libc/time/tz-link.html

1.1Schristos<!DOCTYPE html>
1.1Schristos<html lang="en">
1.1Schristos<head>
1.13Schristos<title>Time zone and daylight saving time data</title>
1.1Schristos<meta charset="UTF-8">
1.2Schristos<style>
1.2Schristospre {margin-left: 2em; white-space: pre-wrap;}
1.2Schristos</style>
1.1Schristos</head>
1.1Schristos<body>
1.13Schristos<h1>Time zone and daylight saving time data</h1>
1.1Schristos<p>
1.1Schristos<a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
1.1Schristos<a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
1.1Schristosrules are controlled by individual
1.1Schristosgovernments. They are sometimes changed with little notice, and their
1.1Schristoshistories and planned futures are often recorded only fitfully. Here
1.1Schristosis a summary of attempts to organize and record relevant data in this
1.1Schristosarea.
1.1Schristos</p>
1.6Schristos  <h3>Outline</h3>
1.6Schristos  <nav>
1.6Schristos    <ul>
1.6Schristos      <li>The <code><abbr>tz</abbr></code> database product and process
1.6Schristos	<ul>
1.6Schristos	  <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
1.6Schristos	  <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
1.6Schristos	  <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
1.14Schristos	  <li><a href="#coordinating">Coordinating with governments and distributors</a></li>
1.6Schristos	  <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
1.6Schristos	</ul>
1.6Schristos      </li>
1.6Schristos      <li>Uses of the <code><abbr>tz</abbr></code> database
1.6Schristos	<ul>
1.6Schristos	  <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
1.6Schristos	  <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
1.6Schristos	  <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
1.6Schristos	  <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
1.6Schristos	  <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
1.6Schristos	</ul>
1.6Schristos      </li>
1.6Schristos      <li>Related data
1.6Schristos	<ul>
1.6Schristos	  <li><a href="#other-dbs">Other time zone databases</a></li>
1.6Schristos	  <li><a href="#maps">Maps</a></li>
1.6Schristos	  <li><a href="#boundaries">Time zone boundaries</a></li>
1.6Schristos	</ul>
1.6Schristos      </li>
1.6Schristos      <li>Timekeeping concepts
1.6Schristos	<ul>
1.6Schristos	  <li><a href="#civil">Civil time concepts and history</a></li>
1.6Schristos	  <li><a href="#national">National histories of legal time</a></li>
1.7Schristos	  <li><a href="#costs">Costs and benefits of time shifts</a></li>
1.6Schristos	  <li><a href="#precision">Precision timekeeping</a></li>
1.6Schristos	  <li><a href="#notation">Time notation</a></li>
1.6Schristos	</ul>
1.6Schristos      </li>
1.13Schristos      <li><a href="#see-also">See also</a></li>
1.6Schristos    </ul>
1.6Schristos  </nav>
1.6Schristos
1.6Schristos<section>
1.1Schristos<h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
1.1Schristos<p>
1.1SchristosThe <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
1.1Schristostime zone database contains code and data
1.1Schristosthat represent the history of local time
1.1Schristosfor many representative locations around the globe.
1.1SchristosIt is updated periodically to reflect changes made by political bodies
1.1Schristosto time zone boundaries and daylight saving rules.
1.1SchristosThis database (known as <code><abbr>tz</abbr></code>,
1.1Schristos<code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
1.1Schristosis used by several implementations,
1.1Schristosincluding
1.1Schristos<a href="https://www.gnu.org/software/libc/">the
1.20Schristos<abbr title="GNU’s Not Unix">GNU</abbr>
1.1SchristosC Library</a> (used in
1.1Schristos<a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
1.1Schristos<a href="https://www.android.com">Android</a>,
1.1Schristos<a href="https://www.freebsd.org">Free<abbr
1.1Schristostitle="Berkeley Software Distribution">BSD</abbr></a>,
1.1Schristos<a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
1.1Schristos<a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
1.17Schristos<a href="https://www.chromium.org/chromium-os/">ChromiumOS</a>,
1.1Schristos<a href="https://cygwin.com">Cygwin</a>,
1.7Schristos<a href="https://mariadb.org">MariaDB</a>,
1.17Schristos<a href="https://musl.libc.org">musl libc</a>,
1.1Schristos<a href="https://www.mysql.com">MySQL</a>,
1.1Schristos<a href="https://en.wikipedia.org/wiki/WebOS"><abbr
1.1Schristostitle="Web Operating System">webOS</abbr></a>,
1.7Schristos<a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
1.1Schristostitle="Advanced Interactive eXecutive">AIX</abbr></a>,
1.6Schristos<a href="https://www.apple.com/ios"><abbr
1.1Schristostitle="iPhone OS">iOS</abbr></a>,
1.6Schristos<a href="https://www.apple.com/macos">macOS</a>,
1.1Schristos<a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
1.6Schristos<a href="https://www.vmssoftware.com">Open<abbr
1.1Schristostitle="Virtual Memory System">VMS</abbr></a>,
1.20Schristos<a href="https://www.oracle.com/database/">Oracle Database</a>,
1.20Schristos<a href="https://www.oracle.com/solaris">Oracle Solaris</a>,
1.20Schristosand <a href="https://blackberry.qnx.com/en">QNX</a>.</p>
1.1Schristos<p>
1.2SchristosEach main entry in the database represents a <dfn>timezone</dfn>
1.2Schristosfor a set of civil-time clocks that have all agreed since 1970.
1.2SchristosTimezones are typically identified by continent or ocean and then by the
1.2Schristosname of the largest city within the region containing the clocks.
1.1SchristosFor example, <code>America/New_York</code>
1.1Schristosrepresents most of the <abbr title="United States">US</abbr> eastern time zone;
1.1Schristos<code>America/Phoenix</code> represents most of Arizona, which
1.2Schristosuses mountain time without daylight saving time (<abbr>DST</abbr>);
1.1Schristos<code>America/Detroit</code> represents most of Michigan, which uses
1.1Schristoseastern time but with different <abbr>DST</abbr> rules in 1975;
1.1Schristosand other entries represent smaller regions like Starke County,
1.1SchristosIndiana, which switched from central to eastern time in 1991
1.1Schristosand switched back in 2006.
1.17SchristosTo use the database on a <a
1.1Schristoshref="https://en.wikipedia.org/wiki/POSIX"><abbr
1.17Schristostitle="Portable Operating System Interface">POSIX</abbr>.1-2024</a>
1.1Schristosimplementation set the <code><abbr>TZ</abbr></code>
1.20Schristosenvironment variable to the location’s full name,
1.1Schristose.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
1.1Schristos<p>
1.2SchristosAssociated with each timezone is a history of offsets from
1.1Schristos<a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
1.1SchristosTime</a> (<abbr>UT</abbr>), which is <a
1.1Schristoshref="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
1.1SchristosTime</a> (<abbr>GMT</abbr>) with days beginning at midnight;
1.2Schristosfor timestamps after 1960 this is more precisely <a
1.1Schristoshref="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
1.1SchristosUniversal Time</a> (<abbr>UTC</abbr>).
1.1SchristosThe database also records when daylight saving time was in use,
1.1Schristosalong with some time zone abbreviations such as <abbr>EST</abbr>
1.1Schristosfor Eastern Standard Time in the <abbr>US</abbr>.</p>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.1Schristos<h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
1.1Schristos<p>
1.1SchristosThe following <a
1.1Schristoshref="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
1.20Schristosthe latest release’s two
1.1Schristos<a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
1.1Schristosto a <abbr>GNU</abbr>/Linux or similar host.</p>
1.2Schristos<pre><code>mkdir tzdb
1.1Schristoscd tzdb
1.1Schristos<a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
1.1Schristoswget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
1.1Schristos<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 -
1.1Schristosgzip -dc tzdata-latest.tar.gz | tar -xf -
1.1Schristos</code></pre>
1.1Schristos<p>Alternatively, the following shell commands download the same
1.1Schristosrelease in a single-tarball format containing extra data
1.1Schristosuseful for regression testing:</p>
1.2Schristos<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>
1.1Schristos<a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
1.1Schristos</code></pre>
1.1Schristos<p>These commands use convenience links to the latest release
1.1Schristosof the <code><abbr>tz</abbr></code> database hosted by the
1.1Schristos<a href="https://www.iana.org/time-zones">Time Zone Database website</a>
1.1Schristosof the <a href="https://www.iana.org">Internet Assigned Numbers
1.1SchristosAuthority (IANA)</a>.
