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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).
1701.1SchristosSince version 2016h, each release has contained a text file named
1711.1Schristos"<samp>version</samp>" whose first (and currently only) line is the version.
1721.7SchristosOlder releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
1731.7Schristosand are also available in an
1741.1Schristos<a href="ftp://ftp.iana.org/tz/releases/"><abbr
1751.1Schristostitle="File Transfer Protocol">FTP</abbr> directory</a> via a
1761.1Schristosless-secure protocol.</p>
1771.1Schristos<p>Alternatively, a development repository of code and data can be
1781.1Schristosretrieved from <a href="https://github.com">GitHub</a> via the shell
1791.1Schristoscommand:</p>
1801.2Schristos<pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
1811.1Schristos</code></pre>
1821.1Schristos<p>
1831.1SchristosSince version 2012e, each release has been tagged in development repositories.
1841.1SchristosUntagged commits are less well tested and probably contain
1851.1Schristosmore errors.</p>
1861.1Schristos<p>
1871.1SchristosAfter obtaining the code and data files, see the
1881.1Schristos<code>README</code> file for what to do next.
1891.1SchristosThe code lets you compile the <code><abbr>tz</abbr></code> source files into
1901.2Schristosmachine-readable binary files, one for each location. The binary files
1911.5Schristosare in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
1921.5Schristosspecified by <a href="https://tools.ietf.org/html/8536">Internet
1931.5Schristos<abbr>RFC</abbr> 8536</a>.
1941.2SchristosThe code also lets
1951.2Schristosyou read a <abbr>TZif</abbr> file and interpret timestamps for that
1961.1Schristoslocation.</p>
1971.6Schristos</section>
1981.6Schristos
1991.6Schristos<section>
2001.1Schristos<h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
2011.1Schristos<p>
2021.1SchristosThe <code><abbr>tz</abbr></code> code and data
2031.1Schristosare by no means authoritative. If you find errors, please
2041.1Schristossend changes to <a href="mailto:tz@iana.org"><code>tz@iana.org</code></a>,
2051.1Schristosthe time zone mailing list. You can also <a
2061.1Schristoshref="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
2071.1Schristosand browse the <a
2081.1Schristoshref="https://mm.icann.org/pipermail/tz/">archive of old
2091.5Schristosmessages</a>.
2101.5Schristos<a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
2111.5Schristosdiscussions</a> and corresponding data changes can be
2121.5Schristosgenerated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
2131.5Schristos</p>
2141.1Schristos<p>
2151.1SchristosIf your government plans to change its time zone boundaries or
2161.1Schristosdaylight saving rules, inform <code>tz@iana.org</code> well in
2171.1Schristosadvance, as this will coordinate updates to many cell phones,
2181.8Schristoscomputers, and other devices around the world.
2191.8SchristosThe change should be officially announced at least a year before it affects
2201.8Schristoshow clocks operate; otherwise, there is a good chance that some
2211.8Schristosclocks will operate incorrectly after the change, due
2221.1Schristosto delays in propagating updates to software and data. The shorter
2231.1Schristosthe notice, the more likely clock problems will arise; see "<a
2241.1Schristoshref="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
2251.1Schristosthe Timing of Time Zone Changes</a>" for examples.
2261.8SchristosThe <code><abbr>tz</abbr></code> data can represent planned changes
2271.8Schristosfar into the future, and a long-planned change can easily be reverted
2281.8Schristosor otherwise altered with a year's notice before the change would have
2291.8Schristosaffected clocks.
2301.1Schristos</p>
2311.1Schristos<p>
2321.1SchristosChanges to the <code><abbr>tz</abbr></code> code and data are often
2331.1Schristospropagated to clients via operating system updates, so
2341.1Schristosclient <code><abbr>tz</abbr></code> data can often be corrected by
2351.1Schristosapplying these updates. With GNU/Linux and similar systems, if your
2361.1Schristosmaintenance provider has not yet adopted the
2371.1Schristoslatest <code><abbr>tz</abbr></code> data, you can often short-circuit
2381.1Schristosthe process by tailoring the generic instructions in
2391.1Schristosthe <code><abbr>tz</abbr> README</code> file and installing the latest
2401.1Schristosdata yourself. System-specific instructions for installing the
2411.1Schristoslatest <code><abbr>tz</abbr></code> data have also been published
2421.1Schristosfor <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
2431.1Schristos<a
2441.1Schristoshref="https://play.google.com/store/apps/details?id=com.google.android.timezone.data">Android</a>,
2451.1Schristos<a
2461.1Schristoshref="http://userguide.icu-project.org/datetime/timezone"><abbr
2471.1Schristostitle="International Components for Unicode">ICU</abbr></a>,
2481.9Schristos<a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr>
2491.9SchristosJDK</a>,
2501.9Schristos<a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
2511.1Schristoshref="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
2521.9Schristos<a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
2531.9Schristoshref="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html#usage">OpenJDK/Oracle JDK</a>.
2541.1Schristos</p>
2551.1Schristos<p>Sources for the <code><abbr>tz</abbr></code> database are
2561.1Schristos<a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
2571.1Schristostitle="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
2581.1Schristos<a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
2591.1Schristoswith lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
2601.1Schristostitle="linefeed">LF</abbr></a>,
2611.1Schristoswhich can be modified by common text editors such
2621.1Schristosas <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
2631.1Schristos<a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
2641.1Schristos<a href="https://www.vim.org">vim</a>.
2651.1SchristosSpecialized source-file editing can be done via the
2661.1Schristos<a href="https://packagecontrol.io/packages/zoneinfo">Sublime
2671.1Schristoszoneinfo</a> package for <a
2681.1Schristoshref="https://www.sublimetext.com">Sublime Text</a> and the <a
2691.1Schristoshref="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
2701.1Schristoszoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
2711.1SchristosStudio Code</a>.
2721.1Schristos</p>
2731.1Schristos<p>
2741.1SchristosFor further information about updates, please see
2751.1Schristos<a href="https://tools.ietf.org/html/rfc6557">Procedures for
2761.1SchristosMaintaining the Time Zone Database</a> (Internet <abbr
2771.1Schristostitle="Request For Comments">RFC</abbr> 6557). More detail can be
2781.7Schristosfound in <a href="theory.html">Theory and pragmatics of the
2791.7Schristos<code><abbr>tz</abbr></code> code and data</a>.
2801.2Schristos<a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
2811.2Schristosdisplays changes between recent <code><abbr>tzdb</abbr></code> versions.
