libc/time/tz-link.html

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<body>
<h1>Time zone and daylight saving time data</h1>
<p>
<a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
<a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
rules are controlled by 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="#coordinating">Coordinating with governments and distributors</a></li>
	  <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
	</ul>
      </li>
      <li>Uses of the <code><abbr>tz</abbr></code> database
	<ul>
	  <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
	  <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
	  <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
	  <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
	  <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
	</ul>
      </li>
      <li>Related data
	<ul>
	  <li><a href="#other-dbs">Other time zone databases</a></li>
	  <li><a href="#maps">Maps</a></li>
	  <li><a href="#boundaries">Time zone boundaries</a></li>
	</ul>
      </li>
      <li>Timekeeping concepts
	<ul>
	  <li><a href="#civil">Civil time concepts and history</a></li>
	  <li><a href="#national">National histories of legal time</a></li>
	  <li><a href="#costs">Costs and benefits of time shifts</a></li>
	  <li><a href="#precision">Precision timekeeping</a></li>
	  <li><a href="#notation">Time notation</a></li>
	</ul>
      </li>
      <li><a href="#see-also">See also</a></li>
    </ul>
  </nav>

<section>
<h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
<p>
The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
time zone database contains code and 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/">ChromiumOS</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://musl.libc.org">musl libc</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://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 a <a
href="https://en.wikipedia.org/wiki/POSIX"><abbr
title="Portable Operating System Interface">POSIX</abbr>.1-2024</a>
implementation set the <code><abbr>TZ</abbr></code>
environment variable to the location's full name,
e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
<p>
Associated with each timezone is a history of offsets from
<a href="https://en.wikipedia.org/wiki/Universal_Time">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 2022a, 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
"<code>version</code>" 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 format specified by
<a href="https://www.rfc-editor.org/rfc/9636">The
Time Zone Information Format (<abbr>TZif</abbr>)</a>
(Internet <abbr title="Request For Comments">RFC</abbr> 9636).
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
email changes to <a href="mailto:tz@iana.org"><code>tz@iana.org</code></a>,
the time zone mailing list. See
<a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing
list's main page</a> to subscribe or to browse its archive of old messages.
<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>
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/support/pages/aix-time-zone-olson-tzdata-updates"><abbr>AIX</abbr></a>,
<a
href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
<a
href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
title="International Components for Unicode">ICU</abbr></a>,
<a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
JDK</a>,
<a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <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">OpenJDK/Oracle JDK</a>.
</p>
<p>Since version 2013a,
sources for the <code><abbr>tz</abbr></code> database have been
<a href="https://en.wikipedia.org/wiki/UTF-8"><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://gedit-technology.github.io/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://www.rfc-editor.org/rfc/rfc6557">Procedures for
Maintaining the Time Zone Database</a> (Internet <abbr>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="coordinating">Coordinating with governments and distributors</h2>
<p>
As discussed in
"<a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How
Time Zones Are Coordinated</a>", the time zone database relies on
collaboration among governments, the time zone database volunteer
community, and data distributors downstream.
<p>
If your government plans to change its time zone boundaries or
daylight saving rules, please send email to <a
href="mailto:tz@iana.org"><code>tz@iana.org</code></a> well in advance,
as this will lessen confusion and will coordinate updates to many cell phones,
computers, and other devices around the world.
In your email, please cite the legislation or regulation that specifies
the change, so that it can be checked for details such as the exact times
when clock transitions occur.
It is OK if a rule change is planned to affect clocks
far into the future, as 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>
There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.
However, typically a release occurs every few months.
Many downstream timezone data distributors wait for
a <code><abbr>tzdb</abbr></code> release before they produce an update
to time zone behavior in consumer devices and software products.
After a release, various parties must integrate, test,
and roll out an update before <a
href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.
These updates can be expensive, for both the <a
href="https://en.wikipedia.org/wiki/Quality_assurance">quality
assurance</a> process and the overall cost of shipping and installing
updates to each device's copy of <code><abbr>tzdb</abbr></code>.
Updates may be batched with other updates and may take substantial
time to reach end users after a release.
Older devices may no longer be supported and thus may never be updated,
which means they will continue to use out-of-date rules.</p>
<p>
For these reasons any rule change should be promulgated at least a
year before it affects how clocks operate; otherwise, there is a good
chance that many clocks will be wrong due to delays in propagating updates,
and that residents will be confused or even actively resist the change.
The shorter the notice, the more likely clock problems will arise; see "<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>
</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="https://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://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="https://www.wx-now.com">Weather Now</a> and
<a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
</ul>
</section>

