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<!DOCTYPE html><html lang="en"><head><title>Time zone and daylight saving time data</title><meta charset="UTF-8"><meta name="viewport" content="width=device-width, initial-scale=1"><style>dd {margin-left: 1.3rem;}pre {margin-left: 1.3rem; overflow: auto;}ul {padding-left: 1.3rem;}</style></head><body><h1>Time zone and daylight saving time data</h1><p><a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and<a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>rules are controlled by individualgovernments. They are sometimes changed with little notice, and theirhistories and planned futures are often recorded only fitfully. Hereis a summary of attempts to organize and record relevant data in thisarea.</p>  <nav>    <ul>      <li>The <code><abbr>tz</abbr></code> database product and process	<ul>	  <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>	  <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>	  <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>	  <li><a href="#coordinating">Coordinating with governments and distributors</a></li>	  <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>	</ul>      </li>      <li>Uses of the <code><abbr>tz</abbr></code> database	<ul>	  <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>	  <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>	  <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>	  <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>	  <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>	</ul>      </li>      <li>Related data	<ul>	  <li><a href="#other-dbs">Other time zone databases</a></li>	  <li><a href="#maps">Maps</a></li>	  <li><a href="#boundaries">Time zone boundaries</a></li>	</ul>      </li>      <li>Timekeeping concepts	<ul>	  <li><a href="#civil">Civil time concepts and history</a></li>	  <li><a href="#national">National histories of legal time</a></li>	  <li><a href="#costs">Costs and benefits of time shifts</a></li>	  <li><a href="#precision">Precision timekeeping</a></li>	  <li><a href="#notation">Time notation</a></li>	</ul>      </li>      <li><a href="#see-also">See also</a></li>    </ul>  </nav> <section><h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2><p>The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>time zone database contains code and datathat represent the history of local timefor many representative locations around the globe.It is updated periodically to reflect changes made by political bodiesto time zone boundaries and daylight saving rules.This database (known as <code><abbr>tz</abbr></code>,<code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)is used by several implementations,including<a href="https://www.gnu.org/software/libc/">the<abbr title="GNU’s Not Unix">GNU</abbr>C Library</a> (used in<a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),<a href="https://www.android.com">Android</a>,<a href="https://www.freebsd.org">Free<abbrtitle="Berkeley Software Distribution">BSD</abbr></a>,<a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,<a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,<a href="https://www.chromium.org/chromium-os/">ChromiumOS</a>,<a href="https://cygwin.com">Cygwin</a>,<a href="https://mariadb.org">MariaDB</a>,<a href="https://musl.libc.org">musl libc</a>,<a href="https://www.mysql.com">MySQL</a>,<a href="https://en.wikipedia.org/wiki/WebOS"><abbrtitle="Web Operating System">webOS</abbr></a>,<a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbrtitle="Advanced Interactive eXecutive">AIX</abbr></a>,<a href="https://www.apple.com/os/ios/"><abbrtitle="iPhone OS">iOS</abbr></a>,<a href="https://www.apple.com/os/macos/">macOS</a>,<a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,<a href="https://vmssoftware.com">Open<abbrtitle="Virtual Memory System">VMS</abbr></a>,<a href="https://www.oracle.com/database/">Oracle Database</a>,<a href="https://www.oracle.com/solaris/solaris11/">Oracle Solaris</a>,and <a href="https://qnx.software/en">QNX</a>.</p><p>Each main entry in the database represents a <dfn>timezone</dfn>for a set of civil-time clocks that have all agreed since 1970.Timezones are typically identified by continent or ocean and then by thename of the largest city within the region containing the clocks.For example, <code>America/New_York</code>represents most of the <abbr title="United States">US</abbr> eastern time zone;<code>America/Phoenix</code> represents most of Arizona, whichuses mountain time without daylight saving time (<abbr>DST</abbr>);<code>America/Detroit</code> represents most of Michigan, which useseastern time but with different <abbr>DST</abbr> rules in 1975;and other entries represent smaller regions like Starke County,Indiana, which switched from central to eastern time in 1991and switched back in 2006.To use the database on a <ahref="https://en.wikipedia.org/wiki/POSIX"><abbrtitle="Portable Operating System Interface">POSIX</abbr>.1-2024</a>implementation set the <code><abbr>TZ</abbr></code>environment variable to the location’s full name,e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p><p>Associated with each timezone is a history of offsets from<a href="https://en.wikipedia.org/wiki/Universal_Time">UniversalTime</a> (<abbr>UT</abbr>), which is <ahref="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich MeanTime</a> (<abbr>GMT</abbr>) with days beginning at midnight;for timestamps after 1960 this is more precisely <ahref="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">CoordinatedUniversal Time</a> (<abbr>UTC</abbr>).The database also records when daylight saving time was in use,along with some time zone abbreviations such as <abbr>EST</abbr>for Eastern Standard Time in the <abbr>US</abbr>.</p></section> <section><h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2><p>The following <ahref="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands downloadthe latest release’s two<a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>to a <abbr>GNU</abbr>/Linux or similar host.</p><pre><code>mkdir tzdbcd tzdb<a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gzwget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz<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 -gzip -dc tzdata-latest.tar.gz | tar -xf -</code></pre><p>Alternatively, the following shell commands download the samerelease in a single-tarball format containing extra datauseful for regression testing:</p><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><a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -</code></pre><p>These commands use convenience links to the latest releaseof the <code><abbr>tz</abbr></code> database hosted by the<a href="https://www.iana.org/time-zones">Time Zone Database website</a>of the <a href="https://www.iana.org">Internet Assigned NumbersAuthority (IANA)</a>.Older releases are in files named<code>tzcode<var>V</var>.tar.gz</code>,<code>tzdata<var>V</var>.tar.gz</code>, and<code>tzdb-<var>V</var>.tar.lz</code>,where <code><var>V</var></code> is the version.Since 1996, each version has been a four-digit year followed bylower-case letter (<samp>a</samp> through <samp>z</samp>,then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>through <samp>zzz</samp>, and so on).Since version 2022a, each release has been distributed in<a href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/pax.html#tag_20_94_13_06">POSIXustar interchange format</a>, compressed as described above;older releases use a nearly compatible format.Since version 2016h, each release has contained a text file named“<code>version</code>” whose first (and currently only) line is the version.