2821.1Schristos</p>
2831.6Schristos</section>
2841.6Schristos
2851.6Schristos<section>
2861.1Schristos<h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
2871.1Schristos<ul>
2881.1Schristos<li>The article
2891.1Schristos<a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
2901.1Schristosan encyclopedic summary.</li>
2911.1Schristos<li><a href="tz-how-to.html">How to Read the
2921.1Schristostz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
2931.1Schristosdatabase format.</li>
2941.1Schristos<li><a
2951.1Schristoshref="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
2961.1Schristosliterary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
2971.1Schristosdatabase's style.</li>
2981.1Schristos</ul>
2991.6Schristos</section>
3001.6Schristos
3011.6Schristos<section>
3021.1Schristos<h2 id="web">Web sites using recent versions of the
3031.1Schristos<code><abbr>tz</abbr></code> database</h2>
3041.1Schristos<p>
3051.1SchristosThese are listed roughly in ascending order of complexity and fanciness.
3061.1Schristos</p>
3071.1Schristos<ul>
3081.1Schristos<li><a href="https://time.is">Time.is</a> shows locations'
3091.1Schristostime and zones.</li>
3101.1Schristos<li><a href="https://www.timejones.com">TimeJones.com</a>,
3111.1Schristos<a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
3121.7Schristos<a href="https://www.worldclock.com">The World Clock</a>
3131.1Schristosare time zone converters.</li>
3141.8Schristos<li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
3151.8Schristospublishes <code><abbr>tzdb</abbr></code>-derived data in
3161.8Schristos<a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
3171.8Schristostitle="comma-separated values">CSV</abbr></a> and
3181.8Schristosin <a href="https://en.wikipedia.org/wiki/SQL"><abbr
3191.8Schristostitle="Structured Query Language">SQL</abbr></a> form.</li>
3201.1Schristos<li><a
3211.6Schristoshref="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
3221.1Schristoslets you see the <code><abbr>TZ</abbr></code> values directly.</li>
3231.1Schristos<li><a
3241.1Schristoshref="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
3251.1SchristosTime in 1000 Places</a> uses descriptions of the values.</li>
3261.6Schristos<li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
3271.1Schristosuses a pulldown menu.</li>
3281.7Schristos<li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
3291.1Schristostimezone information for all countries</a> displays tables of DST rules.
3301.1Schristos<li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
3311.1SchristosWorldwide</a> lets you sort zone names and convert times.</li>
3321.1Schristos<li><a href="https://24timezones.com">24TimeZones</a> has a world
3331.1Schristostime map and a time converter.</li>
3341.1Schristos<li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
3351.1Schristoscalculates the current time difference between locations.</li>
3361.1Schristos<li><a href="http://www.wx-now.com">Weather Now</a> and
3371.1Schristos<a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
3381.1Schristos</ul>
3391.6Schristos</section>
3401.6Schristos
3411.6Schristos<section>
3421.1Schristos<h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
3431.1Schristos<ul>
3441.1Schristos<li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
3451.1Schristos<a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
3461.1SchristosDistribution Service (tzdist) working group</a> defined <a
3471.1Schristoshref="https://tools.ietf.org/html/rfc7808">TZDIST</a>
3481.1Schristos(Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
3491.1Schristosalong with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
3501.1Schristos(Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
3511.2Schristostransferring time zone data by reference.
3521.8Schristos<a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
3531.8Schristosimplementations</a> are available.
3541.2SchristosThe <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
3551.9Schristosmailing list</a> discusses possible extensions.</li>
3561.1Schristos<li>The <a href="https://tools.ietf.org/html/rfc5545">
3571.1SchristosInternet Calendaring and Scheduling Core Object Specification
3581.1Schristos(iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
3591.1Schristoscovers time zone
3601.1Schristosdata; see its VTIMEZONE calendar component.
3611.1SchristosThe iCalendar format requires specialized parsers and generators; a
3621.1Schristosvariant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
3631.1Schristos(Internet <abbr>RFC</abbr> 6321) uses
3641.1Schristos<a href="https://www.w3.org/XML/"><abbr
3651.1Schristostitle="Extensible Markup Language">XML</abbr></a> format, and a variant
3661.1Schristos<a href="https://tools.ietf.org/html/rfc7265">jCal</a>
3671.1Schristos(Internet <abbr>RFC</abbr> 7265)
3681.1Schristosuses <a href="https://www.json.org"><abbr
3691.1Schristostitle="JavaScript Object Notation">JSON</abbr></a> format.</li>
3701.1Schristos</ul>
3711.6Schristos</section>
3721.6Schristos
3731.6Schristos<section>
3741.1Schristos<h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
3751.7Schristos<p>Although some of these do not fully support
3761.7Schristos<code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
3771.7Schristosdistributions you can generally work around compatibility problems by
3781.7Schristosrunning the command <code>make rearguard_tarballs</code> and compiling
3791.7Schristosfrom the resulting tarballs instead.</p>
3801.1Schristos<ul>
3811.1Schristos<li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
3821.1Schristoshref="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
3831.1Schristosprogram that compiles
3841.1Schristos<code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
3851.1SchristosVzic is freely
3861.1Schristosavailable under the <a
3871.1Schristoshref="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
3881.1SchristosGeneral Public License (<abbr
3891.1Schristostitle="General Public License">GPL</abbr>)</a>.</li>
3901.1Schristos<li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
3911.1Schristosdatabase conversion utility</a> is like Vzic, except for the <a
3921.6Schristoshref="https://dotnet.microsoft.com">.NET framework</a>
3931.1Schristosand with a <abbr>BSD</abbr>-style license.</li>
3941.1Schristos<li><a
3951.4Schristoshref="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
3961.1Schristoscontains a script <code>parse_olson</code> that compiles
3971.1Schristos<code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
3981.1Schristosmodules. It is part of the Perl <a
3991.4Schristoshref="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
4001.4Schristoswhich is freely
4011.1Schristosavailable under both the <abbr>GPL</abbr> and the Perl Artistic
4021.1SchristosLicense. DateTime::TimeZone also contains a script
4031.1Schristos<code>tests_from_zdump</code> that generates test cases for each clock
4041.1Schristostransition in the <code><abbr>tz</abbr></code> database.</li>
4051.1Schristos<li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
4061.1SchristosDatabase Parser</a> is a
4071.1Schristos<a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
4081.9Schristosruntime library with <a
4091.9Schristoshref="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
4101.9Schristosadopted by
4111.6Schristos<a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
4121.9Schristosthe current iteration of the C++ standard.
4131.1SchristosIt is freely available under the
4141.1Schristos<abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
4151.1Schristos<li><a id="ICU" href="http://site.icu-project.org">International Components for
4161.1SchristosUnicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
4171.1Schristoshref="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
4181.1Schristoslibraries for internationalization that
4191.1Schristoshas a compiler from <code><abbr>tz</abbr></code> source
4201.1Schristosand from <abbr title="Common Locale Data Repository">CLDR</abbr> data
4211.1Schristos(mentioned <a href="#CLDR">below</a>)
4221.1Schristosinto an <abbr>ICU</abbr>-specific format.
