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41.1Schristos<title>Sources for time zone and daylight saving time data</title>
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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>,
2481.1Schristos<a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr></a>
2491.1Schristosand <a
2501.4Schristoshref="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a>
2511.4SchristosJava, <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
2521.1Schristoshref="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
2531.1Schristosand <a
2541.1Schristoshref="https://nodatime.org/userguide/tzdb">Noda Time</a> (see below).
2551.1Schristos</p>
2561.1Schristos<p>Sources for the <code><abbr>tz</abbr></code> database are
2571.1Schristos<a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
2581.1Schristostitle="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
2591.1Schristos<a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
2601.1Schristoswith lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
2611.1Schristostitle="linefeed">LF</abbr></a>,
2621.1Schristoswhich can be modified by common text editors such
2631.1Schristosas <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
2641.1Schristos<a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
2651.1Schristos<a href="https://www.vim.org">vim</a>.
2661.1SchristosSpecialized source-file editing can be done via the
2671.1Schristos<a href="https://packagecontrol.io/packages/zoneinfo">Sublime
2681.1Schristoszoneinfo</a> package for <a
2691.1Schristoshref="https://www.sublimetext.com">Sublime Text</a> and the <a
2701.1Schristoshref="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
2711.1Schristoszoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
2721.1SchristosStudio Code</a>.
2731.1Schristos</p>
2741.1Schristos<p>
2751.1SchristosFor further information about updates, please see
2761.1Schristos<a href="https://tools.ietf.org/html/rfc6557">Procedures for
2771.1SchristosMaintaining the Time Zone Database</a> (Internet <abbr
2781.1Schristostitle="Request For Comments">RFC</abbr> 6557). More detail can be
2791.7Schristosfound in <a href="theory.html">Theory and pragmatics of the
2801.7Schristos<code><abbr>tz</abbr></code> code and data</a>.
2811.2Schristos<a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
2821.2Schristosdisplays changes between recent <code><abbr>tzdb</abbr></code> versions.
2831.1Schristos</p>
2841.6Schristos</section>
2851.6Schristos
2861.6Schristos<section>
2871.1Schristos<h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
2881.1Schristos<ul>
2891.1Schristos<li>The article
2901.1Schristos<a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
2911.1Schristosan encyclopedic summary.</li>
2921.1Schristos<li><a href="tz-how-to.html">How to Read the
2931.1Schristostz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
2941.1Schristosdatabase format.</li>
2951.1Schristos<li><a
2961.1Schristoshref="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
2971.1Schristosliterary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
2981.1Schristosdatabase's style.</li>
2991.1Schristos</ul>
3001.6Schristos</section>
3011.6Schristos
3021.6Schristos<section>
3031.1Schristos<h2 id="web">Web sites using recent versions of the
3041.1Schristos<code><abbr>tz</abbr></code> database</h2>
3051.1Schristos<p>
3061.1SchristosThese are listed roughly in ascending order of complexity and fanciness.
3071.1Schristos</p>
3081.1Schristos<ul>
3091.1Schristos<li><a href="https://time.is">Time.is</a> shows locations'
3101.1Schristostime and zones.</li>
3111.1Schristos<li><a href="https://www.timejones.com">TimeJones.com</a>,
3121.1Schristos<a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
3131.7Schristos<a href="https://www.worldclock.com">The World Clock</a>
3141.1Schristosare time zone converters.</li>
3151.8Schristos<li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
3161.8Schristospublishes <code><abbr>tzdb</abbr></code>-derived data in
3171.8Schristos<a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
3181.8Schristostitle="comma-separated values">CSV</abbr></a> and
3191.8Schristosin <a href="https://en.wikipedia.org/wiki/SQL"><abbr
3201.8Schristostitle="Structured Query Language">SQL</abbr></a> form.</li>
3211.1Schristos<li><a
3221.6Schristoshref="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
3231.1Schristoslets you see the <code><abbr>TZ</abbr></code> values directly.</li>
3241.1Schristos<li><a
3251.1Schristoshref="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
3261.1SchristosTime in 1000 Places</a> uses descriptions of the values.</li>
3271.6Schristos<li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
3281.1Schristosuses a pulldown menu.</li>
3291.7Schristos<li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
3301.1Schristostimezone information for all countries</a> displays tables of DST rules.
3311.1Schristos<li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
3321.1SchristosWorldwide</a> lets you sort zone names and convert times.</li>
3331.1Schristos<li><a href="https://24timezones.com">24TimeZones</a> has a world
3341.1Schristostime map and a time converter.</li>
3351.1Schristos<li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
3361.1Schristoscalculates the current time difference between locations.</li>
3371.1Schristos<li><a href="http://www.wx-now.com">Weather Now</a> and
3381.1Schristos<a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
3391.1Schristos</ul>
3401.6Schristos</section>
3411.6Schristos
3421.6Schristos<section>
3431.1Schristos<h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
3441.1Schristos<ul>
3451.1Schristos<li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
3461.1Schristos<a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
3471.1SchristosDistribution Service (tzdist) working group</a> defined <a
3481.1Schristoshref="https://tools.ietf.org/html/rfc7808">TZDIST</a>
3491.1Schristos(Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
3501.1Schristosalong with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
3511.1Schristos(Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
3521.2Schristostransferring time zone data by reference.
3531.8Schristos<a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
3541.8Schristosimplementations</a> are available.