<a href="https://ftp.iana.org/tz/releases/">Older archived releases areavailable</a> via<a href="https://en.wikipedia.org/wiki/HTTPS"><abbrtitle="Hypertext Transfer Protocol Secure">HTTPS</abbr></a>,<a href="https://en.wikipedia.org/wiki/Rsync"><abbrtitle="remote sync">rsync</abbr></a>, and<a href="https://en.wikipedia.org/wiki/FTP"><abbrtitle="File Transfer Protocol">FTP</abbr></a>.<p>Alternatively, a development repository of code and data can beretrieved from <a href="https://github.com">GitHub</a> via the shellcommand:</p><pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a></code></pre><p>Since version 2012e, each release has been tagged in development repositories.Untagged commits are less well tested and probably containmore errors.</p><p>After obtaining the code and data files, see the<code>README</code> file for what to do next.The code lets you compile the <code><abbr>tz</abbr></code> source files intomachine-readable binary files, one for each location. The binary filesare in a special format specified by<a href="https://www.rfc-editor.org/rfc/rfc9636">TheTime Zone Information Format (<abbr>TZif</abbr>)</a>(Internet <abbr title="Request For Comments">RFC</abbr> 9636).The code also letsyou read a <abbr>TZif</abbr> file and interpret timestamps for thatlocation.</p></section> <section><h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2><p>The <code><abbr>tz</abbr></code> code and dataare by no means authoritative. If you find errors, pleaseemail changes to <a href="mailto:tz@iana.org"><code>tz@iana.org</code></a>,the time zone mailing list.The mailing list and its archives are public,so please do not send confidential information.See<a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailinglist’s main page</a> to subscribe or to browse its archive of old messages.<a href="https://tzdata-meta.timtimeonline.com">Metadata for mailing listdiscussions</a> and corresponding data changes can begenerated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.</p><p>Changes to the <code><abbr>tz</abbr></code> code and data are oftenpropagated to clients via operating system updates, soclient <code><abbr>tz</abbr></code> data can often be corrected byapplying these updates. With GNU/Linux and similar systems, if yourmaintenance provider has not yet adopted thelatest <code><abbr>tz</abbr></code> data, you can often short-circuitthe process by tailoring the generic instructions inthe <code><abbr>tz</abbr> README</code> file and installing the latestdata yourself. System-specific instructions for installing thelatest <code><abbr>tz</abbr></code> data have also been publishedfor <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates"><abbr>AIX</abbr></a>,<ahref="https://source.android.com/docs/core/permissions/timezone-rules">Android</a>,<ahref="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbrtitle="International Components for Unicode">ICU</abbr></a>,<a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>JDK</a>,<a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <ahref="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,<a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <ahref="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.</p><p>Since version 2013a,sources for the <code><abbr>tz</abbr></code> database have been<a href="https://en.wikipedia.org/wiki/UTF-8"><abbrtitle="Unicode Transformation Format 8-bit">UTF-8</abbr></a><a href="https://en.wikipedia.org/wiki/Text_file">text files</a>with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbrtitle="linefeed">LF</abbr></a>,which can be modified by common text editors suchas <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,<a href="https://gedit-text-editor.org">gedit</a>, and<a href="https://www.vim.org">vim</a>.Specialized source-file editing can be done via the<a href="https://packagecontrol.io/packages/zoneinfo">Sublimezoneinfo</a> package for <ahref="https://www.sublimetext.com">Sublime Text</a> and the <ahref="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCodezoneinfo</a> extension for <a href="https://code.visualstudio.com">VisualStudio Code</a>.</p><p>For further information about updates, please see<a href="https://www.rfc-editor.org/rfc/rfc6557">Procedures forMaintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557).More detail can befound in <a href="theory.html">Theory and pragmatics of the<code><abbr>tz</abbr></code> code and data</a>.<a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>displays changes between recent <code><abbr>tzdb</abbr></code> versions.</p></section> <section><h2 id="coordinating">Coordinating with governments and distributors</h2><p>As discussed in“<a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">HowTime Zones Are Coordinated</a>”, the time zone database relies oncollaboration among governments, the time zone database volunteercommunity, and data distributors downstream.<p>If your government plans to change its time zone boundaries ordaylight saving rules, please send email as described in“<a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a>”.Do this well in advance,as this will lessen confusion and will coordinate updates to many cell phones,computers, and other devices around the world.In your email, please cite the legislation or regulation that specifiesthe change, so that it can be checked for details such as the exact timeswhen clock transitions occur.It is OK if a rule change is planned to affect clocksfar into the future, as a long-planned change can easily be revertedor otherwise altered with a year’s notice before the change would haveaffected clocks.</p><p>There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.However, typically a release occurs every few months.Many downstream timezone data distributors wait fora <code><abbr>tzdb</abbr></code> release before they produce an updateto time zone behavior in consumer devices and software products.After a release, various parties must integrate, test,and roll out an update before <ahref="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.These updates can be expensive, for both the <ahref="https://en.wikipedia.org/wiki/Quality_assurance">qualityassurance</a> process and the overall cost of shipping and installingupdates to each device’s copy of <code><abbr>tzdb</abbr></code>.Updates may be batched with other updates and may take substantialtime to reach end users after a release.Older devices may no longer be supported and thus may never be updated,which means they will continue to use out-of-date rules.</p><p>For these reasons any rule change should be promulgated at least ayear before it affects how clocks operate; otherwise, there is a goodchance that many clocks will be wrong due to delays in propagating updates,and that residents will be confused or even actively resist the change.The shorter the notice, the more likely clock problems will arise; see “<ahref="https://codeofmatt.com/on-the-timing-of-time-zone-changes/">Onthe Timing of Time Zone Changes</a>” for examples.