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diff --git a/tz/tz-link.html b/tz/tz-link.html new file mode 100644 index 0000000..d2661ad --- /dev/null +++ b/tz/tz-link.html @@ -0,0 +1,1184 @@ +<!DOCTYPE html> +<html lang="en"> +<head> +<title>Sources for time zone and daylight saving time data</title> +<meta charset="UTF-8"> +<style> +pre {margin-left: 2em; white-space: pre-wrap;} +</style> +</head> +<body> +<h1>Sources for 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 individual +governments. They are sometimes changed with little notice, and their +histories and planned futures are often recorded only fitfully. Here +is a summary of attempts to organize and record relevant data in this +area. +</p> + <h3>Outline</h3> + <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="#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> + <li><a href="#see-also">See also</a></li> + </ul> + </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 data +that represent the history of local time +for many representative locations around the globe. +It is updated periodically to reflect changes made by political bodies +to 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<abbr +title="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">Chromium OS</a>, +<a href="https://cygwin.com">Cygwin</a>, +<a href="https://mariadb.org">MariaDB</a>, +<a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>, +<a href="https://www.mysql.com">MySQL</a>, +<a href="https://en.wikipedia.org/wiki/WebOS"><abbr +title="Web Operating System">webOS</abbr></a>, +<a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr +title="Advanced Interactive eXecutive">AIX</abbr></a>, +<a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>, +<a href="https://www.apple.com/ios"><abbr +title="iPhone OS">iOS</abbr></a>, +<a href="https://www.apple.com/macos">macOS</a>, +<a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>, +<a href="https://www.vmssoftware.com">Open<abbr +title="Virtual Memory System">VMS</abbr></a>, +<a href="https://www.oracle.com/database/">Oracle Database</a>, and +<a href="https://www.oracle.com/solaris">Oracle Solaris</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 the +name 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, which +uses mountain time without daylight saving time (<abbr>DST</abbr>); +<code>America/Detroit</code> represents most of Michigan, which uses +eastern 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 1991 +and switched back in 2006. +To use the database on an extended <a +href="https://en.wikipedia.org/wiki/POSIX"><abbr +title="Portable Operating System Interface">POSIX</abbr></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">Universal +Time</a> (<abbr>UT</abbr>), which is <a +href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean +Time</a> (<abbr>GMT</abbr>) with days beginning at midnight; +for timestamps after 1960 this is more precisely <a +href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated +Universal 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 <a +href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download +the 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 tzdb +cd tzdb +<a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz +wget 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 same +release in a single-tarball format containing extra data +useful 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 release +of 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 Numbers +Authority (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 by +lower-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 1999g, each release has been distributed in +<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX +ustar 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 +"<samp>version</samp>" whose first (and currently only) line is the version. +Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>, +and are also available in an +<a href="ftp://ftp.iana.org/tz/releases/"><abbr +title="File Transfer Protocol">FTP</abbr> directory</a> via a +less-secure protocol.</p> +<p>Alternatively, a development repository of code and data can be +retrieved from <a href="https://github.com">GitHub</a> via the shell +command:</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 contain +more 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 into +machine-readable binary files, one for each location. The binary files +are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>) +specified by <a href="https://tools.ietf.org/html/8536">Internet +<abbr>RFC</abbr> 8536</a>. +The code also lets +you read a <abbr>TZif</abbr> file and interpret timestamps for that +location.</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 data +are by no means authoritative. If you find errors, please +send changes to <a href="mailto:tz@iana.org"><code>tz@iana.org</code></a>, +the time zone mailing list. You can also <a +href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it +and browse the <a +href="https://mm.icann.org/pipermail/tz/">archive of old +messages</a>. +<a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list +discussions</a> and corresponding data changes can be +generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>. +</p> +<p> +If your government plans to change its time zone boundaries or +daylight saving rules, inform <code>tz@iana.org</code> well in +advance, as this will coordinate updates to many cell phones, +computers, and other devices around the world. +The change should be officially announced at least a year before it affects +how clocks operate; otherwise, there is a good chance that some +clocks will operate incorrectly after the change, due +to