3551.2SchristosThe <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
3561.6Schristosmailing list</a> discussed the Internet draft <a
3571.1Schristosid="TZDIST-Geolocate"
3581.2Schristoshref="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate">TZDIST
3591.6SchristosGeolocate Extension</a> (now expired), which let a client determine its timezone
3601.1Schristosfrom its geographic location using a <a
3611.5Schristoshref="https://tools.ietf.org/html/rfc5870">'geo' URI</a>.</li>
3621.1Schristos<li>The <a href="https://tools.ietf.org/html/rfc5545">
3631.1SchristosInternet Calendaring and Scheduling Core Object Specification
3641.1Schristos(iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
3651.1Schristoscovers time zone
3661.1Schristosdata; see its VTIMEZONE calendar component.
3671.1SchristosThe iCalendar format requires specialized parsers and generators; a
3681.1Schristosvariant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
3691.1Schristos(Internet <abbr>RFC</abbr> 6321) uses
3701.1Schristos<a href="https://www.w3.org/XML/"><abbr
3711.1Schristostitle="Extensible Markup Language">XML</abbr></a> format, and a variant
3721.1Schristos<a href="https://tools.ietf.org/html/rfc7265">jCal</a>
3731.1Schristos(Internet <abbr>RFC</abbr> 7265)
3741.1Schristosuses <a href="https://www.json.org"><abbr
3751.1Schristostitle="JavaScript Object Notation">JSON</abbr></a> format.</li>
3761.1Schristos</ul>
3771.6Schristos</section>
3781.6Schristos
3791.6Schristos<section>
3801.1Schristos<h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
3811.7Schristos<p>Although some of these do not fully support
3821.7Schristos<code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
3831.7Schristosdistributions you can generally work around compatibility problems by
3841.7Schristosrunning the command <code>make rearguard_tarballs</code> and compiling
3851.7Schristosfrom the resulting tarballs instead.</p>
3861.1Schristos<ul>
3871.1Schristos<li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
3881.1Schristoshref="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
3891.1Schristosprogram that compiles
3901.1Schristos<code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
3911.1SchristosVzic is freely
3921.1Schristosavailable under the <a
3931.1Schristoshref="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
3941.1SchristosGeneral Public License (<abbr
3951.1Schristostitle="General Public License">GPL</abbr>)</a>.</li>
3961.1Schristos<li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
3971.1Schristosdatabase conversion utility</a> is like Vzic, except for the <a
3981.6Schristoshref="https://dotnet.microsoft.com">.NET framework</a>
3991.1Schristosand with a <abbr>BSD</abbr>-style license.</li>
4001.1Schristos<li><a
4011.4Schristoshref="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
4021.1Schristoscontains a script <code>parse_olson</code> that compiles
4031.1Schristos<code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
4041.1Schristosmodules. It is part of the Perl <a
4051.4Schristoshref="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
4061.4Schristoswhich is freely
4071.1Schristosavailable under both the <abbr>GPL</abbr> and the Perl Artistic
4081.1SchristosLicense. DateTime::TimeZone also contains a script
4091.1Schristos<code>tests_from_zdump</code> that generates test cases for each clock
4101.1Schristostransition in the <code><abbr>tz</abbr></code> database.</li>
4111.1Schristos<li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
4121.1SchristosDatabase Parser</a> is a
4131.1Schristos<a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
4141.7Schristosruntime library with API <a
4151.7Schristoshref="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">adopted
4161.7Schristosinto the draft standard</a> for
4171.6Schristos<a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
4181.6Schristosthe next iteration of the C++ standard.
4191.1SchristosIt is freely available under the
4201.1Schristos<abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
4211.1Schristos<li><a id="ICU" href="http://site.icu-project.org">International Components for
4221.1SchristosUnicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
4231.1Schristoshref="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
4241.1Schristoslibraries for internationalization that
4251.1Schristoshas a compiler from <code><abbr>tz</abbr></code> source
4261.1Schristosand from <abbr title="Common Locale Data Repository">CLDR</abbr> data
4271.1Schristos(mentioned <a href="#CLDR">below</a>)
4281.1Schristosinto an <abbr>ICU</abbr>-specific format.
4291.1Schristos<abbr>ICU</abbr> is freely available under a
4301.1Schristos<abbr>BSD</abbr>-style license.</li>
4311.1Schristos<li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
4321.1Schristosthe <a href="https://elixir-lang.org">Elixir</a> language downloads
4331.7Schristosand compiles <code><abbr>tz</abbr></code> source and exposes <abbr
4341.1Schristostitle="Application Program Interface">API</abbr>s for use. It is
4351.1Schristosfreely available under the <abbr>MIT</abbr> license.</li>
4361.1Schristos<li>Java-based compilers and libraries include:
4371.1Schristos<ul>
4381.1Schristos<li>The <a
4391.4Schristoshref="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
4401.1Schristostool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
4411.1SchristosOracle Java.</li>
4421.1Schristos<li>The <a
4431.4Schristoshref="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
4441.6SchristosSE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
4451.4Schristoshref="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
4461.1Schristoswhich is freely available under a <abbr>BSD</abbr>-style license.</li>
4471.4Schristos<li><a href="https://www.joda.org/joda-time/">Joda-Time &ndash; Java date
4481.1Schristosand time <abbr>API</abbr></a> contains a class
4491.1Schristos<code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
4501.1Schristos<code><abbr>tz</abbr></code> source into a binary format. It inspired
4511.1SchristosJava 8 <code>java.time</code>, which its users should migrate to once
4521.1Schristosthey can assume Java 8 or later. It is available under the <a
4531.1Schristoshref="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
4541.7Schristos<li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a>,
4551.7Schristos<a href="https://github.com/akashche/tzdbgen">tzdbgen</a>, and <a
4561.7Schristoshref="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
4571.7Schristosare other alternatives to TZUpdater. IANA Updater's license is unclear;
4581.7Schristosthe others are licensed under the <abbr>GPL</abbr>.</li>
4591.4Schristos<li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
4601.4Schristostime library for Android</a> and
4611.4Schristos<a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
4621.4Schristostime and interval library for Java</a> compile
4631.4Schristos<code><abbr>tz</abbr></code> source into a binary format.