</p></section> <section><h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2><ul><li>The article<a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> isan encyclopedic summary.</li><li><a href="tz-how-to.html">How to Read thetz Database Source Files</a> explains the <code><abbr>tz</abbr></code>database format.</li><li><ahref="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">Aliterary appreciation of the Olson/Zoneinfo/tz database</a> comments on thedatabase’s style.</li><li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:managing time in the internet</a> analyzes the database longitudinally.</li></ul></section> <section><h2 id="web">Web sites using recent versions of the<code><abbr>tz</abbr></code> database</h2><p>These are listed roughly in ascending order of complexity and fanciness.</p><ul><li><a href="https://time.is">Time.is</a> shows locations’time and zones.</li><li><a href="https://www.timejones.com">TimeJones.com</a>,<a href="https://timezoneconverterapp.com">Time Zone Converter</a> and<a href="https://www.worldclock.com">The World Clock</a>are time zone converters.</li><li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>publishes <code><abbr>tzdb</abbr></code>-derived data in<a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbrtitle="comma-separated values">CSV</abbr></a> andin <a href="https://en.wikipedia.org/wiki/SQL"><abbrtitle="Structured Query Language">SQL</abbr></a> form.</li><li><ahref="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>lets you see the <code><abbr>TZ</abbr></code> values directly.</li><li><ahref="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">CurrentTime in 1000 Places</a> uses descriptions of the values.</li><li><a href="https://www.timeanddate.com/worldclock/">The World Clock –Worldwide</a> lets you sort zone names and convert times.</li><li><a href="https://24timezones.com">24TimeZones</a> has a worldtime map and a time converter.</li><li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>calculates the current time difference between locations.</li><li><a href="https://www.wx-now.com">Weather Now</a> and<a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li></ul></section> <section><h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2><ul><li><a href="https://www.rfc-editor.org/rfc/rfc7808">Time ZoneData Distribution Service</a> (TZDIST, Internet <abbr>RFC</abbr> 7808)is associated with<a href="https://www.rfc-editor.org/rfc/rfc7809">CalDAV</a>(Internet <abbr>RFC</abbr> 7809), a calendar access protocol fortransferring time zone data by reference.<a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDISTimplementations</a> are available.</li><li>The <a href="https://www.rfc-editor.org/rfc/rfc5545">iCalendar format</a>(Internet <abbr>RFC</abbr> 5445)covers time zonedata; see its VTIMEZONE calendar component.The iCalendar format requires specialized parsers and generators; avariant <a href="https://www.rfc-editor.org/rfc/rfc6321">xCal</a>(Internet <abbr>RFC</abbr> 6321) uses<a href="https://www.w3.org/XML/"><abbrtitle="Extensible Markup Language">XML</abbr></a> format, and a variant<a href="https://www.rfc-editor.org/rfc/rfc7265">jCal</a>(Internet <abbr>RFC</abbr> 7265)uses <a href="https://www.json.org/json-en.html"><abbrtitle="JavaScript Object Notation">JSON</abbr></a> format.</li></ul></section> <section><h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2><p>Although some of these do not fully support<code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>distributions you can generally work around compatibility problems byrunning the command <code>make rearguard_tarballs</code> and compilingfrom the resulting tarballs instead.</p><ul><li><a href="https://github.com/libical/vzic">Vzic</a> is a <ahref="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>program that compiles<code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.Vzic is freelyavailable under the <ahref="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>General Public License (<abbrtitle="General Public License">GPL</abbr>)</a>.</li><li><ahref="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>contains a script <code>parse_olson</code> that compiles<code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>modules. It is part of the Perl <ahref="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,which is freelyavailable under both the <abbr>GPL</abbr> and the Perl ArtisticLicense. DateTime::TimeZone also contains a script<code>tests_from_zdump</code> that generates test cases for each clocktransition in the <code><abbr>tz</abbr></code> database.</li><li>The <a href="https://howardhinnant.github.io/date/tz.html">Time ZoneDatabase Parser</a> is a<a href="https://en.wikipedia.org/wiki/C++">C++</a> parser andruntime library with a <ahref="https://en.cppreference.com/cpp/chrono"><code>std::chrono</code> API</a>that is a standard part of C++.It is freely available under the<abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li><li><a id="ICU" href="https://icu.unicode.org">International Components forUnicode (<abbr>ICU</abbr>)</a> contains C/C++ and <ahref="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a>libraries for internationalization thathas a compiler from <code><abbr>tz</abbr></code> sourceand from <abbr title="Common Locale Data Repository">CLDR</abbr> data(mentioned <a href="#CLDR">below</a>)into an <abbr>ICU</abbr>-specific format.<abbr>ICU</abbr> is freely available under a<abbr>BSD</abbr>-style license.</li><li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package forthe <a href="https://elixir-lang.org">Elixir</a> language downloadsand compiles <code><abbr>tz</abbr></code> source and exposes <abbrtitle="Application Program Interface">API</abbr>s for use. It isfreely available under the <abbr>MIT</abbr> license.</li><li>Java-based compilers and libraries include:<ul><li>The <ahref="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdatertool</a> compiles <code><abbr>tz</abbr></code> source into the format used by<a href="https://openjdk.org">OpenJDK</a> and<a href="https://jdk.java.net">Oracle JDK</a>.Although its source code is proprietary, its executable is available under the<a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SETimezone Updater License Agreement</a>.</li><li>The <ahref="https://www.oracle.com/technical-resources/articles/java/jf14-date-time.html">JavaSE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <ahref="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,which is freely available under a <abbr>BSD</abbr>-style license.</li><li><a href="https://www.joda.org/joda-time/">Joda-Time – Java dateand time <abbr>API</abbr></a> contains a class<code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles<code><abbr>tz</abbr></code> source into a binary format. It inspiredJava 8 <code>java.time</code>, which its users should migrate to oncethey can assume Java 8 or later. It is available under the <ahref="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li><li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <ahref="https://www.azul.com/products/components/ziupdater-time-zone-tool/">ZIUpdater</a>are alternatives to TZUpdater. IANA Updater’s license is unclear;ZIUpdater is licensed under the <abbr>GPL</abbr>.