<section>
<h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
<ul>
<li>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://www.rfc-editor.org/rfc/rfc7808">TZDIST</a>
(Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
along with <a href="https://www.rfc-editor.org/rfc/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://www.rfc-editor.org/rfc/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://www.rfc-editor.org/rfc/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://www.rfc-editor.org/rfc/rfc7265">jCal</a>
(Internet <abbr>RFC</abbr> 7265)
uses <a href="https://www.json.org/json-en.html"><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://github.com/libical/vzic">Vzic</a> is a <a
href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>
program that compiles
<code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
Vzic is 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++">C++</a> parser and
runtime library with a <a
href="https://en.cppreference.com/w/cpp/chrono"><code>std::chrono</code> API</a>
that is a standard part of C++.
It is freely available under the
<abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
<li><a id="ICU" href="https://icu.unicode.org">International Components for
Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
href="https://en.wikipedia.org/wiki/Java_(programming_language)">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.org">OpenJDK</a> and
<a href="https://jdk.java.net">Oracle JDK</a>.
Although its source code is proprietary, its executable is available under the
<a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java 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 &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> 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 &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>Many modern
<a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
runtimes support <code><abbr>tz</abbr></code> natively via the
<code>timeZone</code> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><code>Intl.DateTimeFormat</code></a>.
This can be used as-is or with most of the following libraries,
many of which also support runtimes lacking the <code>timeZone</code> option.
<ul>
<li>The <a
href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code>
timezone polyfill</a>
is freely available under a <abbr>BSD</abbr>-style license.</li>
<li>The <a href="https://date-fns.org">date-fns</a>
library manipulates timezone-aware timestamps in browsers 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 <code>Intl</code> API.
It is freely available under the <abbr>MIT</abbr> license.</li>
<li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is 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"><code>Temporal</code>
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 <code>Date</code> 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> &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="https://pythonhosted.org/pytz/">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.
In code that can assume Python 3.6 or later it is largely superseded; see <a
href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
The Fastest Footgun in the West</a>.</li>
<li><a href="https://tzinfo.github.io">TZInfo &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="https://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>The
<a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code>
package</a> contains Python code
to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map
<code><abbr>tz</abbr></code> settings to proleptic TZ approximations.
For example, it maps <code>"Africa/Cairo"</code>
to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>,
an approximation valid for Cairo timestamps from 2023 on.
This can help porting to platforms that support only proleptic TZ.
The package is freely available under the MIT license.</li>
<li><a href="https://github.com/derickr/timelib">Timelib</a> is a 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/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/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>, <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 &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>
</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&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/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 borders appear to be quite accurate.
Its main web page lists more than twenty libraries
for looking up a timezone name from a GPS coordinate.</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://manifold.net/info/freestuff.shtml">Manifold Software
&ndash; GIS and Database Tools</a> includes a Manifold-format map of
world time zone boundaries circa 2007, 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://webspace.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://parlinfo.aph.gov.au/parlInfo/download/library/prspub/359V6/upload_binary/359v60.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://robinfo.oma.be/nl/astro-info/tijd/"
hreflang="nl">Dutch</a> and <a
href="https://robinfo.oma.be/fr/astro-info/heure/"
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.php" 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><a
href="https://tz.cs.huji.ac.il">Israel Timezone Files</a>
lists official time-change announcements and laws since 1940,
almost all in Hebrew.</dd>
<dt>Malaysia</dt>
<dd>See Singapore <a href="#Singapore">below</a>.</dd>
<dt>Mexico</dt>
<dd>The Investigation and Analysis Service of the Mexican Library of
Congress has published a <a
href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
<dt>Netherlands</dt>
<dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"
hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
covers the history of local time in the Netherlands from ancient times.</dd>
<dt>New Zealand</dt>
<dd>The Department of Internal Affairs maintains a brief <a
href="https://www.dia.govt.nz/Daylight-Saving-History">History of
Daylight Saving</a>.</dd>
<dt>Palestine</dt>
<dd>The Ministry of Telecom and IT publishes a <a
href="https://mtit.pna.ps/home/TimeZone"
hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
<dt>Portugal</dt>
<dd>The Lisbon Astronomical Observatory publishes a
<a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history 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
official written time zone boundaries, and its <a
href="https://geodata.bts.gov/datasets/usdot::time-zones/about">Time
Zones dataset</a> maps current boundaries.
These boundaries are only for standard time, so the current map puts
all of Arizona in one time zone even though part of Arizona
observes <abbr>DST</abbr> and part does not.</dd>
<dt>Uruguay</dt>
<dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
Navy (SOHMA) publishes an annual <a
href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
(in Spanish)</a>.</dd>
</dl>
</section>