1.1SchristosOlder releases are in files named
1.1Schristos<code>tzcode<var>V</var>.tar.gz</code>,
1.1Schristos<code>tzdata<var>V</var>.tar.gz</code>, and
1.1Schristos<code>tzdb-<var>V</var>.tar.lz</code>,
1.1Schristoswhere <code><var>V</var></code> is the version.
1.1SchristosSince 1996, each version has been a four-digit year followed by
1.1Schristoslower-case letter (<samp>a</samp> through <samp>z</samp>,
1.1Schristosthen <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
1.1Schristosthrough <samp>zzz</samp>, and so on).
1.13SchristosSince version 2022a, each release has been distributed in
1.20Schristos<a href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/pax.html#tag_20_94_13_06">POSIX
1.10Schristosustar interchange format</a>, compressed as described above;
1.14Schristosolder releases use a nearly compatible format.
1.1SchristosSince version 2016h, each release has contained a text file named
1.20Schristos“<code>version</code>” whose first (and currently only) line is the version.
1.20Schristos<a href="https://ftp.iana.org/tz/releases/">Older archived releases are
1.20Schristosavailable</a> via
1.20Schristos<a href="https://en.wikipedia.org/wiki/HTTPS"><abbr
1.20Schristostitle="Hypertext Transfer Protocol Secure">HTTPS</abbr></a>,
1.20Schristos<a href="https://en.wikipedia.org/wiki/Rsync"><abbr
1.20Schristostitle="remote sync">rsync</abbr></a>, and
1.20Schristos<a href="https://en.wikipedia.org/wiki/FTP"><abbr
1.20Schristostitle="File Transfer Protocol">FTP</abbr></a>.
1.20Schristos<p>Alternatively, a development repository of code and data can bem
1.1Schristosretrieved from <a href="https://github.com">GitHub</a> via the shell
1.1Schristoscommand:</p>
1.2Schristos<pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
1.1Schristos</code></pre>
1.1Schristos<p>
1.1SchristosSince version 2012e, each release has been tagged in development repositories.
1.1SchristosUntagged commits are less well tested and probably contain
1.1Schristosmore errors.</p>
1.1Schristos<p>
1.1SchristosAfter obtaining the code and data files, see the
1.1Schristos<code>README</code> file for what to do next.
1.1SchristosThe code lets you compile the <code><abbr>tz</abbr></code> source files into
1.2Schristosmachine-readable binary files, one for each location. The binary files
1.17Schristosare in a special format specified by
1.20Schristos<a href="https://www.rfc-editor.org/rfc/rfc9636">The
1.17SchristosTime Zone Information Format (<abbr>TZif</abbr>)</a>
1.18Schristos(Internet <abbr title="Request For Comments">RFC</abbr> 9636).
1.2SchristosThe code also lets
1.2Schristosyou read a <abbr>TZif</abbr> file and interpret timestamps for that
1.1Schristoslocation.</p>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.1Schristos<h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
1.1Schristos<p>
1.1SchristosThe <code><abbr>tz</abbr></code> code and data
1.1Schristosare by no means authoritative. If you find errors, please
1.17Schristosemail changes to <a href="mailto:tz@iana.org"><code>tz@iana.org</code></a>,
1.20Schristosthe time zone mailing list.
1.20SchristosThe mailing list and its archives are public,
1.20Schristosso please do not send confidential information.
1.20SchristosSee
1.17Schristos<a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing
1.20Schristoslist’s main page</a> to subscribe or to browse its archive of old messages.
1.17Schristos<a href="https://tzdata-meta.timtimeonline.com">Metadata for mailing list
1.5Schristosdiscussions</a> and corresponding data changes can be
1.5Schristosgenerated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
1.5Schristos</p>
1.1Schristos<p>
1.1SchristosChanges to the <code><abbr>tz</abbr></code> code and data are often
1.1Schristospropagated to clients via operating system updates, so
1.1Schristosclient <code><abbr>tz</abbr></code> data can often be corrected by
1.1Schristosapplying these updates. With GNU/Linux and similar systems, if your
1.1Schristosmaintenance provider has not yet adopted the
1.1Schristoslatest <code><abbr>tz</abbr></code> data, you can often short-circuit
1.1Schristosthe process by tailoring the generic instructions in
1.1Schristosthe <code><abbr>tz</abbr> README</code> file and installing the latest
1.1Schristosdata yourself. System-specific instructions for installing the
1.1Schristoslatest <code><abbr>tz</abbr></code> data have also been published
1.17Schristosfor <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates"><abbr>AIX</abbr></a>,
1.1Schristos<a
1.10Schristoshref="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
1.1Schristos<a
1.11Schristoshref="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
1.1Schristostitle="International Components for Unicode">ICU</abbr></a>,
1.11Schristos<a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
1.9SchristosJDK</a>,
1.9Schristos<a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
1.1Schristoshref="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
1.9Schristos<a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
1.11Schristoshref="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
1.1Schristos</p>
1.12Schristos<p>Since version 2013a,
1.12Schristossources for the <code><abbr>tz</abbr></code> database have been
1.1Schristos<a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
1.1Schristostitle="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
1.1Schristos<a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
1.1Schristoswith lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
1.1Schristostitle="linefeed">LF</abbr></a>,
1.1Schristoswhich can be modified by common text editors such
1.1Schristosas <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
1.20Schristos<a href="https://gedit-text-editor.org">gedit</a>, and
1.1Schristos<a href="https://www.vim.org">vim</a>.
1.1SchristosSpecialized source-file editing can be done via the
1.1Schristos<a href="https://packagecontrol.io/packages/zoneinfo">Sublime
1.1Schristoszoneinfo</a> package for <a
1.1Schristoshref="https://www.sublimetext.com">Sublime Text</a> and the <a
1.1Schristoshref="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
1.1Schristoszoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
1.1SchristosStudio Code</a>.
1.1Schristos</p>
1.1Schristos<p>
1.1SchristosFor further information about updates, please see
1.18Schristos<a href="https://www.rfc-editor.org/rfc/rfc6557">Procedures for
1.17SchristosMaintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557).
1.17SchristosMore detail can be
1.7Schristosfound in <a href="theory.html">Theory and pragmatics of the
1.7Schristos<code><abbr>tz</abbr></code> code and data</a>.
1.2Schristos<a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
1.2Schristosdisplays changes between recent <code><abbr>tzdb</abbr></code> versions.
1.1Schristos</p>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.14Schristos<h2 id="coordinating">Coordinating with governments and distributors</h2>
1.14Schristos<p>
1.14SchristosAs discussed in
1.20Schristos“<a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How
1.20SchristosTime Zones Are Coordinated</a>”, the time zone database relies on
1.14Schristoscollaboration among governments, the time zone database volunteer
1.14Schristoscommunity, and data distributors downstream.
1.14Schristos<p>
1.14SchristosIf your government plans to change its time zone boundaries or
1.20Schristosdaylight saving rules, please send email as described in
1.20Schristos"<a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a>".
1.20SchristosDo this well in advance,
1.14Schristosas this will lessen confusion and will coordinate updates to many cell phones,
1.14Schristoscomputers, and other devices around the world.
1.14SchristosIn your email, please cite the legislation or regulation that specifies
1.14Schristosthe change, so that it can be checked for details such as the exact times
1.14Schristoswhen clock transitions occur.
1.14SchristosIt is OK if a rule change is planned to affect clocks
1.14Schristosfar into the future, as a long-planned change can easily be reverted
1.20Schristosor otherwise altered with a year’s notice before the change would have
1.14Schristosaffected clocks.</p>
1.14Schristos<p>
1.14SchristosThere is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.
1.14SchristosHowever, typically a release occurs every few months.
1.14SchristosMany downstream timezone data distributors wait for
1.14Schristosa <code><abbr>tzdb</abbr></code> release before they produce an update
1.14Schristosto time zone behavior in consumer devices and software products.
1.14SchristosAfter a release, various parties must integrate, test,
1.14Schristosand roll out an update before <a
1.14Schristoshref="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.
1.14SchristosThese updates can be expensive, for both the <a
1.14Schristoshref="https://en.wikipedia.org/wiki/Quality_assurance">quality
1.14Schristosassurance</a> process and the overall cost of shipping and installing
1.20Schristosupdates to each device’s copy of <code><abbr>tzdb</abbr></code>.
1.14SchristosUpdates may be batched with other updates and may take substantial
1.14Schristostime to reach end users after a release.