</li><li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date andtime library for Android</a> and<a href="https://github.com/MenoData/Time4J">Time4J: Advanced date,time and interval library for Java</a> compile<code><abbr>tz</abbr></code> source into a binary format.Time4A is available under the Apache License and Time4J isavailable under the <ahref="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> LesserGeneral Public License (<abbr title="Lesser General PublicLicense">LGPL</abbr>)</a>.</li><li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers andJava-based libraries.</li></ul><li><a href="https://nodatime.org">Noda Time – Date andtime <abbr>API</abbr> for .NET</a>is like Joda-Time and Time4J, but for the .NET framework instead of Java.It is freely available under the Apache License.</li><li>Many modern<a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>runtimes support <code><abbr>tz</abbr></code> natively via the<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>.This can be used as-is or with most of the following libraries,many of which also support runtimes lacking the <code>timeZone</code> option.<ul><li>The <ahref="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code>timezone polyfill</a>is freely available under a <abbr>BSD</abbr>-style license.</li><li>The <a href="https://date-fns.org">date-fns</a>library manipulates timezone-aware timestamps in browsers andin <a href="https://nodejs.org/en/">Node.js</a>.It is freely available under the <abbr>MIT</abbr> license.</li><li><a href="https://github.com/iamkun/dayjs">Day.js</a> is aminimalist replacement for the date and time API ofthe <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> datemanipulation library.It is freely available under the <abbr>MIT</abbr> license.</li><li><a href="https://moment.github.io/luxon/">Luxon</a> improvestimezone support for the <code>Intl</code> API.It is freely available under the <abbr>MIT</abbr> license.</li><li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is aMoment.js plugin.It is freely available under the <abbr>MIT</abbr> license.</li><li><a href="https://github.com/bigeasy/timezone">Timezone</a> is aJavaScript library that supports date arithmetic that is time zoneaware. It is freely available under the <abbr>MIT</abbr> license.</li><li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>supports live <code><abbr>tzdb</abbr></code> updates,astronomical and atomic time, a command-line interface,and full <ahref="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.Its companion <ahref="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>can generate <abbr>TZif</abbr> and other files, and a companion website<a href="https://tzexplorer.org">Timezone Database Explorer</a> lets youconvert timestamps, view transition histories, and download code and data.It is freely available under the <abbr>MIT</abbr> license.</li></ul>The proposed <ahref="https://github.com/tc39/proposal-temporal"><code>Temporal</code>objects</a> let programs access an abstract view of<code><abbr>tzdb</abbr></code> data, and are designed to replace <ahref="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript’sproblematic <code>Date</code> objects</a> when working with dates and times.<li><a href="https://github.com/JuliaTime">JuliaTime</a> contains acompiler from <code><abbr>tz</abbr></code> source into<a href="https://julialang.org">Julia</a>. It is freely availableunder the <abbr>MIT</abbr> license.</li><li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –<abbr>IANA</abbr> Time Zone Database for Delphi/<abbrtitle="Free Pascal Compiler">FPC</abbr></a>compiles from <code><abbr>tz</abbr></code> source into<a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>and <ahref="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.It is freely available under a <abbr>BSD</abbr>-style license.</li><li><a href="https://pythonhosted.org/pytz/">pytz – World TimezoneDefinitions for Python</a> compiles <code><abbr>tz</abbr></code> source into<a href="https://www.python.org">Python</a>.It is freely available under a <abbr>BSD</abbr>-style license.In code that can assume Python 3.6 or later it is largely superseded; see <ahref="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:The Fastest Footgun in the West</a>.</li><li><a href="https://tzinfo.github.io">TZInfo –Ruby Timezone Library</a>compiles <code><abbr>tz</abbr></code> source into<a href="https://www.ruby-lang.org/en/">Ruby</a>.It is freely available under the <abbr>MIT</abbr> license.</li><li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/TimeLibrary</a> isa <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> classlibrary that compiles <code><abbr>tz</abbr></code> source into a timezone repository whose formatis either proprietary or an <abbr>XML</abbr>-encodedrepresentation.</li><li><a id="Tcl" href="https://www.tcl-lang.org">Tcl</a>contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>source into text files, along with a runtime that can read thosefiles. Tcl is freely available under a <abbr>BSD</abbr>-stylelicense.</li></ul></section> <section><h2 id="TZif">Other <abbr>TZif</abbr> readers</h2><ul><li>The <ahref="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> CLibrary</a>has an independent, thread-safe implementation ofa <abbr>TZif</abbr> file reader.This library is freely available under the LGPLand is widely used in <abbr>GNU</abbr>/Linux systems.</li><li><a href="https://www.gnome.org">GNOME</a>’s<a href="https://docs.gtk.org/glib/">GLib</a> hasa <abbr>TZif</abbr> file reader written in C thatcreates a <code>GTimeZone</code> object representing setsof <abbr>UT</abbr> offsets.It is freely available under the <abbr>LGPL</abbr>.</li><li>The<a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>’s<code>baltzo::TimeZoneUtil</code> component contains a C++implementation of a <abbr>TZif</abbr> file reader. It is freely available underthe Apache License.</li><li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++library that translates between <abbr>UT</abbr> and civil time andcan read <abbr>TZif</abbr> files. It is freely available under the ApacheLicense.</li><li>The <a href="https://go.dev">Go programming language</a>has a <abbr>TZif</abbr> file reader <ahref="https://pkg.go.dev/time#LoadLocationFromTZData"><code>LoadLocationFromTZData</code></a>.</li><li>The<a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code>package</a> contains Python codeto generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map<code><abbr>tz</abbr></code> settings to proleptic TZ approximations.For example, it maps <code>"Africa/Cairo"</code>to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>,an approximation valid for Cairo timestamps from 2023 on.This can help porting to platforms that support only proleptic TZ.The package is freely available under the MIT license.</li><li><a href="https://github.com/derickr/timelib">Timelib</a> is a Clibrary that reads <abbr>TZif</abbr> files and convertstimestamps from one time zone or format to another.It is used by <a href="https://www.php.net"><abbrtitle="PHP: Hypertext Preprocessor">PHP</abbr></a>,<a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,and <a href="https://www.mongodb.com">MongoDB</a>.It is freely available under the <abbr>MIT</abbr> license.</li><li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a<abbr>TZif</abbr> file reader.