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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>,
921.6Schristos<a href="https://www.apple.com/ios"><abbr
931.1Schristostitle="iPhone OS">iOS</abbr></a>,
941.6Schristos<a href="https://www.apple.com/macos">macOS</a>,
951.1Schristos<a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
961.6Schristos<a href="https://www.vmssoftware.com">Open<abbr
971.1Schristostitle="Virtual Memory System">VMS</abbr></a>,
981.6Schristos<a href="https://www.oracle.com/database/">Oracle Database</a>, and
991.1Schristos<a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
1001.1Schristos<p>
1011.2SchristosEach main entry in the database represents a <dfn>timezone</dfn>
1021.2Schristosfor a set of civil-time clocks that have all agreed since 1970.
1031.2SchristosTimezones are typically identified by continent or ocean and then by the
1041.2Schristosname of the largest city within the region containing the clocks.
1051.1SchristosFor example, <code>America/New_York</code>
1061.1Schristosrepresents most of the <abbr title="United States">US</abbr> eastern time zone;
1071.1Schristos<code>America/Phoenix</code> represents most of Arizona, which
1081.2Schristosuses mountain time without daylight saving time (<abbr>DST</abbr>);
1091.1Schristos<code>America/Detroit</code> represents most of Michigan, which uses
1101.1Schristoseastern time but with different <abbr>DST</abbr> rules in 1975;
1111.1Schristosand other entries represent smaller regions like Starke County,
1121.1SchristosIndiana, which switched from central to eastern time in 1991
1131.1Schristosand switched back in 2006.
1141.1SchristosTo use the database on an extended <a
1151.1Schristoshref="https://en.wikipedia.org/wiki/POSIX"><abbr
1161.1Schristostitle="Portable Operating System Interface">POSIX</abbr></a>
1171.1Schristosimplementation set the <code><abbr>TZ</abbr></code>
1181.1Schristosenvironment variable to the location's full name,
1191.1Schristose.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
1201.1Schristos<p>
1211.2SchristosAssociated with each timezone is a history of offsets from
1221.1Schristos<a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
1231.1SchristosTime</a> (<abbr>UT</abbr>), which is <a
1241.1Schristoshref="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
1251.1SchristosTime</a> (<abbr>GMT</abbr>) with days beginning at midnight;
1261.2Schristosfor timestamps after 1960 this is more precisely <a
1271.1Schristoshref="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
1281.1SchristosUniversal Time</a> (<abbr>UTC</abbr>).
1291.1SchristosThe database also records when daylight saving time was in use,
1301.1Schristosalong with some time zone abbreviations such as <abbr>EST</abbr>
1311.1Schristosfor Eastern Standard Time in the <abbr>US</abbr>.</p>
1321.6Schristos</section>
1331.6Schristos
1341.6Schristos<section>
1351.1Schristos<h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
1361.1Schristos<p>
1371.1SchristosThe following <a
1381.1Schristoshref="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
1391.1Schristosthe latest release's two
1401.1Schristos<a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
1411.1Schristosto a <abbr>GNU</abbr>/Linux or similar host.</p>
1421.2Schristos<pre><code>mkdir tzdb
1431.1Schristoscd tzdb
1441.1Schristos<a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
1451.1Schristoswget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
1461.1Schristos<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 -
1471.1Schristosgzip -dc tzdata-latest.tar.gz | tar -xf -
1481.1Schristos</code></pre>
1491.1Schristos<p>Alternatively, the following shell commands download the same
1501.1Schristosrelease in a single-tarball format containing extra data
1511.1Schristosuseful for regression testing:</p>
1521.2Schristos<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>
1531.1Schristos<a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
1541.1Schristos</code></pre>
1551.1Schristos<p>These commands use convenience links to the latest release
1561.1Schristosof the <code><abbr>tz</abbr></code> database hosted by the
1571.1Schristos<a href="https://www.iana.org/time-zones">Time Zone Database website</a>
1581.1Schristosof the <a href="https://www.iana.org">Internet Assigned Numbers
1591.1SchristosAuthority (IANA)</a>.
1601.1SchristosOlder releases are in files named
1611.1Schristos<code>tzcode<var>V</var>.tar.gz</code>,
1621.1Schristos<code>tzdata<var>V</var>.tar.gz</code>, and
1631.1Schristos<code>tzdb-<var>V</var>.tar.lz</code>,
1641.1Schristoswhere <code><var>V</var></code> is the version.
1651.1SchristosSince 1996, each version has been a four-digit year followed by
1661.1Schristoslower-case letter (<samp>a</samp> through <samp>z</samp>,
1671.1Schristosthen <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
1681.1Schristosthrough <samp>zzz</samp>, and so on).
1691.10SchristosSince version 1999g, each release has been distributed in
1701.10Schristos<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
1711.10Schristosustar interchange format</a>, compressed as described above;
1721.10Schristosolder releases use a nearly-compatible format.
1731.1SchristosSince version 2016h, each release has contained a text file named
1741.1Schristos"<samp>version</samp>" whose first (and currently only) line is the version.
1751.7SchristosOlder releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
1761.7Schristosand are also available in an
1771.1Schristos<a href="ftp://ftp.iana.org/tz/releases/"><abbr
1781.1Schristostitle="File Transfer Protocol">FTP</abbr> directory</a> via a
1791.1Schristosless-secure protocol.</p>
1801.1Schristos<p>Alternatively, a development repository of code and data can be
1811.1Schristosretrieved from <a href="https://github.com">GitHub</a> via the shell
1821.1Schristoscommand:</p>
1831.2Schristos<pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
1841.1Schristos</code></pre>
1851.1Schristos<p>
1861.1SchristosSince version 2012e, each release has been tagged in development repositories.
1871.1SchristosUntagged commits are less well tested and probably contain
1881.1Schristosmore errors.</p>
1891.1Schristos<p>
1901.1SchristosAfter obtaining the code and data files, see the
1911.1Schristos<code>README</code> file for what to do next.