1.14SchristosOlder devices may no longer be supported and thus may never be updated,
1.14Schristoswhich means they will continue to use out-of-date rules.</p>
1.14Schristos<p>
1.14SchristosFor these reasons any rule change should be promulgated at least a
1.14Schristosyear before it affects how clocks operate; otherwise, there is a good
1.14Schristoschance that many clocks will be wrong due to delays in propagating updates,
1.14Schristosand that residents will be confused or even actively resist the change.
1.20SchristosThe shorter the notice, the more likely clock problems will arise; see “<a
1.14Schristoshref="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
1.20Schristosthe Timing of Time Zone Changes</a>” for examples.
1.14Schristos</p>
1.14Schristos</section>
1.14Schristos
1.14Schristos<section>
1.1Schristos<h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
1.1Schristos<ul>
1.1Schristos<li>The article
1.1Schristos<a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
1.1Schristosan encyclopedic summary.</li>
1.1Schristos<li><a href="tz-how-to.html">How to Read the
1.1Schristostz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
1.1Schristosdatabase format.</li>
1.1Schristos<li><a
1.1Schristoshref="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
1.1Schristosliterary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
1.20Schristosdatabase’s style.</li>
1.10Schristos<li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
1.10Schristosmanaging time in the internet</a> analyzes the database longitudinally.</li>
1.1Schristos</ul>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.1Schristos<h2 id="web">Web sites using recent versions of the
1.1Schristos<code><abbr>tz</abbr></code> database</h2>
1.1Schristos<p>
1.1SchristosThese are listed roughly in ascending order of complexity and fanciness.
1.1Schristos</p>
1.1Schristos<ul>
1.20Schristos<li><a href="https://time.is">Time.is</a> shows locations’
1.1Schristostime and zones.</li>
1.1Schristos<li><a href="https://www.timejones.com">TimeJones.com</a>,
1.1Schristos<a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
1.7Schristos<a href="https://www.worldclock.com">The World Clock</a>
1.1Schristosare time zone converters.</li>
1.8Schristos<li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
1.8Schristospublishes <code><abbr>tzdb</abbr></code>-derived data in
1.8Schristos<a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
1.8Schristostitle="comma-separated values">CSV</abbr></a> and
1.8Schristosin <a href="https://en.wikipedia.org/wiki/SQL"><abbr
1.8Schristostitle="Structured Query Language">SQL</abbr></a> form.</li>
1.1Schristos<li><a
1.6Schristoshref="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
1.1Schristoslets you see the <code><abbr>TZ</abbr></code> values directly.</li>
1.1Schristos<li><a
1.13Schristoshref="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
1.1SchristosTime in 1000 Places</a> uses descriptions of the values.</li>
1.20Schristos<li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
1.1SchristosWorldwide</a> lets you sort zone names and convert times.</li>
1.1Schristos<li><a href="https://24timezones.com">24TimeZones</a> has a world
1.1Schristostime map and a time converter.</li>
1.1Schristos<li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
1.1Schristoscalculates the current time difference between locations.</li>
1.13Schristos<li><a href="https://www.wx-now.com">Weather Now</a> and
1.13Schristos<a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
1.1Schristos</ul>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.1Schristos<h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
1.1Schristos<ul>
1.20Schristos<li><a href="https://www.rfc-editor.org/rfc/rfc7808">Time Zone
1.20SchristosData Distribution Service</a> (TZDIST, Internet <abbr>RFC</abbr> 7808)
1.20Schristosis associated with
1.20Schristos<a href="https://www.rfc-editor.org/rfc/rfc7809">CalDAV</a>
1.1Schristos(Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
1.2Schristostransferring time zone data by reference.
1.8Schristos<a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
1.20Schristosimplementations</a> are available.</li>
1.20Schristos<li>The <a href="https://www.rfc-editor.org/rfc/rfc5545">iCalendar format</a>
1.20Schristos(Internet <abbr>RFC</abbr> 5445)
1.1Schristoscovers time zone
1.1Schristosdata; see its VTIMEZONE calendar component.
1.1SchristosThe iCalendar format requires specialized parsers and generators; a
1.18Schristosvariant <a href="https://www.rfc-editor.org/rfc/rfc6321">xCal</a>
1.1Schristos(Internet <abbr>RFC</abbr> 6321) uses
1.1Schristos<a href="https://www.w3.org/XML/"><abbr
1.1Schristostitle="Extensible Markup Language">XML</abbr></a> format, and a variant
1.18Schristos<a href="https://www.rfc-editor.org/rfc/rfc7265">jCal</a>
1.1Schristos(Internet <abbr>RFC</abbr> 7265)
1.17Schristosuses <a href="https://www.json.org/json-en.html"><abbr
1.1Schristostitle="JavaScript Object Notation">JSON</abbr></a> format.</li>
1.1Schristos</ul>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.1Schristos<h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
1.7Schristos<p>Although some of these do not fully support
1.7Schristos<code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
1.7Schristosdistributions you can generally work around compatibility problems by
1.7Schristosrunning the command <code>make rearguard_tarballs</code> and compiling
1.7Schristosfrom the resulting tarballs instead.</p>
1.1Schristos<ul>
1.17Schristos<li><a href="https://github.com/libical/vzic">Vzic</a> is a <a
1.14Schristoshref="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>
1.1Schristosprogram that compiles
1.1Schristos<code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
1.1SchristosVzic is freely
1.1Schristosavailable under the <a
1.1Schristoshref="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
1.1SchristosGeneral Public License (<abbr
1.1Schristostitle="General Public License">GPL</abbr>)</a>.</li>
1.1Schristos<li><a
1.4Schristoshref="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
1.1Schristoscontains a script <code>parse_olson</code> that compiles
1.1Schristos<code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
1.1Schristosmodules. It is part of the Perl <a
1.4Schristoshref="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
1.4Schristoswhich is freely
1.1Schristosavailable under both the <abbr>GPL</abbr> and the Perl Artistic
1.1SchristosLicense. DateTime::TimeZone also contains a script
1.1Schristos<code>tests_from_zdump</code> that generates test cases for each clock
1.1Schristostransition in the <code><abbr>tz</abbr></code> database.</li>
1.1Schristos<li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
1.1SchristosDatabase Parser</a> is a
1.14Schristos<a href="https://en.wikipedia.org/wiki/C++">C++</a> parser and
1.17Schristosruntime library with a <a
1.17Schristoshref="https://en.cppreference.com/w/cpp/chrono"><code>std::chrono</code> API</a>
1.17Schristosthat is a standard part of C++.
1.1SchristosIt is freely available under the
1.1Schristos<abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
1.13Schristos<li><a id="ICU" href="https://icu.unicode.org">International Components for
1.1SchristosUnicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
1.14Schristoshref="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a>
1.1Schristoslibraries for internationalization that
1.1Schristoshas a compiler from <code><abbr>tz</abbr></code> source
1.1Schristosand from <abbr title="Common Locale Data Repository">CLDR</abbr> data
1.1Schristos(mentioned <a href="#CLDR">below</a>)
1.1Schristosinto an <abbr>ICU</abbr>-specific format.
1.1Schristos<abbr>ICU</abbr> is freely available under a
1.1Schristos<abbr>BSD</abbr>-style license.</li>
1.1Schristos<li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
1.1Schristosthe <a href="https://elixir-lang.org">Elixir</a> language downloads
1.7Schristosand compiles <code><abbr>tz</abbr></code> source and exposes <abbr
1.1Schristostitle="Application Program Interface">API</abbr>s for use. It is
1.1Schristosfreely available under the <abbr>MIT</abbr> license.</li>
1.1Schristos<li>Java-based compilers and libraries include:
1.1Schristos<ul>
1.1Schristos<li>The <a
1.9Schristoshref="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
1.1Schristostool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
1.17Schristos<a href="https://openjdk.org">OpenJDK</a> and
1.17Schristos<a href="https://jdk.java.net">Oracle JDK</a>.
1.9SchristosAlthough its source code is proprietary, its executable is available under the
1.9Schristos<a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
1.9SchristosTimezone Updater License Agreement</a>.</li>
1.1Schristos<li>The <a
1.4Schristoshref="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
1.6SchristosSE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
1.4Schristoshref="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
1.1Schristoswhich is freely available under a <abbr>BSD</abbr>-style license.</li>
1.20Schristos<li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
1.1Schristosand time <abbr>API</abbr></a> contains a class
1.1Schristos<code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
1.1Schristos<code><abbr>tz</abbr></code> source into a binary format. It inspired
1.1SchristosJava 8 <code>java.time</code>, which its users should migrate to once
1.1Schristosthey can assume Java 8 or later. It is available under the <a
1.1Schristoshref="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
1.9Schristos<li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
1.7Schristoshref="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
1.20Schristosare alternatives to TZUpdater. IANA Updater’s license is unclear;
1.9SchristosZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
1.4Schristos<li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
1.4Schristostime library for Android</a> and
1.17Schristos<a href="https://github.com/MenoData/Time4J">Time4J: Advanced date,
1.4Schristostime and interval library for Java</a> compile
1.4Schristos<code><abbr>tz</abbr></code> source into a binary format.