</li><li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">DateTime::TimeZone::Tzfile</a>is a <abbr>TZif</abbr> file reader written in Perl.It is freely available under the same terms as Perl(dual <abbr>GPL</abbr> and Artistic license).</li><li>Python has a <a id="python-zoneinfo"href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>class</a> that reads <abbr>TZif</abbr> data and creates objectsthat represent <code><abbr>tzdb</abbr></code> timezones.Python is freely available under the<a href="https://docs.python.org/3/license.html">Python Software FoundationLicense</a>.A companion <a id="pypi-tzdata" href="https://pypi.org">PyPI</a> module<a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>supplies TZif data if the underlying system data cannot be found;it is freely available under the Apache License.</li><li>Thepublic-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>library contains a Python tool thatconverts <abbr>TZif</abbr> data into<abbr>JSON</abbr>-format data suitable for usein its JavaScript library for time zone conversion. Dates before 1970are not supported.</li><li>The <ahref="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>package contains <a href="https://www.haskell.org">Haskell</a> code thatparses and uses <abbr>TZif</abbr> data. It is freelyavailable under a <abbr>BSD</abbr>-style license.</li></ul></section> <section><h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2><ul><li><a href="https://foxclocks.org">FoxClocks</a>is an extension for <a href="https://www.google.com/chrome/">GoogleChrome</a>, <ahref="https://www.firefox.com/en-US/">Firefox</a> and <ahref="https://www.thunderbird.net/en-US/">Thunderbird</a>.It displays multiple clocks in the application window, and has a mappinginterface to <a href="https://earth.google.com/web/">Google Earth</a>.It is freely available under the <abbr>GPL</abbr>.</li><li>Microsoft Windows 8.1and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>data (mentioned <a href="#CLDR">below</a>) used by the<a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /<a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes<a href="https://learn.microsoft.com/en-us/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and<a href="https://learn.microsoft.com/en-us/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.<a id="System.TimeZoneInfo"href="https://learn.microsoft.com/en-us/archive/blogs/bclteam/exploring-windows-time-zones-with-system-timezoneinfo-josh-free">ExploringWindows Time Zones with <code>System.TimeZoneInfo</code></a> describesthe older, proprietary method of Microsoft Windows 2000 and later,which stores time zone data in the<a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The<ahref="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone→ Tzid table</a> or <ahref="https://github.com/unicode-org/cldr/blob/main/common/supplemental/windowsZones.xml"><abbr>XML</abbr>file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDsto <code><abbr>tz</abbr></code> names.These mappings can be performed programmatically via the<a href="https://github.com/mattjohnsonpint/TimeZoneConverter">TimeZoneConverter</a>.NET library,or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.<li><ahref="https://www.oracle.com/java/">OracleJava</a> contains a copy of a subset of a recent<code><abbr>tz</abbr></code> database in aJava-specific format.</li></ul></section> <section><h2 id="other-dbs">Other time zone databases</h2><ul><li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>is Astrodienst’s Web version of Shanks and Pottenger’s out-of-printtime zone history atlases<a href="https://search.worldcat.org/title/468828649">for the US</a> and<a href="https://search.worldcat.org/title/76950459">for the world</a>.Although these extensive atlases<a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">weresources for much of the older <code><abbr>tz</abbr></code> data</a>,they are unreliable as Shanks appears to haveguessed many <abbr>UT</abbr> offsets and transitions. The atlases cite nosources and do not indicate which entries are guesswork.</li><li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database inits own <code>tztab</code>(4) format.</li><li>Microsoft Windows has proprietary data mentioned<a href="#System.TimeZoneInfo">above</a>.</li><li><a href="https://www.worldtimeserver.com">World Time Server</a>is another time zone database.</li><li>The <ahref="https://www.iata.org/en/publications/manuals/standard-schedules-information">StandardSchedules Information Manual</a> of theInternational Air Transport Associationgives current time zone rules for airports served by commercial aviation.</li></ul></section> <section><h2 id="maps">Maps</h2><ul><li><a href="https://www.worldtimezone.com">World Time Zone Mapwith current time</a>has several fancy time zone maps; it covers Russia particularly well.</li><li><ahref="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">Howmuch is time wrong around the world?</a> maps the difference betweenmean solar and standard time, highlighting areas such as western Chinawhere the two differ greatly. It’s a bit out of date, unfortunately.</li><li>The<ahref="https://maps.lib.utexas.edu/maps/world.html">Perry–CastañedaLibrary Map Collection</a> of the University of Texas at Austin hascopies of old maps taken from <ahref="https://en.wikipedia.org/wiki/The_World_Factbook"><em>TheWorld Factbook</em></a>, formerly published by the<a href="https://www.cia.gov">US Central Intelligence Agency</a>.Although the maps’ pictorial quality is good,the maps do not indicate daylight saving time.</li></ul></section> <section><h2 id="boundaries">Time zone boundaries</h2><p>Geographical boundaries between timezones are availablefrom several <ahref="https://en.wikipedia.org/wiki/Internet_geolocation">Internetgeolocation</a>services and other sources.</p><ul><li><a href="https://github.com/evansiroky/timezone-boundary-builder">TimezoneBoundary Builder</a> extracts<a href="https://www.openstreetmap.org">Open Street Map</a> data to buildboundaries of <code><abbr>tzdb</abbr></code> timezones.Its code is freely available under the <abbr>MIT</abbr> license, andits data entries are freely available under the<a href="https://opendatacommons.org/licenses/odbl/">Open Data CommonsOpen Database License</a>. The borders appear to be quite accurate.Its main web page lists more than twenty librariesfor looking up a timezone name from a GPS coordinate.</li><li>Free access via a network API, if you register a key, is provided bythe <ahref="https://www.geonames.org/export/web-services.html#timezone">GeoNamesTimezone web service</a>, the <ahref="https://developers.google.com/maps/documentation/timezone/overview">GoogleMaps Time Zone API</a>, andthe <a href="https://timezonedb.com/api">TimeZoneDB API</a>.Commercial network API access is providedby <a href="https://askgeo.com">AskGeo</a>and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.</li><li>“<ahref="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">Howto get a time zone from a location using latitude and longitudecoordinates?</a>” discusses other geolocation possibilities.</li><li><a href="https://statoids.com/statoids.html">AdministrativeDivisions of Countries (“Statoids”)</a> listspolitical subdivision data related to time zones.</li><li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software– GIS and Database Tools</a> includes a Manifold-format map ofworld time zone boundaries circa 2007, distributed under the<abbr>GPL</abbr>.