4231.1Schristos<abbr>ICU</abbr> is freely available under a
4241.1Schristos<abbr>BSD</abbr>-style license.</li>
4251.1Schristos<li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
4261.1Schristosthe <a href="https://elixir-lang.org">Elixir</a> language downloads
4271.7Schristosand compiles <code><abbr>tz</abbr></code> source and exposes <abbr
4281.1Schristostitle="Application Program Interface">API</abbr>s for use. It is
4291.1Schristosfreely available under the <abbr>MIT</abbr> license.</li>
4301.1Schristos<li>Java-based compilers and libraries include:
4311.1Schristos<ul>
4321.1Schristos<li>The <a
4331.9Schristoshref="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
4341.1Schristostool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
4351.9Schristos<a href="https://openjdk.java.net/">OpenJDK</a> and
4361.9Schristos<a href="https://jdk.java.net/">Oracle JDK</a>.
4371.9SchristosAlthough its source code is proprietary, its executable is available under the
4381.9Schristos<a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
4391.9SchristosTimezone Updater License Agreement</a>.</li>
4401.1Schristos<li>The <a
4411.4Schristoshref="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
4421.6SchristosSE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
4431.4Schristoshref="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
4441.1Schristoswhich is freely available under a <abbr>BSD</abbr>-style license.</li>
4451.4Schristos<li><a href="https://www.joda.org/joda-time/">Joda-Time &ndash; Java date
4461.1Schristosand time <abbr>API</abbr></a> contains a class
4471.1Schristos<code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
4481.1Schristos<code><abbr>tz</abbr></code> source into a binary format. It inspired
4491.1SchristosJava 8 <code>java.time</code>, which its users should migrate to once
4501.1Schristosthey can assume Java 8 or later. It is available under the <a
4511.1Schristoshref="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
4521.9Schristos<li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
4531.7Schristoshref="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
4541.9Schristosare alternatives to TZUpdater. IANA Updater's license is unclear;
4551.9SchristosZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
4561.4Schristos<li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
4571.4Schristostime library for Android</a> and
4581.4Schristos<a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
4591.4Schristostime and interval library for Java</a> compile
4601.4Schristos<code><abbr>tz</abbr></code> source into a binary format.
4611.4SchristosTime4A is available under the Apache License and Time4J is
4621.1Schristosavailable under the <a
4631.1Schristoshref="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
4641.1SchristosGeneral Public License (<abbr title="Lesser General Public
4651.1SchristosLicense">LGPL</abbr>)</a>.</li>
4661.1Schristos<li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
4671.1SchristosJava-based libraries.</li>
4681.1Schristos</ul>
4691.1Schristos<li><a href="https://nodatime.org">Noda Time &ndash; Date and
4701.1Schristostime <abbr>API</abbr> for .NET</a>
4711.4Schristosis like Joda-Time and Time4J, but for the .NET framework instead of Java.
4721.4SchristosIt is freely available under the Apache License.</li>
4731.8Schristos<li>Many modern
4741.8Schristos<a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
4751.8Schristosruntimes support <code><abbr>tz</abbr></code> natively via the
4761.8Schristos<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>.
4771.8SchristosOn other runtimes, you can use one of the following.
4781.1Schristos<ul>
4791.8Schristos<li>The <a
4801.8Schristoshref="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
4811.8Schristostimezone polyfill</a>
4821.8Schristosis freely available under a <abbr>BSD</abbr>-style license.</li>
4831.1Schristos<li><a
4841.1Schristoshref="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
4851.1Schristoscompiles time zone data into a compact form designed for
4861.1SchristosJavaScript. It is freely available under a combination of
4871.1Schristosthe <abbr>MIT</abbr> license and the Apache License.</li>
4881.1Schristos<li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
4891.1Schristosplugin for the <a href="https://momentjs.com">Moment.js</a> date
4901.1Schristosmanipulation library. It is freely available under the <abbr>MIT</abbr>
4911.1Schristoslicense.</li>
4921.8Schristos<li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
4931.8SchristosJavaScript library that supports date arithmetic that is time zone
4941.8Schristosaware. It is freely available under the <abbr>MIT</abbr> license.</li>
4951.1Schristos<li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
4961.1Schristos<abbr>API</abbr> is upward compatible with standard JavaScript
4971.1SchristosDates. It is freely available under the Apache License.</li>
4981.1Schristos</ul>
4991.1Schristos<li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
5001.1Schristoscompiler from <code><abbr>tz</abbr></code> source into
5011.1Schristos<a href="https://julialang.org/">Julia</a>. It is freely available
5021.1Schristosunder the <abbr>MIT</abbr> license.</li>
5031.6Schristos<li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> &ndash;
5041.6Schristos<abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
5051.6Schristostitle="Free Pascal Compiler">FPC</abbr></a>
5061.2Schristoscompiles from <code><abbr>tz</abbr></code> source into
5071.2Schristos<a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
5081.2Schristosas compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
5091.2Schristosand <a
5101.2Schristoshref="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
5111.2SchristosIt is freely available under a <abbr>BSD</abbr>-style license.</li>
5121.1Schristos<li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
5131.1SchristosDefinitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
5141.1Schristos<a href="https://www.python.org">Python</a>.
5151.9SchristosIt is freely available under a <abbr>BSD</abbr>-style license.
5161.9SchristosIn code that can assume Python 3.9 or later it is superseded by
5171.9Schristos<a href="#python-zoneinfo"><code>zoneinfo.ZoneInfo</code></a>
5181.9Schristosand the <a href="#pypi-tzdata"><code>tzdata</code> Python package</a>.</li>
5191.1Schristos<li><a href="https://tzinfo.github.io">TZInfo &ndash;
5201.1SchristosRuby Timezone Library</a>
5211.1Schristoscompiles <code><abbr>tz</abbr></code> source into
5221.1Schristos<a href="https://www.ruby-lang.org/en/">Ruby</a>.
5231.1SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
5241.1Schristos<li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
5251.1SchristosLibrary</a> is
5261.1Schristosa <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
5271.1Schristoslibrary that compiles <code><abbr>tz</abbr></code> source into a time
5281.1Schristoszone repository whose format
5291.1Schristosis either proprietary or an <abbr>XML</abbr>-encoded
5301.1Schristosrepresentation.</li>
5311.1Schristos<li><a id="Tcl" href="https://tcl.tk">Tcl</a>
5321.1Schristoscontains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
5331.1Schristossource into text files, along with a runtime that can read those
5341.1Schristosfiles. Tcl is freely available under a <abbr>BSD</abbr>-style
5351.1Schristoslicense.</li>
5361.1Schristos</ul>
5371.6Schristos</section>
5381.6Schristos
5391.6Schristos<section>
5401.2Schristos<h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
5411.1Schristos<ul>
5421.1Schristos<li>The <a
5431.1Schristoshref="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
5441.1SchristosLibrary</a>
5451.1Schristoshas an independent, thread-safe implementation of
5461.2Schristosa <abbr>TZif</abbr> file reader.
