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      3   1.1  christos <head>
      4   1.1  christos <title>Sources for time zone and daylight saving time data</title>
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     10   1.1  christos <body>
     11   1.1  christos <h1>Sources for time zone and daylight saving time data</h1>
     12   1.1  christos <p>
     13   1.1  christos <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
     14   1.1  christos <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
     15   1.1  christos rules are controlled by individual
     16   1.1  christos governments. They are sometimes changed with little notice, and their
     17   1.1  christos histories and planned futures are often recorded only fitfully. Here
     18   1.1  christos is a summary of attempts to organize and record relevant data in this
     19   1.1  christos area.
     20   1.1  christos </p>
     21   1.6  christos   <h3>Outline</h3>
     22   1.6  christos   <nav>
     23   1.6  christos     <ul>
     24   1.6  christos       <li>The <code><abbr>tz</abbr></code> database product and process
     25   1.6  christos 	<ul>
     26   1.6  christos 	  <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
     27   1.6  christos 	  <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
     28   1.6  christos 	  <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
     29   1.6  christos 	  <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
     30   1.6  christos 	</ul>
     31   1.6  christos       </li>
     32   1.6  christos       <li>Uses of the <code><abbr>tz</abbr></code> database
     33   1.6  christos 	<ul>
     34   1.6  christos 	  <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
     35   1.6  christos 	  <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
     36   1.6  christos 	  <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
     37   1.6  christos 	  <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
     38   1.6  christos 	  <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
     39   1.6  christos 	</ul>
     40   1.6  christos       </li>
     41   1.6  christos       <li>Related data
     42   1.6  christos 	<ul>
     43   1.6  christos 	  <li><a href="#other-dbs">Other time zone databases</a></li>
     44   1.6  christos 	  <li><a href="#maps">Maps</a></li>
     45   1.6  christos 	  <li><a href="#boundaries">Time zone boundaries</a></li>
     46   1.6  christos 	</ul>
     47   1.6  christos       </li>
     48   1.6  christos       <li>Timekeeping concepts
     49   1.6  christos 	<ul>
     50   1.6  christos 	  <li><a href="#civil">Civil time concepts and history</a></li>
     51   1.6  christos 	  <li><a href="#national">National histories of legal time</a></li>
     52   1.7  christos 	  <li><a href="#costs">Costs and benefits of time shifts</a></li>
     53   1.6  christos 	  <li><a href="#precision">Precision timekeeping</a></li>
     54   1.6  christos 	  <li><a href="#notation">Time notation</a></li>
     55   1.6  christos 	  <li><a href="#see-also">See also</a></li>
     56   1.6  christos 	</ul>
     57   1.6  christos       </li>
     58   1.6  christos     </ul>
     59   1.6  christos   </nav>
     60   1.6  christos 
     61   1.6  christos <section>
     62   1.1  christos <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
     63   1.1  christos <p>
     64   1.1  christos The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
     65   1.1  christos time zone database contains code and data
     66   1.1  christos that represent the history of local time
     67   1.1  christos for many representative locations around the globe.
     68   1.1  christos It is updated periodically to reflect changes made by political bodies
     69   1.1  christos to time zone boundaries and daylight saving rules.
     70   1.1  christos This database (known as <code><abbr>tz</abbr></code>,
     71   1.1  christos <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
     72   1.1  christos is used by several implementations,
     73   1.1  christos including
     74   1.1  christos <a href="https://www.gnu.org/software/libc/">the
     75   1.1  christos <abbr title="GNU's Not Unix">GNU</abbr>
     76   1.1  christos C Library</a> (used in
     77   1.1  christos <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
     78   1.1  christos <a href="https://www.android.com">Android</a>,
     79   1.1  christos <a href="https://www.freebsd.org">Free<abbr
     80   1.1  christos title="Berkeley Software Distribution">BSD</abbr></a>,
     81   1.1  christos <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
     82   1.1  christos <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
     83   1.1  christos <a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
     84   1.1  christos <a href="https://cygwin.com">Cygwin</a>,
     85   1.7  christos <a href="https://mariadb.org">MariaDB</a>,
     86   1.1  christos <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
     87   1.1  christos <a href="https://www.mysql.com">MySQL</a>,
     88   1.1  christos <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
     89   1.1  christos title="Web Operating System">webOS</abbr></a>,
     90   1.7  christos <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
     91   1.1  christos title="Advanced Interactive eXecutive">AIX</abbr></a>,
     92   1.6  christos <a href="https://www.apple.com/ios"><abbr
     93   1.1  christos title="iPhone OS">iOS</abbr></a>,
     94   1.6  christos <a href="https://www.apple.com/macos">macOS</a>,
     95   1.1  christos <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
     96   1.6  christos <a href="https://www.vmssoftware.com">Open<abbr
     97   1.1  christos title="Virtual Memory System">VMS</abbr></a>,
     98   1.6  christos <a href="https://www.oracle.com/database/">Oracle Database</a>, and
     99   1.1  christos <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
    100   1.1  christos <p>
    101   1.2  christos Each main entry in the database represents a <dfn>timezone</dfn>
    102   1.2  christos for a set of civil-time clocks that have all agreed since 1970.
    103   1.2  christos Timezones are typically identified by continent or ocean and then by the
    104   1.2  christos name of the largest city within the region containing the clocks.
    105   1.1  christos For example, <code>America/New_York</code>
    106   1.1  christos represents most of the <abbr title="United States">US</abbr> eastern time zone;
    107   1.1  christos <code>America/Phoenix</code> represents most of Arizona, which
    108   1.2  christos uses mountain time without daylight saving time (<abbr>DST</abbr>);
    109   1.1  christos <code>America/Detroit</code> represents most of Michigan, which uses
    110   1.1  christos eastern time but with different <abbr>DST</abbr> rules in 1975;
    111   1.1  christos and other entries represent smaller regions like Starke County,
    112   1.1  christos Indiana, which switched from central to eastern time in 1991
    113   1.1  christos and switched back in 2006.
    114   1.1  christos To use the database on an extended <a
    115   1.1  christos href="https://en.wikipedia.org/wiki/POSIX"><abbr
    116   1.1  christos title="Portable Operating System Interface">POSIX</abbr></a>
    117   1.1  christos implementation set the <code><abbr>TZ</abbr></code>
    118   1.1  christos environment variable to the location's full name,
    119   1.1  christos e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
    120   1.1  christos <p>
    121   1.2  christos Associated with each timezone is a history of offsets from
    122   1.1  christos <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
    123   1.1  christos Time</a> (<abbr>UT</abbr>), which is <a
    124   1.1  christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
    125   1.1  christos Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
    126   1.2  christos for timestamps after 1960 this is more precisely <a
    127   1.1  christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
    128   1.1  christos Universal Time</a> (<abbr>UTC</abbr>).