4641.4SchristosTime4A is available under the Apache License and Time4J is
4651.1Schristosavailable under the <a
4661.1Schristoshref="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
4671.1SchristosGeneral Public License (<abbr title="Lesser General Public
4681.1SchristosLicense">LGPL</abbr>)</a>.</li>
4691.1Schristos<li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
4701.1SchristosJava-based libraries.</li>
4711.1Schristos</ul>
4721.1Schristos<li><a href="https://nodatime.org">Noda Time &ndash; Date and
4731.1Schristostime <abbr>API</abbr> for .NET</a>
4741.4Schristosis like Joda-Time and Time4J, but for the .NET framework instead of Java.
4751.4SchristosIt is freely available under the Apache License.</li>
4761.8Schristos<li>Many modern
4771.8Schristos<a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
4781.8Schristosruntimes support <code><abbr>tz</abbr></code> natively via the
4791.8Schristos<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>.
4801.8SchristosOn other runtimes, you can use one of the following.
4811.1Schristos<ul>
4821.8Schristos<li>The <a
4831.8Schristoshref="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
4841.8Schristostimezone polyfill</a>
4851.8Schristosis freely available under a <abbr>BSD</abbr>-style license.</li>
4861.1Schristos<li><a
4871.1Schristoshref="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
4881.1Schristoscompiles time zone data into a compact form designed for
4891.1SchristosJavaScript. It is freely available under a combination of
4901.1Schristosthe <abbr>MIT</abbr> license and the Apache License.</li>
4911.1Schristos<li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
4921.1Schristosplugin for the <a href="https://momentjs.com">Moment.js</a> date
4931.1Schristosmanipulation library. It is freely available under the <abbr>MIT</abbr>
4941.1Schristoslicense.</li>
4951.8Schristos<li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
4961.8SchristosJavaScript library that supports date arithmetic that is time zone
4971.8Schristosaware. It is freely available under the <abbr>MIT</abbr> license.</li>
4981.1Schristos<li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
4991.1Schristos<abbr>API</abbr> is upward compatible with standard JavaScript
5001.1SchristosDates. It is freely available under the Apache License.</li>
5011.1Schristos</ul>
5021.1Schristos<li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
5031.1Schristoscompiler from <code><abbr>tz</abbr></code> source into
5041.1Schristos<a href="https://julialang.org/">Julia</a>. It is freely available
5051.1Schristosunder the <abbr>MIT</abbr> license.</li>
5061.6Schristos<li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> &ndash;
5071.6Schristos<abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
5081.6Schristostitle="Free Pascal Compiler">FPC</abbr></a>
5091.2Schristoscompiles from <code><abbr>tz</abbr></code> source into
5101.2Schristos<a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
5111.2Schristosas compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
5121.2Schristosand <a
5131.2Schristoshref="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
5141.2SchristosIt is freely available under a <abbr>BSD</abbr>-style license.</li>
5151.1Schristos<li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
5161.1SchristosDefinitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
5171.1Schristos<a href="https://www.python.org">Python</a>.
5181.1SchristosIt is freely available under a <abbr>BSD</abbr>-style license.</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>
5821.8Schristos<li>Python has a <a
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>.
5891.8SchristosA companion <a 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><a
6261.1Schristoshref="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
6271.1Schristosclock (intclock)</a> is a clock that displays multiple time zones on
6281.1Schristos<abbr>GNU</abbr>/Linux and similar systems. It is freely available
6291.1Schristosunder the <abbr>GPL</abbr>.</li>
6301.1Schristos<li>Microsoft Windows 8.1
6311.1Schristosand later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
6321.3Schristosdata (mentioned <a href="#CLDR">below</a>) used by the
6331.3Schristos<a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
6341.3Schristos<a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
6351.3Schristos<a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
6361.3Schristos<a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
6371.1Schristos<a id="System.TimeZoneInfo"
6381.1Schristoshref="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
6391.1SchristosWindows Time Zones with <code>System.TimeZoneInfo</code></a> describes
6401.1Schristosthe older, proprietary method of Microsoft Windows 2000 and later,
6411.1Schristoswhich stores time zone data in the
6421.1Schristos<a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
6431.1Schristos<a
6441.1Schristoshref="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
6451.1SchristosTzid table</a> or <a
6461.6Schristoshref="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
6471.1Schristosfile</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
6481.1Schristosto <code><abbr>tz</abbr></code> names.
6491.3SchristosThese mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
6501.3Schristosor the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
6511.1Schristos<li><a
6521.1Schristoshref="https://www.oracle.com/java/index.html">Oracle
6531.1SchristosJava</a> contains a copy of a subset of a recent
6541.1Schristos<code><abbr>tz</abbr></code> database in a
6551.1SchristosJava-specific format.</li>
6561.2Schristos<li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone
6571.1SchristosMaster</a> is a Microsoft Windows clock program that can automatically
6581.4Schristosdownload, compile and use <code><abbr>tz</abbr></code> releases.