5471.1SchristosThis library is freely available under the LGPL
5481.1Schristosand is widely used in <abbr>GNU</abbr>/Linux systems.</li>
5491.1Schristos<li><a href="https://www.gnome.org">GNOME</a>'s
5501.1Schristos<a href="https://developer.gnome.org/glib/">GLib</a> has
5511.2Schristosa <abbr>TZif</abbr> file reader written in C that
5521.1Schristoscreates a <code>GTimeZone</code> object representing sets
5531.1Schristosof <abbr>UT</abbr> offsets.
5541.1SchristosIt is freely available under the <abbr>LGPL</abbr>.</li>
5551.1Schristos<li>The
5561.1Schristos<a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
5571.1Schristos<code>baltzo::TimeZoneUtil</code> component contains a C++
5581.2Schristosimplementation of a <abbr>TZif</abbr> file reader. It is freely available under
5591.1Schristosthe Apache License.</li>
5601.1Schristos<li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
5611.1Schristoslibrary that translates between <abbr>UT</abbr> and civil time and
5621.2Schristoscan read <abbr>TZif</abbr> files. It is freely available under the Apache
5631.1SchristosLicense.</li>
5641.1Schristos<li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
5651.2Schristosis a <abbr>TZif</abbr> file reader written in Java.
5661.1SchristosIt is freely available under the <abbr>LGPL</abbr>.</li>
5671.1Schristos<li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
5681.2Schristoslibrary that reads <abbr>TZif</abbr> files and converts
5691.2Schristostimestamps from one time zone or format to another.
5701.1SchristosIt is used by <a href="https://secure.php.net"><abbr
5711.1Schristostitle="PHP: Hypertext Preprocessor">PHP</abbr></a>,
5721.1Schristos<a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
5731.1Schristosand <a href="https://www.mongodb.com">MongoDB</a>.
5741.1SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
5751.1Schristos<li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
5761.2Schristos<abbr>TZif</abbr> file reader.</li>
5771.4Schristos<li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
5781.1SchristosDateTime::TimeZone::Tzfile</a>
5791.2Schristosis a <abbr>TZif</abbr> file reader written in Perl.
5801.1SchristosIt is freely available under the same terms as Perl
5811.1Schristos(dual <abbr>GPL</abbr> and Artistic license).</li>
5821.9Schristos<li>Python has a <a id="python-zoneinfo"
5831.8Schristoshref="https://docs.python.org/3.9/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
5841.8Schristosclass</a> that reads <abbr>TZif</abbr> data and creates objects
5851.8Schristosthat represent <code><abbr>tzdb</abbr></code> timezones.
5861.8SchristosPython is freely available under the
5871.8Schristos<a href="https://docs.python.org/3.9/license.html">Python Software Foundation
5881.8SchristosLicense</a>.
5891.9SchristosA companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
5901.8Schristos<a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
5911.8Schristossupplies TZif data if the underlying system data cannot be found;
5921.8Schristosit is freely available under the Apache License.</li>
5931.1Schristos<li>The
5941.1Schristospublic-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
5951.1Schristoslibrary contains a Python tool that
5961.2Schristosconverts <abbr>TZif</abbr> data into
5971.1Schristos<abbr>JSON</abbr>-format data suitable for use
5981.1Schristosin its JavaScript library for time zone conversion. Dates before 1970
5991.1Schristosare not supported.</li>
6001.1Schristos<li>The <a
6011.1Schristoshref="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
6021.1Schristospackage contains <a href="https://www.haskell.org">Haskell</a> code that
6031.2Schristosparses and uses <abbr>TZif</abbr> data. It is freely
6041.1Schristosavailable under a <abbr>BSD</abbr>-style license.</li>
6051.1Schristos</ul>
6061.6Schristos</section>
6071.6Schristos
6081.6Schristos<section>
6091.1Schristos<h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
6101.1Schristos<ul>
6111.1Schristos<li><a href="https://foxclocks.org">FoxClocks</a>
6121.1Schristosis an extension for <a href="https://www.google.com/chrome/">Google
6131.1SchristosChrome</a> and for <a
6141.1Schristoshref="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
6151.1SchristosToolkit</a> applications like <a
6161.1Schristoshref="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
6171.1Schristoshref="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
6181.1SchristosIt displays multiple clocks in the application window, and has a mapping
6191.1Schristosinterface to <a href="https://www.google.com/earth/">Google Earth</a>.
6201.1SchristosIt is freely available under the <abbr>GPL</abbr>.</li>
6211.1Schristos<li><a href="https://golang.org">Go programming language</a>
6221.1Schristosimplementations contain a copy of a 32-bit subset of a recent
6231.1Schristos<code><abbr>tz</abbr></code> database in a
6241.1SchristosGo-specific format.</li>
6251.1Schristos<li>Microsoft Windows 8.1
6261.1Schristosand later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
6271.3Schristosdata (mentioned <a href="#CLDR">below</a>) used by the
6281.3Schristos<a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
6291.3Schristos<a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
6301.3Schristos<a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
6311.3Schristos<a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
6321.1Schristos<a id="System.TimeZoneInfo"
6331.1Schristoshref="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
6341.1SchristosWindows Time Zones with <code>System.TimeZoneInfo</code></a> describes
6351.1Schristosthe older, proprietary method of Microsoft Windows 2000 and later,
6361.1Schristoswhich stores time zone data in the
6371.1Schristos<a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
6381.1Schristos<a
6391.1Schristoshref="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
6401.1SchristosTzid table</a> or <a
6411.6Schristoshref="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
6421.1Schristosfile</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
6431.1Schristosto <code><abbr>tz</abbr></code> names.
6441.3SchristosThese mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
6451.3Schristosor the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
6461.1Schristos<li><a
6471.1Schristoshref="https://www.oracle.com/java/index.html">Oracle
6481.1SchristosJava</a> contains a copy of a subset of a recent
6491.1Schristos<code><abbr>tz</abbr></code> database in a
6501.1SchristosJava-specific format.</li>
6511.2Schristos<li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone
6521.1SchristosMaster</a> is a Microsoft Windows clock program that can automatically
6531.4Schristosdownload, compile and use <code><abbr>tz</abbr></code> releases.