</li><li>A ship within the <ahref="https://en.wikipedia.org/wiki/Territorial_waters">territorialwaters</a> of any nation uses that nation’s time. In internationalwaters, time zone boundaries are meridians 15° apart, except that<abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°wide and are separated bythe 180° meridian (not by the International Date Line, which isfor land and territorial waters only). A captain can change ship’sclocks any time after entering a new time zone; midnight changes arecommon.</li></ul></section> <section><h2 id="civil">Civil time concepts and history</h2><ul><li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">AWalk through Time</a>surveys the evolution of timekeeping.</li><li>The history of daylight saving time is surveyed in <ahref="https://www.webexhibits.org/daylightsaving/">About DaylightSaving Time – History, rationale, laws &amp; dates</a> and summarized in<a href="http://seizethedaylight.com/dst/">A BriefHistory of Daylight Saving Time</a>.</li><li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">TimeLords</a> discusses how authoritarians manipulate civil time.</li><li><a href="https://www.w3.org/TR/timezone/">Working with Timeand Time Zones</a>contains guidelines and best practices for software applications thatdeal with civil time.</li><li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History ofthe International Date Line</a> tells the story of the most importanttime zone boundary.</li><li><a href="https://statoids.com/tconcept.html">Basic TimeZone Concepts</a> discusses terminological issues behind time zones.</li></ul></section> <section><h2 id="national">National histories of legal time</h2><dl><dt>Australia</dt><dd>The Bureau of Meteorology publishes a list of <ahref="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">ImplementationDates of Daylight Savings Time within Australia</a>.</dd><dt>Belgium</dt><dd>The Royal Observatory of Belgium maintains a table of time inBelgium (in<a href="https://robinfo.oma.be/nl/astro-info/tijd/"hreflang="nl">Dutch</a> and <ahref="https://robinfo.oma.be/fr/astro-info/heure/"hreflang="fr">French</a>).</dd><dt>Brazil</dt><dd>The Time Service Department of the National Observatoryrecords <a href="http://pcdsh01.on.br/DecHV.html"hreflang="pt-BR">Brazil’s daylight saving time decrees (inPortuguese)</a>.</dd><dt>Canada</dt><dd>National Research Council Canada publishes currentand some older information about <ahref="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">timezones and daylight saving time</a>.</dd><dt>Chile</dt><dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a<a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history ofChile’s official time (in Spanish)</a>.</dd><dt>China</dt><dd>The Hong Kong Observatory maintains a<a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of summer time in Hong Kong</a>,and Macau’s Meteorological and Geophysical Bureau maintains a <ahref="https://www.smg.gov.mo/en/subpage/224/page/174">similarhistory for Macau</a>.Unfortunately the latter is incomplete and has errors.</dd><dt>Czech Republic</dt><dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"hreflang="cs">When daylight saving time starts and ends (in Czech)</a>summarizes and cites historical <abbr>DST</abbr> regulations.</dd><dt>Germany</dt><dd>The National Institute for Science and Technology maintains the <ahref="https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisationof Legal Time in Germany</a>.</dd><dt>Israel</dt><dd><ahref="https://tz.cs.huji.ac.il">Israel Timezone Files</a>lists official time-change announcements and laws since 1940,almost all in Hebrew.</dd><dt>Malaysia</dt><dd>See Singapore <a href="#Singapore">below</a>.</dd><dt>Mexico</dt><dd>The Investigation and Analysis Service of the Mexican Library ofCongress has published a <ahref="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"hreflang="es">history of Mexican local time (in Spanish)</a>.</dd><dt>Netherlands</dt><dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"hreflang="nl">Legal time in the Netherlands (in Dutch)</a>covers the history of local time in the Netherlands from ancient times.</dd><dt>New Zealand</dt><dd>The Department of Internal Affairs maintains a brief <ahref="https://www.govt.nz/browse/recreation-and-the-environment/daylight-saving/history-of-daylight-saving-in-nz/">Historyof Daylight Saving in NZ</a>.</dd><dt>Palestine</dt><dd>The Ministry of Telecom and Digital Economy publishes a <ahref="https://mtde.gov.ps/home/TimeZone"hreflang="ar">history of clock changes (in Arabic)</a>.</dd><dt>Portugal</dt><dd>The Lisbon Astronomical Observatory publishes a<a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history oflegal time (in Portuguese)</a>.</dd><dt>Singapore</dt><dd><a id="Singapore"href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Whyis Singapore in the “Wrong” Time Zone?</a> details thehistory of legal time in Singapore and Malaysia.</dd><dt>United Kingdom</dt><dd><ahref="https://www.polyomino.org.uk/british-time/">History oflegal time in Britain</a> discusses in detail the countrywith perhaps the best-documented history of clock adjustments.</dd><dt>United States</dt><dd>The Department of Transportation’s <ahref="https://www.transportation.gov/regulations/recent-time-zone-proceedings">RecentTime Zone Proceedings</a> lists changes toofficial written time zone boundaries, and its <ahref="https://geodata.bts.gov/datasets/usdot::time-zones/about">TimeZones dataset</a> maps current boundaries.These boundaries are only for standard time, so the current map putsall of Arizona in one time zone even though part of Arizonaobserves <abbr>DST</abbr> and part does not.</dd><dt>Uruguay</dt><dd>The Oceanography, Hydrography, and Meteorology Service of the UruguayanNavy (SOHMA) publishes an annual <ahref="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac(in Spanish)</a>.</dd></dl></section> <section><h2 id="costs">Costs and benefits of time shifts</h2><p>Various sources argue for and against daylight saving time and timezone shifts, and many scientific studies have been conducted. Thissection summarizes reviews and position statements based onscientific literature in the area.</p><ul><li>Carey RN, Sarma KM.<a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact ofdaylight saving time on road traffic collision risk: a systematicreview</a>.<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>.This reviews research literature and concludes that the evidenceneither supports nor refutes road safety benefits fromshifts in time zones.</li><li>Havranek T, Herman D, Irsova D.Does daylight saving save electricity? A meta-analysis.<em>Energy J.</em> 2018;39(2):35–61.doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.This analyzes research literature and concludes, “Electricity savingsare larger for countries farther away from the equator, whilesubtropical regions consume more electricity because of <abbr>DST</abbr>.”</li><li>Neumann P, von Blanckenburg K. <ahref="https://journals.sagepub.com/doi/full/10.1177/0961463X241310562">Whattime will it be? A comprehensive literature review on daylight saving time</a>.<em>Time Soc</em>. 