6591.4SchristosThe Basic version is free.</li>
6601.1Schristos<li><a
6611.1Schristoshref="http://veladg.com/velaterra.html">VelaTerra</a> is
6621.1Schristosa macOS program. Its developers
6631.1Schristos<a href="http://veladg.com/tzoffer.html">offer free
6641.1Schristoslicenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
6651.1Schristos</ul>
6661.6Schristos</section>
6671.6Schristos
6681.6Schristos<section>
6691.1Schristos<h2 id="other-dbs">Other time zone databases</h2>
6701.1Schristos<ul>
6711.1Schristos<li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
6721.4Schristosis Astrodienst's Web version of Shanks and Pottenger's out-of-print
6731.4Schristostime zone history atlases
6741.4Schristos<a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
6751.4Schristos<a href="https://www.worldcat.org/oclc/76950459">for the world</a>,
6761.4Schristosnow published in <a
6771.1Schristoshref="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a>
6781.1Schristosform by <a href="https://astrocom.com">ACS-Starcrafts</a>.
6791.4SchristosAlthough these extensive atlases
6801.4Schristos<a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
6811.4Schristossources for much of the older <code><abbr>tz</abbr></code> data</a>,
6821.4Schristosthey are unreliable as Shanks appears to have
6831.1Schristosguessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
6841.1Schristossources and do not indicate which entries are guesswork.</li>
6851.1Schristos<li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
6861.1Schristosits own <code>tztab</code>(4) format.</li>
6871.1Schristos<li>Microsoft Windows has proprietary data mentioned
6881.1Schristos<a href="#System.TimeZoneInfo">above</a>.</li>
6891.1Schristos<li><a href="https://www.worldtimeserver.com">World Time Server</a>
6901.1Schristosis another time zone database.</li>
6911.1Schristos<li>The <a
6921.1Schristoshref="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
6931.1SchristosSchedules Information Manual</a> of the
6941.1SchristosInternational Air Transport Association
6951.1Schristosgives current time zone rules for airports served by commercial aviation.</li>
6961.1Schristos</ul>
6971.6Schristos</section>
6981.6Schristos
6991.6Schristos<section>
7001.1Schristos<h2 id="maps">Maps</h2>
7011.1Schristos<ul>
7021.1Schristos<li>The <a href="https://www.cia.gov/index.html">United States Central
7031.1SchristosIntelligence Agency (<abbr
7041.1Schristostitle="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
7051.1Schristoshref="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
7061.1Schristoszone map</a>; the
7071.1Schristos<a
7081.6Schristoshref="https://legacy.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
7091.1SchristosLibrary Map Collection</a>
7101.1Schristosof the University of Texas at Austin has copies of
7111.1Schristosrecent editions.
7121.1SchristosThe pictorial quality is good,
7131.1Schristosbut the maps do not indicate daylight saving time,
7141.1Schristosand parts of the data are a few years out of date.</li>
7151.6Schristos<li><a href="https://www.worldtimezone.com">World Time Zone Map
7161.6Schristoswith current time</a>
7171.1Schristoshas several fancy time zone maps; it covers Russia particularly well.
7181.1SchristosThe maps' pictorial quality is not quite as good as the
7191.1Schristos<abbr>CIA</abbr>'s
7201.1Schristosbut the maps are more up to date.</li>
7211.1Schristos<li><a
7221.1Schristoshref="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
7231.1Schristosmuch is time wrong around the world?</a> maps the difference between
7241.1Schristosmean solar and standard time, highlighting areas such as western China
7251.1Schristoswhere the two differ greatly. It's a bit out of date, unfortunately.</li>
7261.1Schristos</ul>
7271.6Schristos</section>
7281.6Schristos
7291.6Schristos<section>
7301.1Schristos<h2 id="boundaries">Time zone boundaries</h2>
7311.2Schristos<p>Geographical boundaries between timezones are available
7321.1Schristosfrom several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a>
7331.1Schristosservices and other sources.</p>
7341.1Schristos<ul>
7351.1Schristos<li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
7361.1SchristosBoundary Builder</a> extracts
7371.1Schristos<a href="https://www.openstreetmap.org">Open Street Map</a> data to build
7381.2Schristosboundaries of <code><abbr>tzdb</abbr></code> timezones.
7391.1SchristosIts code is freely available under the <abbr>MIT</abbr> license, and
7401.1Schristosits data entries are freely available under the
7411.1Schristos<a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
7421.1SchristosOpen Database License</a>. The maps' borders appear to be quite accurate.</li>
7431.1Schristos<li>Programmatic interfaces that map geographical coordinates via tz_world to
7441.2Schristos<code><abbr>tzdb</abbr></code> timezones include:
7451.1Schristos<ul>
7461.1Schristos<li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
7471.1Schristoswritten in <a
7481.1Schristoshref="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
7491.1Schristosand is freely available under the <abbr>MIT</abbr> license.</li>
7501.1Schristos<li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
7511.1Schristosis written in Go and is freely available under the Apache License.</li>
7521.1Schristos<li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
7531.1Schristosin both Java and
7541.1Schristos<a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
7551.1Schristosform, is freely available under the MIT license.</li>
7561.1Schristos<li>For <a href="https://nodejs.org/en/">Node.js</a>,
7571.1Schristosthe <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
7581.1Schristosis freely available under the MIT license, and
7591.1Schristosthe <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
7601.1Schristosis in the public domain.</li>
7611.1Schristos<li>The <a
7621.1Schristoshref="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
7631.1Schristoslibrary for Python is freely available under the MIT license.