    129   1.1  christos The database also records when daylight saving time was in use,
    130   1.1  christos along with some time zone abbreviations such as <abbr>EST</abbr>
    131   1.1  christos for Eastern Standard Time in the <abbr>US</abbr>.</p>
    132   1.6  christos </section>
    133   1.6  christos 
    134   1.6  christos <section>
    135   1.1  christos <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
    136   1.1  christos <p>
    137   1.1  christos The following <a
    138   1.1  christos href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
    139   1.1  christos the latest release's two
    140   1.1  christos <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
    141   1.1  christos to a <abbr>GNU</abbr>/Linux or similar host.</p>
    142   1.2  christos <pre><code>mkdir tzdb
    143   1.1  christos cd tzdb
    144   1.1  christos <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
    145   1.1  christos wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
    146   1.1  christos <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 -
    147   1.1  christos gzip -dc tzdata-latest.tar.gz | tar -xf -
    148   1.1  christos </code></pre>
    149   1.1  christos <p>Alternatively, the following shell commands download the same
    150   1.1  christos release in a single-tarball format containing extra data
    151   1.1  christos useful for regression testing:</p>
    152   1.2  christos <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>
    153   1.1  christos <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
    154   1.1  christos </code></pre>
    155   1.1  christos <p>These commands use convenience links to the latest release
    156   1.1  christos of the <code><abbr>tz</abbr></code> database hosted by the
    157   1.1  christos <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
    158   1.1  christos of the <a href="https://www.iana.org">Internet Assigned Numbers
    159   1.1  christos Authority (IANA)</a>.
    160   1.1  christos Older releases are in files named
    161   1.1  christos <code>tzcode<var>V</var>.tar.gz</code>,
    162   1.1  christos <code>tzdata<var>V</var>.tar.gz</code>, and
    163   1.1  christos <code>tzdb-<var>V</var>.tar.lz</code>,
    164   1.1  christos where <code><var>V</var></code> is the version.
    165   1.1  christos Since 1996, each version has been a four-digit year followed by
    166   1.1  christos lower-case letter (<samp>a</samp> through <samp>z</samp>,
    167   1.1  christos then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
    168   1.1  christos through <samp>zzz</samp>, and so on).
    169  1.10  christos Since version 1999g, each release has been distributed in
    170  1.10  christos <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
    171  1.10  christos ustar interchange format</a>, compressed as described above;
    172  1.10  christos older releases use a nearly-compatible format.
    173   1.1  christos Since version 2016h, each release has contained a text file named
    174   1.1  christos "<samp>version</samp>" whose first (and currently only) line is the version.
    175   1.7  christos Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
    176   1.7  christos and are also available in an
    177   1.1  christos <a href="ftp://ftp.iana.org/tz/releases/"><abbr
    178   1.1  christos title="File Transfer Protocol">FTP</abbr> directory</a> via a
    179   1.1  christos less-secure protocol.</p>
    180   1.1  christos <p>Alternatively, a development repository of code and data can be
    181   1.1  christos retrieved from <a href="https://github.com">GitHub</a> via the shell
    182   1.1  christos command:</p>
    183   1.2  christos <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
    184   1.1  christos </code></pre>
    185   1.1  christos <p>
    186   1.1  christos Since version 2012e, each release has been tagged in development repositories.
    187   1.1  christos Untagged commits are less well tested and probably contain
    188   1.1  christos more errors.</p>
    189   1.1  christos <p>
    190   1.1  christos After obtaining the code and data files, see the
    191   1.1  christos <code>README</code> file for what to do next.
    192   1.1  christos The code lets you compile the <code><abbr>tz</abbr></code> source files into
    193   1.2  christos machine-readable binary files, one for each location. The binary files
    194   1.5  christos are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
    195   1.5  christos specified by <a href="https://tools.ietf.org/html/8536">Internet
    196   1.5  christos <abbr>RFC</abbr> 8536</a>.
    197   1.2  christos The code also lets
    198   1.2  christos you read a <abbr>TZif</abbr> file and interpret timestamps for that
    199   1.1  christos location.</p>
    200   1.6  christos </section>
    201   1.6  christos 
    202   1.6  christos <section>
    203   1.1  christos <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    204   1.1  christos <p>
    205   1.1  christos The <code><abbr>tz</abbr></code> code and data
    206   1.1  christos are by no means authoritative. If you find errors, please
    207   1.1  christos send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    208   1.1  christos the time zone mailing list. You can also <a
    209   1.1  christos href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
    210   1.1  christos and browse the <a
    211   1.1  christos href="https://mm.icann.org/pipermail/tz/">archive of old
    212   1.5  christos messages</a>.
    213   1.5  christos <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
    214   1.5  christos discussions</a> and corresponding data changes can be
    215   1.5  christos generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
    216   1.5  christos </p>
    217   1.1  christos <p>
    218   1.1  christos If your government plans to change its time zone boundaries or
    219   1.1  christos daylight saving rules, inform <code>tz (a] iana.org</code> well in
    220   1.1  christos advance, as this will coordinate updates to many cell phones,
    221   1.8  christos computers, and other devices around the world.
    222   1.8  christos The change should be officially announced at least a year before it affects
    223   1.8  christos how clocks operate; otherwise, there is a good chance that some
    224   1.8  christos clocks will operate incorrectly after the change, due
    225   1.1  christos to delays in propagating updates to software and data. The shorter
    226   1.1  christos the notice, the more likely clock problems will arise; see "<a
    227   1.1  christos href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
    228   1.1  christos the Timing of Time Zone Changes</a>" for examples.
    229   1.8  christos The <code><abbr>tz</abbr></code> data can represent planned changes
    230   1.8  christos far into the future, and a long-planned change can easily be reverted
    231   1.8  christos or otherwise altered with a year's notice before the change would have
    232   1.8  christos affected clocks.
    233   1.1  christos </p>
    234   1.1  christos <p>
    235   1.1  christos Changes to the <code><abbr>tz</abbr></code> code and data are often
    236   1.1  christos propagated to clients via operating system updates, so
    237   1.1  christos client <code><abbr>tz</abbr></code> data can often be corrected by
    238   1.1  christos applying these updates. With GNU/Linux and similar systems, if your
    239   1.1  christos maintenance provider has not yet adopted the
    240   1.1  christos latest <code><abbr>tz</abbr></code> data, you can often short-circuit
    241   1.1  christos the process by tailoring the generic instructions in
    242   1.1  christos the <code><abbr>tz</abbr> README</code> file and installing the latest
    243   1.1  christos data yourself. System-specific instructions for installing the
    244   1.1  christos latest <code><abbr>tz</abbr></code> data have also been published
    245  1.11  christos for <a href="https://developer.ibm.com/articles/au-aix-olson-time-zone/"><abbr>AIX</abbr></a>,
    246   1.1  christos <a
    247  1.10  christos href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
    248   1.1  christos <a
    249  1.11  christos href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
    250   1.1  christos title="International Components for Unicode">ICU</abbr></a>,
    251  1.11  christos <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
    252   1.9  christos JDK</a>,
    253   1.9  christos <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    254   1.1  christos href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    255   1.9  christos <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
    256  1.11  christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
    257   1.1  christos </p>
    258   1.1  christos <p>Sources for the <code><abbr>tz</abbr></code> database are
    259   1.1  christos <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
    260   1.1  christos title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
    261   1.1  christos <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
    262   1.1  christos with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
    263   1.1  christos title="linefeed">LF</abbr></a>,
    264   1.1  christos which can be modified by common text editors such
    265   1.1  christos as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
    266   1.1  christos <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
    267   1.1  christos <a href="https://www.vim.org">vim</a>.
