="https://tools.ietf.org/html/rfc4833">Timezone
10071.1SchristosOptions for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
10081.1Schristos(Internet <abbr>RFC</abbr> 4833)
10091.1Schristosspecifies a <a
10101.1Schristoshref="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
10111.1Schristosoption for a server to configure
10121.1Schristosa client's time zone and daylight saving settings automatically.</li>
10131.1Schristos<li><a
10141.1Schristoshref="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
10151.1SchristosTimes</a> explains more abstruse astronomical time scales like
10161.1Schristos<abbr title="Terrestrial Dynamic Time">TDT</abbr>,
10171.1Schristos<abbr title="Geocentric Coordinate Time">TCG</abbr>, and
10181.1Schristos<abbr title="Barycentric Dynamic Time">TDB</abbr>.
10191.1Schristos<a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
10201.1SchristosScales</a> goes into more detail, particularly for historical variants.</li>
10211.1Schristos<li>The <a href="https://www.iau.org"><abbr
10221.1Schristostitle="International Astronomical Union">IAU</abbr></a>'s <a
10231.1Schristoshref="http://www.iausofa.org"><abbr
10241.1Schristostitle="Standards Of Fundamental Astronomy">SOFA</abbr></a>
10251.1Schristoscollection contains C and <a
10261.1Schristoshref="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
10271.1Schristoscode for converting among time scales like
10281.1Schristos<abbr title="International Atomic Time">TAI</abbr>,
10291.1Schristos<abbr>TDB</abbr>, <abbr>TDT</abbr> and
10301.1Schristos<abbr>UTC</abbr>.</li>
10311.1Schristos<li><a
10321.1Schristoshref="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
10331.1Schristos&ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
10341.1Schristosdiverse local time
10351.1Schristosscales used by each landed mission on Mars.</li>
10361.1Schristos<li><a href="http://leapsecond.com">LeapSecond.com</a> is
10371.1Schristosdedicated not only to leap seconds but to precise time and frequency
10381.1Schristosin general. It covers the state of the art in amateur timekeeping, and
10391.1Schristoshow the art has progressed over the past few decades.</li>
10401.7Schristos<li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
10411.7Schristoshref="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
10421.7Schristosand time-signal emissions</a>, International Telecommunication Union &ndash;
10431.7SchristosRadiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
10441.1Schristos<li><a
10451.1Schristoshref="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
10461.1Schristostitle="International Earth Rotation and Reference Systems Service">IERS</abbr>
10471.1SchristosBulletins</a> contains official publications of the International
10481.1SchristosEarth Rotation and Reference Systems Service, which decides when leap
10491.4Schristosseconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
10501.9Schristosvia an optional "<code>right</code>" configuration where a computer's internal
10511.9Schristos<code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
10521.9Schristosas opposed to the default "<code>posix</code>" configuration
10531.9Schristoswhere the internal clock ignores leap seconds.
10541.9SchristosThe two configurations agree for timestamps starting with 1972-01-01 00:00:00
10551.9Schristos<abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge for
10561.9Schristostimestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,
10571.9Schristoswhich corresponds to the first leap second
10581.9Schristos1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
10591.9Schristosand to
10601.9Schristos1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
10611.9SchristosIn practice the two configurations also agree for timestamps before
10621.9Schristos1972 even though the historical situation is messy, partly because
10631.9Schristosneither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
10641.9Schristosis well-defined for sufficiently-old timestamps.</li>
10651.1Schristos<li><a href="https://developers.google.com/time/smear">Leap Smear</a>
10661.1Schristosdiscusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
10671.1Schristosleap second so that they disagree with <abbr>UTC</abbr> by at most a
10681.1Schristoshalf second, even though every <abbr>POSIX</abbr> minute has exactly
10691.4Schristossixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
10701.1Schristos"<code>posix</code>" configuration, is <a
10711.1Schristoshref="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
10721.1Schristosthe <abbr>NTP</abbr> reference implementation, and is used by major
10731.7Schristoscloud service providers. However, according to
10741.7Schristos<a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">&sect;3.7.1 of
10751.7SchristosNetwork Time Protocol Best Current Practices</a>
10761.7Schristos(Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
10771.9Schristosapplications requiring accurate <abbr>UTC</abbr> or civil time,
10781.7Schristosand is intended for use only in single, well-controlled environments.</li>
10791.1Schristos<li>The <a
10801.1Schristoshref="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
10811.1SchristosSecond Discussion List</a> covers <a
10821.1Schristoshref="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
10831.1Schristosand Klepczynski's 1999 proposal to discontinue leap seconds</a>,
10841.1Schristosdiscussed further in
10851.1Schristos<a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
10861.1Schristosleap second: its history and possible future</a>.