7641.1Schristos<li>The <a
7651.1Schristoshref="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
7661.1Schristoslibrary for Ruby is freely available under the MIT license.</li>
7671.1Schristos</ul></li>
7681.1Schristos<li>Free access via a network API, if you register a key, is provided by
7691.4Schristosthe <a
7701.4Schristoshref="https://www.geonames.org/export/web-services.html#timezone">GeoNames
7711.4SchristosTimezone web service</a>, the <a
7721.4Schristoshref="https://developers.google.com/maps/documentation/timezone/intro">Google
7731.4SchristosMaps Time Zone API</a>, and
7741.8Schristosthe <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
7751.1SchristosCommercial network API access is provided
7761.1Schristosby <a href="https://askgeo.com">AskGeo</a>
7771.1Schristosand <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
7781.1Schristos</li>
7791.1Schristos<li>"<a
7801.1Schristoshref="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
7811.1Schristosto get a time zone from a location using latitude and longitude
7821.1Schristoscoordinates?</a>" discusses other geolocation possibilities.</li>
7831.1Schristos<li><a href="http://statoids.com/statoids.html">Administrative
7841.1SchristosDivisions of Countries ("Statoids")</a> lists
7851.1Schristospolitical subdivision data related to time zones.</li>
7861.7Schristos<li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
7871.1Schristoszone boundaries for multizone countries</a> summarizes legal
7881.1Schristosboundaries between time zones within countries.</li>
7891.1Schristos<li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
7901.1Schristos&ndash; GIS and Database Tools</a> includes a Manifold-format map of
7911.1Schristosworld time zone boundaries distributed under the
7921.1Schristos<abbr>GPL</abbr>.</li>
7931.1Schristos<li>A ship within the <a
7941.1Schristoshref="https://en.wikipedia.org/wiki/Territorial_waters">territorial
7951.1Schristoswaters</a> of any nation uses that nation's time. In international
7961.1Schristoswaters, time zone boundaries are meridians 15&deg; apart, except that
7971.1Schristos<abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
7981.1Schristoswide and are separated by
7991.1Schristosthe 180&deg; meridian (not by the International Date Line, which is
8001.1Schristosfor land and territorial waters only). A captain can change ship's
8011.1Schristosclocks any time after entering a new time zone; midnight changes are
8021.1Schristoscommon.</li>
8031.1Schristos</ul>
8041.6Schristos</section>
8051.6Schristos
8061.6Schristos<section>
8071.1Schristos<h2 id="civil">Civil time concepts and history</h2>
8081.1Schristos<ul>
8091.1Schristos<li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
8101.1SchristosWalk through Time</a>
8111.1Schristossurveys the evolution of timekeeping.</li>
8121.4Schristos<li>The history of daylight saving time is surveyed in <a
8131.4Schristoshref="http://www.webexhibits.org/daylightsaving/">About Daylight
8141.4SchristosSaving Time &ndash; History, rationale, laws &amp; dates</a> and summarized in
8151.4Schristos<a href="http://seizethedaylight.com/dst/">A Brief
8161.4SchristosHistory of Daylight Saving Time</a>.</li>
8171.4Schristos<li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
8181.4SchristosLords</a> discusses how authoritarians manipulate civil time.</li>
8191.1Schristos<li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
8201.1Schristoscontains guidelines and best practices for software applications that
8211.1Schristosdeal with civil time.</li>
8221.1Schristos<li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
8231.1Schristosthe International Date Line</a> tells the story of the most important
8241.1Schristostime zone boundary.</li>
8251.1Schristos<li><a href="http://statoids.com/tconcept.html">Basic Time
8261.1SchristosZone Concepts</a> discusses terminological issues behind time zones.</li>
8271.1Schristos</ul>
8281.6Schristos</section>
8291.6Schristos
8301.6Schristos<section>
8311.1Schristos<h2 id="national">National histories of legal time</h2>
8321.1Schristos<dl>
8331.1Schristos<dt>Australia</dt>
8341.6Schristos<dd>The Parliamentary Library commissioned a <a
8351.1Schristoshref="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
8361.1Schristospaper on daylight saving time in Australia</a>.
8371.1SchristosThe Bureau of Meteorology publishes a list of <a
8381.1Schristoshref="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
8391.1SchristosDates of Daylight Savings Time within Australia</a>.</dd>
8401.1Schristos<dt>Belgium</dt>
8411.7Schristos<dd>The Royal Observatory of Belgium maintains a table of time in
8421.7SchristosBelgium (in
8431.7Schristos<a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
8441.7Schristoshreflang="nl">Dutch</a> and <a
8451.7Schristoshref="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
8461.7Schristoshreflang="fr">French</a>).</dd>
8471.1Schristos<dt>Brazil</dt>
8481.1Schristos<dd>The Time Service Department of the National Observatory
8491.1Schristosrecords <a href="http://pcdsh01.on.br/DecHV.html"
8501.1Schristoshreflang="pt-BR">Brazil's daylight saving time decrees (in
8511.1SchristosPortuguese)</a>.</dd>
8521.1Schristos<dt>Canada</dt>
8531.1Schristos<dd>National Research Council Canada publishes current
8541.1Schristosand some older information about <a
8551.6Schristoshref="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
8561.6Schristoszones and daylight saving time</a>.</dd>
8571.1Schristos<dt>Chile</dt>
8581.1Schristos<dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
8591.4Schristos<a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
8601.1SchristosChile's official time (in Spanish)</a>.</dd>
8611.3Schristos<dt>China</dt>
8621.3Schristos<dd>The Hong Kong Observatory maintains a
8631.7Schristos<a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
8641.3Schristos summer time in Hong Kong</a>,
8651.3Schristosand Macau's Meteorological and Geophysical Bureau maintains a <a
8661.7Schristoshref="https://www.smg.gov.mo/en/subpage/224/page/174">similar
8671.3Schristoshistory for Macau</a>.