1.4SchristosTime4A is available under the Apache License and Time4J is
1.1Schristosavailable under the <a
1.1Schristoshref="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
1.1SchristosGeneral Public License (<abbr title="Lesser General Public
1.1SchristosLicense">LGPL</abbr>)</a>.</li>
1.1Schristos<li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
1.1SchristosJava-based libraries.</li>
1.1Schristos</ul>
1.20Schristos<li><a href="https://nodatime.org">Noda Time – Date and
1.1Schristostime <abbr>API</abbr> for .NET</a>
1.4Schristosis like Joda-Time and Time4J, but for the .NET framework instead of Java.
1.4SchristosIt is freely available under the Apache License.</li>
1.8Schristos<li>Many modern
1.8Schristos<a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
1.8Schristosruntimes support <code><abbr>tz</abbr></code> natively via the
1.16Schristos<code>timeZone</code> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><code>Intl.DateTimeFormat</code></a>.
1.10SchristosThis can be used as-is or with most of the following libraries,
1.16Schristosmany of which also support runtimes lacking the <code>timeZone</code> option.
1.1Schristos<ul>
1.8Schristos<li>The <a
1.16Schristoshref="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code>
1.8Schristostimezone polyfill</a>
1.8Schristosis freely available under a <abbr>BSD</abbr>-style license.</li>
1.17Schristos<li>The <a href="https://date-fns.org">date-fns</a>
1.10Schristoslibrary manipulates timezone-aware timestamps in browsers and
1.10Schristosin <a href="https://nodejs.org/en/">Node.js</a>.
1.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
1.10Schristos<li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
1.10Schristosminimalist replacement for the date and time API of
1.10Schristosthe <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
1.10Schristosmanipulation library.
1.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
1.10Schristos<li><a href="https://moment.github.io/luxon/">Luxon</a> improves
1.16Schristostimezone support for the <code>Intl</code> API.
1.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
1.1Schristos<li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
1.10SchristosMoment.js plugin.
1.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
1.8Schristos<li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
1.8SchristosJavaScript library that supports date arithmetic that is time zone
1.8Schristosaware. It is freely available under the <abbr>MIT</abbr> license.</li>
1.10Schristos<li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
1.10Schristossupports live <code><abbr>tzdb</abbr></code> updates,
1.10Schristosastronomical and atomic time, a command-line interface,
1.10Schristosand full <a
1.10Schristoshref="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
1.10SchristosIts companion <a
1.10Schristoshref="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
1.10Schristoscan generate <abbr>TZif</abbr> and other files, and a companion website
1.10Schristos<a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
1.10Schristosconvert timestamps, view transition histories, and download code and data.
1.10SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
1.1Schristos</ul>
1.10SchristosThe proposed <a
1.16Schristoshref="https://github.com/tc39/proposal-temporal"><code>Temporal</code>
1.10Schristosobjects</a> let programs access an abstract view of
1.10Schristos<code><abbr>tzdb</abbr></code> data, and are designed to replace <a
1.20Schristoshref="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript’s
1.16Schristosproblematic <code>Date</code> objects</a> when working with dates and times.
1.17Schristos<li><a href="https://github.com/JuliaTime">JuliaTime</a> contains a
1.1Schristoscompiler from <code><abbr>tz</abbr></code> source into
1.17Schristos<a href="https://julialang.org">Julia</a>. It is freely available
1.1Schristosunder the <abbr>MIT</abbr> license.</li>
1.20Schristos<li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
1.6Schristos<abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
1.6Schristostitle="Free Pascal Compiler">FPC</abbr></a>
1.2Schristoscompiles from <code><abbr>tz</abbr></code> source into
1.2Schristos<a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
1.2Schristosas compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
1.2Schristosand <a
1.2Schristoshref="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
1.2SchristosIt is freely available under a <abbr>BSD</abbr>-style license.</li>
1.20Schristos<li><a href="https://pythonhosted.org/pytz/">pytz – World Timezone
1.1SchristosDefinitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
1.1Schristos<a href="https://www.python.org">Python</a>.
1.9SchristosIt is freely available under a <abbr>BSD</abbr>-style license.
1.11SchristosIn code that can assume Python 3.6 or later it is largely superseded; see <a
1.11Schristoshref="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
1.11SchristosThe Fastest Footgun in the West</a>.</li>
1.20Schristos<li><a href="https://tzinfo.github.io">TZInfo –
1.1SchristosRuby Timezone Library</a>
1.1Schristoscompiles <code><abbr>tz</abbr></code> source into
1.1Schristos<a href="https://www.ruby-lang.org/en/">Ruby</a>.
1.1SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
1.13Schristos<li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
1.1SchristosLibrary</a> is
1.1Schristosa <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
1.1Schristoslibrary that compiles <code><abbr>tz</abbr></code> source into a time
1.1Schristoszone repository whose format
1.1Schristosis either proprietary or an <abbr>XML</abbr>-encoded
1.1Schristosrepresentation.</li>
1.1Schristos<li><a id="Tcl" href="https://tcl.tk">Tcl</a>
1.1Schristoscontains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
1.1Schristossource into text files, along with a runtime that can read those
1.1Schristosfiles. Tcl is freely available under a <abbr>BSD</abbr>-style
1.1Schristoslicense.</li>
1.1Schristos</ul>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.2Schristos<h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
1.1Schristos<ul>
1.1Schristos<li>The <a
1.1Schristoshref="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
1.1SchristosLibrary</a>
1.1Schristoshas an independent, thread-safe implementation of
1.2Schristosa <abbr>TZif</abbr> file reader.
1.1SchristosThis library is freely available under the LGPL
1.1Schristosand is widely used in <abbr>GNU</abbr>/Linux systems.</li>
1.20Schristos<li><a href="https://www.gnome.org">GNOME</a>’s
1.1Schristos<a href="https://developer.gnome.org/glib/">GLib</a> has
1.2Schristosa <abbr>TZif</abbr> file reader written in C that
1.1Schristoscreates a <code>GTimeZone</code> object representing sets
1.1Schristosof <abbr>UT</abbr> offsets.
1.1SchristosIt is freely available under the <abbr>LGPL</abbr>.</li>
1.1Schristos<li>The
1.20Schristos<a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>’s
1.1Schristos<code>baltzo::TimeZoneUtil</code> component contains a C++
1.2Schristosimplementation of a <abbr>TZif</abbr> file reader. It is freely available under
1.1Schristosthe Apache License.</li>
1.1Schristos<li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
1.1Schristoslibrary that translates between <abbr>UT</abbr> and civil time and
1.2Schristoscan read <abbr>TZif</abbr> files. It is freely available under the Apache
1.1SchristosLicense.</li>
1.20Schristos<li>The <a href="https://golang.org">Go programming language</a>
1.20Schristoshas a <abbr>TZif</abbr> file reader <a
1.20Schristoshref="https://pkg.go.dev/time#LoadLocationFromTZData"><code>LoadLocationFromTZData</code></a>.</li>
1.16Schristos<li>The
1.16Schristos<a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code>
1.16Schristospackage</a> contains Python code
1.16Schristosto generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map
1.17Schristos<code><abbr>tz</abbr></code> settings to proleptic TZ approximations.
1.16SchristosFor example, it maps <code>"Africa/Cairo"</code>
1.16Schristosto <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>,
1.16Schristosan approximation valid for Cairo timestamps from 2023 on.
1.17SchristosThis can help porting to platforms that support only proleptic TZ.
1.16SchristosThe package is freely available under the MIT license.</li>
1.1Schristos<li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
1.2Schristoslibrary that reads <abbr>TZif</abbr> files and converts
1.2Schristostimestamps from one time zone or format to another.
1.1SchristosIt is used by <a href="https://secure.php.net"><abbr
1.1Schristostitle="PHP: Hypertext Preprocessor">PHP</abbr></a>,
1.1Schristos<a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
1.1Schristosand <a href="https://www.mongodb.com">MongoDB</a>.
1.1SchristosIt is freely available under the <abbr>MIT</abbr> license.</li>
1.1Schristos<li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
1.2Schristos<abbr>TZif</abbr> file reader.</li>
1.4Schristos<li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
1.1SchristosDateTime::TimeZone::Tzfile</a>
1.2Schristosis a <abbr>TZif</abbr> file reader written in Perl.