delays in propagating updates to software and data. The shorter +the notice, the more likely clock problems will arise; see "<a +href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On +the Timing of Time Zone Changes</a>" for examples. +The <code><abbr>tz</abbr></code> data can represent planned changes +far into the future, and a long-planned change can easily be reverted +or otherwise altered with a year's notice before the change would have +affected clocks. +</p> +<p> +Changes to the <code><abbr>tz</abbr></code> code and data are often +propagated to clients via operating system updates, so +client <code><abbr>tz</abbr></code> data can often be corrected by +applying these updates. With GNU/Linux and similar systems, if your +maintenance provider has not yet adopted the +latest <code><abbr>tz</abbr></code> data, you can often short-circuit +the process by tailoring the generic instructions in +the <code><abbr>tz</abbr> README</code> file and installing the latest +data yourself. System-specific instructions for installing the +latest <code><abbr>tz</abbr></code> data have also been published +for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>, +<a +href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>, +<a +href="http://userguide.icu-project.org/datetime/timezone"><abbr +title="International Components for Unicode">ICU</abbr></a>, +<a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr> +JDK</a>, +<a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a +href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>, +<a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a +href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html#usage">OpenJDK/Oracle JDK</a>. +</p> +<p>Sources for the <code><abbr>tz</abbr></code> database are +<a href="https://en.wikipedia.org/wiki/UTF-8"><abbr +title="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"><abbr +title="linefeed">LF</abbr></a>, +which can be modified by common text editors such +as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>, +<a href="https://wiki.gnome.org/Apps/Gedit">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">Sublime +zoneinfo</a> package for <a +href="https://www.sublimetext.com">Sublime Text</a> and the <a +href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode +zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual +Studio Code</a>. +</p> +<p> +For further information about updates, please see +<a href="https://tools.ietf.org/html/rfc6557">Procedures for +Maintaining the Time Zone Database</a> (Internet <abbr +title="Request For Comments">RFC</abbr> 6557). More detail can be +found 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="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> is +an encyclopedic summary.</li> +<li><a href="tz-how-to.html">How to Read the +tz Database Source Files</a> explains the <code><abbr>tz</abbr></code> +database format.</li> +<li><a +href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A +literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the +database'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"><abbr +title="comma-separated values">CSV</abbr></a> and +in <a href="https://en.wikipedia.org/wiki/SQL"><abbr +title="Structured Query Language">SQL</abbr></a> form.</li> +<li><a +href="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><a +href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current +Time in 1000 Places</a> uses descriptions of the values.</li> +<li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a> +uses a pulldown menu.</li> +<li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete +timezone information for all countries</a> +displays tables of <abbr>DST</abbr> rules. +<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 world +time 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="http://www.wx-now.com">Weather Now</a> and +<a href="http://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>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s +<a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data +Distribution Service (tzdist) working group</a> defined <a +href="https://tools.ietf.org/html/rfc7808">TZDIST</a> +(Internet <abbr>RFC</abbr> 7808), a time zone data distribution service, +along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a> +(Internet <abbr>RFC</abbr> 7809), a calendar access protocol for +transferring time zone data by reference. +<a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST +implementations</a> are available. +The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis +mailing list</a> discusses possible extensions.</li> +<li>The <a href="https://tools.ietf.org/html/rfc5545"> +Internet Calendaring and Scheduling Core Object Specification +(iCalendar)</a> (Internet <abbr>RFC</abbr> 5445) +covers time zone +data; see its VTIMEZONE calendar component. +The iCalendar format requires specialized parsers and generators; a +variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a> +(Internet <abbr>RFC</abbr> 6321) uses +<a href="https://www.w3.org/XML/"><abbr +title="Extensible Markup Language">XML</abbr></a> format, and a variant +<a href="https://tools.ietf.org/html/rfc7265">jCal</a> +(Internet <abbr>RFC</abbr> 7265) +uses <a href="https://www.json.org"><abbr +title="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 by +running the command <code>make rearguard_tarballs</code> and compiling +from the resulting tarballs instead.</p> +<ul> +<li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a +href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a> +program that compiles +<code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files. +Vzic is freely +available under the <a +href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr> +General Public License (<abbr +title="General Public License">GPL</abbr>)</a>.