10871.1Schristos<a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
10881.1Schristosmight be redefined
10891.1Schristoswithout Leap Seconds</a> gives pointers on this
10901.1Schristoscontentious issue, which was active until 2015 and could become active
10911.1Schristosagain.</li>
10921.1Schristos</ul>
10931.6Schristos</section>
10941.6Schristos
10951.6Schristos<section>
10961.1Schristos<h2 id="notation">Time notation</h2>
10971.1Schristos<ul>
10981.1Schristos<li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
10991.1SchristosRepository (<abbr>CLDR</abbr>) Project</a> has localizations for time
11001.1Schristoszone names, abbreviations, identifiers, and formats. For example, it
11011.1Schristoscontains French translations for "Eastern European Summer Time",
11021.1Schristos"<abbr title="Eastern European Summer Time">EEST</abbr>", and
11031.1Schristos"Bucharest". Its
11041.1Schristos<a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
11051.1Schristoscharts</a> show these values for many locales. Data values are available in
11061.1Schristosboth <abbr title="Locale Data Markup Language">LDML</abbr>
11071.1Schristos(an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
11081.1Schristos<li>
11091.1Schristos<a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
11101.6Schristosthe international standard date and time notation</a> covers
11111.1Schristos<a
11121.6Schristoshref="https://www.iso.org/standard/70907.html"><em><abbr
11131.6Schristostitle="International Organization for Standardization">ISO</abbr>
11141.6Schristos8601-1:2019 &ndash; Date and time &ndash; Representations for information
11151.6Schristosinterchange &ndash; Part 1: Basic rules</em></a>.</li>
11161.1Schristos<li>
11171.6Schristos<a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
11181.1SchristosSchema: Datatypes &ndash; dateTime</a> specifies a format inspired by
11191.1Schristos<abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
11201.1Schristos<li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">&sect;3.3 of
11211.1SchristosInternet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
11221.1Schristosspecifies the time notation used in email and <a
11231.1Schristoshref="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
11241.1Schristosheaders.</li>
11251.1Schristos<li>
11261.1Schristos<a href="https://tools.ietf.org/html/rfc3339">Date and Time
11271.1Schristoson the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
11281.1Schristosspecifies an <abbr>ISO</abbr> 8601
11291.1Schristosprofile for use in new Internet
11301.1Schristosprotocols.</li>
11311.1Schristos<li>
11321.7Schristos<a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
11331.1SchristosFormats on the Web</a> surveys web- and Internet-oriented date and time
11341.1Schristosformats.</li>
11351.1Schristos<li>Alphabetic time zone abbreviations should not be used as unique
11361.1Schristosidentifiers for <abbr>UT</abbr> offsets as they are ambiguous in
11371.1Schristospractice. For example, in English-speaking North America
11381.1Schristos"<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
11391.1Schristosbut in China it denotes 8 hours ahead of <abbr>UT</abbr>,
11401.1Schristosand French-speaking North Americans prefer
11411.1Schristos"<abbr title="Heure Normale du Centre">HNC</abbr>" to
11421.1Schristos"<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
11431.2Schristosdatabase contains English abbreviations for many timestamps;
11441.1Schristosunfortunately some of these abbreviations were merely the database maintainers'
11451.1Schristosinventions, and these have been removed when possible.</li>
11461.1Schristos<li>Numeric time zone abbreviations typically count hours east of
11471.1Schristos<abbr>UT</abbr>, e.g., +09 for Japan and
11481.1Schristos&minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
11491.1Schristos<code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
11501.1SchristosFor example, one might use <code><abbr>TZ</abbr>="<abbr
11511.1Schristostitle="Japan Standard Time">JST</abbr>-9"</code> and
11521.1Schristos<code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
11531.1Schristosfor Japan and Hawaii, respectively. If the
11541.1Schristos<code><abbr>tz</abbr></code> database is available, it is usually better to use
11551.1Schristossettings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
11561.1Schristos<code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
11571.2Schristosconfusion, handle old timestamps better, and insulate you better from
11581.1Schristosany future changes to the rules. One should never set
11591.1Schristos<abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
11601.1Schristos<code>"GMT-9"</code>, though, since this would incorrectly imply that
11611.1Schristoslocal time is nine hours ahead of <abbr>UT</abbr> and the time zone
11621.1Schristosis called "<abbr>GMT</abbr>".</li>
11631.1Schristos</ul>
11641.6Schristos</section>
11651.6Schristos
11661.6Schristos<section>
11671.1Schristos<h2 id="see-also">See also</h2>
11681.1Schristos<ul>
11691.7Schristos<li><a href="theory.html">Theory and pragmatics of the
11701.7Schristos<code><abbr>tz</abbr></code> code and data</a></li>
11711.1Schristos<li><a href="tz-art.html">Time and the Arts</a></li>
11721.1Schristos</ul>
11731.6Schristos</section>
11741.6Schristos
11751.6Schristos<footer>
11761.1Schristos<hr>
11771.1SchristosThis web page is in the public domain, so clarified as of
11781.1Schristos2009-05-17 by Arthur David Olson.
11791.1Schristos<br>
11801.1SchristosPlease send corrections to this web page to the
11811.1Schristos<a href="mailto:tz@iana.org">time zone mailing list</a>.
11821.6Schristos</footer>
11831.1Schristos</body>
11841.1Schristos</html>
1185