6541.4SchristosThe Basic version is free.</li>
6551.1Schristos<li><a
6561.1Schristoshref="http://veladg.com/velaterra.html">VelaTerra</a> is
6571.1Schristosa macOS program. Its developers
6581.1Schristos<a href="http://veladg.com/tzoffer.html">offer free
6591.1Schristoslicenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
6601.1Schristos</ul>
6611.6Schristos</section>
6621.6Schristos
6631.6Schristos<section>
6641.1Schristos<h2 id="other-dbs">Other time zone databases</h2>
6651.1Schristos<ul>
6661.1Schristos<li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
6671.4Schristosis Astrodienst's Web version of Shanks and Pottenger's out-of-print
6681.4Schristostime zone history atlases
6691.4Schristos<a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
6701.9Schristos<a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
6711.4SchristosAlthough these extensive atlases
6721.4Schristos<a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
6731.4Schristossources for much of the older <code><abbr>tz</abbr></code> data</a>,
6741.4Schristosthey are unreliable as Shanks appears to have
6751.1Schristosguessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
6761.1Schristossources and do not indicate which entries are guesswork.</li>
6771.1Schristos<li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
6781.1Schristosits own <code>tztab</code>(4) format.</li>
6791.1Schristos<li>Microsoft Windows has proprietary data mentioned
6801.1Schristos<a href="#System.TimeZoneInfo">above</a>.</li>
6811.1Schristos<li><a href="https://www.worldtimeserver.com">World Time Server</a>
6821.1Schristosis another time zone database.</li>
6831.1Schristos<li>The <a
6841.1Schristoshref="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
6851.1SchristosSchedules Information Manual</a> of the
6861.1SchristosInternational Air Transport Association
6871.1Schristosgives current time zone rules for airports served by commercial aviation.</li>
6881.1Schristos</ul>
6891.6Schristos</section>
6901.6Schristos
6911.6Schristos<section>
6921.1Schristos<h2 id="maps">Maps</h2>
6931.1Schristos<ul>
6941.9Schristos<li>The <a
6951.9Schristoshref="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
6961.9Schristosand Regional Maps section</a> of <em>The World Factbook</em>, published by the
6971.9Schristos<a href="https://www.cia.gov">US Central Intelligence
6981.9SchristosAgency (<abbr
6991.9Schristostitle="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
7001.9Schristoszone map; the
7011.1Schristos<a
7021.6Schristoshref="https://legacy.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
7031.1SchristosLibrary Map Collection</a>
7041.1Schristosof the University of Texas at Austin has copies of
7051.1Schristosrecent editions.
7061.1SchristosThe pictorial quality is good,
7071.1Schristosbut the maps do not indicate daylight saving time,
7081.1Schristosand parts of the data are a few years out of date.</li>
7091.6Schristos<li><a href="https://www.worldtimezone.com">World Time Zone Map
7101.6Schristoswith current time</a>
7111.1Schristoshas several fancy time zone maps; it covers Russia particularly well.
7121.1SchristosThe maps' pictorial quality is not quite as good as the
7131.1Schristos<abbr>CIA</abbr>'s
7141.1Schristosbut the maps are more up to date.</li>
7151.1Schristos<li><a
7161.1Schristoshref="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
7171.1Schristosmuch is time wrong around the world?</a> maps the difference between
7181.1Schristosmean solar and standard time, highlighting areas such as western China
7191.1Schristoswhere the two differ greatly. It's a bit out of date, unfortunately.</li>
7201.1Schristos</ul>
7211.6Schristos</section>
7221.6Schristos
7231.6Schristos<section>
7241.1Schristos<h2 id="boundaries">Time zone boundaries</h2>
7251.2Schristos<p>Geographical boundaries between timezones are available
7261.9Schristosfrom several <a
7271.9Schristoshref="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
7281.9Schristosgeolocation</a>
7291.1Schristosservices and other sources.</p>
7301.1Schristos<ul>
7311.1Schristos<li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
7321.1SchristosBoundary Builder</a> extracts
7331.1Schristos<a href="https://www.openstreetmap.org">Open Street Map</a> data to build
7341.2Schristosboundaries of <code><abbr>tzdb</abbr></code> timezones.
7351.1SchristosIts code is freely available under the <abbr>MIT</abbr> license, and
7361.1Schristosits data entries are freely available under the
7371.1Schristos<a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
7381.1SchristosOpen Database License</a>. The maps' borders appear to be quite accurate.</li>
7391.1Schristos<li>Programmatic interfaces that map geographical coordinates via tz_world to
7401.2Schristos<code><abbr>tzdb</abbr></code> timezones include:
7411.1Schristos<ul>
7421.1Schristos<li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
7431.1Schristoswritten in <a
7441.1Schristoshref="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
7451.1Schristosand is freely available under the <abbr>MIT</abbr> license.</li>
7461.1Schristos<li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
7471.1Schristosis written in Go and is freely available under the Apache License.</li>
7481.1Schristos<li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
7491.1Schristosin both Java and
7501.1Schristos<a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
7511.1Schristosform, is freely available under the MIT license.</li>
7521.1Schristos<li>For <a href="https://nodejs.org/en/">Node.js</a>,
7531.1Schristosthe <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
7541.1Schristosis freely available under the MIT license, and
7551.1Schristosthe <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
7561.1Schristosis in the public domain.</li>
7571.1Schristos<li>The <a
7581.1Schristoshref="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
7591.1Schristoslibrary for Python is freely available under the MIT license.