</li> +<li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz +database conversion utility</a> is like Vzic, except for the <a +href="https://dotnet.microsoft.com">.NET framework</a> +and with a <abbr>BSD</abbr>-style license.</li> +<li><a +href="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 <a +href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>, +which is freely +available under both the <abbr>GPL</abbr> and the Perl Artistic +License. DateTime::TimeZone also contains a script +<code>tests_from_zdump</code> that generates test cases for each clock +transition in the <code><abbr>tz</abbr></code> database.</li> +<li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone +Database Parser</a> is a +<a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and +runtime library with <a +href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a> +adopted by +<a href="https://en.wikipedia.org/wiki/C++20">C++20</a>, +the current iteration of the C++ standard. +It is freely available under the +<abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li> +<li><a id="ICU" href="http://site.icu-project.org">International Components for +Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a +href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a> +libraries for internationalization that +has a compiler from <code><abbr>tz</abbr></code> source +and 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 for +the <a href="https://elixir-lang.org">Elixir</a> language downloads +and compiles <code><abbr>tz</abbr></code> source and exposes <abbr +title="Application Program Interface">API</abbr>s for use. It is +freely available under the <abbr>MIT</abbr> license.</li> +<li>Java-based compilers and libraries include: +<ul> +<li>The <a +href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater +tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by +<a href="https://openjdk.java.net/">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 SE +Timezone Updater License Agreement</a>.</li> +<li>The <a +href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java +SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a +href="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 date +and 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 inspired +Java 8 <code>java.time</code>, which its users should migrate to once +they can assume Java 8 or later. It is available under the <a +href="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 <a +href="https://www.azul.com/products/open-source-tools/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 and +time 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 is +available under the <a +href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser +General Public License (<abbr title="Lesser General Public +License">LGPL</abbr>)</a>.</li> +<li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and +Java-based libraries.</li> +</ul> +<li><a href="https://nodatime.org">Noda Time – Date and +time <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 +<samp>timeZone</samp> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><samp>Intl.DateTimeFormat</samp></a>. +This can be used as-is or with most of the following libraries, +many of which also support runtimes lacking the <samp>timeZone</samp> option. +<ul> +<li>The <a +href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp> +timezone polyfill</a> +is freely available under a <abbr>BSD</abbr>-style license.</li> +<li><a +href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a> +compiles time zone data into a compact form designed for +JavaScript. It is freely available under a combination of +the <abbr>MIT</abbr> license and the Apache License.</li> +<li>The <a href="https://date-fns.org/">date-fns</a> +library manipulates timezone-aware timestamps in browsers and +in <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 a +minimalist replacement for the date and time API of +the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date +manipulation library. +It is freely available under the <abbr>MIT</abbr> license.</li> +<li><a href="https://moment.github.io/luxon/">Luxon</a> improves +timezone support for the <samp>Intl</samp> API. +It is freely available under the <abbr>MIT</abbr> license.</li> +<li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a +Moment.js plugin. +It is freely available under the <abbr>MIT</abbr> license.</li> +<li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a +JavaScript library that supports date arithmetic that is time zone +aware. 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 <a +href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>. +Its companion <a +href="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 you +convert timestamps, view transition histories, and download code and data. +It is freely available under the <abbr>MIT</abbr> license.</li> +</ul> +The proposed <a +href="https://github.com/tc39/proposal-temporal"><samp>Temporal</samp> +objects</a> let programs access an abstract view of +<code><abbr>tzdb</abbr></code> data, and are designed to replace <a +href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's +problematic <samp>Date</samp> objects</a> when working with dates and times. +<li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a +compiler from <code><abbr>tz</abbr></code> source into +<a href="https://julialang.org/">Julia</a>. It is freely available +under 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/<abbr +title="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 <a +href="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="http://pytz.sourceforge.net">pytz – World Timezone +Definitions 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 3.9 or later it is superseded by +<a href="#python-zoneinfo"><code>zoneinfo.ZoneInfo</code></a> +and the <a href="#pypi-tzdata"><code>tzdata</code> Python package</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="http://www.squeaksource.com/Chronos/">Chronos Date/Time +Library</a> is +a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class +library that compiles <code><abbr>tz</abbr></code> source into a time +zone repository whose format +is either proprietary or an <abbr>XML</abbr>-encoded +representation.