1921.1SchristosThe code lets you compile the <code><abbr>tz</abbr></code> source files into
1931.2Schristosmachine-readable binary files, one for each location. The binary files
1941.5Schristosare in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
1951.5Schristosspecified by <a href="https://tools.ietf.org/html/8536">Internet
1961.5Schristos<abbr>RFC</abbr> 8536</a>.
1971.2SchristosThe code also lets
1981.2Schristosyou read a <abbr>TZif</abbr> file and interpret timestamps for that
1991.1Schristoslocation.</p>
2001.6Schristos</section>
2011.6Schristos
2021.6Schristos<section>
2031.1Schristos<h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
2041.1Schristos<p>
2051.1SchristosThe <code><abbr>tz</abbr></code> code and data
2061.1Schristosare by no means authoritative. If you find errors, please
2071.1Schristossend changes to <a href="mailto:tz@iana.org"><code>tz@iana.org</code></a>,
2081.1Schristosthe time zone mailing list. You can also <a
2091.1Schristoshref="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
2101.1Schristosand browse the <a
2111.1Schristoshref="https://mm.icann.org/pipermail/tz/">archive of old
2121.5Schristosmessages</a>.
2131.5Schristos<a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
2141.5Schristosdiscussions</a> and corresponding data changes can be
2151.5Schristosgenerated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
2161.5Schristos</p>
2171.1Schristos<p>
2181.1SchristosIf your government plans to change its time zone boundaries or
2191.1Schristosdaylight saving rules, inform <code>tz@iana.org</code> well in
2201.1Schristosadvance, as this will coordinate updates to many cell phones,
2211.8Schristoscomputers, and other devices around the world.
2221.8SchristosThe change should be officially announced at least a year before it affects
2231.8Schristoshow clocks operate; otherwise, there is a good chance that some
2241.8Schristosclocks will operate incorrectly after the change, due
2251.1Schristosto delays in propagating updates to software and data. The shorter
2261.1Schristosthe notice, the more likely clock problems will arise; see "<a
2271.1Schristoshref="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
2281.1Schristosthe Timing of Time Zone Changes</a>" for examples.
2291.8SchristosThe <code><abbr>tz</abbr></code> data can represent planned changes
2301.8Schristosfar into the future, and a long-planned change can easily be reverted
2311.8Schristosor otherwise altered with a year's notice before the change would have
2321.8Schristosaffected clocks.
2331.1Schristos</p>
2341.1Schristos<p>
2351.1SchristosChanges to the <code><abbr>tz</abbr></code> code and data are often
2361.1Schristospropagated to clients via operating system updates, so
2371.1Schristosclient <code><abbr>tz</abbr></code> data can often be corrected by
2381.1Schristosapplying these updates. With GNU/Linux and similar systems, if your
2391.1Schristosmaintenance provider has not yet adopted the
2401.1Schristoslatest <code><abbr>tz</abbr></code> data, you can often short-circuit
2411.1Schristosthe process by tailoring the generic instructions in
2421.1Schristosthe <code><abbr>tz</abbr> README</code> file and installing the latest
2431.1Schristosdata yourself. System-specific instructions for installing the
2441.1Schristoslatest <code><abbr>tz</abbr></code> data have also been published
2451.11Schristosfor <a href="https://developer.ibm.com/articles/au-aix-olson-time-zone/"><abbr>AIX</abbr></a>,
2461.1Schristos<a
2471.10Schristoshref="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
2481.1Schristos<a
2491.11Schristoshref="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
2501.1Schristostitle="International Components for Unicode">ICU</abbr></a>,
2511.11Schristos<a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
2521.9SchristosJDK</a>,
2531.9Schristos<a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
2541.1Schristoshref="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
2551.9Schristos<a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
2561.11Schristoshref="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
2571.1Schristos</p>
2581.12Schristos<p>Since version 2013a,
2591.12Schristossources for the <code><abbr>tz</abbr></code> database have been
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>
4911.10Schristos<li>The <a href="https://date-fns.org/">date-fns</a>
4921.10Schristoslibrary manipulates timezone-aware timestamps in browsers and
4931.10Schristosin <a href="https://nodejs.org/en/">Node.js</a>.
4941.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
4951.10Schristos<li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
4961.10Schristosminimalist replacement for the date and time API of
4971.10Schristosthe <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
4981.10Schristosmanipulation library.
4991.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
5001.10Schristos<li><a href="https://moment.github.io/luxon/">Luxon</a> improves
5011.10Schristostimezone support for the <samp>Intl</samp> API.
5021.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
5031.1Schristos<li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
5041.10SchristosMoment.js plugin.
5051.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
5061.8Schristos<li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
5071.8SchristosJavaScript library that supports date arithmetic that is time zone
5081.8Schristosaware. It is freely available under the <abbr>MIT</abbr> license.</li>
5091.10Schristos<li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
5101.10Schristossupports live <code><abbr>tzdb</abbr></code> updates,
5111.10Schristosastronomical and atomic time, a command-line interface,
5121.10Schristosand full <a
5131.10Schristoshref="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
5141.10SchristosIts companion <a
5151.10Schristoshref="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
5161.10Schristoscan generate <abbr>TZif</abbr> and other files, and a companion website
5171.10Schristos<a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
5181.10Schristosconvert timestamps, view transition histories, and download code and data.
5191.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
5201.1Schristos</ul>
5211.10SchristosThe proposed <a
5221.10Schristoshref="https://github.com/tc39/proposal-temporal"><samp>Temporal</samp>
5231.10Schristosobjects</a> let programs access an abstract view of
5241.10Schristos<code><abbr>tzdb</abbr></code> data, and are designed to replace <a
5251.10Schristoshref="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
5261.10Schristosproblematic <samp>Date</samp> objects</a> when working with dates and times.