1.1SchristosIt is freely available under the same terms as Perl
1.1Schristos(dual <abbr>GPL</abbr> and Artistic license).</li>
1.9Schristos<li>Python has a <a id="python-zoneinfo"
1.11Schristoshref="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
1.8Schristosclass</a> that reads <abbr>TZif</abbr> data and creates objects
1.8Schristosthat represent <code><abbr>tzdb</abbr></code> timezones.
1.8SchristosPython is freely available under the
1.11Schristos<a href="https://docs.python.org/3/license.html">Python Software Foundation
1.8SchristosLicense</a>.
1.17SchristosA companion <a id="pypi-tzdata" href="https://pypi.org">PyPI</a> module
1.8Schristos<a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
1.8Schristossupplies TZif data if the underlying system data cannot be found;
1.8Schristosit is freely available under the Apache License.</li>
1.1Schristos<li>The
1.1Schristospublic-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
1.1Schristoslibrary contains a Python tool that
1.2Schristosconverts <abbr>TZif</abbr> data into
1.1Schristos<abbr>JSON</abbr>-format data suitable for use
1.1Schristosin its JavaScript library for time zone conversion. Dates before 1970
1.1Schristosare not supported.</li>
1.1Schristos<li>The <a
1.1Schristoshref="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
1.1Schristospackage contains <a href="https://www.haskell.org">Haskell</a> code that
1.2Schristosparses and uses <abbr>TZif</abbr> data. It is freely
1.1Schristosavailable under a <abbr>BSD</abbr>-style license.</li>
1.1Schristos</ul>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.1Schristos<h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
1.1Schristos<ul>
1.1Schristos<li><a href="https://foxclocks.org">FoxClocks</a>
1.1Schristosis an extension for <a href="https://www.google.com/chrome/">Google
1.16SchristosChrome</a>, <a
1.1Schristoshref="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
1.1Schristoshref="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
1.1SchristosIt displays multiple clocks in the application window, and has a mapping
1.1Schristosinterface to <a href="https://www.google.com/earth/">Google Earth</a>.
1.1SchristosIt is freely available under the <abbr>GPL</abbr>.</li>
1.1Schristos<li>Microsoft Windows 8.1
1.1Schristosand later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
1.3Schristosdata (mentioned <a href="#CLDR">below</a>) used by the
1.3Schristos<a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
1.3Schristos<a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
1.3Schristos<a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
1.3Schristos<a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
1.1Schristos<a id="System.TimeZoneInfo"
1.1Schristoshref="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
1.1SchristosWindows Time Zones with <code>System.TimeZoneInfo</code></a> describes
1.1Schristosthe older, proprietary method of Microsoft Windows 2000 and later,
1.1Schristoswhich stores time zone data in the
1.1Schristos<a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
1.1Schristos<a
1.20Schristoshref="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone
1.20Schristos→ Tzid table</a> or <a
1.6Schristoshref="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
1.1Schristosfile</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
1.1Schristosto <code><abbr>tz</abbr></code> names.
1.3SchristosThese mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
1.3Schristosor the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
1.1Schristos<li><a
1.1Schristoshref="https://www.oracle.com/java/index.html">Oracle
1.1SchristosJava</a> contains a copy of a subset of a recent
1.1Schristos<code><abbr>tz</abbr></code> database in a
1.1SchristosJava-specific format.</li>
1.1Schristos</ul>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.1Schristos<h2 id="other-dbs">Other time zone databases</h2>
1.1Schristos<ul>
1.1Schristos<li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
1.20Schristosis Astrodienst’s Web version of Shanks and Pottenger’s out-of-print
1.4Schristostime zone history atlases
1.4Schristos<a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
1.9Schristos<a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
1.4SchristosAlthough these extensive atlases
1.4Schristos<a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
1.4Schristossources for much of the older <code><abbr>tz</abbr></code> data</a>,
1.4Schristosthey are unreliable as Shanks appears to have
1.1Schristosguessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
1.1Schristossources and do not indicate which entries are guesswork.</li>
1.1Schristos<li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
1.1Schristosits own <code>tztab</code>(4) format.</li>
1.1Schristos<li>Microsoft Windows has proprietary data mentioned
1.1Schristos<a href="#System.TimeZoneInfo">above</a>.</li>
1.1Schristos<li><a href="https://www.worldtimeserver.com">World Time Server</a>
1.1Schristosis another time zone database.</li>
1.1Schristos<li>The <a
1.1Schristoshref="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
1.1SchristosSchedules Information Manual</a> of the
1.1SchristosInternational Air Transport Association
1.1Schristosgives current time zone rules for airports served by commercial aviation.</li>
1.1Schristos</ul>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.1Schristos<h2 id="maps">Maps</h2>
1.1Schristos<ul>
1.9Schristos<li>The <a
1.9Schristoshref="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
1.9Schristosand Regional Maps section</a> of <em>The World Factbook</em>, published by the
1.9Schristos<a href="https://www.cia.gov">US Central Intelligence
1.9SchristosAgency (<abbr
1.9Schristostitle="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
1.9Schristoszone map; the
1.1Schristos<a
1.20Schristoshref="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda
1.1SchristosLibrary Map Collection</a>
1.1Schristosof the University of Texas at Austin has copies of
1.1Schristosrecent editions.
1.1SchristosThe pictorial quality is good,
1.1Schristosbut the maps do not indicate daylight saving time,
1.1Schristosand parts of the data are a few years out of date.</li>
1.6Schristos<li><a href="https://www.worldtimezone.com">World Time Zone Map
1.6Schristoswith current time</a>
1.1Schristoshas several fancy time zone maps; it covers Russia particularly well.
1.20SchristosThe maps’ pictorial quality is not quite as good as the <abbr>CIA</abbr>’s
1.1Schristosbut the maps are more up to date.</li>
1.1Schristos<li><a
1.1Schristoshref="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
1.1Schristosmuch is time wrong around the world?</a> maps the difference between
1.1Schristosmean solar and standard time, highlighting areas such as western China
1.20Schristoswhere the two differ greatly. It’s a bit out of date, unfortunately.</li>
1.1Schristos</ul>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.1Schristos<h2 id="boundaries">Time zone boundaries</h2>
1.2Schristos<p>Geographical boundaries between timezones are available
1.9Schristosfrom several <a
1.9Schristoshref="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
1.9Schristosgeolocation</a>
1.1Schristosservices and other sources.</p>
1.1Schristos<ul>
1.1Schristos<li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
1.1SchristosBoundary Builder</a> extracts
1.1Schristos<a href="https://www.openstreetmap.org">Open Street Map</a> data to build
1.2Schristosboundaries of <code><abbr>tzdb</abbr></code> timezones.
1.1SchristosIts code is freely available under the <abbr>MIT</abbr> license, and
1.1Schristosits data entries are freely available under the
1.1Schristos<a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
1.16SchristosOpen Database License</a>. The borders appear to be quite accurate.