</li> +<li><a id="Tcl" href="https://tcl.tk">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 those +files. Tcl is freely available under a <abbr>BSD</abbr>-style +license.</li> +</ul> +</section> + +<section> +<h2 id="TZif">Other <abbr>TZif</abbr> readers</h2> +<ul> +<li>The <a +href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C +Library</a> +has an independent, thread-safe implementation of +a <abbr>TZif</abbr> file reader. +This library is freely available under the LGPL +and is widely used in <abbr>GNU</abbr>/Linux systems.</li> +<li><a href="https://www.gnome.org">GNOME</a>'s +<a href="https://developer.gnome.org/glib/">GLib</a> has +a <abbr>TZif</abbr> file reader written in C that +creates a <code>GTimeZone</code> object representing sets +of <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 under +the 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 and +can read <abbr>TZif</abbr> files. It is freely available under the Apache +License.</li> +<li><a href="https://github.com/derickr/timelib">Timelib</a> is a C +library that reads <abbr>TZif</abbr> files and converts +timestamps from one time zone or format to another. +It is used by <a href="https://secure.php.net"><abbr +title="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/3.9/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code> +class</a> that reads <abbr>TZif</abbr> data and creates objects +that represent <code><abbr>tzdb</abbr></code> timezones. +Python is freely available under the +<a href="https://docs.python.org/3.9/license.html">Python Software Foundation +License</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>The +public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a> +library contains a Python tool that +converts <abbr>TZif</abbr> data into +<abbr>JSON</abbr>-format data suitable for use +in its JavaScript library for time zone conversion. Dates before 1970 +are not supported.</li> +<li>The <a +href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a> +package contains <a href="https://www.haskell.org">Haskell</a> code that +parses and uses <abbr>TZif</abbr> data. It is freely +available 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/">Google +Chrome</a> and for <a +href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla +Toolkit</a> applications like <a +href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a +href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>. +It displays multiple clocks in the application window, and has a mapping +interface to <a href="https://www.google.com/earth/">Google Earth</a>. +It is freely available under the <abbr>GPL</abbr>.</li> +<li><a href="https://golang.org">Go programming language</a> +implementations contain a copy of a 32-bit subset of a recent +<code><abbr>tz</abbr></code> database in a +Go-specific format.</li> +<li>Microsoft Windows 8.1 +and 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://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and +<a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>. +<a id="System.TimeZoneInfo" +href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring +Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes +the 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 +<a +href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone → +Tzid table</a> or <a +href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr> +file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs +to <code><abbr>tz</abbr></code> names. +These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library, +or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>. +<li><a +href="https://www.oracle.com/java/index.html">Oracle +Java</a> contains a copy of a subset of a recent +<code><abbr>tz</abbr></code> database in a +Java-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-print +time zone history atlases +<a href="https://www.worldcat.org/oclc/468828649">for the US</a> and +<a href="https://www.worldcat.org/oclc/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/">were +sources for much of the older <code><abbr>tz</abbr></code> data</a>, +they are unreliable as Shanks appears to have +guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no +sources 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 in +its 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 <a +href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard +Schedules Information Manual</a> of the +International Air Transport Association +gives current time zone rules for airports served by commercial aviation.</li> +</ul> +</section> + +<section> +<h2 id="maps">Maps</h2> +<ul> +<li>The <a +href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World +and Regional Maps section</a> of <em>The World Factbook</em>, published by the +<a href="https://www.cia.gov">US Central Intelligence +Agency (<abbr +title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time +zone map; the +<a +href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda +Library Map Collection</a> +of the University of Texas at Austin has copies of +recent editions. +The pictorial quality is good, +but the maps do not indicate daylight saving time, +and parts of the data are a few years out of date.