5271.1Schristos<li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
5281.1Schristoscompiler from <code><abbr>tz</abbr></code> source into
5291.1Schristos<a href="https://julialang.org/">Julia</a>. It is freely available
5301.1Schristosunder the <abbr>MIT</abbr> license.</li>
5311.6Schristos<li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> &ndash;
5321.6Schristos<abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
5331.6Schristostitle="Free Pascal Compiler">FPC</abbr></a>
5341.2Schristoscompiles from <code><abbr>tz</abbr></code> source into
5351.2Schristos<a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
5361.2Schristosas compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
5371.2Schristosand <a
5381.2Schristoshref="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
5391.2SchristosIt is freely available under a <abbr>BSD</abbr>-style license.</li>
5401.1Schristos<li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
5411.1SchristosDefinitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
5421.1Schristos<a href="https://www.python.org">Python</a>.
5431.9SchristosIt is freely available under a <abbr>BSD</abbr>-style license.
5441.11SchristosIn code that can assume Python 3.6 or later it is largely superseded; see <a
5451.11Schristoshref="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
5461.11SchristosThe Fastest Footgun in the West</a>.</li>
5471.1Schristos<li><a href="https://tzinfo.github.io">TZInfo &ndash;
5481.1SchristosRuby Timezone Library</a>
5491.1Schristoscompiles <code><abbr>tz</abbr></code> source into
5501.1Schristos<a href="https://www.ruby-lang.org/en/">Ruby</a>.
5511.1SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
5521.1Schristos<li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
5531.1SchristosLibrary</a> is
5541.1Schristosa <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
5551.1Schristoslibrary that compiles <code><abbr>tz</abbr></code> source into a time
5561.1Schristoszone repository whose format
5571.1Schristosis either proprietary or an <abbr>XML</abbr>-encoded
5581.1Schristosrepresentation.</li>
5591.1Schristos<li><a id="Tcl" href="https://tcl.tk">Tcl</a>
5601.1Schristoscontains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
5611.1Schristossource into text files, along with a runtime that can read those
5621.1Schristosfiles. Tcl is freely available under a <abbr>BSD</abbr>-style
5631.1Schristoslicense.</li>
5641.1Schristos</ul>
5651.6Schristos</section>
5661.6Schristos
5671.6Schristos<section>
5681.2Schristos<h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
5691.1Schristos<ul>
5701.1Schristos<li>The <a
5711.1Schristoshref="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
5721.1SchristosLibrary</a>
5731.1Schristoshas an independent, thread-safe implementation of
5741.2Schristosa <abbr>TZif</abbr> file reader.
5751.1SchristosThis library is freely available under the LGPL
5761.1Schristosand is widely used in <abbr>GNU</abbr>/Linux systems.</li>
5771.1Schristos<li><a href="https://www.gnome.org">GNOME</a>'s
5781.1Schristos<a href="https://developer.gnome.org/glib/">GLib</a> has
5791.2Schristosa <abbr>TZif</abbr> file reader written in C that
5801.1Schristoscreates a <code>GTimeZone</code> object representing sets
5811.1Schristosof <abbr>UT</abbr> offsets.
5821.1SchristosIt is freely available under the <abbr>LGPL</abbr>.</li>
5831.1Schristos<li>The
5841.1Schristos<a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
5851.1Schristos<code>baltzo::TimeZoneUtil</code> component contains a C++
5861.2Schristosimplementation of a <abbr>TZif</abbr> file reader. It is freely available under
5871.1Schristosthe Apache License.</li>
5881.1Schristos<li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
5891.1Schristoslibrary that translates between <abbr>UT</abbr> and civil time and
5901.2Schristoscan read <abbr>TZif</abbr> files. It is freely available under the Apache
5911.1SchristosLicense.</li>
5921.1Schristos<li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
5931.2Schristoslibrary that reads <abbr>TZif</abbr> files and converts
5941.2Schristostimestamps from one time zone or format to another.
5951.1SchristosIt is used by <a href="https://secure.php.net"><abbr
5961.1Schristostitle="PHP: Hypertext Preprocessor">PHP</abbr></a>,
5971.1Schristos<a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
5981.1Schristosand <a href="https://www.mongodb.com">MongoDB</a>.
5991.1SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
6001.1Schristos<li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
6011.2Schristos<abbr>TZif</abbr> file reader.</li>
6021.4Schristos<li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
6031.1SchristosDateTime::TimeZone::Tzfile</a>
6041.2Schristosis a <abbr>TZif</abbr> file reader written in Perl.
6051.1SchristosIt is freely available under the same terms as Perl
6061.1Schristos(dual <abbr>GPL</abbr> and Artistic license).</li>
6071.9Schristos<li>Python has a <a id="python-zoneinfo"
6081.11Schristoshref="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
6091.8Schristosclass</a> that reads <abbr>TZif</abbr> data and creates objects
6101.8Schristosthat represent <code><abbr>tzdb</abbr></code> timezones.
6111.8SchristosPython is freely available under the
6121.11Schristos<a href="https://docs.python.org/3/license.html">Python Software Foundation
6131.8SchristosLicense</a>.
6141.9SchristosA companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
6151.8Schristos<a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
6161.8Schristossupplies TZif data if the underlying system data cannot be found;
6171.8Schristosit is freely available under the Apache License.</li>
6181.1Schristos<li>The
6191.1Schristospublic-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
6201.1Schristoslibrary contains a Python tool that
6211.2Schristosconverts <abbr>TZif</abbr> data into
6221.1Schristos<abbr>JSON</abbr>-format data suitable for use
6231.1Schristosin its JavaScript library for time zone conversion. Dates before 1970
6241.1Schristosare not supported.</li>
6251.1Schristos<li>The <a
6261.1Schristoshref="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
6271.1Schristospackage contains <a href="https://www.haskell.org">Haskell</a> code that
6281.2Schristosparses and uses <abbr>TZif</abbr> data. It is freely
6291.1Schristosavailable under a <abbr>BSD</abbr>-style license.</li>
6301.1Schristos</ul>
6311.6Schristos</section>
6321.6Schristos
6331.6Schristos<section>
6341.1Schristos<h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
6351.1Schristos<ul>
6361.1Schristos<li><a href="https://foxclocks.org">FoxClocks</a>
6371.1Schristosis an extension for <a href="https://www.google.com/chrome/">Google
6381.1SchristosChrome</a> and for <a
6391.11Schristoshref="https://wiki.mozilla.org/Modules/Toolkit">Mozilla
6401.1SchristosToolkit</a> applications like <a
6411.1Schristoshref="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
6421.1Schristoshref="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
6431.1SchristosIt displays multiple clocks in the application window, and has a mapping
6441.1Schristosinterface to <a href="https://www.google.com/earth/">Google Earth</a>.