<section>
<h2 id="costs">Costs and benefits of time shifts</h2>
<p>Various sources argue for and against daylight saving time and 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>In 2022 the American Medical Association issued a
<a href="https://www.ama-assn.org/press-center/press-releases/ama-calls-permanent-standard-time">statement
supporting permanent standard time</a> on health grounds.</li>
<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.iaee.org/en/publications/ejarticle.aspx?id=3051">Does
daylight saving save electricity? A meta-analysis</a>.
<em>Energy J.</em> 2018;39(2):35&ndash;61.
doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
This analyzes research literature and concludes, "Electricity savings
are larger for countries farther away from the equator, while
subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
<li>Malow BA. <a
href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time
to abolish the clock change and adopt permanent
standard time in the United States:
a Sleep Research Society position statement</a>.
<em>Sleep.</em> 2022;45(12):zsac236.
doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
After reviewing the scientific literature, the Sleep Research Society
advocates permanent standard time due to its health benefits.
<li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
<a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
is the optimal choice for health and safety:
an American Academy of Sleep Medicine position statement</a>.
<em>J Clin Sleep Med.</em> 2024;20(1):121&ndash;125.
doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
The AASM argues for permanent standard time due to health and safety risks
and economic costs of both <abbr>DST</abbr> transitions and
permanent <abbr>DST</abbr>.</li>
<li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
<a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why
should we abolish Daylight Saving Time?</a>
<em>J Biol Rhythms.</em> 2019;34(3):227&ndash;230.
doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
The Society for Research on Biological Rhythms
opposes <abbr>DST</abbr> changes and permanent <abbr>DST</abbr>,
and advocates that governments adopt
"permanent Standard Time for the health and safety of their citizens".</li>
</ul>
</section>

<section>
<h2 id="precision">Precision timekeeping</h2>
<ul>
<li><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="https://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://www.rfc-editor.org/rfc/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.ucolick.org/~sla/leapsecs/timescales.html">Time
Scales</a> describes astronomical time scales like
<abbr title="Terrestrial Dynamic Time">TDT</abbr>,
<abbr title="Geocentric Coordinate Time">TCG</abbr>, and
<abbr title="Barycentric Dynamic Time">TDB</abbr>.
<li>The <a href="https://www.iau.org"><abbr
title="International Astronomical Union">IAU</abbr></a>'s <a
href="https://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>. It is freely available under the
<a href="https://www.iausofa.org/tandc.html">SOFA license</a>.</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 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&thinsp;072&thinsp;000) and diverge for
timestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;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://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The
<abbr>NTP</abbr> Leap Second File</a> covers the text file
<code>leap-seconds.list</code>, which lists the currently known leap seconds.
The <abbr>IERS</abbr> maintains this file, and a copy is distributed by
<code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like
<a href="https://www.ntp.org">classic
<code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a>
and <a href="https://ntpsec.org">NTPsec</a>.
The <code><abbr>tz</abbr></code> database also distributes leap second
information in a differently-formatted <code>leapseconds</code> text file,
as well as in the "<code>right</code>" configuration in binary form; for
example, <code>right/UTC</code> can be used
by <a href="https://chrony-project.org"><code>chrony</code></a>,
another <abbr>NTP</abbr> implementation.</li>
<li><a href="https://developers.google.com/time/smear">Leap Smear</a>
discusses how to gradually adjust <abbr>POSIX</abbr> clocks near 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 abovementioned <abbr>NTP</abbr> implementations, <a
href="https://github.com/google/unsmear">supports</a> conversion between
<abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
cloud service providers. However, according to
<a href="https://www.rfc-editor.org/rfc/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 <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.
The General Conference on Weights and Measures
<a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>
to discontinue the use of leap seconds by 2035, and requested that no
discontinuous adjustments be made to UTC for at least a century.
The World Radiocommunication Conference <a
href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved
in 2023</a> to cooperate with this process.
<a href="https://www.preprints.org/manuscript/202406.0043/v1">A proposal
to change the leap-second adjustments to Coordinated Universal Time</a>
(doi:<a href="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a>)
would replace leap seconds with 13-second leap smears occurring once per
decade until 2100, with leap smears after that gradually increasing in size.
However, there is still no consensus on whether this is the best way
to replace leap seconds.
</li>
</ul>
</section>

<section>
<h2 id="notation">Time notation</h2>
<ul>
<li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale 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://www.rfc-editor.org/rfc/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://www.rfc-editor.org/rfc/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.
An extension, <a href="https://www.rfc-editor.org/rfc/rfc9557">Date
and Time on the Internet: Timestamps with Additional Information</a>
(Internet <abbr>RFC</abbr> 9557) extends this profile
to let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp
via suffixes like "<code>[Asia/Tokyo]</code>".
<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, <abbr>POSIX</abbr> proleptic
<code><abbr>TZ</abbr></code> settings use 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>
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