libc/time/tz-link.html

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<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 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. With
less than a year's notice there is a good chance that some
computer-based 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.
</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://play.google.com/store/apps/details?id=com.google.android.timezone.data">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></a>
and <a
href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a>
Java, <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>,
and <a
href="https://nodatime.org/userguide/tzdb">Noda Time</a> (see below).
</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>
</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://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 DST rules.
<li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
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.
The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
mailing list</a> discussed the Internet draft <a
id="TZDIST-Geolocate"
href="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate">TZDIST
Geolocate Extension</a> (now expired), which let a client determine its timezone
from its geographic location using a <a
href="https://tools.ietf.org/html/rfc5870">'geo' URI</a>.</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 &ndash; 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 API <a
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">adopted
into the draft standard</a> for
<a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
the next 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/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
Oracle Java.</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 &ndash; 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>,
<a href="https://github.com/akashche/tzdbgen">tzdbgen</a>, and <a
href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
are other alternatives to TZUpdater. IANA Updater's license is unclear;
the others are 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 &ndash; 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><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based
compilers and libraries include:
<ul>
<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><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
plugin for the <a href="https://momentjs.com">Moment.js</a> date
manipulation library. It is freely available under the <abbr>MIT</abbr>
license.</li>
<li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
<abbr>API</abbr> is upward compatible with standard JavaScript
Dates. It is freely available under the Apache License.</li>
</ul>
<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> &ndash;
<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 &ndash; 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.</li>
<li><a href="https://tzinfo.github.io">TZInfo &ndash;
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="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
is a <abbr>TZif</abbr> file reader written in Java.
It is freely available under the <abbr>LGPL</abbr>.</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><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>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>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><a
href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
clock (intclock)</a> is a clock that displays multiple time zones on
<abbr>GNU</abbr>/Linux and similar systems. It is freely available
under the <abbr>GPL</abbr>.</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 &rarr;
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>
<li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone
Master</a> is a Microsoft Windows clock program that can automatically
download, compile and use <code><abbr>tz</abbr></code> releases.
The Basic version is free.</li>
<li><a
href="http://veladg.com/velaterra.html">VelaTerra</a> is
a macOS program. Its developers
<a href="http://veladg.com/tzoffer.html">offer free
licenses</a> to <code><abbr>tz</abbr></code> contributors.</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>,
now published in <a
href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a>
form by <a href="https://astrocom.com">ACS-Starcrafts</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/index.html">United States Central
Intelligence Agency (<abbr
title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
zone map</a>; the
<a
href="https://legacy.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
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/Geolocation">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 <a href="https://nodejs.org/en/">Node.js</a>,
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">Time Zone Database &amp; 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
&ndash; 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&deg; apart, except that
<abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
wide and are separated by
the 180&deg; 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 &ndash; History, rationale, laws &amp; 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 DST 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>Italy</dt>
<dd>The National Institute of Metrological Research publishes a
<a href="http://oldsite.inrim.it/res/tf/ora_legale_i.shtml" hreflang="it">table
of civil time (in Italian)</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>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 of 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 DST."</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 &ndash; 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 DST and concludes,
"In summary, the scientific literature strongly argues against the
switching between DST and Standard Time and even more so against
adopting DST 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
&ndash; 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 &ndash;
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, as opposed to the
default "<code>posix</code>" configuration.</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">&sect;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 UTC 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 &ndash; Date and time &ndash; Representations for information
interchange &ndash; Part 1: Basic rules</em></a>.</li>
<li>
<a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
Schema: Datatypes &ndash; 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">&sect;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 &amp; 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
&minus;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>

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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>.
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