6451.1SchristosIt is freely available under the <abbr>GPL</abbr>.</li>
6461.1Schristos<li><a href="https://golang.org">Go programming language</a>
6471.1Schristosimplementations contain a copy of a 32-bit subset of a recent
6481.1Schristos<code><abbr>tz</abbr></code> database in a
6491.1SchristosGo-specific format.</li>
6501.1Schristos<li>Microsoft Windows 8.1
6511.1Schristosand later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
6521.3Schristosdata (mentioned <a href="#CLDR">below</a>) used by the
6531.3Schristos<a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
6541.3Schristos<a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
6551.3Schristos<a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
6561.3Schristos<a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
6571.1Schristos<a id="System.TimeZoneInfo"
6581.1Schristoshref="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
6591.1SchristosWindows Time Zones with <code>System.TimeZoneInfo</code></a> describes
6601.1Schristosthe older, proprietary method of Microsoft Windows 2000 and later,
6611.1Schristoswhich stores time zone data in the
6621.1Schristos<a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
6631.1Schristos<a
6641.1Schristoshref="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
6651.1SchristosTzid table</a> or <a
6661.6Schristoshref="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
6671.1Schristosfile</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
6681.1Schristosto <code><abbr>tz</abbr></code> names.
6691.3SchristosThese mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
6701.3Schristosor the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
6711.1Schristos<li><a
6721.1Schristoshref="https://www.oracle.com/java/index.html">Oracle
6731.1SchristosJava</a> contains a copy of a subset of a recent
6741.1Schristos<code><abbr>tz</abbr></code> database in a
6751.1SchristosJava-specific format.</li>
6761.1Schristos</ul>
6771.6Schristos</section>
6781.6Schristos
6791.6Schristos<section>
6801.1Schristos<h2 id="other-dbs">Other time zone databases</h2>
6811.1Schristos<ul>
6821.1Schristos<li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
6831.4Schristosis Astrodienst's Web version of Shanks and Pottenger's out-of-print
6841.4Schristostime zone history atlases
6851.4Schristos<a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
6861.9Schristos<a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
6871.4SchristosAlthough these extensive atlases
6881.4Schristos<a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
6891.4Schristossources for much of the older <code><abbr>tz</abbr></code> data</a>,
6901.4Schristosthey are unreliable as Shanks appears to have
6911.1Schristosguessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
6921.1Schristossources and do not indicate which entries are guesswork.</li>
6931.1Schristos<li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
6941.1Schristosits own <code>tztab</code>(4) format.</li>
6951.1Schristos<li>Microsoft Windows has proprietary data mentioned
6961.1Schristos<a href="#System.TimeZoneInfo">above</a>.</li>
6971.1Schristos<li><a href="https://www.worldtimeserver.com">World Time Server</a>
6981.1Schristosis another time zone database.</li>
6991.1Schristos<li>The <a
7001.1Schristoshref="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
7011.1SchristosSchedules Information Manual</a> of the
7021.1SchristosInternational Air Transport Association
7031.1Schristosgives current time zone rules for airports served by commercial aviation.</li>
7041.1Schristos</ul>
7051.6Schristos</section>
7061.6Schristos
7071.6Schristos<section>
7081.1Schristos<h2 id="maps">Maps</h2>
7091.1Schristos<ul>
7101.9Schristos<li>The <a
7111.9Schristoshref="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
7121.9Schristosand Regional Maps section</a> of <em>The World Factbook</em>, published by the
7131.9Schristos<a href="https://www.cia.gov">US Central Intelligence
7141.9SchristosAgency (<abbr
7151.9Schristostitle="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
7161.9Schristoszone map; the
7171.1Schristos<a
7181.6Schristoshref="https://legacy.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
7191.1SchristosLibrary Map Collection</a>
7201.1Schristosof the University of Texas at Austin has copies of
7211.1Schristosrecent editions.
7221.1SchristosThe pictorial quality is good,
7231.1Schristosbut the maps do not indicate daylight saving time,
7241.1Schristosand parts of the data are a few years out of date.</li>
7251.6Schristos<li><a href="https://www.worldtimezone.com">World Time Zone Map
7261.6Schristoswith current time</a>
7271.1Schristoshas several fancy time zone maps; it covers Russia particularly well.
7281.1SchristosThe maps' pictorial quality is not quite as good as the
7291.1Schristos<abbr>CIA</abbr>'s
7301.1Schristosbut the maps are more up to date.</li>
7311.1Schristos<li><a
7321.1Schristoshref="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
7331.1Schristosmuch is time wrong around the world?</a> maps the difference between
7341.1Schristosmean solar and standard time, highlighting areas such as western China
7351.1Schristoswhere the two differ greatly. It's a bit out of date, unfortunately.</li>
7361.1Schristos</ul>
7371.6Schristos</section>
7381.6Schristos
7391.6Schristos<section>
7401.1Schristos<h2 id="boundaries">Time zone boundaries</h2>
7411.2Schristos<p>Geographical boundaries between timezones are available
7421.9Schristosfrom several <a
7431.9Schristoshref="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
7441.9Schristosgeolocation</a>
7451.1Schristosservices and other sources.</p>
7461.1Schristos<ul>
7471.1Schristos<li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
7481.1SchristosBoundary Builder</a> extracts
7491.1Schristos<a href="https://www.openstreetmap.org">Open Street Map</a> data to build
7501.2Schristosboundaries of <code><abbr>tzdb</abbr></code> timezones.