    268   1.1  christos Specialized source-file editing can be done via the
    269   1.1  christos <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
    270   1.1  christos zoneinfo</a> package for <a
    271   1.1  christos href="https://www.sublimetext.com">Sublime Text</a> and the <a
    272   1.1  christos href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
    273   1.1  christos zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
    274   1.1  christos Studio Code</a>.
    275   1.1  christos </p>
    276   1.1  christos <p>
    277   1.1  christos For further information about updates, please see
    278   1.1  christos <a href="https://tools.ietf.org/html/rfc6557">Procedures for
    279   1.1  christos Maintaining the Time Zone Database</a> (Internet <abbr
    280   1.1  christos title="Request For Comments">RFC</abbr> 6557). More detail can be
    281   1.7  christos found in <a href="theory.html">Theory and pragmatics of the
    282   1.7  christos <code><abbr>tz</abbr></code> code and data</a>.
    283   1.2  christos <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
    284   1.2  christos displays changes between recent <code><abbr>tzdb</abbr></code> versions.
    285   1.1  christos </p>
    286   1.6  christos </section>
    287   1.6  christos 
    288   1.6  christos <section>
    289   1.1  christos <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
    290   1.1  christos <ul>
    291   1.1  christos <li>The article
    292   1.1  christos <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
    293   1.1  christos an encyclopedic summary.</li>
    294   1.1  christos <li><a href="tz-how-to.html">How to Read the
    295   1.1  christos tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
    296   1.1  christos database format.</li>
    297   1.1  christos <li><a
    298   1.1  christos href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
    299   1.1  christos literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
    300   1.1  christos database's style.</li>
    301  1.10  christos <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
    302  1.10  christos managing time in the internet</a> analyzes the database longitudinally.</li>
    303   1.1  christos </ul>
    304   1.6  christos </section>
    305   1.6  christos 
    306   1.6  christos <section>
    307   1.1  christos <h2 id="web">Web sites using recent versions of the
    308   1.1  christos <code><abbr>tz</abbr></code> database</h2>
    309   1.1  christos <p>
    310   1.1  christos These are listed roughly in ascending order of complexity and fanciness.
    311   1.1  christos </p>
    312   1.1  christos <ul>
    313   1.1  christos <li><a href="https://time.is">Time.is</a> shows locations'
    314   1.1  christos time and zones.</li>
    315   1.1  christos <li><a href="https://www.timejones.com">TimeJones.com</a>,
    316   1.1  christos <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
    317   1.7  christos <a href="https://www.worldclock.com">The World Clock</a>
    318   1.1  christos are time zone converters.</li>
    319   1.8  christos <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
    320   1.8  christos publishes <code><abbr>tzdb</abbr></code>-derived data in
    321   1.8  christos <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
    322   1.8  christos title="comma-separated values">CSV</abbr></a> and
    323   1.8  christos in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
    324   1.8  christos title="Structured Query Language">SQL</abbr></a> form.</li>
    325   1.1  christos <li><a
    326   1.6  christos href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
    327   1.1  christos lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
    328   1.1  christos <li><a
    329   1.1  christos href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
    330   1.1  christos Time in 1000 Places</a> uses descriptions of the values.</li>
    331   1.6  christos <li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
    332   1.1  christos uses a pulldown menu.</li>
    333   1.7  christos <li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
    334  1.10  christos timezone information for all countries</a>
    335  1.10  christos displays tables of <abbr>DST</abbr> rules.
    336   1.1  christos <li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
    337   1.1  christos Worldwide</a> lets you sort zone names and convert times.</li>
    338   1.1  christos <li><a href="https://24timezones.com">24TimeZones</a> has a world
    339   1.1  christos time map and a time converter.</li>
    340   1.1  christos <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    341   1.1  christos calculates the current time difference between locations.</li>
    342   1.1  christos <li><a href="http://www.wx-now.com">Weather Now</a> and
    343   1.1  christos <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    344   1.1  christos </ul>
    345   1.6  christos </section>
    346   1.6  christos 
    347   1.6  christos <section>
    348   1.1  christos <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    349   1.1  christos <ul>
    350   1.1  christos <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
    351   1.1  christos <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
    352   1.1  christos Distribution Service (tzdist) working group</a> defined <a
    353   1.1  christos href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
    354   1.1  christos (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
    355   1.1  christos along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
    356   1.1  christos (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    357   1.2  christos transferring time zone data by reference.
    358   1.8  christos <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
    359   1.8  christos implementations</a> are available.
    360   1.2  christos The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
    361   1.9  christos mailing list</a> discusses possible extensions.</li>
    362   1.1  christos <li>The <a href="https://tools.ietf.org/html/rfc5545">
    363   1.1  christos Internet Calendaring and Scheduling Core Object Specification
    364   1.1  christos (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
    365   1.1  christos covers time zone
    366   1.1  christos data; see its VTIMEZONE calendar component.