7601.1Schristos<li>The <a
7611.1Schristoshref="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
7621.1Schristoslibrary for Ruby is freely available under the MIT license.</li>
7631.1Schristos</ul></li>
7641.1Schristos<li>Free access via a network API, if you register a key, is provided by
7651.4Schristosthe <a
7661.4Schristoshref="https://www.geonames.org/export/web-services.html#timezone">GeoNames
7671.4SchristosTimezone web service</a>, the <a
7681.4Schristoshref="https://developers.google.com/maps/documentation/timezone/intro">Google
7691.4SchristosMaps Time Zone API</a>, and
7701.8Schristosthe <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
7711.1SchristosCommercial network API access is provided
7721.1Schristosby <a href="https://askgeo.com">AskGeo</a>
7731.1Schristosand <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
7741.1Schristos</li>
7751.1Schristos<li>"<a
7761.1Schristoshref="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
7771.1Schristosto get a time zone from a location using latitude and longitude
7781.1Schristoscoordinates?</a>" discusses other geolocation possibilities.</li>
7791.1Schristos<li><a href="http://statoids.com/statoids.html">Administrative
7801.1SchristosDivisions of Countries ("Statoids")</a> lists
7811.1Schristospolitical subdivision data related to time zones.</li>
7821.7Schristos<li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
7831.1Schristoszone boundaries for multizone countries</a> summarizes legal
7841.1Schristosboundaries between time zones within countries.</li>
7851.1Schristos<li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
7861.1Schristos&ndash; GIS and Database Tools</a> includes a Manifold-format map of
7871.1Schristosworld time zone boundaries distributed under the
7881.1Schristos<abbr>GPL</abbr>.</li>
7891.1Schristos<li>A ship within the <a
7901.1Schristoshref="https://en.wikipedia.org/wiki/Territorial_waters">territorial
7911.1Schristoswaters</a> of any nation uses that nation's time. In international
7921.1Schristoswaters, time zone boundaries are meridians 15&deg; apart, except that
7931.1Schristos<abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
7941.1Schristoswide and are separated by
7951.1Schristosthe 180&deg; meridian (not by the International Date Line, which is
7961.1Schristosfor land and territorial waters only). A captain can change ship's
7971.1Schristosclocks any time after entering a new time zone; midnight changes are
7981.1Schristoscommon.</li>
7991.1Schristos</ul>
8001.6Schristos</section>
8011.6Schristos
8021.6Schristos<section>
8031.1Schristos<h2 id="civil">Civil time concepts and history</h2>
8041.1Schristos<ul>
8051.1Schristos<li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
8061.1SchristosWalk through Time</a>
8071.1Schristossurveys the evolution of timekeeping.</li>
8081.4Schristos<li>The history of daylight saving time is surveyed in <a
8091.4Schristoshref="http://www.webexhibits.org/daylightsaving/">About Daylight
8101.4SchristosSaving Time &ndash; History, rationale, laws &amp; dates</a> and summarized in
8111.4Schristos<a href="http://seizethedaylight.com/dst/">A Brief
8121.4SchristosHistory of Daylight Saving Time</a>.</li>
8131.4Schristos<li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
8141.4SchristosLords</a> discusses how authoritarians manipulate civil time.</li>
8151.1Schristos<li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
8161.1Schristoscontains guidelines and best practices for software applications that
8171.1Schristosdeal with civil time.</li>
8181.1Schristos<li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
8191.1Schristosthe International Date Line</a> tells the story of the most important
8201.1Schristostime zone boundary.</li>
8211.1Schristos<li><a href="http://statoids.com/tconcept.html">Basic Time
8221.1SchristosZone Concepts</a> discusses terminological issues behind time zones.</li>
8231.1Schristos</ul>
8241.6Schristos</section>
8251.6Schristos
8261.6Schristos<section>
8271.1Schristos<h2 id="national">National histories of legal time</h2>
8281.1Schristos<dl>
8291.1Schristos<dt>Australia</dt>
8301.6Schristos<dd>The Parliamentary Library commissioned a <a
8311.1Schristoshref="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
8321.1Schristospaper on daylight saving time in Australia</a>.
8331.1SchristosThe Bureau of Meteorology publishes a list of <a
8341.1Schristoshref="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
8351.1SchristosDates of Daylight Savings Time within Australia</a>.</dd>
8361.1Schristos<dt>Belgium</dt>
8371.7Schristos<dd>The Royal Observatory of Belgium maintains a table of time in
8381.7SchristosBelgium (in
8391.7Schristos<a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
8401.7Schristoshreflang="nl">Dutch</a> and <a
8411.7Schristoshref="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
8421.7Schristoshreflang="fr">French</a>).</dd>
8431.1Schristos<dt>Brazil</dt>
8441.1Schristos<dd>The Time Service Department of the National Observatory
8451.1Schristosrecords <a href="http://pcdsh01.on.br/DecHV.html"
8461.1Schristoshreflang="pt-BR">Brazil's daylight saving time decrees (in
8471.1SchristosPortuguese)</a>.</dd>
8481.1Schristos<dt>Canada</dt>
8491.1Schristos<dd>National Research Council Canada publishes current
8501.1Schristosand some older information about <a
8511.6Schristoshref="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
8521.6Schristoszones and daylight saving time</a>.</dd>
8531.1Schristos<dt>Chile</dt>
8541.1Schristos<dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
8551.4Schristos<a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
8561.1SchristosChile's official time (in Spanish)</a>.</dd>
8571.3Schristos<dt>China</dt>
8581.3Schristos<dd>The Hong Kong Observatory maintains a
8591.7Schristos<a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
8601.3Schristos summer time in Hong Kong</a>,
8611.3Schristosand Macau's Meteorological and Geophysical Bureau maintains a <a
8621.7Schristoshref="https://www.smg.gov.mo/en/subpage/224/page/174">similar
8631.3Schristoshistory for Macau</a>.
8641.3SchristosUnfortunately the latter is incomplete and has errors.</dd>
8651.1Schristos<dt>Czech Republic</dt>
8661.1Schristos<dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
8671.1Schristoshreflang="cs">When daylight saving time starts and ends (in Czech)</a>
8681.1Schristossummarizes and cites historical DST regulations.</dd>
8691.1Schristos<dt>Germany</dt>
8701.1Schristos<dd>The National Institute for Science and Technology maintains the <a
8711.1Schristoshref="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
8721.1Schristosof Legal Time in Germany</a>.</dd>
8731.1Schristos<dt>Israel</dt>
8741.1Schristos<dd>The Interior Ministry periodically issues <a
8751.1Schristoshref="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
8761.1Schristoshreflang="he">announcements (in Hebrew)</a>.</dd>
8771.2Schristos<dt>Malaysia</dt>
8781.2Schristos<dd>See Singapore <a href="#Singapore">below</a>.</dd>
8791.1Schristos<dt>Mexico</dt>
8801.1Schristos<dd>The Investigation and Analysis Service of the Mexican Library of
8811.1SchristosCongress has published a <a
8821.1Schristoshref="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
8831.1Schristoshreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
8841.1Schristos<dt>Netherlands</dt>
8851.1Schristos<dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
8861.1Schristoshreflang="nl">Legal time in the Netherlands (in Dutch)</a>
8871.1Schristoscovers the history of local time in the Netherlands from ancient times.</dd>
8881.1Schristos<dt>New Zealand</dt>
8891.1Schristos<dd>The Department of Internal Affairs maintains a brief <a
8901.1Schristoshref="https://www.dia.govt.nz/Daylight-Saving-History">History of
8911.6SchristosDaylight Saving</a>.</dd>
8921.1Schristos<dt>Singapore</dt>
8931.1Schristos<dd><a id="Singapore"
8941.7Schristoshref="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
8951.1Schristosis Singapore in the "Wrong" Time Zone?</a> details the
8961.1Schristoshistory of legal time in Singapore and Malaysia.</dd>
8971.1Schristos<dt>United Kingdom</dt>
8981.1Schristos<dd><a
8991.1Schristoshref="https://www.polyomino.org.uk/british-time/">History of
9001.1Schristoslegal time in Britain</a> discusses in detail the country
9011.6Schristoswith perhaps the best-documented history of clock adjustments.</dd>
9021.1Schristos<dt>United States</dt>
9031.1Schristos<dd>The Department of Transportation's <a
9041.1Schristoshref="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
9051.1SchristosTime Zone Proceedings</a> lists changes to time zone boundaries.</dd>
9061.1Schristos<dt>Uruguay</dt>
9071.1Schristos<dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
9081.1SchristosNavy (SOHMA) publishes an annual <a
9091.7Schristoshref="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
9101.1Schristos(in Spanish)</a>.</dd>
9111.1Schristos</dl>
9121.6Schristos</section>
9131.6Schristos
9141.6Schristos<section>
9151.7Schristos<h2 id="costs">Costs and benefits of time shifts</h2>
9161.7Schristos<p>Various sources argue for and against daylight saving time and time
9171.7Schristoszone shifts, and many scientific studies have been conducted. This
9181.8Schristossection summarizes reviews and position statements based on
9191.8Schristosscientific literature in the area.</p>
9201.7Schristos<ul>
9211.7Schristos<li>Carey RN, Sarma KM.