7511.1SchristosIts code is freely available under the <abbr>MIT</abbr> license, and
7521.1Schristosits data entries are freely available under the
7531.1Schristos<a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
7541.1SchristosOpen Database License</a>. The maps' borders appear to be quite accurate.</li>
7551.1Schristos<li>Programmatic interfaces that map geographical coordinates via tz_world to
7561.2Schristos<code><abbr>tzdb</abbr></code> timezones include:
7571.1Schristos<ul>
7581.1Schristos<li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
7591.1Schristoswritten in <a
7601.1Schristoshref="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
7611.1Schristosand is freely available under the <abbr>MIT</abbr> license.</li>
7621.1Schristos<li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
7631.1Schristosis written in Go and is freely available under the Apache License.</li>
7641.1Schristos<li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
7651.1Schristosin both Java and
7661.1Schristos<a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
7671.1Schristosform, is freely available under the MIT license.</li>
7681.10Schristos<li>For Node.js,
7691.1Schristosthe <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
7701.1Schristosis freely available under the MIT license, and
7711.1Schristosthe <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
7721.1Schristosis in the public domain.</li>
7731.1Schristos<li>The <a
7741.1Schristoshref="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
7751.1Schristoslibrary for Python is freely available under the MIT license.
7761.1Schristos<li>The <a
7771.1Schristoshref="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
7781.1Schristoslibrary for Ruby is freely available under the MIT license.</li>
7791.1Schristos</ul></li>
7801.1Schristos<li>Free access via a network API, if you register a key, is provided by
7811.4Schristosthe <a
7821.4Schristoshref="https://www.geonames.org/export/web-services.html#timezone">GeoNames
7831.4SchristosTimezone web service</a>, the <a
7841.4Schristoshref="https://developers.google.com/maps/documentation/timezone/intro">Google
7851.4SchristosMaps Time Zone API</a>, and
7861.8Schristosthe <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
7871.1SchristosCommercial network API access is provided
7881.1Schristosby <a href="https://askgeo.com">AskGeo</a>
7891.1Schristosand <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
7901.1Schristos</li>
7911.1Schristos<li>"<a
7921.1Schristoshref="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
7931.1Schristosto get a time zone from a location using latitude and longitude
7941.1Schristoscoordinates?</a>" discusses other geolocation possibilities.</li>
7951.1Schristos<li><a href="http://statoids.com/statoids.html">Administrative
7961.1SchristosDivisions of Countries ("Statoids")</a> lists
7971.1Schristospolitical subdivision data related to time zones.</li>
7981.7Schristos<li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
7991.1Schristoszone boundaries for multizone countries</a> summarizes legal
8001.1Schristosboundaries between time zones within countries.</li>
8011.1Schristos<li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
8021.1Schristos&ndash; GIS and Database Tools</a> includes a Manifold-format map of
8031.1Schristosworld time zone boundaries distributed under the
8041.1Schristos<abbr>GPL</abbr>.</li>
8051.1Schristos<li>A ship within the <a
8061.1Schristoshref="https://en.wikipedia.org/wiki/Territorial_waters">territorial
8071.1Schristoswaters</a> of any nation uses that nation's time. In international
8081.1Schristoswaters, time zone boundaries are meridians 15&deg; apart, except that
8091.1Schristos<abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
8101.1Schristoswide and are separated by
8111.1Schristosthe 180&deg; meridian (not by the International Date Line, which is
8121.1Schristosfor land and territorial waters only). A captain can change ship's
8131.1Schristosclocks any time after entering a new time zone; midnight changes are
8141.1Schristoscommon.</li>
8151.1Schristos</ul>
8161.6Schristos</section>
8171.6Schristos
8181.6Schristos<section>
8191.1Schristos<h2 id="civil">Civil time concepts and history</h2>
8201.1Schristos<ul>
8211.1Schristos<li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
8221.1SchristosWalk through Time</a>
8231.1Schristossurveys the evolution of timekeeping.</li>
8241.4Schristos<li>The history of daylight saving time is surveyed in <a
8251.4Schristoshref="http://www.webexhibits.org/daylightsaving/">About Daylight
8261.4SchristosSaving Time &ndash; History, rationale, laws &amp; dates</a> and summarized in
8271.4Schristos<a href="http://seizethedaylight.com/dst/">A Brief
8281.4SchristosHistory of Daylight Saving Time</a>.</li>
8291.4Schristos<li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
8301.4SchristosLords</a> discusses how authoritarians manipulate civil time.</li>
8311.1Schristos<li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
8321.1Schristoscontains guidelines and best practices for software applications that
8331.1Schristosdeal with civil time.</li>
8341.1Schristos<li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
8351.1Schristosthe International Date Line</a> tells the story of the most important
8361.1Schristostime zone boundary.</li>
8371.1Schristos<li><a href="http://statoids.com/tconcept.html">Basic Time
8381.1SchristosZone Concepts</a> discusses terminological issues behind time zones.</li>
8391.1Schristos</ul>
8401.6Schristos</section>
8411.6Schristos
8421.6Schristos<section>
8431.1Schristos<h2 id="national">National histories of legal time</h2>
8441.1Schristos<dl>
8451.1Schristos<dt>Australia</dt>
8461.6Schristos<dd>The Parliamentary Library commissioned a <a
8471.1Schristoshref="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
8481.1Schristospaper on daylight saving time in Australia</a>.