    367   1.1  christos The iCalendar format requires specialized parsers and generators; a
    368   1.1  christos variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
    369   1.1  christos (Internet <abbr>RFC</abbr> 6321) uses
    370   1.1  christos <a href="https://www.w3.org/XML/"><abbr
    371   1.1  christos title="Extensible Markup Language">XML</abbr></a> format, and a variant
    372   1.1  christos <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
    373   1.1  christos (Internet <abbr>RFC</abbr> 7265)
    374   1.1  christos uses <a href="https://www.json.org"><abbr
    375   1.1  christos title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    376   1.1  christos </ul>
    377   1.6  christos </section>
    378   1.6  christos 
    379   1.6  christos <section>
    380   1.1  christos <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    381   1.7  christos <p>Although some of these do not fully support
    382   1.7  christos <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
    383   1.7  christos distributions you can generally work around compatibility problems by
    384   1.7  christos running the command <code>make rearguard_tarballs</code> and compiling
    385   1.7  christos from the resulting tarballs instead.</p>
    386   1.1  christos <ul>
    387   1.1  christos <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
    388   1.1  christos href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
    389   1.1  christos program that compiles
    390   1.1  christos <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    391   1.1  christos Vzic is freely
    392   1.1  christos available under the <a
    393   1.1  christos href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    394   1.1  christos General Public License (<abbr
    395   1.1  christos title="General Public License">GPL</abbr>)</a>.</li>
    396   1.1  christos <li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
    397   1.1  christos database conversion utility</a> is like Vzic, except for the <a
    398   1.6  christos href="https://dotnet.microsoft.com">.NET framework</a>
    399   1.1  christos and with a <abbr>BSD</abbr>-style license.</li>
    400   1.1  christos <li><a
    401   1.4  christos href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
    402   1.1  christos contains a script <code>parse_olson</code> that compiles
    403   1.1  christos <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
    404   1.1  christos modules. It is part of the Perl <a
    405   1.4  christos href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
    406   1.4  christos which is freely
    407   1.1  christos available under both the <abbr>GPL</abbr> and the Perl Artistic
    408   1.1  christos License. DateTime::TimeZone also contains a script
    409   1.1  christos <code>tests_from_zdump</code> that generates test cases for each clock
    410   1.1  christos transition in the <code><abbr>tz</abbr></code> database.</li>
    411   1.1  christos <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
    412   1.1  christos Database Parser</a> is a
    413   1.1  christos <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
    414   1.9  christos runtime library with <a
    415   1.9  christos href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
    416   1.9  christos adopted by
    417   1.6  christos <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
    418   1.9  christos the current iteration of the C++ standard.
    419   1.1  christos It is freely available under the
    420   1.1  christos <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
    421   1.1  christos <li><a id="ICU" href="http://site.icu-project.org">International Components for
    422   1.1  christos Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
    423   1.1  christos href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
    424   1.1  christos libraries for internationalization that
    425   1.1  christos has a compiler from <code><abbr>tz</abbr></code> source
    426   1.1  christos and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
    427   1.1  christos (mentioned <a href="#CLDR">below</a>)
    428   1.1  christos into an <abbr>ICU</abbr>-specific format.
    429   1.1  christos <abbr>ICU</abbr> is freely available under a
    430   1.1  christos <abbr>BSD</abbr>-style license.</li>
    431   1.1  christos <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
    432   1.1  christos the <a href="https://elixir-lang.org">Elixir</a> language downloads
    433   1.7  christos and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
    434   1.1  christos title="Application Program Interface">API</abbr>s for use. It is
    435   1.1  christos freely available under the <abbr>MIT</abbr> license.</li>
    436   1.1  christos <li>Java-based compilers and libraries include:
    437   1.1  christos <ul>
    438   1.1  christos <li>The <a
    439   1.9  christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
    440   1.1  christos tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
    441   1.9  christos <a href="https://openjdk.java.net/">OpenJDK</a> and
    442   1.9  christos <a href="https://jdk.java.net/">Oracle JDK</a>.
    443   1.9  christos Although its source code is proprietary, its executable is available under the
    444   1.9  christos <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
    445   1.9  christos Timezone Updater License Agreement</a>.</li>
    446   1.1  christos <li>The <a
    447   1.4  christos href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
    448   1.6  christos SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
    449   1.4  christos href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
    450   1.1  christos which is freely available under a <abbr>BSD</abbr>-style license.</li>
    451   1.4  christos <li><a href="https://www.joda.org/joda-time/">Joda-Time &ndash; Java date
    452   1.1  christos and time <abbr>API</abbr></a> contains a class
    453   1.1  christos <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
    454   1.1  christos <code><abbr>tz</abbr></code> source into a binary format. It inspired
    455   1.1  christos Java 8 <code>java.time</code>, which its users should migrate to once
    456   1.1  christos they can assume Java 8 or later. It is available under the <a
    457   1.1  christos href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
    458   1.9  christos <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
    459   1.7  christos href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
    460   1.9  christos are alternatives to TZUpdater. IANA Updater's license is unclear;
    461   1.9  christos ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
    462   1.4  christos <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
    463   1.4  christos time library for Android</a> and
    464   1.4  christos <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
    465   1.4  christos time and interval library for Java</a> compile
    466   1.4  christos <code><abbr>tz</abbr></code> source into a binary format.
    467   1.4  christos Time4A is available under the Apache License and Time4J is
    468   1.1  christos available under the <a
    469   1.1  christos href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
    470   1.1  christos General Public License (<abbr title="Lesser General Public
    471   1.1  christos License">LGPL</abbr>)</a>.</li>
    472   1.1  christos <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
    473   1.1  christos Java-based libraries.</li>
    474   1.1  christos </ul>
    475   1.1  christos <li><a href="https://nodatime.org">Noda Time &ndash; Date and
    476   1.1  christos time <abbr>API</abbr> for .NET</a>
    477   1.4  christos is like Joda-Time and Time4J, but for the .NET framework instead of Java.
    478   1.4  christos It is freely available under the Apache License.</li>
    479   1.8  christos <li>Many modern
    480   1.8  christos <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
    481   1.8  christos runtimes support <code><abbr>tz</abbr></code> natively via the
    482   1.8  christos <samp>timeZone</samp> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><samp>Intl.DateTimeFormat</samp></a>.
    483  1.10  christos This can be used as-is or with most of the following libraries,
    484  1.10  christos many of which also support runtimes lacking the <samp>timeZone</samp> option.
    485   1.1  christos <ul>
    486   1.8  christos <li>The <a
    487   1.8  christos href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
    488   1.8  christos timezone polyfill</a>
    489   1.8  christos is freely available under a <abbr>BSD</abbr>-style license.</li>
    490  1.10  christos <li>The <a href="https://date-fns.org/">date-fns</a>
    491  1.10  christos library manipulates timezone-aware timestamps in browsers and
    492  1.10  christos in <a href="https://nodejs.org/en/">Node.js</a>.
    493  1.10  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    494  1.10  christos <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
    495  1.10  christos minimalist replacement for the date and time API of
    496  1.10  christos the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
    497  1.10  christos manipulation library.
    498  1.10  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    499  1.10  christos <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
    500  1.10  christos timezone support for the <samp>Intl</samp> API.
    501  1.10  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    502   1.1  christos <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    503  1.10  christos Moment.js plugin.
    504  1.10  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    505   1.8  christos <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    506   1.8  christos JavaScript library that supports date arithmetic that is time zone
    507   1.8  christos aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    508  1.10  christos <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
    509  1.10  christos supports live <code><abbr>tzdb</abbr></code> updates,
    510  1.10  christos astronomical and atomic time, a command-line interface,
    511  1.10  christos and full <a
    512  1.10  christos href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
    513  1.10  christos Its companion <a
    514  1.10  christos href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
    515  1.10  christos can generate <abbr>TZif</abbr> and other files, and a companion website
    516  1.10  christos <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
    517  1.10  christos convert timestamps, view transition histories, and download code and data.