1.16SchristosIts main web page lists more than twenty libraries
1.16Schristosfor looking up a timezone name from a GPS coordinate.</li>
1.1Schristos<li>Free access via a network API, if you register a key, is provided by
1.4Schristosthe <a
1.4Schristoshref="https://www.geonames.org/export/web-services.html#timezone">GeoNames
1.4SchristosTimezone web service</a>, the <a
1.4Schristoshref="https://developers.google.com/maps/documentation/timezone/intro">Google
1.4SchristosMaps Time Zone API</a>, and
1.8Schristosthe <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
1.1SchristosCommercial network API access is provided
1.1Schristosby <a href="https://askgeo.com">AskGeo</a>
1.1Schristosand <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
1.1Schristos</li>
1.20Schristos<li>“<a
1.1Schristoshref="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
1.1Schristosto get a time zone from a location using latitude and longitude
1.20Schristoscoordinates?</a>” discusses other geolocation possibilities.</li>
1.1Schristos<li><a href="http://statoids.com/statoids.html">Administrative
1.20SchristosDivisions of Countries (“Statoids”)</a> lists
1.1Schristospolitical subdivision data related to time zones.</li>
1.13Schristos<li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
1.20Schristos– GIS and Database Tools</a> includes a Manifold-format map of
1.16Schristosworld time zone boundaries circa 2007, distributed under the
1.1Schristos<abbr>GPL</abbr>.</li>
1.1Schristos<li>A ship within the <a
1.1Schristoshref="https://en.wikipedia.org/wiki/Territorial_waters">territorial
1.20Schristoswaters</a> of any nation uses that nation’s time. In international
1.20Schristoswaters, time zone boundaries are meridians 15° apart, except that
1.20Schristos<abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
1.1Schristoswide and are separated by
1.20Schristosthe 180° meridian (not by the International Date Line, which is
1.20Schristosfor land and territorial waters only). A captain can change ship’s
1.1Schristosclocks any time after entering a new time zone; midnight changes are
1.1Schristoscommon.</li>
1.1Schristos</ul>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.1Schristos<h2 id="civil">Civil time concepts and history</h2>
1.1Schristos<ul>
1.1Schristos<li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
1.1SchristosWalk through Time</a>
1.1Schristossurveys the evolution of timekeeping.</li>
1.4Schristos<li>The history of daylight saving time is surveyed in <a
1.4Schristoshref="http://www.webexhibits.org/daylightsaving/">About Daylight
1.20SchristosSaving Time – History, rationale, laws &amp; dates</a> and summarized in
1.4Schristos<a href="http://seizethedaylight.com/dst/">A Brief
1.4SchristosHistory of Daylight Saving Time</a>.</li>
1.4Schristos<li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
1.4SchristosLords</a> discusses how authoritarians manipulate civil time.</li>
1.20Schristos<li><a href="https://www.w3.org/TR/timezone/">Working with Time
1.20Schristosand Time Zones</a>
1.1Schristoscontains guidelines and best practices for software applications that
1.1Schristosdeal with civil time.</li>
1.16Schristos<li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of
1.1Schristosthe International Date Line</a> tells the story of the most important
1.1Schristostime zone boundary.</li>
1.1Schristos<li><a href="http://statoids.com/tconcept.html">Basic Time
1.1SchristosZone Concepts</a> discusses terminological issues behind time zones.</li>
1.1Schristos</ul>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.1Schristos<h2 id="national">National histories of legal time</h2>
1.1Schristos<dl>
1.1Schristos<dt>Australia</dt>
1.20Schristos<dd>The Bureau of Meteorology publishes a list of <a
1.1Schristoshref="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
1.1SchristosDates of Daylight Savings Time within Australia</a>.</dd>
1.1Schristos<dt>Belgium</dt>
1.7Schristos<dd>The Royal Observatory of Belgium maintains a table of time in
1.7SchristosBelgium (in
1.16Schristos<a href="https://robinfo.oma.be/nl/astro-info/tijd/"
1.7Schristoshreflang="nl">Dutch</a> and <a
1.16Schristoshref="https://robinfo.oma.be/fr/astro-info/heure/"
1.7Schristoshreflang="fr">French</a>).</dd>
1.1Schristos<dt>Brazil</dt>
1.1Schristos<dd>The Time Service Department of the National Observatory
1.1Schristosrecords <a href="http://pcdsh01.on.br/DecHV.html"
1.20Schristoshreflang="pt-BR">Brazil’s daylight saving time decrees (in
1.1SchristosPortuguese)</a>.</dd>
1.1Schristos<dt>Canada</dt>
1.1Schristos<dd>National Research Council Canada publishes current
1.1Schristosand some older information about <a
1.6Schristoshref="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
1.6Schristoszones and daylight saving time</a>.</dd>
1.1Schristos<dt>Chile</dt>
1.1Schristos<dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
1.11Schristos<a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
1.20SchristosChile’s official time (in Spanish)</a>.</dd>
1.3Schristos<dt>China</dt>
1.3Schristos<dd>The Hong Kong Observatory maintains a
1.7Schristos<a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
1.3Schristos summer time in Hong Kong</a>,
1.20Schristosand Macau’s Meteorological and Geophysical Bureau maintains a <a
1.7Schristoshref="https://www.smg.gov.mo/en/subpage/224/page/174">similar
1.3Schristoshistory for Macau</a>.
1.3SchristosUnfortunately the latter is incomplete and has errors.</dd>
1.1Schristos<dt>Czech Republic</dt>
1.1Schristos<dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
1.1Schristoshreflang="cs">When daylight saving time starts and ends (in Czech)</a>
1.10Schristossummarizes and cites historical <abbr>DST</abbr> regulations.</dd>
1.1Schristos<dt>Germany</dt>
1.1Schristos<dd>The National Institute for Science and Technology maintains the <a
1.1Schristoshref="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
1.1Schristosof Legal Time in Germany</a>.</dd>
1.1Schristos<dt>Israel</dt>
1.17Schristos<dd><a
1.17Schristoshref="https://tz.cs.huji.ac.il">Israel Timezone Files</a>
1.17Schristoslists official time-change announcements and laws since 1940,
1.17Schristosalmost all in Hebrew.</dd>
1.2Schristos<dt>Malaysia</dt>
1.2Schristos<dd>See Singapore <a href="#Singapore">below</a>.</dd>
1.1Schristos<dt>Mexico</dt>
1.1Schristos<dd>The Investigation and Analysis Service of the Mexican Library of
1.1SchristosCongress has published a <a
1.13Schristoshref="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
1.1Schristoshreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
1.1Schristos<dt>Netherlands</dt>
1.16Schristos<dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"
1.1Schristoshreflang="nl">Legal time in the Netherlands (in Dutch)</a>
1.1Schristoscovers the history of local time in the Netherlands from ancient times.</dd>
1.1Schristos<dt>New Zealand</dt>
1.1Schristos<dd>The Department of Internal Affairs maintains a brief <a
1.1Schristoshref="https://www.dia.govt.nz/Daylight-Saving-History">History of
1.6SchristosDaylight Saving</a>.</dd>
1.14Schristos<dt>Palestine</dt>
1.20Schristos<dd>The Ministry of Telecom and Digital Economy publishes a <a
1.20Schristoshref="https://mtde.gov.ps/home/TimeZone"
1.14Schristoshreflang="ar">history of clock changes (in Arabic)</a>.</dd>
1.10Schristos<dt>Portugal</dt>
1.10Schristos<dd>The Lisbon Astronomical Observatory publishes a
1.10Schristos<a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
1.10Schristoslegal time (in Portuguese)</a>.</dd>
1.1Schristos<dt>Singapore</dt>
1.1Schristos<dd><a id="Singapore"
1.7Schristoshref="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
1.20Schristosis Singapore in the “Wrong” Time Zone?</a> details the
1.1Schristoshistory of legal time in Singapore and Malaysia.</dd>
1.1Schristos<dt>United Kingdom</dt>
1.1Schristos<dd><a
1.1Schristoshref="https://www.polyomino.org.uk/british-time/">History of
1.1Schristoslegal time in Britain</a> discusses in detail the country
1.6Schristoswith perhaps the best-documented history of clock adjustments.</dd>
1.1Schristos<dt>United States</dt>
1.20Schristos<dd>The Department of Transportation’s <a
1.1Schristoshref="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
1.18SchristosTime Zone Proceedings</a> lists changes to
1.18Schristosofficial written time zone boundaries, and its <a
1.18Schristoshref="https://geodata.bts.gov/datasets/usdot::time-zones/about">Time
1.18SchristosZones dataset</a> maps current boundaries.
1.18SchristosThese boundaries are only for standard time, so the current map puts
1.18Schristosall of Arizona in one time zone even though part of Arizona
1.18Schristosobserves <abbr>DST</abbr> and part does not.</dd>
1.1Schristos<dt>Uruguay</dt>
1.1Schristos<dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
1.1SchristosNavy (SOHMA) publishes an annual <a
1.13Schristoshref="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
1.1Schristos(in Spanish)</a>.</dd>
1.1Schristos</dl>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.7Schristos<h2 id="costs">Costs and benefits of time shifts</h2>
1.7Schristos<p>Various sources argue for and against daylight saving time and time
1.7Schristoszone shifts, and many scientific studies have been conducted. This
1.8Schristossection summarizes reviews and position statements based on
1.8Schristosscientific literature in the area.</p>
1.7Schristos<ul>
1.7Schristos<li>Carey RN, Sarma KM.
1.7Schristos<a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
1.7Schristosdaylight saving time on road traffic collision risk: a systematic
1.7Schristosreview</a>.
1.7Schristos<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>.
1.7SchristosThis reviews research literature and concludes that the evidence
1.7Schristosneither supports nor refutes road safety benefits from
1.7Schristosshifts in time zones.</li>
1.7Schristos<li>Havranek T, Herman D, Irsova D.
1.20SchristosDoes daylight saving save electricity? A meta-analysis.
1.20Schristos<em>Energy J.</em> 2018;39(2):35–61.