8681.3SchristosUnfortunately the latter is incomplete and has errors.</dd>
8691.1Schristos<dt>Czech Republic</dt>
8701.1Schristos<dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
8711.1Schristoshreflang="cs">When daylight saving time starts and ends (in Czech)</a>
8721.1Schristossummarizes and cites historical DST regulations.</dd>
8731.1Schristos<dt>Germany</dt>
8741.1Schristos<dd>The National Institute for Science and Technology maintains the <a
8751.1Schristoshref="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
8761.1Schristosof Legal Time in Germany</a>.</dd>
8771.1Schristos<dt>Israel</dt>
8781.1Schristos<dd>The Interior Ministry periodically issues <a
8791.1Schristoshref="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
8801.1Schristoshreflang="he">announcements (in Hebrew)</a>.</dd>
8811.1Schristos<dt>Italy</dt>
8821.6Schristos<dd>The National Institute of Metrological Research publishes a
8831.4Schristos<a href="http://oldsite.inrim.it/res/tf/ora_legale_i.shtml" hreflang="it">table
8841.4Schristosof civil time (in Italian)</a>.</dd>
8851.2Schristos<dt>Malaysia</dt>
8861.2Schristos<dd>See Singapore <a href="#Singapore">below</a>.</dd>
8871.1Schristos<dt>Mexico</dt>
8881.1Schristos<dd>The Investigation and Analysis Service of the Mexican Library of
8891.1SchristosCongress has published a <a
8901.1Schristoshref="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
8911.1Schristoshreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
8921.1Schristos<dt>Netherlands</dt>
8931.1Schristos<dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
8941.1Schristoshreflang="nl">Legal time in the Netherlands (in Dutch)</a>
8951.1Schristoscovers the history of local time in the Netherlands from ancient times.</dd>
8961.1Schristos<dt>New Zealand</dt>
8971.1Schristos<dd>The Department of Internal Affairs maintains a brief <a
8981.1Schristoshref="https://www.dia.govt.nz/Daylight-Saving-History">History of
8991.6SchristosDaylight Saving</a>.</dd>
9001.1Schristos<dt>Singapore</dt>
9011.1Schristos<dd><a id="Singapore"
9021.7Schristoshref="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
9031.1Schristosis Singapore in the "Wrong" Time Zone?</a> details the
9041.1Schristoshistory of legal time in Singapore and Malaysia.</dd>
9051.1Schristos<dt>United Kingdom</dt>
9061.1Schristos<dd><a
9071.1Schristoshref="https://www.polyomino.org.uk/british-time/">History of
9081.1Schristoslegal time in Britain</a> discusses in detail the country
9091.6Schristoswith perhaps the best-documented history of clock adjustments.</dd>
9101.1Schristos<dt>United States</dt>
9111.1Schristos<dd>The Department of Transportation's <a
9121.1Schristoshref="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
9131.1SchristosTime Zone Proceedings</a> lists changes to time zone boundaries.</dd>
9141.1Schristos<dt>Uruguay</dt>
9151.1Schristos<dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
9161.1SchristosNavy (SOHMA) publishes an annual <a
9171.7Schristoshref="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
9181.1Schristos(in Spanish)</a>.</dd>
9191.1Schristos</dl>
9201.6Schristos</section>
9211.6Schristos
9221.6Schristos<section>
9231.7Schristos<h2 id="costs">Costs and benefits of time shifts</h2>
9241.7Schristos<p>Various sources argue for and against daylight saving time and time
9251.7Schristoszone shifts, and many scientific studies have been conducted. This
9261.8Schristossection summarizes reviews and position statements based on
9271.8Schristosscientific literature in the area.</p>
9281.7Schristos<ul>
9291.7Schristos<li>Carey RN, Sarma KM.
9301.7Schristos<a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
9311.7Schristosdaylight saving time on road traffic collision risk: a systematic
9321.7Schristosreview</a>.
9331.7Schristos<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>.
9341.7SchristosThis reviews research literature and concludes that the evidence
9351.7Schristosneither supports nor refutes road safety benefits from
9361.7Schristosshifts in time zones.</li>
9371.7Schristos<li>Havranek T, Herman D, Irsova D.
9381.7Schristos<a href="https://www.econstor.eu/handle/10419/174191">Does daylight
9391.7Schristossaving save electricity? A meta-analysis</a>. <em>Energy J.</em>
9401.7Schristos2018;39(2).
9411.7Schristosdoi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
9421.7SchristosThis analyzes research literature and concludes, "Electricity savings
9431.7Schristosare larger for countries farther away from the equator, while
9441.7Schristossubtropical regions consume more electricity because of DST."</li>
9451.8Schristos<li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
9461.8Schristos<a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
9471.8Schristosan American Academy of Sleep Medicine position statement</a>.
9481.8Schristos<em>J Clin Sleep Med.</em>
9491.8Schristos2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
9501.8SchristosThis argues for permanent standard time due to health risks of both
9511.8SchristosDST transitions and permanent DST.</li>
9521.7Schristos<li>Roenneberg T, Winnebeck EC, Klerman EB.