    518  1.10  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    519   1.1  christos </ul>
    520  1.10  christos The proposed <a
    521  1.10  christos href="https://github.com/tc39/proposal-temporal"><samp>Temporal</samp>
    522  1.10  christos objects</a> let programs access an abstract view of
    523  1.10  christos <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
    524  1.10  christos href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
    525  1.10  christos problematic <samp>Date</samp> objects</a> when working with dates and times.
    526   1.1  christos <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
    527   1.1  christos compiler from <code><abbr>tz</abbr></code> source into
    528   1.1  christos <a href="https://julialang.org/">Julia</a>. It is freely available
    529   1.1  christos under the <abbr>MIT</abbr> license.</li>
    530   1.6  christos <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> &ndash;
    531   1.6  christos <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
    532   1.6  christos title="Free Pascal Compiler">FPC</abbr></a>
    533   1.2  christos compiles from <code><abbr>tz</abbr></code> source into
    534   1.2  christos <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
    535   1.2  christos as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
    536   1.2  christos and <a
    537   1.2  christos href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
    538   1.2  christos It is freely available under a <abbr>BSD</abbr>-style license.</li>
    539   1.1  christos <li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
    540   1.1  christos Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    541   1.1  christos <a href="https://www.python.org">Python</a>.
    542   1.9  christos It is freely available under a <abbr>BSD</abbr>-style license.
    543  1.11  christos In code that can assume Python 3.6 or later it is largely superseded; see <a
    544  1.11  christos href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
    545  1.11  christos The Fastest Footgun in the West</a>.</li>
    546   1.1  christos <li><a href="https://tzinfo.github.io">TZInfo &ndash;
    547   1.1  christos Ruby Timezone Library</a>
    548   1.1  christos compiles <code><abbr>tz</abbr></code> source into
    549   1.1  christos <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    550   1.1  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    551   1.1  christos <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
    552   1.1  christos Library</a> is
    553   1.1  christos a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    554   1.1  christos library that compiles <code><abbr>tz</abbr></code> source into a time
    555   1.1  christos zone repository whose format
    556   1.1  christos is either proprietary or an <abbr>XML</abbr>-encoded
    557   1.1  christos representation.</li>
    558   1.1  christos <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
    559   1.1  christos contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    560   1.1  christos source into text files, along with a runtime that can read those
    561   1.1  christos files. Tcl is freely available under a <abbr>BSD</abbr>-style
    562   1.1  christos license.</li>
    563   1.1  christos </ul>
    564   1.6  christos </section>
    565   1.6  christos 
    566   1.6  christos <section>
    567   1.2  christos <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
    568   1.1  christos <ul>
    569   1.1  christos <li>The <a
    570   1.1  christos href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
    571   1.1  christos Library</a>
    572   1.1  christos has an independent, thread-safe implementation of
    573   1.2  christos a <abbr>TZif</abbr> file reader.
    574   1.1  christos This library is freely available under the LGPL
    575   1.1  christos and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    576   1.1  christos <li><a href="https://www.gnome.org">GNOME</a>'s
    577   1.1  christos <a href="https://developer.gnome.org/glib/">GLib</a> has
    578   1.2  christos a <abbr>TZif</abbr> file reader written in C that
    579   1.1  christos creates a <code>GTimeZone</code> object representing sets
    580   1.1  christos of <abbr>UT</abbr> offsets.
    581   1.1  christos It is freely available under the <abbr>LGPL</abbr>.</li>
    582   1.1  christos <li>The
    583   1.1  christos <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
    584   1.1  christos <code>baltzo::TimeZoneUtil</code> component contains a C++
    585   1.2  christos implementation of a <abbr>TZif</abbr> file reader. It is freely available under
    586   1.1  christos the Apache License.</li>
    587   1.1  christos <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    588   1.1  christos library that translates between <abbr>UT</abbr> and civil time and
    589   1.2  christos can read <abbr>TZif</abbr> files. It is freely available under the Apache
    590   1.1  christos License.</li>
    591   1.1  christos <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    592   1.2  christos library that reads <abbr>TZif</abbr> files and converts
    593   1.2  christos timestamps from one time zone or format to another.
    594   1.1  christos It is used by <a href="https://secure.php.net"><abbr
    595   1.1  christos title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    596   1.1  christos <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    597   1.1  christos and <a href="https://www.mongodb.com">MongoDB</a>.
    598   1.1  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    599   1.1  christos <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    600   1.2  christos <abbr>TZif</abbr> file reader.</li>
    601   1.4  christos <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
    602   1.1  christos DateTime::TimeZone::Tzfile</a>
    603   1.2  christos is a <abbr>TZif</abbr> file reader written in Perl.
    604   1.1  christos It is freely available under the same terms as Perl
    605   1.1  christos (dual <abbr>GPL</abbr> and Artistic license).</li>
    606   1.9  christos <li>Python has a <a id="python-zoneinfo"
    607  1.11  christos href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
    608   1.8  christos class</a> that reads <abbr>TZif</abbr> data and creates objects
    609   1.8  christos that represent <code><abbr>tzdb</abbr></code> timezones.
    610   1.8  christos Python is freely available under the
    611  1.11  christos <a href="https://docs.python.org/3/license.html">Python Software Foundation
    612   1.8  christos License</a>.
    613   1.9  christos A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
    614   1.8  christos <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
    615   1.8  christos supplies TZif data if the underlying system data cannot be found;
    616   1.8  christos it is freely available under the Apache License.</li>
    617   1.1  christos <li>The
    618   1.1  christos public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    619   1.1  christos library contains a Python tool that
    620   1.2  christos converts <abbr>TZif</abbr> data into
    621   1.1  christos <abbr>JSON</abbr>-format data suitable for use
    622   1.1  christos in its JavaScript library for time zone conversion. Dates before 1970
    623   1.1  christos are not supported.</li>
    624   1.1  christos <li>The <a
    625   1.1  christos href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    626   1.1  christos package contains <a href="https://www.haskell.org">Haskell</a> code that
    627   1.2  christos parses and uses <abbr>TZif</abbr> data. It is freely
    628   1.1  christos available under a <abbr>BSD</abbr>-style license.</li>
    629   1.1  christos </ul>
    630   1.6  christos </section>
    631   1.6  christos 
    632   1.6  christos <section>
    633   1.1  christos <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    634   1.1  christos <ul>
    635   1.1  christos <li><a href="https://foxclocks.org">FoxClocks</a>
    636   1.1  christos is an extension for <a href="https://www.google.com/chrome/">Google
    637   1.1  christos Chrome</a> and for <a
    638  1.11  christos href="https://wiki.mozilla.org/Modules/Toolkit">Mozilla
    639   1.1  christos Toolkit</a> applications like <a
    640   1.1  christos href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
    641   1.1  christos href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
    642   1.1  christos It displays multiple clocks in the application window, and has a mapping
    643   1.1  christos interface to <a href="https://www.google.com/earth/">Google Earth</a>.