</li> +<li><a href="https://www.worldtimezone.com">World Time Zone Map +with current time</a> +has several fancy time zone maps; it covers Russia particularly well. +The maps' pictorial quality is not quite as good as the +<abbr>CIA</abbr>'s +but the maps are more up to date.</li> +<li><a +href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How +much is time wrong around the world?</a> maps the difference between +mean solar and standard time, highlighting areas such as western China +where the two differ greatly. It's a bit out of date, unfortunately.</li> +</ul> +</section> + +<section> +<h2 id="boundaries">Time zone boundaries</h2> +<p>Geographical boundaries between timezones are available +from several <a +href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet +geolocation</a> +services and other sources.</p> +<ul> +<li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone +Boundary Builder</a> extracts +<a href="https://www.openstreetmap.org">Open Street Map</a> data to build +boundaries of <code><abbr>tzdb</abbr></code> timezones. +Its code is freely available under the <abbr>MIT</abbr> license, and +its data entries are freely available under the +<a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons +Open Database License</a>. The maps' borders appear to be quite accurate.</li> +<li>Programmatic interfaces that map geographical coordinates via tz_world to +<code><abbr>tzdb</abbr></code> timezones include: +<ul> +<li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is +written in <a +href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a> +and is freely available under the <abbr>MIT</abbr> license.</li> +<li>The <a href="https://github.com/bradfitz/latlong">latlong package</a> +is written in Go and is freely available under the Apache License.</li> +<li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>, +in both Java and +<a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a> +form, is freely available under the MIT license.</li> +<li>For Node.js, +the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a> +is freely available under the MIT license, and +the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a> +is in the public domain.</li> +<li>The <a +href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a> +library for Python is freely available under the MIT license. +<li>The <a +href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a> +library for Ruby is freely available under the MIT license.</li> +</ul></li> +<li>Free access via a network API, if you register a key, is provided by +the <a +href="https://www.geonames.org/export/web-services.html#timezone">GeoNames +Timezone web service</a>, the <a +href="https://developers.google.com/maps/documentation/timezone/intro">Google +Maps Time Zone API</a>, and +the <a href="https://timezonedb.com/api">TimeZoneDB API</a>. +Commercial network API access is provided +by <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>"<a +href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How +to get a time zone from a location using latitude and longitude +coordinates?</a>" discusses other geolocation possibilities.</li> +<li><a href="http://statoids.com/statoids.html">Administrative +Divisions of Countries ("Statoids")</a> lists +political subdivision data related to time zones.</li> +<li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time +zone boundaries for multizone countries</a> summarizes legal +boundaries between time zones within countries.</li> +<li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software +– GIS and Database Tools</a> includes a Manifold-format map of +world time zone boundaries distributed under the +<abbr>GPL</abbr>.</li> +<li>A ship within the <a +href="https://en.wikipedia.org/wiki/Territorial_waters">territorial +waters</a> of any nation uses that nation's time. In international +waters, 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 by +the 180° meridian (not by the International Date Line, which is +for land and territorial waters only). A captain can change ship's +clocks any time after entering a new time zone; midnight changes are +common.</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">A +Walk through Time</a> +surveys the evolution of timekeeping.</li> +<li>The history of daylight saving time is surveyed in <a +href="http://www.webexhibits.org/daylightsaving/">About Daylight +Saving Time – History, rationale, laws & dates</a> and summarized in +<a href="http://seizethedaylight.com/dst/">A Brief +History of Daylight Saving Time</a>.</li> +<li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time +Lords</a> discusses how authoritarians manipulate civil time.</li> +<li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a> +contains guidelines and best practices for software applications that +deal with civil time.</li> +<li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of +the International Date Line</a> tells the story of the most important +time zone boundary.</li> +<li><a href="http://statoids.com/tconcept.html">Basic Time +Zone 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 Parliamentary Library commissioned a <a +href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research +paper on daylight saving time in Australia</a>. +The Bureau of Meteorology publishes a list of <a +href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation +Dates of Daylight Savings Time within Australia</a>.