9531.7Schristos<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
9541.7Schristossaving time and artificial time zones &ndash; a battle between
9551.7Schristosbiological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
9561.7Schristosdoi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
9571.7SchristosThis reviews evidence about the health effects of DST and concludes,
9581.7Schristos"In summary, the scientific literature strongly argues against the
9591.7Schristosswitching between DST and Standard Time and even more so against
9601.7Schristosadopting DST permanently."</li>
9611.7Schristos</ul>
9621.7Schristos</section>
9631.7Schristos
9641.7Schristos<section>
9651.1Schristos<h2 id="precision">Precision timekeeping</h2>
9661.1Schristos<ul>
9671.1Schristos<li><a
9681.1Schristoshref="http://leapsecond.com/hpan/an1289.pdf">The
9691.1SchristosScience of Timekeeping</a> is a thorough introduction
9701.1Schristosto the theory and practice of precision timekeeping.</li>
9711.1Schristos<li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
9721.1SchristosTime 2016</a> contains several freely-readable papers.</li>
9731.1Schristos<li><a href="http://www.ntp.org"><abbr
9741.1Schristostitle="Network Time Protocol">NTP</abbr>: The Network
9751.1SchristosTime Protocol</a> (Internet <abbr>RFC</abbr> 5905)
9761.1Schristosdiscusses how to synchronize clocks of
9771.1SchristosInternet hosts.</li>
9781.2Schristos<li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
9791.2Schristosfamily of software algorithms can achieve accuracy to a few tens of
9801.2Schristosnanoseconds in scalable server farms without special hardware.</li>
9811.1Schristos<li>The <a
9821.1Schristoshref="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
9831.1SchristosTime Protocol</a> (<abbr
9841.1Schristostitle="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
9851.2Schristoscan achieve submicrosecond clock accuracy on a local area network
9861.2Schristoswith special-purpose hardware.</li>
9871.1Schristos<li><a
9881.1Schristoshref="https://tools.ietf.org/html/rfc4833">Timezone
9891.1SchristosOptions for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
9901.1Schristos(Internet <abbr>RFC</abbr> 4833)
9911.1Schristosspecifies a <a
9921.1Schristoshref="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
9931.1Schristosoption for a server to configure
9941.1Schristosa client's time zone and daylight saving settings automatically.</li>
9951.1Schristos<li><a
9961.1Schristoshref="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
9971.1SchristosTimes</a> explains more abstruse astronomical time scales like
9981.1Schristos<abbr title="Terrestrial Dynamic Time">TDT</abbr>,
9991.1Schristos<abbr title="Geocentric Coordinate Time">TCG</abbr>, and
10001.1Schristos<abbr title="Barycentric Dynamic Time">TDB</abbr>.
10011.1Schristos<a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
10021.1SchristosScales</a> goes into more detail, particularly for historical variants.</li>
10031.1Schristos<li>The <a href="https://www.iau.org"><abbr
10041.1Schristostitle="International Astronomical Union">IAU</abbr></a>'s <a
10051.1Schristoshref="http://www.iausofa.org"><abbr
10061.1Schristostitle="Standards Of Fundamental Astronomy">SOFA</abbr></a>
10071.1Schristoscollection contains C and <a
10081.1Schristoshref="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
10091.1Schristoscode for converting among time scales like
10101.1Schristos<abbr title="International Atomic Time">TAI</abbr>,
10111.1Schristos<abbr>TDB</abbr>, <abbr>TDT</abbr> and
10121.1Schristos<abbr>UTC</abbr>.</li>
10131.1Schristos<li><a
10141.1Schristoshref="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
10151.1Schristos&ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
10161.1Schristosdiverse local time
10171.1Schristosscales used by each landed mission on Mars.</li>
10181.1Schristos<li><a href="http://leapsecond.com">LeapSecond.com</a> is
10191.1Schristosdedicated not only to leap seconds but to precise time and frequency
10201.1Schristosin general. It covers the state of the art in amateur timekeeping, and
10211.1Schristoshow the art has progressed over the past few decades.</li>
10221.7Schristos<li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
10231.7Schristoshref="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
10241.7Schristosand time-signal emissions</a>, International Telecommunication Union &ndash;
10251.7SchristosRadiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
10261.1Schristos<li><a
10271.1Schristoshref="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
10281.1Schristostitle="International Earth Rotation and Reference Systems Service">IERS</abbr>
10291.1SchristosBulletins</a> contains official publications of the International
10301.1SchristosEarth Rotation and Reference Systems Service, which decides when leap
10311.4Schristosseconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
10321.1Schristosvia an optional "<code>right</code>" configuration, as opposed to the
10331.1Schristosdefault "<code>posix</code>" configuration.</li>
10341.1Schristos<li><a href="https://developers.google.com/time/smear">Leap Smear</a>
10351.1Schristosdiscusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
10361.1Schristosleap second so that they disagree with <abbr>UTC</abbr> by at most a
10371.1Schristoshalf second, even though every <abbr>POSIX</abbr> minute has exactly
10381.4Schristossixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
10391.1Schristos"<code>posix</code>" configuration, is <a
10401.1Schristoshref="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
10411.1Schristosthe <abbr>NTP</abbr> reference implementation, and is used by major
10421.7Schristoscloud service providers. However, according to
10431.7Schristos<a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">&sect;3.7.1 of
10441.7SchristosNetwork Time Protocol Best Current Practices</a>
10451.7Schristos(Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
10461.7Schristosapplications requiring accurate UTC or civil time,
10471.7Schristosand is intended for use only in single, well-controlled environments.</li>
10481.1Schristos<li>The <a
10491.1Schristoshref="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
10501.1SchristosSecond Discussion List</a> covers <a
10511.1Schristoshref="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
10521.1Schristosand Klepczynski's 1999 proposal to discontinue leap seconds</a>,
10531.1Schristosdiscussed further in
10541.1Schristos<a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
10551.1Schristosleap second: its history and possible future</a>.