1.7Schristosdoi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
1.20SchristosThis analyzes research literature and concludes, “Electricity savings
1.7Schristosare larger for countries farther away from the equator, while
1.20Schristossubtropical regions consume more electricity because of <abbr>DST</abbr>.”</li>
1.20Schristos<li>Neumann P, von Blanckenburg K. <a
1.20Schristoshref="https://journals.sagepub.com/doi/full/10.1177/0961463X241310562">What
1.20Schristostime will it be? A comprehensive literature review on daylight saving time</a>.
1.20Schristos<em>Time Soc</em>. 2025-01-21.
1.20Schristosdoi:<a href="https://doi.org/10.1177/0961463X241310562">10.1177/0961463X241310562</a>.
1.20SchristosThis reviews DST’s effects on electricity, health, crime, road safety,
1.20Schristosand the economy, focusing on research since 2010, and concludes that
1.20Schristosyear-round standard time is preferable overall.
1.20Schristos</ul>
1.20Schristos
1.20Schristos<p>The following medical societies have taken positions on the
1.20Schristosadvisability of clock shifts:</p>
1.20Schristos
1.20Schristos<ul>
1.20Schristos<li>In 2022 the American Medical Association issued a
1.20Schristos<a href="https://www.ama-assn.org/press-center/press-releases/ama-calls-permanent-standard-time">statement
1.20Schristossupporting permanent standard time</a> on health grounds.</li>
1.20Schristos<li>Crawford MR, Winnebeck EC, von Schantz M <em>et al</em>.
1.20Schristos<a href="https://onlinelibrary.wiley.com/doi/10.1111/jsr.14352">The
1.20SchristosBritish Sleep Society position statement on Daylight Saving Time in the UK</a>.
1.20Schristos<em>J Sleep Res.</em> 2025;34(3):e14352.
1.20Schristosdoi:<a href="https://doi.org/10.1111/jsr.14352">10.1111/jsr.14352</a>.
1.20SchristosThis recommends that the UK abolish DST for health reasons.</li>
1.16Schristos<li>Malow BA. <a
1.16Schristoshref="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time
1.16Schristosto abolish the clock change and adopt permanent
1.16Schristosstandard time in the United States:
1.16Schristosa Sleep Research Society position statement</a>.
1.15Schristos<em>Sleep.</em> 2022;45(12):zsac236.
1.15Schristosdoi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
1.15SchristosAfter reviewing the scientific literature, the Sleep Research Society
1.20Schristosadvocates permanent standard time due to its health benefits.</li>
1.15Schristos<li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
1.15Schristos<a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
1.15Schristosis the optimal choice for health and safety:
1.8Schristosan American Academy of Sleep Medicine position statement</a>.
1.20Schristos<em>J Clin Sleep Med.</em> 2024;20(1):121–125.
1.15Schristosdoi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
1.15SchristosThe AASM argues for permanent standard time due to health and safety risks
1.15Schristosand economic costs of both <abbr>DST</abbr> transitions and
1.15Schristospermanent <abbr>DST</abbr>.</li>
1.13Schristos<li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
1.16Schristos<a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why
1.13Schristosshould we abolish Daylight Saving Time?</a>
1.20Schristos<em>J Biol Rhythms.</em> 2019;34(3):227–230.
1.13Schristosdoi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
1.15SchristosThe Society for Research on Biological Rhythms
1.18Schristosopposes <abbr>DST</abbr> changes and permanent <abbr>DST</abbr>,
1.18Schristosand advocates that governments adopt
1.20Schristos“permanent Standard Time for the health and safety of their citizens”.</li>
1.7Schristos</ul>
1.7Schristos</section>
1.7Schristos
1.7Schristos<section>
1.1Schristos<h2 id="precision">Precision timekeeping</h2>
1.1Schristos<ul>
1.1Schristos<li><a
1.1Schristoshref="http://leapsecond.com/hpan/an1289.pdf">The
1.1SchristosScience of Timekeeping</a> is a thorough introduction
1.1Schristosto the theory and practice of precision timekeeping.</li>
1.1Schristos<li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
1.14SchristosTime 2016</a> contains several freely readable papers.</li>
1.13Schristos<li><a href="https://www.ntp.org"><abbr
1.1Schristostitle="Network Time Protocol">NTP</abbr>: The Network
1.1SchristosTime Protocol</a> (Internet <abbr>RFC</abbr> 5905)
1.1Schristosdiscusses how to synchronize clocks of
1.1SchristosInternet hosts.</li>
1.2Schristos<li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
1.2Schristosfamily of software algorithms can achieve accuracy to a few tens of
1.2Schristosnanoseconds in scalable server farms without special hardware.</li>
1.1Schristos<li>The <a
1.1Schristoshref="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
1.1SchristosTime Protocol</a> (<abbr
1.1Schristostitle="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
1.2Schristoscan achieve submicrosecond clock accuracy on a local area network
1.2Schristoswith special-purpose hardware.</li>
1.1Schristos<li><a
1.18Schristoshref="https://www.rfc-editor.org/rfc/rfc4833">Timezone
1.1SchristosOptions for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
1.1Schristos(Internet <abbr>RFC</abbr> 4833)
1.1Schristosspecifies a <a
1.1Schristoshref="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
1.1Schristosoption for a server to configure
1.20Schristosa client’s time zone and daylight saving settings automatically.</li>
1.11Schristos<li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
1.11SchristosScales</a> describes astronomical time scales like
1.1Schristos<abbr title="Terrestrial Dynamic Time">TDT</abbr>,
1.1Schristos<abbr title="Geocentric Coordinate Time">TCG</abbr>, and
1.1Schristos<abbr title="Barycentric Dynamic Time">TDB</abbr>.
1.1Schristos<li>The <a href="https://www.iau.org"><abbr
1.20Schristostitle="International Astronomical Union">IAU</abbr></a>’s <a
1.13Schristoshref="https://www.iausofa.org"><abbr
1.1Schristostitle="Standards Of Fundamental Astronomy">SOFA</abbr></a>
1.1Schristoscollection contains C and <a
1.1Schristoshref="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
1.1Schristoscode for converting among time scales like
1.1Schristos<abbr title="International Atomic Time">TAI</abbr>,
1.1Schristos<abbr>TDB</abbr>, <abbr>TDT</abbr> and
1.13Schristos<abbr>UTC</abbr>. It is freely available under the
1.13Schristos<a href="https://www.iausofa.org/tandc.html">SOFA license</a>.</li>
1.1Schristos<li><a
1.1Schristoshref="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
1.20Schristos– Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
1.1Schristosdiverse local time
1.1Schristosscales used by each landed mission on Mars.</li>
1.1Schristos<li><a href="http://leapsecond.com">LeapSecond.com</a> is
1.1Schristosdedicated not only to leap seconds but to precise time and frequency
1.1Schristosin general. It covers the state of the art in amateur timekeeping, and
1.1Schristoshow the art has progressed over the past few decades.</li>
1.7Schristos<li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
1.7Schristoshref="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
1.20Schristosand time-signal emissions</a>, International Telecommunication Union –
1.7SchristosRadiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
1.1Schristos<li><a
1.1Schristoshref="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
1.1Schristostitle="International Earth Rotation and Reference Systems Service">IERS</abbr>
1.1SchristosBulletins</a> contains official publications of the International
1.1SchristosEarth Rotation and Reference Systems Service, which decides when leap
1.20Schristosseconds occur.
1.20SchristosThe <code><abbr>tz</abbr></code> code and data support leap seconds
1.20Schristosvia an optional <code>"right"</code> configuration where a computer’s internal
1.9Schristos<code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
1.20Schristosas opposed to the default <code>"posix"</code> configuration
1.9Schristoswhere the internal clock ignores leap seconds.
1.9SchristosThe two configurations agree for timestamps starting with 1972-01-01 00:00:00
1.9Schristos<abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge for
1.9Schristostimestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,
1.9Schristoswhich corresponds to the first leap second
1.20Schristos1972-06-30 23:59:60 <abbr>UTC</abbr> in the <code>"right"</code> configuration,
1.9Schristosand to
1.20Schristos1972-07-01 00:00:00 <abbr>UTC</abbr> in the <code>"posix"</code> configuration.
1.9SchristosIn practice the two configurations also agree for timestamps before
1.9Schristos1972 even though the historical situation is messy, partly because
1.9Schristosneither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
1.14Schristosis well-defined for sufficiently old timestamps.</li>
1.17Schristos<li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The
1.17Schristos<abbr>NTP</abbr> Leap Second File</a> covers the text file
1.17Schristos<code>leap-seconds.list</code>, which lists the currently known leap seconds.