    644   1.1  christos It is freely available under the <abbr>GPL</abbr>.</li>
    645   1.1  christos <li><a href="https://golang.org">Go programming language</a>
    646   1.1  christos implementations contain a copy of a 32-bit subset of a recent
    647   1.1  christos <code><abbr>tz</abbr></code> database in a
    648   1.1  christos Go-specific format.</li>
    649   1.1  christos <li>Microsoft Windows 8.1
    650   1.1  christos and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    651   1.3  christos data (mentioned <a href="#CLDR">below</a>) used by the
    652   1.3  christos <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    653   1.3  christos <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    654   1.3  christos <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    655   1.3  christos <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
    656   1.1  christos <a id="System.TimeZoneInfo"
    657   1.1  christos href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
    658   1.1  christos Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    659   1.1  christos the older, proprietary method of Microsoft Windows 2000 and later,
    660   1.1  christos which stores time zone data in the
    661   1.1  christos <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    662   1.1  christos <a
    663   1.1  christos href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
    664   1.1  christos Tzid table</a> or <a
    665   1.6  christos href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    666   1.1  christos file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    667   1.1  christos to <code><abbr>tz</abbr></code> names.
    668   1.3  christos These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
    669   1.3  christos or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
    670   1.1  christos <li><a
    671   1.1  christos href="https://www.oracle.com/java/index.html">Oracle
    672   1.1  christos Java</a> contains a copy of a subset of a recent
    673   1.1  christos <code><abbr>tz</abbr></code> database in a
    674   1.1  christos Java-specific format.</li>
    675   1.1  christos </ul>
    676   1.6  christos </section>
    677   1.6  christos 
    678   1.6  christos <section>
    679   1.1  christos <h2 id="other-dbs">Other time zone databases</h2>
    680   1.1  christos <ul>
    681   1.1  christos <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    682   1.4  christos is Astrodienst's Web version of Shanks and Pottenger's out-of-print
    683   1.4  christos time zone history atlases
    684   1.4  christos <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
    685   1.9  christos <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
    686   1.4  christos Although these extensive atlases
    687   1.4  christos <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
    688   1.4  christos sources for much of the older <code><abbr>tz</abbr></code> data</a>,
    689   1.4  christos they are unreliable as Shanks appears to have
    690   1.1  christos guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    691   1.1  christos sources and do not indicate which entries are guesswork.</li>
    692   1.1  christos <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    693   1.1  christos its own <code>tztab</code>(4) format.</li>
    694   1.1  christos <li>Microsoft Windows has proprietary data mentioned
    695   1.1  christos <a href="#System.TimeZoneInfo">above</a>.</li>
    696   1.1  christos <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    697   1.1  christos is another time zone database.</li>
    698   1.1  christos <li>The <a
    699   1.1  christos href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
    700   1.1  christos Schedules Information Manual</a> of the
    701   1.1  christos International Air Transport Association
    702   1.1  christos gives current time zone rules for airports served by commercial aviation.</li>
    703   1.1  christos </ul>
    704   1.6  christos </section>
    705   1.6  christos 
    706   1.6  christos <section>
    707   1.1  christos <h2 id="maps">Maps</h2>
    708   1.1  christos <ul>
    709   1.9  christos <li>The <a
    710   1.9  christos href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
    711   1.9  christos and Regional Maps section</a> of <em>The World Factbook</em>, published by the
    712   1.9  christos <a href="https://www.cia.gov">US Central Intelligence
    713   1.9  christos Agency (<abbr
    714   1.9  christos title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
    715   1.9  christos zone map; the
    716   1.1  christos <a
    717   1.6  christos href="https://legacy.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
    718   1.1  christos Library Map Collection</a>
    719   1.1  christos of the University of Texas at Austin has copies of
    720   1.1  christos recent editions.
    721   1.1  christos The pictorial quality is good,
    722   1.1  christos but the maps do not indicate daylight saving time,
    723   1.1  christos and parts of the data are a few years out of date.</li>
    724   1.6  christos <li><a href="https://www.worldtimezone.com">World Time Zone Map
    725   1.6  christos with current time</a>
    726   1.1  christos has several fancy time zone maps; it covers Russia particularly well.
    727   1.1  christos The maps' pictorial quality is not quite as good as the
    728   1.1  christos <abbr>CIA</abbr>'s
    729   1.1  christos but the maps are more up to date.</li>
    730   1.1  christos <li><a
    731   1.1  christos href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    732   1.1  christos much is time wrong around the world?</a> maps the difference between
    733   1.1  christos mean solar and standard time, highlighting areas such as western China
    734   1.1  christos where the two differ greatly. It's a bit out of date, unfortunately.</li>
    735   1.1  christos </ul>
    736   1.6  christos </section>
    737   1.6  christos 
    738   1.6  christos <section>
    739   1.1  christos <h2 id="boundaries">Time zone boundaries</h2>
    740   1.2  christos <p>Geographical boundaries between timezones are available
    741   1.9  christos from several <a
    742   1.9  christos href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
    743   1.9  christos geolocation</a>
    744   1.1  christos services and other sources.</p>
    745   1.1  christos <ul>
    746   1.1  christos <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    747   1.1  christos Boundary Builder</a> extracts
    748   1.1  christos <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    749   1.2  christos boundaries of <code><abbr>tzdb</abbr></code> timezones.
    750   1.1  christos Its code is freely available under the <abbr>MIT</abbr> license, and
    751   1.1  christos its data entries are freely available under the
    752   1.1  christos <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    753   1.1  christos Open Database License</a>. The maps' borders appear to be quite accurate.</li>
    754   1.1  christos <li>Programmatic interfaces that map geographical coordinates via tz_world to
    755   1.2  christos <code><abbr>tzdb</abbr></code> timezones include:
    756   1.1  christos <ul>
    757   1.1  christos <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
    758   1.1  christos written in <a
    759   1.1  christos href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
    760   1.1  christos and is freely available under the <abbr>MIT</abbr> license.</li>
    761   1.1  christos <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
    762   1.1  christos is written in Go and is freely available under the Apache License.</li>
    763   1.1  christos <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
    764   1.1  christos in both Java and
    765   1.1  christos <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
    766   1.1  christos form, is freely available under the MIT license.</li>
    767  1.10  christos <li>For Node.js,
    768   1.1  christos the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
    769   1.1  christos is freely available under the MIT license, and
    770   1.1  christos the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
    771   1.1  christos is in the public domain.</li>
    772   1.1  christos <li>The <a
    773   1.1  christos href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
    774   1.1  christos library for Python is freely available under the MIT license.