9221.7Schristos<a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
9231.7Schristosdaylight saving time on road traffic collision risk: a systematic
9241.7Schristosreview</a>.
9251.7Schristos<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>.
9261.7SchristosThis reviews research literature and concludes that the evidence
9271.7Schristosneither supports nor refutes road safety benefits from
9281.7Schristosshifts in time zones.</li>
9291.7Schristos<li>Havranek T, Herman D, Irsova D.
9301.7Schristos<a href="https://www.econstor.eu/handle/10419/174191">Does daylight
9311.7Schristossaving save electricity? A meta-analysis</a>. <em>Energy J.</em>
9321.7Schristos2018;39(2).
9331.7Schristosdoi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
9341.7SchristosThis analyzes research literature and concludes, "Electricity savings
9351.7Schristosare larger for countries farther away from the equator, while
9361.7Schristossubtropical regions consume more electricity because of DST."</li>
9371.8Schristos<li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
9381.8Schristos<a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
9391.8Schristosan American Academy of Sleep Medicine position statement</a>.
9401.8Schristos<em>J Clin Sleep Med.</em>
9411.8Schristos2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
9421.8SchristosThis argues for permanent standard time due to health risks of both
9431.8SchristosDST transitions and permanent DST.</li>
9441.7Schristos<li>Roenneberg T, Winnebeck EC, Klerman EB.
9451.7Schristos<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
9461.7Schristossaving time and artificial time zones &ndash; a battle between
9471.7Schristosbiological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
9481.7Schristosdoi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
9491.7SchristosThis reviews evidence about the health effects of DST and concludes,
9501.7Schristos"In summary, the scientific literature strongly argues against the
9511.7Schristosswitching between DST and Standard Time and even more so against
9521.7Schristosadopting DST permanently."</li>
9531.7Schristos</ul>
9541.7Schristos</section>
9551.7Schristos
9561.7Schristos<section>
9571.1Schristos<h2 id="precision">Precision timekeeping</h2>
9581.1Schristos<ul>
9591.1Schristos<li><a
9601.1Schristoshref="http://leapsecond.com/hpan/an1289.pdf">The
9611.1SchristosScience of Timekeeping</a> is a thorough introduction
9621.1Schristosto the theory and practice of precision timekeeping.</li>
9631.1Schristos<li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
9641.1SchristosTime 2016</a> contains several freely-readable papers.</li>
9651.1Schristos<li><a href="http://www.ntp.org"><abbr
9661.1Schristostitle="Network Time Protocol">NTP</abbr>: The Network
9671.1SchristosTime Protocol</a> (Internet <abbr>RFC</abbr> 5905)
9681.1Schristosdiscusses how to synchronize clocks of
9691.1SchristosInternet hosts.</li>
9701.2Schristos<li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
9711.2Schristosfamily of software algorithms can achieve accuracy to a few tens of
9721.2Schristosnanoseconds in scalable server farms without special hardware.</li>
9731.1Schristos<li>The <a
9741.1Schristoshref="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
9751.1SchristosTime Protocol</a> (<abbr
9761.1Schristostitle="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
9771.2Schristoscan achieve submicrosecond clock accuracy on a local area network
9781.2Schristoswith special-purpose hardware.</li>
9791.1Schristos<li><a
9801.1Schristoshref="https://tools.ietf.org/html/rfc4833">Timezone
9811.1SchristosOptions for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
9821.1Schristos(Internet <abbr>RFC</abbr> 4833)
9831.1Schristosspecifies a <a
9841.1Schristoshref="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
9851.1Schristosoption for a server to configure
9861.1Schristosa client's time zone and daylight saving settings automatically.</li>
9871.1Schristos<li><a
9881.1Schristoshref="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
9891.1SchristosTimes</a> explains more abstruse astronomical time scales like
9901.1Schristos<abbr title="Terrestrial Dynamic Time">TDT</abbr>,
9911.1Schristos<abbr title="Geocentric Coordinate Time">TCG</abbr>, and
9921.1Schristos<abbr title="Barycentric Dynamic Time">TDB</abbr>.
9931.1Schristos<a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
9941.1SchristosScales</a> goes into more detail, particularly for historical variants.</li>
9951.1Schristos<li>The <a href="https://www.iau.org"><abbr
9961.1Schristostitle="International Astronomical Union">IAU</abbr></a>'s <a
9971.1Schristoshref="http://www.iausofa.org"><abbr
9981.1Schristostitle="Standards Of Fundamental Astronomy">SOFA</abbr></a>
9991.1Schristoscollection contains C and <a
10001.1Schristoshref="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
10011.1Schristoscode for converting among time scales like
10021.1Schristos<abbr title="International Atomic Time">TAI</abbr>,
10031.1Schristos<abbr>TDB</abbr>, <abbr>TDT</abbr> and
10041.1Schristos<abbr>UTC</abbr>.</li>
10051.1Schristos<li><a
10061.1Schristoshref="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
10071.1Schristos&ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
10081.1Schristosdiverse local time
10091.1Schristosscales used by each landed mission on Mars.</li>
10101.1Schristos<li><a href="http://leapsecond.com">LeapSecond.com</a> is
10111.1Schristosdedicated not only to leap seconds but to precise time and frequency
10121.1Schristosin general. It covers the state of the art in amateur timekeeping, and
10131.1Schristoshow the art has progressed over the past few decades.</li>
10141.7Schristos<li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
10151.7Schristoshref="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
10161.7Schristosand time-signal emissions</a>, International Telecommunication Union &ndash;
10171.7SchristosRadiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
10181.1Schristos<li><a
10191.1Schristoshref="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
10201.1Schristostitle="International Earth Rotation and Reference Systems Service">IERS</abbr>
10211.1SchristosBulletins</a> contains official publications of the International
10221.1SchristosEarth Rotation and Reference Systems Service, which decides when leap
10231.4Schristosseconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
10241.9Schristosvia an optional "<code>right</code>" configuration where a computer's internal
10251.9Schristos<code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
10261.9Schristosas opposed to the default "<code>posix</code>" configuration
10271.9Schristoswhere the internal clock ignores leap seconds.