8491.1SchristosThe Bureau of Meteorology publishes a list of <a
8501.1Schristoshref="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
8511.1SchristosDates of Daylight Savings Time within Australia</a>.</dd>
8521.1Schristos<dt>Belgium</dt>
8531.7Schristos<dd>The Royal Observatory of Belgium maintains a table of time in
8541.7SchristosBelgium (in
8551.7Schristos<a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
8561.7Schristoshreflang="nl">Dutch</a> and <a
8571.7Schristoshref="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
8581.7Schristoshreflang="fr">French</a>).</dd>
8591.1Schristos<dt>Brazil</dt>
8601.1Schristos<dd>The Time Service Department of the National Observatory
8611.1Schristosrecords <a href="http://pcdsh01.on.br/DecHV.html"
8621.1Schristoshreflang="pt-BR">Brazil's daylight saving time decrees (in
8631.1SchristosPortuguese)</a>.</dd>
8641.1Schristos<dt>Canada</dt>
8651.1Schristos<dd>National Research Council Canada publishes current
8661.1Schristosand some older information about <a
8671.6Schristoshref="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
8681.6Schristoszones and daylight saving time</a>.</dd>
8691.1Schristos<dt>Chile</dt>
8701.1Schristos<dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
8711.11Schristos<a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
8721.1SchristosChile's official time (in Spanish)</a>.</dd>
8731.3Schristos<dt>China</dt>
8741.3Schristos<dd>The Hong Kong Observatory maintains a
8751.7Schristos<a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
8761.3Schristos summer time in Hong Kong</a>,
8771.3Schristosand Macau's Meteorological and Geophysical Bureau maintains a <a
8781.7Schristoshref="https://www.smg.gov.mo/en/subpage/224/page/174">similar
8791.3Schristoshistory for Macau</a>.
8801.3SchristosUnfortunately the latter is incomplete and has errors.</dd>
8811.1Schristos<dt>Czech Republic</dt>
8821.1Schristos<dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
8831.1Schristoshreflang="cs">When daylight saving time starts and ends (in Czech)</a>
8841.10Schristossummarizes and cites historical <abbr>DST</abbr> regulations.</dd>
8851.1Schristos<dt>Germany</dt>
8861.1Schristos<dd>The National Institute for Science and Technology maintains the <a
8871.1Schristoshref="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
8881.1Schristosof Legal Time in Germany</a>.</dd>
8891.1Schristos<dt>Israel</dt>
8901.1Schristos<dd>The Interior Ministry periodically issues <a
8911.1Schristoshref="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
8921.1Schristoshreflang="he">announcements (in Hebrew)</a>.</dd>
8931.2Schristos<dt>Malaysia</dt>
8941.2Schristos<dd>See Singapore <a href="#Singapore">below</a>.</dd>
8951.1Schristos<dt>Mexico</dt>
8961.1Schristos<dd>The Investigation and Analysis Service of the Mexican Library of
8971.1SchristosCongress has published a <a
8981.1Schristoshref="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
8991.1Schristoshreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
9001.1Schristos<dt>Netherlands</dt>
9011.1Schristos<dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
9021.1Schristoshreflang="nl">Legal time in the Netherlands (in Dutch)</a>
9031.1Schristoscovers the history of local time in the Netherlands from ancient times.</dd>
9041.1Schristos<dt>New Zealand</dt>
9051.1Schristos<dd>The Department of Internal Affairs maintains a brief <a
9061.1Schristoshref="https://www.dia.govt.nz/Daylight-Saving-History">History of
9071.6SchristosDaylight Saving</a>.</dd>
9081.10Schristos<dt>Portugal</dt>
9091.10Schristos<dd>The Lisbon Astronomical Observatory publishes a
9101.10Schristos<a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
9111.10Schristoslegal time (in Portuguese)</a>.</dd>
9121.1Schristos<dt>Singapore</dt>
9131.1Schristos<dd><a id="Singapore"
9141.7Schristoshref="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
9151.1Schristosis Singapore in the "Wrong" Time Zone?</a> details the
9161.1Schristoshistory of legal time in Singapore and Malaysia.</dd>
9171.1Schristos<dt>United Kingdom</dt>
9181.1Schristos<dd><a
9191.1Schristoshref="https://www.polyomino.org.uk/british-time/">History of
9201.1Schristoslegal time in Britain</a> discusses in detail the country
9211.6Schristoswith perhaps the best-documented history of clock adjustments.</dd>
9221.1Schristos<dt>United States</dt>
9231.1Schristos<dd>The Department of Transportation's <a
9241.1Schristoshref="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
9251.1SchristosTime Zone Proceedings</a> lists changes to time zone boundaries.</dd>
9261.1Schristos<dt>Uruguay</dt>
9271.1Schristos<dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
9281.1SchristosNavy (SOHMA) publishes an annual <a
9291.7Schristoshref="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
9301.1Schristos(in Spanish)</a>.</dd>
9311.1Schristos</dl>
9321.6Schristos</section>
9331.6Schristos
9341.6Schristos<section>
9351.7Schristos<h2 id="costs">Costs and benefits of time shifts</h2>
9361.7Schristos<p>Various sources argue for and against daylight saving time and time
9371.7Schristoszone shifts, and many scientific studies have been conducted. This
9381.8Schristossection summarizes reviews and position statements based on
9391.8Schristosscientific literature in the area.</p>
9401.7Schristos<ul>
9411.7Schristos<li>Carey RN, Sarma KM.
9421.7Schristos<a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
9431.7Schristosdaylight saving time on road traffic collision risk: a systematic
9441.7Schristosreview</a>.
9451.7Schristos<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>.
9461.7SchristosThis reviews research literature and concludes that the evidence
9471.7Schristosneither supports nor refutes road safety benefits from
9481.7Schristosshifts in time zones.</li>
9491.7Schristos<li>Havranek T, Herman D, Irsova D.
9501.7Schristos<a href="https://www.econstor.eu/handle/10419/174191">Does daylight
9511.7Schristossaving save electricity? A meta-analysis</a>. <em>Energy J.</em>
9521.7Schristos2018;39(2).
9531.7Schristosdoi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
9541.7SchristosThis analyzes research literature and concludes, "Electricity savings
9551.7Schristosare larger for countries farther away from the equator, while
9561.10Schristossubtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
9571.8Schristos<li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
9581.8Schristos<a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
9591.8Schristosan American Academy of Sleep Medicine position statement</a>.
9601.8Schristos<em>J Clin Sleep Med.</em>
9611.8Schristos2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
9621.8SchristosThis argues for permanent standard time due to health risks of both
9631.10Schristos<abbr>DST</abbr> transitions and permanent <abbr>DST</abbr>.</li>
9641.7Schristos<li>Roenneberg T, Winnebeck EC, Klerman EB.