</dd> +<dt>Belgium</dt> +<dd>The Royal Observatory of Belgium maintains a table of time in +Belgium (in +<a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html" +hreflang="nl">Dutch</a> and <a +href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html" +hreflang="fr">French</a>).</dd> +<dt>Brazil</dt> +<dd>The Time Service Department of the National Observatory +records <a href="http://pcdsh01.on.br/DecHV.html" +hreflang="pt-BR">Brazil's daylight saving time decrees (in +Portuguese)</a>.</dd> +<dt>Canada</dt> +<dd>National Research Council Canada publishes current +and some older information about <a +href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time +zones 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.html" hreflang="es">history of +Chile'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 <a +href="https://www.smg.gov.mo/en/subpage/224/page/174">similar +history 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 <a +href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation +of Legal Time in Germany</a>.</dd> +<dt>Israel</dt> +<dd>The Interior Ministry periodically issues <a +href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements" +hreflang="he">announcements (in Hebrew)</a>.</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 of +Congress has published a <a +href="http://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://www.staff.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 <a +href="https://www.dia.govt.nz/Daylight-Saving-History">History of +Daylight Saving</a>.</dd> +<dt>Portugal</dt> +<dd>The Lisbon Astronomical Observatory publishes a +<a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of +legal 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">Why +is Singapore in the "Wrong" Time Zone?</a> details the +history of legal time in Singapore and Malaysia.</dd> +<dt>United Kingdom</dt> +<dd><a +href="https://www.polyomino.org.uk/british-time/">History of +legal time in Britain</a> discusses in detail the country +with perhaps the best-documented history of clock adjustments.</dd> +<dt>United States</dt> +<dd>The Department of Transportation's <a +href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent +Time Zone Proceedings</a> lists changes to time zone boundaries.</dd> +<dt>Uruguay</dt> +<dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan +Navy (SOHMA) publishes an annual <a +href="http://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 time +zone shifts, and many scientific studies have been conducted. This +section summarizes reviews and position statements based on +scientific literature in the area.</p> +<ul> +<li>Carey RN, Sarma KM. +<a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of +daylight saving time on road traffic collision risk: a systematic +review</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 evidence +neither supports nor refutes road safety benefits from +shifts in time zones.</li> +<li>Havranek T, Herman D, Irsova D. +<a href="https://www.econstor.eu/handle/10419/174191">Does daylight +saving save electricity? A meta-analysis</a>. <em>Energy J.</em> +2018;39(2). +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 savings +are larger for countries farther away from the equator, while +subtropical regions consume more electricity because of <abbr>DST</abbr>."</li> +<li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>. +<a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time: +an American Academy of Sleep Medicine position statement</a>. +<em>J Clin Sleep Med.</em> +2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>. +This argues for permanent standard time due to health risks of both +<abbr>DST</abbr> transitions and permanent <abbr>DST</abbr>.</li> +<li>Roenneberg T, Winnebeck EC, Klerman EB. +<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight +saving time and artificial time zones – a battle between +biological and social times</a>. <em>Front Physiol.</em> 2019;10:944. +doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>. +This reviews evidence about the health effects of <abbr>DST</abbr> +and concludes, +"In summary, the scientific literature strongly argues against the +switching between <abbr>DST</abbr> and Standard Time and even more so against +adopting <abbr>DST</abbr> permanently."</li> +</ul> +</section> + +<section> +<h2 id="precision">Precision timekeeping</h2> +<ul> +<li><a +href="http://leapsecond.com/hpan/an1289.pdf">The +Science of Timekeeping</a> is a thorough introduction +to the theory and practice of precision timekeeping.</li> +<li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of +Time 2016</a> contains several freely-readable papers.</li> +<li><a href="http://www.ntp.org"><abbr +title="Network Time Protocol">NTP</abbr>: The Network +Time Protocol</a> (Internet <abbr>RFC</abbr> 5905) +discusses how to synchronize clocks of +Internet 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 of +nanoseconds in scalable server farms without special hardware.</li> +<li>The <a +href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision +Time Protocol</a> (<abbr +title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588) +can achieve submicrosecond clock accuracy on a local area network +with special-purpose hardware.</li> +<li><a +href="https://tools.ietf.org/html/rfc4833">Timezone +Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a> +(Internet <abbr>RFC</abbr> 4833) +specifies a <a +href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a> +option for a server to configure +a client's time zone and daylight saving settings automatically.</li> +<li><a +href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical +Times</a> explains more abstruse 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>. +<a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time +Scales</a> goes into more detail, particularly for historical variants.