1.17SchristosThe <abbr>IERS</abbr> maintains this file, and a copy is distributed by
1.17Schristos<code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like
1.17Schristos<a href="https://www.ntp.org">classic
1.17Schristos<code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a>
1.17Schristosand <a href="https://ntpsec.org">NTPsec</a>.
1.17SchristosThe <code><abbr>tz</abbr></code> database also distributes leap second
1.17Schristosinformation in a differently-formatted <code>leapseconds</code> text file,
1.20Schristosas well as in the <code>"right"</code> configuration in binary form; for
1.17Schristosexample, <code>right/UTC</code> can be used
1.17Schristosby <a href="https://chrony-project.org"><code>chrony</code></a>,
1.17Schristosanother <abbr>NTP</abbr> implementation.</li>
1.1Schristos<li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1.1Schristosdiscusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1.1Schristosleap second so that they disagree with <abbr>UTC</abbr> by at most a
1.1Schristoshalf second, even though every <abbr>POSIX</abbr> minute has exactly
1.4Schristossixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1.20Schristos<code>"posix"</code> configuration, is <a
1.1Schristoshref="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
1.17Schristosthe abovementioned <abbr>NTP</abbr> implementations, <a
1.12Schristoshref="https://github.com/google/unsmear">supports</a> conversion between
1.12Schristos<abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
1.7Schristoscloud service providers. However, according to
1.20Schristos<a href="https://www.rfc-editor.org/rfc/rfc8633#section-3.7.1">§3.7.1 of
1.7SchristosNetwork Time Protocol Best Current Practices</a>
1.7Schristos(Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1.9Schristosapplications requiring accurate <abbr>UTC</abbr> or civil time,
1.7Schristosand is intended for use only in single, well-controlled environments.</li>
1.1Schristos<li>The <a
1.1Schristoshref="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
1.1SchristosSecond Discussion List</a> covers <a
1.1Schristoshref="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
1.20Schristosand Klepczynski’s 1999 proposal to discontinue leap seconds</a>,
1.1Schristosdiscussed further in
1.1Schristos<a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1.1Schristosleap second: its history and possible future</a>.
1.1Schristos<a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1.1Schristosmight be redefined
1.1Schristoswithout Leap Seconds</a> gives pointers on this
1.14Schristoscontentious issue.
1.14SchristosThe General Conference on Weights and Measures
1.16Schristos<a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>
1.17Schristosto discontinue the use of leap seconds by 2035, and requested that no
1.17Schristosdiscontinuous adjustments be made to UTC for at least a century.
1.16SchristosThe World Radiocommunication Conference <a
1.16Schristoshref="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved
1.20Schristosin 2023</a> to cooperate with this process. One proposal to implement this
1.20Schristoswould replace leap seconds with seven 13-second leap smears occurring once per
1.17Schristosdecade until 2100, with leap smears after that gradually increasing in size.
1.20SchristosSee:
1.20Schristos<ul>
1.20Schristos<li>Levine J. <a href="https://www.preprints.org/manuscript/202406.0043/v1">A
1.20Schristosproposal to change the leap-second adjustments to
1.20Schristoscoordinated universal time</a>. <em>Metrologia.</em> 2024;61(5):055002. doi:<a
1.20Schristoshref="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a>.</li>
1.20Schristos</ul>
1.17SchristosHowever, there is still no consensus on whether this is the best way
1.17Schristosto replace leap seconds.
1.14Schristos</li>
1.1Schristos</ul>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.1Schristos<h2 id="notation">Time notation</h2>
1.1Schristos<ul>
1.13Schristos<li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
1.1SchristosRepository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1.1Schristoszone names, abbreviations, identifiers, and formats. For example, it
1.20Schristoscontains French translations for “Eastern European Summer Time”,
1.20Schristos“<abbr title="Eastern European Summer Time">EEST</abbr>”, and “Bucharest”. Its
1.1Schristos<a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1.1Schristoscharts</a> show these values for many locales. Data values are available in
1.1Schristosboth <abbr title="Locale Data Markup Language">LDML</abbr>
1.1Schristos(an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1.1Schristos<li>
1.1Schristos<a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1.6Schristosthe international standard date and time notation</a> covers
1.1Schristos<a
1.6Schristoshref="https://www.iso.org/standard/70907.html"><em><abbr
1.6Schristostitle="International Organization for Standardization">ISO</abbr>
1.20Schristos8601-1:2019 – Date and time – Representations for information
1.20Schristosinterchange – Part 1: Basic rules</em></a>.</li>
1.1Schristos<li>
1.6Schristos<a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1.20SchristosSchema: Datatypes – dateTime</a> specifies a format inspired by
1.1Schristos<abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1.20Schristos<li><a href="https://www.rfc-editor.org/rfc/rfc5322#section-3.3">§3.3 of
1.1SchristosInternet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1.1Schristosspecifies the time notation used in email and <a
1.1Schristoshref="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1.1Schristosheaders.</li>
1.1Schristos<li>
1.18Schristos<a href="https://www.rfc-editor.org/rfc/rfc3339">Date and Time
1.1Schristoson the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1.17Schristosspecifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.
1.18SchristosAn extension, <a href="https://www.rfc-editor.org/rfc/rfc9557">Date
1.17Schristosand Time on the Internet: Timestamps with Additional Information</a>
1.17Schristos(Internet <abbr>RFC</abbr> 9557) extends this profile
1.17Schristosto let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp
1.20Schristosvia suffixes like <code>[Asia/Tokyo]</code>.
1.1Schristos<li>
1.7Schristos<a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
1.1SchristosFormats on the Web</a> surveys web- and Internet-oriented date and time
1.1Schristosformats.</li>
1.1Schristos<li>Alphabetic time zone abbreviations should not be used as unique
1.1Schristosidentifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1.1Schristospractice. For example, in English-speaking North America
1.20Schristos“<abbr>CST</abbr>” denotes 6 hours behind <abbr>UT</abbr>,
1.1Schristosbut in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1.1Schristosand French-speaking North Americans prefer
1.20Schristos“<abbr title="Heure Normale du Centre">HNC</abbr>” to
1.20Schristos“<abbr>CST</abbr>”. The <code><abbr>tz</abbr></code>
1.2Schristosdatabase contains English abbreviations for many timestamps;
1.20Schristosunfortunately some of these abbreviations were merely the database maintainers’
1.1Schristosinventions, and these have been removed when possible.</li>
1.1Schristos<li>Numeric time zone abbreviations typically count hours east of
1.1Schristos<abbr>UT</abbr>, e.g., +09 for Japan and
1.20Schristos−10 for Hawaii. However, <abbr>POSIX</abbr> proleptic
1.17Schristos<code><abbr>TZ</abbr></code> settings use the opposite convention.
1.1SchristosFor example, one might use <code><abbr>TZ</abbr>="<abbr
1.1Schristostitle="Japan Standard Time">JST</abbr>-9"</code> and
1.1Schristos<code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1.1Schristosfor Japan and Hawaii, respectively. If the
1.1Schristos<code><abbr>tz</abbr></code> database is available, it is usually better to use
1.1Schristossettings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1.1Schristos<code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1.2Schristosconfusion, handle old timestamps better, and insulate you better from
1.1Schristosany future changes to the rules. One should never set
1.1Schristos<abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1.1Schristos<code>"GMT-9"</code>, though, since this would incorrectly imply that
1.1Schristoslocal time is nine hours ahead of <abbr>UT</abbr> and the time zone
1.20Schristosis called “<abbr>GMT</abbr>”.</li>
1.1Schristos</ul>
1.6Schristos</section>
1.6Schristos
1.6Schristos<section>
1.1Schristos<h2 id="see-also">See also</h2>
1.1Schristos<ul>
1.7Schristos<li><a href="theory.html">Theory and pragmatics of the
1.7Schristos<code><abbr>tz</abbr></code> code and data</a></li>
1.1Schristos<li><a href="tz-art.html">Time and the Arts</a></li>
1.1Schristos</ul>
1.6Schristos</section>
1.6Schristos
1.6Schristos<footer>
1.1Schristos<hr>
1.1SchristosThis web page is in the public domain, so clarified as of
1.1Schristos2009-05-17 by Arthur David Olson.
1.1Schristos<br>
1.1SchristosPlease send corrections to this web page to the
1.1Schristos<a href="mailto:tz@iana.org">time zone mailing list</a>.
1.20SchristosThe mailing list and its archives are public,
1.20Schristosso please do not send confidential information.
1.6Schristos</footer>
1.1Schristos</body>
1.1Schristos</html>