9651.7Schristos<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
9661.7Schristossaving time and artificial time zones &ndash; a battle between
9671.7Schristosbiological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
9681.7Schristosdoi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
9691.10SchristosThis reviews evidence about the health effects of <abbr>DST</abbr>
9701.10Schristosand concludes,
9711.7Schristos"In summary, the scientific literature strongly argues against the
9721.10Schristosswitching between <abbr>DST</abbr> and Standard Time and even more so against
9731.10Schristosadopting <abbr>DST</abbr> permanently."</li>
9741.7Schristos</ul>
9751.7Schristos</section>
9761.7Schristos
9771.7Schristos<section>
9781.1Schristos<h2 id="precision">Precision timekeeping</h2>
9791.1Schristos<ul>
9801.1Schristos<li><a
9811.1Schristoshref="http://leapsecond.com/hpan/an1289.pdf">The
9821.1SchristosScience of Timekeeping</a> is a thorough introduction
9831.1Schristosto the theory and practice of precision timekeeping.</li>
9841.1Schristos<li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
9851.1SchristosTime 2016</a> contains several freely-readable papers.</li>
9861.1Schristos<li><a href="http://www.ntp.org"><abbr
9871.1Schristostitle="Network Time Protocol">NTP</abbr>: The Network
9881.1SchristosTime Protocol</a> (Internet <abbr>RFC</abbr> 5905)
9891.1Schristosdiscusses how to synchronize clocks of
9901.1SchristosInternet hosts.</li>
9911.2Schristos<li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
9921.2Schristosfamily of software algorithms can achieve accuracy to a few tens of
9931.2Schristosnanoseconds in scalable server farms without special hardware.</li>
9941.1Schristos<li>The <a
9951.1Schristoshref="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
9961.1SchristosTime Protocol</a> (<abbr
9971.1Schristostitle="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
9981.2Schristoscan achieve submicrosecond clock accuracy on a local area network
9991.2Schristoswith special-purpose hardware.</li>
10001.1Schristos<li><a
10011.1Schristoshref="https://tools.ietf.org/html/rfc4833">Timezone
10021.1SchristosOptions for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
10031.1Schristos(Internet <abbr>RFC</abbr> 4833)
10041.1Schristosspecifies a <a
10051.1Schristoshref="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
10061.1Schristosoption for a server to configure
10071.1Schristosa client's time zone and daylight saving settings automatically.</li>
10081.11Schristos<li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
10091.11SchristosScales</a> describes astronomical time scales like
10101.1Schristos<abbr title="Terrestrial Dynamic Time">TDT</abbr>,
10111.1Schristos<abbr title="Geocentric Coordinate Time">TCG</abbr>, and
10121.1Schristos<abbr title="Barycentric Dynamic Time">TDB</abbr>.
10131.1Schristos<li>The <a href="https://www.iau.org"><abbr
10141.1Schristostitle="International Astronomical Union">IAU</abbr></a>'s <a
10151.1Schristoshref="http://www.iausofa.org"><abbr
10161.1Schristostitle="Standards Of Fundamental Astronomy">SOFA</abbr></a>
10171.1Schristoscollection contains C and <a
10181.1Schristoshref="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
10191.1Schristoscode for converting among time scales like
10201.1Schristos<abbr title="International Atomic Time">TAI</abbr>,
10211.1Schristos<abbr>TDB</abbr>, <abbr>TDT</abbr> and
10221.1Schristos<abbr>UTC</abbr>.</li>
10231.1Schristos<li><a
10241.1Schristoshref="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
10251.1Schristos&ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
10261.1Schristosdiverse local time
10271.1Schristosscales used by each landed mission on Mars.</li>
10281.1Schristos<li><a href="http://leapsecond.com">LeapSecond.com</a> is
10291.1Schristosdedicated not only to leap seconds but to precise time and frequency
10301.1Schristosin general. It covers the state of the art in amateur timekeeping, and
10311.1Schristoshow the art has progressed over the past few decades.</li>
10321.7Schristos<li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
10331.7Schristoshref="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
10341.7Schristosand time-signal emissions</a>, International Telecommunication Union &ndash;
10351.7SchristosRadiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
10361.1Schristos<li><a
10371.1Schristoshref="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
10381.1Schristostitle="International Earth Rotation and Reference Systems Service">IERS</abbr>
10391.1SchristosBulletins</a> contains official publications of the International
10401.1SchristosEarth Rotation and Reference Systems Service, which decides when leap
10411.4Schristosseconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
10421.9Schristosvia an optional "<code>right</code>" configuration where a computer's internal
10431.9Schristos<code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
10441.9Schristosas opposed to the default "<code>posix</code>" configuration
10451.9Schristoswhere the internal clock ignores leap seconds.
10461.9SchristosThe two configurations agree for timestamps starting with 1972-01-01 00:00:00
10471.9Schristos<abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge for
10481.9Schristostimestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,
10491.9Schristoswhich corresponds to the first leap second
10501.9Schristos1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
10511.9Schristosand to
10521.9Schristos1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
10531.9SchristosIn practice the two configurations also agree for timestamps before
10541.9Schristos1972 even though the historical situation is messy, partly because
10551.9Schristosneither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
10561.9Schristosis well-defined for sufficiently-old timestamps.</li>
10571.1Schristos<li><a href="https://developers.google.com/time/smear">Leap Smear</a>
10581.1Schristosdiscusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
10591.1Schristosleap second so that they disagree with <abbr>UTC</abbr> by at most a
10601.1Schristoshalf second, even though every <abbr>POSIX</abbr> minute has exactly
10611.4Schristossixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
10621.1Schristos"<code>posix</code>" configuration, is <a
10631.1Schristoshref="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
10641.12Schristosthe <abbr>NTP</abbr> reference implementation, <a
10651.12Schristoshref="https://github.com/google/unsmear">supports</a> conversion between
10661.12Schristos<abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
10671.7Schristoscloud service providers. However, according to
10681.7Schristos<a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">&sect;3.7.1 of
10691.7SchristosNetwork Time Protocol Best Current Practices</a>
10701.7Schristos(Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
10711.9Schristosapplications requiring accurate <abbr>UTC</abbr> or civil time,
10721.7Schristosand is intended for use only in single, well-controlled environments.</li>
10731.1Schristos<li>The <a
10741.1Schristoshref="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
10751.1SchristosSecond Discussion List</a> covers <a
10761.1Schristoshref="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
10771.1Schristosand Klepczynski's 1999 proposal to discontinue leap seconds</a>,
10781.1Schristosdiscussed further in
10791.1Schristos<a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
10801.1Schristosleap second: its history and possible future</a>.