    775   1.1  christos <li>The <a
    776   1.1  christos href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
    777   1.1  christos library for Ruby is freely available under the MIT license.</li>
    778   1.1  christos </ul></li>
    779   1.1  christos <li>Free access via a network API, if you register a key, is provided by
    780   1.4  christos the <a
    781   1.4  christos href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
    782   1.4  christos Timezone web service</a>, the <a
    783   1.4  christos href="https://developers.google.com/maps/documentation/timezone/intro">Google
    784   1.4  christos Maps Time Zone API</a>, and
    785   1.8  christos the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
    786   1.1  christos Commercial network API access is provided
    787   1.1  christos by <a href="https://askgeo.com">AskGeo</a>
    788   1.1  christos and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    789   1.1  christos </li>
    790   1.1  christos <li>"<a
    791   1.1  christos href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
    792   1.1  christos to get a time zone from a location using latitude and longitude
    793   1.1  christos coordinates?</a>" discusses other geolocation possibilities.</li>
    794   1.1  christos <li><a href="http://statoids.com/statoids.html">Administrative
    795   1.1  christos Divisions of Countries ("Statoids")</a> lists
    796   1.1  christos political subdivision data related to time zones.</li>
    797   1.7  christos <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
    798   1.1  christos zone boundaries for multizone countries</a> summarizes legal
    799   1.1  christos boundaries between time zones within countries.</li>
    800   1.1  christos <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
    801   1.1  christos &ndash; GIS and Database Tools</a> includes a Manifold-format map of
    802   1.1  christos world time zone boundaries distributed under the
    803   1.1  christos <abbr>GPL</abbr>.</li>
    804   1.1  christos <li>A ship within the <a
    805   1.1  christos href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    806   1.1  christos waters</a> of any nation uses that nation's time. In international
    807   1.1  christos waters, time zone boundaries are meridians 15&deg; apart, except that
    808   1.1  christos <abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
    809   1.1  christos wide and are separated by
    810   1.1  christos the 180&deg; meridian (not by the International Date Line, which is
    811   1.1  christos for land and territorial waters only). A captain can change ship's
    812   1.1  christos clocks any time after entering a new time zone; midnight changes are
    813   1.1  christos common.</li>
    814   1.1  christos </ul>
    815   1.6  christos </section>
    816   1.6  christos 
    817   1.6  christos <section>
    818   1.1  christos <h2 id="civil">Civil time concepts and history</h2>
    819   1.1  christos <ul>
    820   1.1  christos <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    821   1.1  christos Walk through Time</a>
    822   1.1  christos surveys the evolution of timekeeping.</li>
    823   1.4  christos <li>The history of daylight saving time is surveyed in <a
    824   1.4  christos href="http://www.webexhibits.org/daylightsaving/">About Daylight
    825   1.4  christos Saving Time &ndash; History, rationale, laws &amp; dates</a> and summarized in
    826   1.4  christos <a href="http://seizethedaylight.com/dst/">A Brief
    827   1.4  christos History of Daylight Saving Time</a>.</li>
    828   1.4  christos <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
    829   1.4  christos Lords</a> discusses how authoritarians manipulate civil time.</li>
    830   1.1  christos <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
    831   1.1  christos contains guidelines and best practices for software applications that
    832   1.1  christos deal with civil time.</li>
    833   1.1  christos <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
    834   1.1  christos the International Date Line</a> tells the story of the most important
    835   1.1  christos time zone boundary.</li>
    836   1.1  christos <li><a href="http://statoids.com/tconcept.html">Basic Time
    837   1.1  christos Zone Concepts</a> discusses terminological issues behind time zones.</li>
    838   1.1  christos </ul>
    839   1.6  christos </section>
    840   1.6  christos 
    841   1.6  christos <section>
    842   1.1  christos <h2 id="national">National histories of legal time</h2>
    843   1.1  christos <dl>
    844   1.1  christos <dt>Australia</dt>
    845   1.6  christos <dd>The Parliamentary Library commissioned a <a
    846   1.1  christos href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
    847   1.1  christos paper on daylight saving time in Australia</a>.
    848   1.1  christos The Bureau of Meteorology publishes a list of <a
    849   1.1  christos href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    850   1.1  christos Dates of Daylight Savings Time within Australia</a>.</dd>
    851   1.1  christos <dt>Belgium</dt>
    852   1.7  christos <dd>The Royal Observatory of Belgium maintains a table of time in
    853   1.7  christos Belgium (in
    854   1.7  christos <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
    855   1.7  christos hreflang="nl">Dutch</a> and <a
    856   1.7  christos href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
    857   1.7  christos hreflang="fr">French</a>).</dd>
    858   1.1  christos <dt>Brazil</dt>
    859   1.1  christos <dd>The Time Service Department of the National Observatory
    860   1.1  christos records <a href="http://pcdsh01.on.br/DecHV.html"
    861   1.1  christos hreflang="pt-BR">Brazil's daylight saving time decrees (in
    862   1.1  christos Portuguese)</a>.</dd>
    863   1.1  christos <dt>Canada</dt>
    864   1.1  christos <dd>National Research Council Canada publishes current
    865   1.1  christos and some older information about <a
    866   1.6  christos href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
    867   1.6  christos zones and daylight saving time</a>.</dd>
    868   1.1  christos <dt>Chile</dt>
    869   1.1  christos <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    870  1.11  christos <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
    871   1.1  christos Chile's official time (in Spanish)</a>.</dd>
    872   1.3  christos <dt>China</dt>
    873   1.3  christos <dd>The Hong Kong Observatory maintains a
    874   1.7  christos <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
    875   1.3  christos  summer time in Hong Kong</a>,
    876   1.3  christos and Macau's Meteorological and Geophysical Bureau maintains a <a
    877   1.7  christos href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
    878   1.3  christos history for Macau</a>.