10561.1Schristos<a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
10571.1Schristosmight be redefined
10581.1Schristoswithout Leap Seconds</a> gives pointers on this
10591.1Schristoscontentious issue, which was active until 2015 and could become active
10601.1Schristosagain.</li>
10611.1Schristos</ul>
10621.6Schristos</section>
10631.6Schristos
10641.6Schristos<section>
10651.1Schristos<h2 id="notation">Time notation</h2>
10661.1Schristos<ul>
10671.1Schristos<li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
10681.1SchristosRepository (<abbr>CLDR</abbr>) Project</a> has localizations for time
10691.1Schristoszone names, abbreviations, identifiers, and formats. For example, it
10701.1Schristoscontains French translations for "Eastern European Summer Time",
10711.1Schristos"<abbr title="Eastern European Summer Time">EEST</abbr>", and
10721.1Schristos"Bucharest". Its
10731.1Schristos<a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
10741.1Schristoscharts</a> show these values for many locales. Data values are available in
10751.1Schristosboth <abbr title="Locale Data Markup Language">LDML</abbr>
10761.1Schristos(an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
10771.1Schristos<li>
10781.1Schristos<a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
10791.6Schristosthe international standard date and time notation</a> covers
10801.1Schristos<a
10811.6Schristoshref="https://www.iso.org/standard/70907.html"><em><abbr
10821.6Schristostitle="International Organization for Standardization">ISO</abbr>
10831.6Schristos8601-1:2019 &ndash; Date and time &ndash; Representations for information
10841.6Schristosinterchange &ndash; Part 1: Basic rules</em></a>.</li>
10851.1Schristos<li>
10861.6Schristos<a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
10871.1SchristosSchema: Datatypes &ndash; dateTime</a> specifies a format inspired by
10881.1Schristos<abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
10891.1Schristos<li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">&sect;3.3 of
10901.1SchristosInternet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
10911.1Schristosspecifies the time notation used in email and <a
10921.1Schristoshref="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
10931.1Schristosheaders.</li>
10941.1Schristos<li>
10951.1Schristos<a href="https://tools.ietf.org/html/rfc3339">Date and Time
10961.1Schristoson the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
10971.1Schristosspecifies an <abbr>ISO</abbr> 8601
10981.1Schristosprofile for use in new Internet
10991.1Schristosprotocols.</li>
11001.1Schristos<li>
11011.7Schristos<a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
11021.1SchristosFormats on the Web</a> surveys web- and Internet-oriented date and time
11031.1Schristosformats.</li>
11041.1Schristos<li>Alphabetic time zone abbreviations should not be used as unique
11051.1Schristosidentifiers for <abbr>UT</abbr> offsets as they are ambiguous in
11061.1Schristospractice. For example, in English-speaking North America
11071.1Schristos"<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
11081.1Schristosbut in China it denotes 8 hours ahead of <abbr>UT</abbr>,
11091.1Schristosand French-speaking North Americans prefer
11101.1Schristos"<abbr title="Heure Normale du Centre">HNC</abbr>" to
11111.1Schristos"<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
11121.2Schristosdatabase contains English abbreviations for many timestamps;
11131.1Schristosunfortunately some of these abbreviations were merely the database maintainers'
11141.1Schristosinventions, and these have been removed when possible.</li>
11151.1Schristos<li>Numeric time zone abbreviations typically count hours east of
11161.1Schristos<abbr>UT</abbr>, e.g., +09 for Japan and
11171.1Schristos&minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
11181.1Schristos<code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
11191.1SchristosFor example, one might use <code><abbr>TZ</abbr>="<abbr
11201.1Schristostitle="Japan Standard Time">JST</abbr>-9"</code> and
11211.1Schristos<code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
11221.1Schristosfor Japan and Hawaii, respectively. If the
11231.1Schristos<code><abbr>tz</abbr></code> database is available, it is usually better to use
11241.1Schristossettings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
11251.1Schristos<code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
11261.2Schristosconfusion, handle old timestamps better, and insulate you better from
11271.1Schristosany future changes to the rules. One should never set
11281.1Schristos<abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
11291.1Schristos<code>"GMT-9"</code>, though, since this would incorrectly imply that
11301.1Schristoslocal time is nine hours ahead of <abbr>UT</abbr> and the time zone
11311.1Schristosis called "<abbr>GMT</abbr>".</li>
11321.1Schristos</ul>
11331.6Schristos</section>
11341.6Schristos
11351.6Schristos<section>
11361.1Schristos<h2 id="see-also">See also</h2>
11371.1Schristos<ul>
11381.7Schristos<li><a href="theory.html">Theory and pragmatics of the
11391.7Schristos<code><abbr>tz</abbr></code> code and data</a></li>
11401.1Schristos<li><a href="tz-art.html">Time and the Arts</a></li>
11411.1Schristos</ul>
11421.6Schristos</section>
11431.6Schristos
11441.6Schristos<footer>
11451.1Schristos<hr>
11461.1SchristosThis web page is in the public domain, so clarified as of
11471.1Schristos2009-05-17 by Arthur David Olson.
11481.1Schristos<br>
11491.1SchristosPlease send corrections to this web page to the
11501.1Schristos<a href="mailto:tz@iana.org">time zone mailing list</a>.
11511.6Schristos</footer>
11521.1Schristos</body>
11531.1Schristos</html>
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