2025;34(4):684–745.doi:<a href="https://doi.org/10.1177/0961463X241310562">10.1177/0961463X241310562</a>.This reviews <abbr>DST</abbr>’s effects on electricity, health, crime, roadsafety, and the economy, focusing on research since 2010, and concludes thatyear-round standard time is preferable overall.</li><li>Romigi A, Franco V, Scoditti E <em>et al</em>.The effects of daylight saving time and clock time transitions on sleep andsleepiness: a systematic review. <em>Sleep Med Rev.</em> 2025;84:102161. doi:<ahref="https://doi.org/10.1016/j.smrv.2025.102161">10.1016/j.smrv.2025.102161</a>.This reviews <abbr>DST</abbr> and <abbr>DST</abbr> transitions,and concludes that they both harm sleep, health and behavior.</li></ul> <p>The following medical societies have taken positions on theadvisability of clock shifts:</p> <ul><li>In 2022 the American Medical Association issued a<a href="https://www.ama-assn.org/press-center/ama-press-releases/ama-calls-permanent-standard-time">statementsupporting permanent standard time</a> on health grounds.</li><li>Crawford MR, Winnebeck EC, von Schantz M <em>et al</em>.<a href="https://onlinelibrary.wiley.com/doi/10.1111/jsr.14352">TheBritish Sleep Society position statement on Daylight Saving Time in the UK</a>.<em>J Sleep Res.</em> 2025;34(3):e14352.doi:<a href="https://doi.org/10.1111/jsr.14352">10.1111/jsr.14352</a>.This recommends that the UK abolish <abbr>DST</abbr> for health reasons.</li><li>Malow BA. <ahref="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is timeto abolish the clock change and adopt permanentstandard time in the United States:a Sleep Research Society position statement</a>.<em>Sleep.</em> 2022;45(12):zsac236.doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.The Sleep Research Societyadvocates permanent standard time due to its health benefits.</li><li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.<a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard timeis the optimal choice for health and safety:an American Academy of Sleep Medicine position statement</a>.<em>J Clin Sleep Med.</em> 2024;20(1):121–125.doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.The AASM argues for permanent standard time due to health and safety risksand economic costs of both <abbr>DST</abbr> transitions andpermanent <abbr>DST</abbr>.</li><li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.<a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Whyshould we abolish Daylight Saving Time?</a><em>J Biol Rhythms.</em> 2019;34(3):227–230.doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.The Society for Research on Biological Rhythmsopposes <abbr>DST</abbr> changes and permanent <abbr>DST</abbr>,and advocates that governments adopt“permanent Standard Time for the health and safety of their citizens”.</li></ul></section> <section><h2 id="precision">Precision timekeeping</h2><ul><li><ahref="http://leapsecond.com/hpan/an1289.pdf">TheScience of Timekeeping</a> is a thorough introductionto the theory and practice of precision timekeeping.</li><li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science ofTime 2016</a> contains several freely readable papers.</li><li><a href="https://www.ntp.org"><abbrtitle="Network Time Protocol">NTP</abbr>: The NetworkTime Protocol</a> (Internet <abbr>RFC</abbr> 5905)discusses how to synchronize clocks ofInternet hosts.</li><li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>family of software algorithms can achieve accuracy to a few tens ofnanoseconds in scalable server farms without special hardware.</li><li>The <ahref="https://www.nist.gov/el/intelligent-systems-division-73500/ieee-1588">PrecisionTime Protocol</a> (<abbrtitle="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)can achieve submicrosecond clock accuracy on a local area networkwith special-purpose hardware.</li><li><ahref="https://www.rfc-editor.org/rfc/rfc4833">TimezoneOptions for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>(Internet <abbr>RFC</abbr> 4833)specifies a <ahref="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>option for a server to configurea client’s time zone and daylight saving settings automatically.</li><li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">TimeScales</a> describes astronomical time scales like<abbr title="Terrestrial Dynamic Time">TDT</abbr>,<abbr title="Geocentric Coordinate Time">TCG</abbr>, and<abbr title="Barycentric Dynamic Time">TDB</abbr>.<li>The <a href="https://www.iau.org"><abbrtitle="International Astronomical Union">IAU</abbr></a>’s <ahref="https://www.iausofa.org"><abbrtitle="Standards Of Fundamental Astronomy">SOFA</abbr></a>collection contains C and <ahref="https://en.wikipedia.org/wiki/Fortran">Fortran</a>code for converting among time scales like<abbr title="International Atomic Time">TAI</abbr>,<abbr>TDB</abbr>, <abbr>TDT</abbr> and<abbr>UTC</abbr>. It is freely available under the<a href="https://www.iausofa.org/terms-and-conditions">SOFA license</a>.</li><li><ahref="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock– Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and thediverse local timescales used by each landed mission on Mars.</li><li><a href="http://leapsecond.com">LeapSecond.com</a> isdedicated not only to leap seconds but to precise time and frequencyin general. It covers the state of the art in amateur timekeeping, andhow the art has progressed over the past few decades.</li><li>The rules for leap seconds are specified in Annex 1 (Time scales) of <ahref="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequencyand time-signal emissions</a>, International Telecommunication Union –Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li><li><ahref="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbrtitle="International Earth Rotation and Reference Systems Service">IERS</abbr>Bulletins</a> contains official publications of the InternationalEarth Rotation and Reference Systems Service, which decides when leapseconds occur.The <code><abbr>tz</abbr></code> code and data support leap secondsvia an optional <code>"right"</code> configuration where a computer’s internal<code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,as opposed to the default <code>"posix"</code> configurationwhere the internal clock ignores leap seconds.The two configurations agree for timestamps starting with 1972-01-01 00:00:00<abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge fortimestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,which corresponds to the first leap second1972-06-30 23:59:60 <abbr>UTC</abbr> in the <code>"right"</code> configuration,and to1972-07-01 00:00:00 <abbr>UTC</abbr> in the <code>"posix"</code> configuration.In practice the two configurations also agree for timestamps before1972 even though the historical situation is messy, partly becauseneither <abbr>UTC</abbr> nor <abbr>TAI</abbr>is well-defined for sufficiently old timestamps.</li><li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The<abbr>NTP</abbr> Leap Second File</a> covers the text file<code>leap-seconds.list</code>, which lists the currently known leap seconds.The <abbr>IERS</abbr> maintains this file, and a copy is distributed by<code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like<a href="https://www.ntp.org">classic<code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a>and <a href="https://ntpsec.org">NTPsec</a>.The <code><abbr>tz</abbr></code> database also distributes leap secondinformation in a differently-formatted <code>leapseconds</code> text file,as well as in the <code>"right"</code> configuration in binary form; forexample, <code>right/UTC</code> can be usedby <a href="https://chrony-project.org"><code>chrony</code></a>,another <abbr>NTP</abbr> implementation.