</li> +<li>The <a href="https://www.iau.org"><abbr +title="International Astronomical Union">IAU</abbr></a>'s <a +href="http://www.iausofa.org"><abbr +title="Standards Of Fundamental Astronomy">SOFA</abbr></a> +collection contains C and <a +href="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>.</li> +<li><a +href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock +– Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the +diverse local time +scales used by each landed mission on Mars.</li> +<li><a href="http://leapsecond.com">LeapSecond.com</a> is +dedicated not only to leap seconds but to precise time and frequency +in general. It covers the state of the art in amateur timekeeping, and +how the art has progressed over the past few decades.</li> +<li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a +href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency +and time-signal emissions</a>, International Telecommunication Union – +Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li> +<li><a +href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr +title="International Earth Rotation and Reference Systems Service">IERS</abbr> +Bulletins</a> contains official publications of the International +Earth Rotation and Reference Systems Service, which decides when leap +seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds +via 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>" configuration +where 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> 63 072 000) and diverge for +timestamps starting with <code>time_t</code> 78 796 800, +which corresponds to the first leap second +1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration, +and to +1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration. +In practice the two configurations also agree for timestamps before +1972 even though the historical situation is messy, partly because +neither <abbr>UTC</abbr> nor <abbr>TAI</abbr> +is well-defined for sufficiently-old timestamps.</li> +<li><a href="https://developers.google.com/time/smear">Leap Smear</a> +discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a +leap second so that they disagree with <abbr>UTC</abbr> by at most a +half second, even though every <abbr>POSIX</abbr> minute has exactly +sixty seconds. This approach works with the default <code><abbr>tz</abbr></code> +"<code>posix</code>" configuration, is <a +href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by +the <abbr>NTP</abbr> reference implementation, and is used by major +cloud service providers. However, according to +<a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">§3.7.1 of +Network Time Protocol Best Current Practices</a> +(Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for +applications requiring accurate <abbr>UTC</abbr> or civil time, +and is intended for use only in single, well-controlled environments.</li> +<li>The <a +href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap +Second Discussion List</a> covers <a +href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy +and 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">The +leap second: its history and possible future</a>. +<a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr> +might be redefined +without Leap Seconds</a> gives pointers on this +contentious issue, which was active until 2015 and could become active +again.</li> +</ul> +</section> + +<section> +<h2 id="notation">Time notation</h2> +<ul> +<li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data +Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time +zone names, abbreviations, identifiers, and formats. For example, it +contains 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-type +charts</a> show these values for many locales. Data values are available in +both <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 of +the international standard date and time notation</a> covers +<a +href="https://www.iso.org/standard/70907.html"><em><abbr +title="International Organization for Standardization">ISO</abbr> +8601-1:2019 – Date and time – Representations for information +interchange – 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://tools.ietf.org/html/rfc5322#section-3.3">§3.3 of +Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322) +specifies the time notation used in email and <a +href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a> +headers.</li> +<li> +<a href="https://tools.ietf.org/html/rfc3339">Date and Time +on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339) +specifies an <abbr>ISO</abbr> 8601 +profile for use in new Internet +protocols.</li> +<li> +<a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time +Formats on the Web</a> surveys web- and Internet-oriented date and time +formats.</li> +<li>Alphabetic time zone abbreviations should not be used as unique +identifiers for <abbr>UT</abbr> offsets as they are ambiguous in +practice. 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, the <abbr>POSIX</abbr> +<code><abbr>TZ</abbr></code> environment variable uses the opposite convention. +For example, one might use <code><abbr>TZ</abbr>="<abbr +title="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 use +settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and +<code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid +confusion, handle old timestamps better, and insulate you better from +any 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 that +local time is nine hours ahead of <abbr>UT</abbr> and the time zone +is 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 of +2009-05-17 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>. +</footer> +</body> +</html> |