10281.9SchristosThe two configurations agree for timestamps starting with 1972-01-01 00:00:00
10291.9Schristos<abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge for
10301.9Schristostimestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,
10311.9Schristoswhich corresponds to the first leap second
10321.9Schristos1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
10331.9Schristosand to
10341.9Schristos1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
10351.9SchristosIn practice the two configurations also agree for timestamps before
10361.9Schristos1972 even though the historical situation is messy, partly because
10371.9Schristosneither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
10381.9Schristosis well-defined for sufficiently-old timestamps.</li>
10391.1Schristos<li><a href="https://developers.google.com/time/smear">Leap Smear</a>
10401.1Schristosdiscusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
10411.1Schristosleap second so that they disagree with <abbr>UTC</abbr> by at most a
10421.1Schristoshalf second, even though every <abbr>POSIX</abbr> minute has exactly
10431.4Schristossixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
10441.1Schristos"<code>posix</code>" configuration, is <a
10451.1Schristoshref="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
10461.1Schristosthe <abbr>NTP</abbr> reference implementation, and is used by major
10471.7Schristoscloud service providers. However, according to
10481.7Schristos<a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">&sect;3.7.1 of
10491.7SchristosNetwork Time Protocol Best Current Practices</a>
10501.7Schristos(Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
10511.9Schristosapplications requiring accurate <abbr>UTC</abbr> or civil time,
10521.7Schristosand is intended for use only in single, well-controlled environments.</li>
10531.1Schristos<li>The <a
10541.1Schristoshref="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
10551.1SchristosSecond Discussion List</a> covers <a
10561.1Schristoshref="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
10571.1Schristosand Klepczynski's 1999 proposal to discontinue leap seconds</a>,
10581.1Schristosdiscussed further in
10591.1Schristos<a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
10601.1Schristosleap second: its history and possible future</a>.
10611.1Schristos<a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
10621.1Schristosmight be redefined
10631.1Schristoswithout Leap Seconds</a> gives pointers on this
10641.1Schristoscontentious issue, which was active until 2015 and could become active
10651.1Schristosagain.</li>
10661.1Schristos</ul>
10671.6Schristos</section>
10681.6Schristos
10691.6Schristos<section>
10701.1Schristos<h2 id="notation">Time notation</h2>
10711.1Schristos<ul>
10721.1Schristos<li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
10731.1SchristosRepository (<abbr>CLDR</abbr>) Project</a> has localizations for time
10741.1Schristoszone names, abbreviations, identifiers, and formats. For example, it
10751.1Schristoscontains French translations for "Eastern European Summer Time",
10761.1Schristos"<abbr title="Eastern European Summer Time">EEST</abbr>", and
10771.1Schristos"Bucharest". Its
10781.1Schristos<a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
10791.1Schristoscharts</a> show these values for many locales. Data values are available in
10801.1Schristosboth <abbr title="Locale Data Markup Language">LDML</abbr>
10811.1Schristos(an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
10821.1Schristos<li>
10831.1Schristos<a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
10841.6Schristosthe international standard date and time notation</a> covers
10851.1Schristos<a
10861.6Schristoshref="https://www.iso.org/standard/70907.html"><em><abbr
10871.6Schristostitle="International Organization for Standardization">ISO</abbr>
10881.6Schristos8601-1:2019 &ndash; Date and time &ndash; Representations for information
10891.6Schristosinterchange &ndash; Part 1: Basic rules</em></a>.</li>
10901.1Schristos<li>
10911.6Schristos<a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
10921.1SchristosSchema: Datatypes &ndash; dateTime</a> specifies a format inspired by
10931.1Schristos<abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
10941.1Schristos<li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">&sect;3.3 of
10951.1SchristosInternet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
10961.1Schristosspecifies the time notation used in email and <a
10971.1Schristoshref="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
10981.1Schristosheaders.</li>
10991.1Schristos<li>
11001.1Schristos<a href="https://tools.ietf.org/html/rfc3339">Date and Time
11011.1Schristoson the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
11021.1Schristosspecifies an <abbr>ISO</abbr> 8601
11031.1Schristosprofile for use in new Internet
11041.1Schristosprotocols.</li>
11051.1Schristos<li>
11061.7Schristos<a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
11071.1SchristosFormats on the Web</a> surveys web- and Internet-oriented date and time
11081.1Schristosformats.</li>
11091.1Schristos<li>Alphabetic time zone abbreviations should not be used as unique
11101.1Schristosidentifiers for <abbr>UT</abbr> offsets as they are ambiguous in
11111.1Schristospractice. For example, in English-speaking North America
11121.1Schristos"<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
11131.1Schristosbut in China it denotes 8 hours ahead of <abbr>UT</abbr>,
11141.1Schristosand French-speaking North Americans prefer
11151.1Schristos"<abbr title="Heure Normale du Centre">HNC</abbr>" to
11161.1Schristos"<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
11171.2Schristosdatabase contains English abbreviations for many timestamps;
11181.1Schristosunfortunately some of these abbreviations were merely the database maintainers'
11191.1Schristosinventions, and these have been removed when possible.</li>
11201.1Schristos<li>Numeric time zone abbreviations typically count hours east of
11211.1Schristos<abbr>UT</abbr>, e.g., +09 for Japan and
11221.1Schristos&minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
11231.1Schristos<code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
11241.1SchristosFor example, one might use <code><abbr>TZ</abbr>="<abbr
11251.1Schristostitle="Japan Standard Time">JST</abbr>-9"</code> and
11261.1Schristos<code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
11271.1Schristosfor Japan and Hawaii, respectively. If the
11281.1Schristos<code><abbr>tz</abbr></code> database is available, it is usually better to use
11291.1Schristossettings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
11301.1Schristos<code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
11311.2Schristosconfusion, handle old timestamps better, and insulate you better from
11321.1Schristosany future changes to the rules. One should never set
11331.1Schristos<abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
11341.1Schristos<code>"GMT-9"</code>, though, since this would incorrectly imply that
11351.1Schristoslocal time is nine hours ahead of <abbr>UT</abbr> and the time zone
11361.1Schristosis called "<abbr>GMT</abbr>".</li>
11371.1Schristos</ul>
11381.6Schristos</section>
11391.6Schristos
11401.6Schristos<section>
11411.1Schristos<h2 id="see-also">See also</h2>
11421.1Schristos<ul>
11431.7Schristos<li><a href="theory.html">Theory and pragmatics of the
11441.7Schristos<code><abbr>tz</abbr></code> code and data</a></li>
11451.1Schristos<li><a href="tz-art.html">Time and the Arts</a></li>
11461.1Schristos</ul>
11471.6Schristos</section>
11481.6Schristos
11491.6Schristos<footer>
11501.1Schristos<hr>
11511.1SchristosThis web page is in the public domain, so clarified as of
11521.1Schristos2009-05-17 by Arthur David Olson.
11531.1Schristos<br>
11541.1SchristosPlease send corrections to this web page to the
11551.1Schristos<a href="mailto:tz@iana.org">time zone mailing list</a>.
11561.6Schristos</footer>
11571.1Schristos</body>
11581.1Schristos</html>
1159