</li><li><a href="https://developers.google.com/time/smear">Leap Smear</a>discusses how to gradually adjust <abbr>POSIX</abbr> clocks near aleap second so that they disagree with <abbr>UTC</abbr> by at most ahalf second, even though every <abbr>POSIX</abbr> minute has exactlysixty seconds. This approach works with the default <code><abbr>tz</abbr></code><code>"posix"</code> configuration, is <ahref="https://gitlab.com/NTPsec/ntpsec/-/blob/master/docs/leapsmear.adoc">supported</a>by the abovementioned <abbr>NTP</abbr> implementations, <ahref="https://github.com/google/unsmear">supports</a> conversion between<abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by majorcloud service providers. However, according to<a href="https://www.rfc-editor.org/rfc/rfc8633#section-3.7.1">§3.7.1 ofNetwork Time Protocol Best Current Practices</a>(Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable forapplications requiring accurate <abbr>UTC</abbr> or civil time,and is intended for use only in single, well-controlled environments.</li><li>The <ahref="https://groups.io/g/LEAPSECS">LEAPSECS List</a> covers <ahref="https://gge.ext.unb.ca/Resources/gpsworld.november99.pdf">McCarthyand Klepczynski’s 1999 proposal to discontinue leap seconds</a>,discussed further in<a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">Theleap second: its history and possible future</a>.<a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>might be redefinedwithout Leap Seconds</a> gives pointers on thiscontentious issue.The General Conference on Weights and Measures (CGPM)<a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>to discontinue the use of leap seconds by 2035, and requested that nodiscontinuous adjustments be made to UTC for at least a century.The World Radiocommunication Conference <ahref="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolvedin 2023</a> to cooperate with this process. A draft <ahref="https://www.bipm.org/documents/d/guest/cgpm-2026-draft-resolutions">ResolutionC to make continuous UTC effective on 2027-05-20</a>,and thereby discontinue leap seconds,has been scheduled for the 28th CGPM starting 2026-10-13 in Paris.One proposal to implement thiswould replace leap seconds with seven 13-second leap smears occurring once perdecade until 2100, with leap smears after that gradually increasing in size.See:<ul><li>Levine J. <a href="https://tf.nist.gov/general/pdf/3242.pdf">Aproposal to change the leap-second adjustments tocoordinated universal time</a>. <em>Metrologia.</em> 2024;61(5):055002. doi:<ahref="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a>with followups in doi:<ahref="https://doi.org/10.1088/1681-7575/ade314">10.1088/1681-7575/ade314</a>and doi:<ahref="https://doi.org/10.1088/1681-7575/ade315">10.1088/1681-7575/ade315</a>.</li></ul>However, there is still no consensus on whether this is the best wayto replace leap seconds.</li></ul></section> <section><h2 id="notation">Time notation</h2><ul><li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale DataRepository (<abbr>CLDR</abbr>) Project</a> has localizations for timezone names, abbreviations, identifiers, and formats. For example, itcontains French translations for “Eastern European Summer Time”,“<abbr title="Eastern European Summer Time">EEST</abbr>”, and “Bucharest”. Its<a href="https://unicode.org/cldr/charts/latest/by_type/">by-typecharts</a> show these values for many locales. Data values are available inboth <abbr title="Locale Data Markup Language">LDML</abbr>(an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.<li><a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary ofthe international standard date and time notation</a> covers<ahref="https://www.iso.org/standard/70907.html"><em><abbrtitle="International Organization for Standardization">ISO</abbr>8601-1:2019 – Date and time – Representations for informationinterchange – Part 1: Basic rules</em></a>.</li><li><a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>Schema: Datatypes – dateTime</a> specifies a format inspired by<abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li><li><a href="https://www.rfc-editor.org/rfc/rfc5322#section-3.3">§3.3 ofInternet Message Format</a> (Internet <abbr>RFC</abbr> 5322)specifies the time notation used in email and <ahref="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>headers.</li><li><a href="https://www.rfc-editor.org/rfc/rfc3339">Date and Timeon the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.An extension, <a href="https://www.rfc-editor.org/rfc/rfc9557">Dateand Time on the Internet: Timestamps with Additional Information</a>(Internet <abbr>RFC</abbr> 9557) extends this profileto let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestampvia suffixes like <code>[Asia/Tokyo]</code>.<li><a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; TimeFormats on the Web</a> surveys web- and Internet-oriented date and timeformats.</li><li>Alphabetic time zone abbreviations should not be used as uniqueidentifiers for <abbr>UT</abbr> offsets as they are ambiguous inpractice. For example, in English-speaking North America“<abbr>CST</abbr>” denotes 6 hours behind <abbr>UT</abbr>,but in China it denotes 8 hours ahead of <abbr>UT</abbr>,and French-speaking North Americans prefer“<abbr title="Heure Normale du Centre">HNC</abbr>” to“<abbr>CST</abbr>”. The <code><abbr>tz</abbr></code>database contains English abbreviations for many timestamps;unfortunately some of these abbreviations were merely the database maintainers’inventions, and these have been removed when possible.</li><li>Numeric time zone abbreviations typically count hours east of<abbr>UT</abbr>, e.g., +09 for Japan and−10 for Hawaii. However, <abbr>POSIX</abbr> proleptic<code><abbr>TZ</abbr></code> settings use the opposite convention.For example, one might use <code><abbr>TZ</abbr>="<abbrtitle="Japan Standard Time">JST</abbr>-9"</code> and<code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>for Japan and Hawaii, respectively. If the<code><abbr>tz</abbr></code> database is available, it is usually better to usesettings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and<code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoidconfusion, handle old timestamps better, and insulate you better fromany future changes to the rules. One should never set<abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like<code>"GMT-9"</code>, though, since this would incorrectly imply thatlocal time is nine hours ahead of <abbr>UT</abbr> and the time zoneis called “<abbr>GMT</abbr>”.</li></ul></section> <section><h2 id="see-also">See also</h2><ul><li><a href="theory.html">Theory and pragmatics of the<code><abbr>tz</abbr></code> code and data</a></li><li><a href="tz-art.html">Time and the Arts</a></li></ul></section> <footer><hr>This web page is in the public domain, so clarified as of2009-05-17 by Arthur David Olson.<br>Please send corrections to this web page to the<a href="mailto:tz@iana.org">time zone mailing list</a>.The mailing list and its archives are public,so please do not send confidential information.</footer></body></html>