10811.1Schristos<a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
10821.1Schristosmight be redefined
10831.1Schristoswithout Leap Seconds</a> gives pointers on this
10841.1Schristoscontentious issue, which was active until 2015 and could become active
10851.1Schristosagain.</li>
10861.1Schristos</ul>
10871.6Schristos</section>
10881.6Schristos
10891.6Schristos<section>
10901.1Schristos<h2 id="notation">Time notation</h2>
10911.1Schristos<ul>
10921.1Schristos<li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
10931.1SchristosRepository (<abbr>CLDR</abbr>) Project</a> has localizations for time
10941.1Schristoszone names, abbreviations, identifiers, and formats. For example, it
10951.1Schristoscontains French translations for "Eastern European Summer Time",
10961.1Schristos"<abbr title="Eastern European Summer Time">EEST</abbr>", and
10971.1Schristos"Bucharest". Its
10981.1Schristos<a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
10991.1Schristoscharts</a> show these values for many locales. Data values are available in
11001.1Schristosboth <abbr title="Locale Data Markup Language">LDML</abbr>
11011.1Schristos(an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
11021.1Schristos<li>
11031.1Schristos<a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
11041.6Schristosthe international standard date and time notation</a> covers
11051.1Schristos<a
11061.6Schristoshref="https://www.iso.org/standard/70907.html"><em><abbr
11071.6Schristostitle="International Organization for Standardization">ISO</abbr>
11081.6Schristos8601-1:2019 &ndash; Date and time &ndash; Representations for information
11091.6Schristosinterchange &ndash; Part 1: Basic rules</em></a>.</li>
11101.1Schristos<li>
11111.6Schristos<a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
11121.1SchristosSchema: Datatypes &ndash; dateTime</a> specifies a format inspired by
11131.1Schristos<abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
11141.1Schristos<li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">&sect;3.3 of
11151.1SchristosInternet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
11161.1Schristosspecifies the time notation used in email and <a
11171.1Schristoshref="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
11181.1Schristosheaders.</li>
11191.1Schristos<li>
11201.1Schristos<a href="https://tools.ietf.org/html/rfc3339">Date and Time
11211.1Schristoson the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
11221.1Schristosspecifies an <abbr>ISO</abbr> 8601
11231.1Schristosprofile for use in new Internet
11241.1Schristosprotocols.</li>
11251.1Schristos<li>
11261.7Schristos<a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
11271.1SchristosFormats on the Web</a> surveys web- and Internet-oriented date and time
11281.1Schristosformats.</li>
11291.1Schristos<li>Alphabetic time zone abbreviations should not be used as unique
11301.1Schristosidentifiers for <abbr>UT</abbr> offsets as they are ambiguous in
11311.1Schristospractice. For example, in English-speaking North America
11321.1Schristos"<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
11331.1Schristosbut in China it denotes 8 hours ahead of <abbr>UT</abbr>,
11341.1Schristosand French-speaking North Americans prefer
11351.1Schristos"<abbr title="Heure Normale du Centre">HNC</abbr>" to
11361.1Schristos"<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
11371.2Schristosdatabase contains English abbreviations for many timestamps;
11381.1Schristosunfortunately some of these abbreviations were merely the database maintainers'
11391.1Schristosinventions, and these have been removed when possible.</li>
11401.1Schristos<li>Numeric time zone abbreviations typically count hours east of
11411.1Schristos<abbr>UT</abbr>, e.g., +09 for Japan and
11421.1Schristos&minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
11431.1Schristos<code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
11441.1SchristosFor example, one might use <code><abbr>TZ</abbr>="<abbr
11451.1Schristostitle="Japan Standard Time">JST</abbr>-9"</code> and
11461.1Schristos<code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
11471.1Schristosfor Japan and Hawaii, respectively. If the
11481.1Schristos<code><abbr>tz</abbr></code> database is available, it is usually better to use
11491.1Schristossettings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
11501.1Schristos<code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
11511.2Schristosconfusion, handle old timestamps better, and insulate you better from
11521.1Schristosany future changes to the rules. One should never set
11531.1Schristos<abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
11541.1Schristos<code>"GMT-9"</code>, though, since this would incorrectly imply that
11551.1Schristoslocal time is nine hours ahead of <abbr>UT</abbr> and the time zone
11561.1Schristosis called "<abbr>GMT</abbr>".</li>
11571.1Schristos</ul>
11581.6Schristos</section>
11591.6Schristos
11601.6Schristos<section>
11611.1Schristos<h2 id="see-also">See also</h2>
11621.1Schristos<ul>
11631.7Schristos<li><a href="theory.html">Theory and pragmatics of the
11641.7Schristos<code><abbr>tz</abbr></code> code and data</a></li>
11651.1Schristos<li><a href="tz-art.html">Time and the Arts</a></li>
11661.1Schristos</ul>
11671.6Schristos</section>
11681.6Schristos
11691.6Schristos<footer>
11701.1Schristos<hr>
11711.1SchristosThis web page is in the public domain, so clarified as of
11721.1Schristos2009-05-17 by Arthur David Olson.
11731.1Schristos<br>
11741.1SchristosPlease send corrections to this web page to the
11751.1Schristos<a href="mailto:tz@iana.org">time zone mailing list</a>.
11761.6Schristos</footer>
11771.1Schristos</body>
11781.1Schristos</html>
1179