    879   1.3  christos Unfortunately the latter is incomplete and has errors.</dd>
    880   1.1  christos <dt>Czech Republic</dt>
    881   1.1  christos <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    882   1.1  christos hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    883  1.10  christos summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
    884   1.1  christos <dt>Germany</dt>
    885   1.1  christos <dd>The National Institute for Science and Technology maintains the <a
    886   1.1  christos href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    887   1.1  christos of Legal Time in Germany</a>.</dd>
    888   1.1  christos <dt>Israel</dt>
    889   1.1  christos <dd>The Interior Ministry periodically issues <a
    890   1.1  christos href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
    891   1.1  christos hreflang="he">announcements (in Hebrew)</a>.</dd>
    892   1.2  christos <dt>Malaysia</dt>
    893   1.2  christos <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    894   1.1  christos <dt>Mexico</dt>
    895   1.1  christos <dd>The Investigation and Analysis Service of the Mexican Library of
    896   1.1  christos Congress has published a <a
    897   1.1  christos href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    898   1.1  christos hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    899   1.1  christos <dt>Netherlands</dt>
    900   1.1  christos <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    901   1.1  christos hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    902   1.1  christos covers the history of local time in the Netherlands from ancient times.</dd>
    903   1.1  christos <dt>New Zealand</dt>
    904   1.1  christos <dd>The Department of Internal Affairs maintains a brief <a
    905   1.1  christos href="https://www.dia.govt.nz/Daylight-Saving-History">History of
    906   1.6  christos Daylight Saving</a>.</dd>
    907  1.10  christos <dt>Portugal</dt>
    908  1.10  christos <dd>The Lisbon Astronomical Observatory publishes a
    909  1.10  christos <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
    910  1.10  christos legal time (in Portuguese)</a>.</dd>
    911   1.1  christos <dt>Singapore</dt>
    912   1.1  christos <dd><a id="Singapore"
    913   1.7  christos href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
    914   1.1  christos is Singapore in the "Wrong" Time Zone?</a> details the
    915   1.1  christos history of legal time in Singapore and Malaysia.</dd>
    916   1.1  christos <dt>United Kingdom</dt>
    917   1.1  christos <dd><a
    918   1.1  christos href="https://www.polyomino.org.uk/british-time/">History of
    919   1.1  christos legal time in Britain</a> discusses in detail the country
    920   1.6  christos with perhaps the best-documented history of clock adjustments.</dd>
    921   1.1  christos <dt>United States</dt>
    922   1.1  christos <dd>The Department of Transportation's <a
    923   1.1  christos href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    924   1.1  christos Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
    925   1.1  christos <dt>Uruguay</dt>
    926   1.1  christos <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    927   1.1  christos Navy (SOHMA) publishes an annual <a
    928   1.7  christos href="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
    929   1.1  christos (in Spanish)</a>.</dd>
    930   1.1  christos </dl>
    931   1.6  christos </section>
    932   1.6  christos 
    933   1.6  christos <section>
    934   1.7  christos <h2 id="costs">Costs and benefits of time shifts</h2>
    935   1.7  christos <p>Various sources argue for and against daylight saving time and time
    936   1.7  christos zone shifts, and many scientific studies have been conducted. This
    937   1.8  christos section summarizes reviews and position statements based on
    938   1.8  christos scientific literature in the area.</p>
    939   1.7  christos <ul>
    940   1.7  christos <li>Carey RN, Sarma KM.
    941   1.7  christos <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
    942   1.7  christos daylight saving time on road traffic collision risk: a systematic
    943   1.7  christos review</a>.
    944   1.7  christos <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>.
    945   1.7  christos This reviews research literature and concludes that the evidence
    946   1.7  christos neither supports nor refutes road safety benefits from
    947   1.7  christos shifts in time zones.</li>
    948   1.7  christos <li>Havranek T, Herman D, Irsova D.
    949   1.7  christos <a href="https://www.econstor.eu/handle/10419/174191">Does daylight
    950   1.7  christos saving save electricity? A meta-analysis</a>. <em>Energy J.</em>
    951   1.7  christos 2018;39(2).
    952   1.7  christos doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
    953   1.7  christos This analyzes research literature and concludes, "Electricity savings
    954   1.7  christos are larger for countries farther away from the equator, while
    955  1.10  christos subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
    956   1.8  christos <li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
    957   1.8  christos <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
    958   1.8  christos an American Academy of Sleep Medicine position statement</a>.
    959   1.8  christos <em>J Clin Sleep Med.</em>
    960   1.8  christos 2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
    961   1.8  christos This argues for permanent standard time due to health risks of both
    962  1.10  christos <abbr>DST</abbr> transitions and permanent <abbr>DST</abbr>.</li>
    963   1.7  christos <li>Roenneberg T, Winnebeck EC, Klerman EB.
    964   1.7  christos <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
    965   1.7  christos saving time and artificial time zones &ndash; a battle between
    966   1.7  christos biological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
    967   1.7  christos doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
    968  1.10  christos This reviews evidence about the health effects of <abbr>DST</abbr>
    969  1.10  christos and concludes,
    970   1.7  christos "In summary, the scientific literature strongly argues against the
    971  1.10  christos switching between <abbr>DST</abbr> and Standard Time and even more so against
    972  1.10  christos adopting <abbr>DST</abbr> permanently."</li>
    973   1.7  christos </ul>
    974   1.7  christos </section>
    975   1.7  christos 
    976   1.7  christos <section>
    977   1.1  christos <h2 id="precision">Precision timekeeping</h2>
    978   1.1  christos <ul>
    979   1.1  christos <li><a
    980   1.1  christos href="http://leapsecond.com/hpan/an1289.pdf">The
    981   1.1  christos Science of Timekeeping</a> is a thorough introduction
    982   1.1  christos to the theory and practice of precision timekeeping.</li>
    983   1.1  christos <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
    984   1.1  christos Time 2016</a> contains several freely-readable papers.</li>
    985   1.1  christos <li><a href="http://www.ntp.org"><abbr
    986   1.1  christos title="Network Time Protocol">NTP</abbr>: The Network
    987   1.1  christos Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
    988   1.1  christos discusses how to synchronize clocks of
    989   1.1  christos Internet hosts.</li>
    990   1.2  christos <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
    991   1.2  christos family of software algorithms can achieve accuracy to a few tens of
    992   1.2  christos nanoseconds in scalable server farms without special hardware.</li>
    993   1.1  christos <li>The <a
    994   1.1  christos href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
    995   1.1  christos Time Protocol</a> (<abbr
    996   1.1  christos title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
    997   1.2  christos can achieve submicrosecond clock accuracy on a local area network
    998   1.2  christos with special-purpose hardware.</li>
    999   1.1  christos <li><a
   1000   1.1  christos href="https://tools.ietf.org/html/rfc4833">Timezone
   1001   1.1  christos Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
   1002   1.1  christos (Internet <abbr>RFC</abbr> 4833)
   1003   1.1  christos specifies a <a
   1004   1.1  christos href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
   1005   1.1  christos option for a server to configure
   1006   1.1  christos a client's time zone and daylight saving settings automatically.</li>
   1007  1.11  christos <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
   1008  1.11  christos Scales</a> describes astronomical time scales like
   1009   1.1  christos <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
   1010   1.1  christos <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
   1011   1.1  christos <abbr title="Barycentric Dynamic Time">TDB</abbr>.
   1012   1.1  christos <li>The <a href="https://www.iau.org"><abbr
   1013   1.1  christos title="International Astronomical Union">IAU</abbr></a>'s <a
   1014   1.1  christos href="http://www.iausofa.org"><abbr
   1015   1.1  christos title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
   1016   1.1  christos collection contains C and <a
   1017   1.1  christos href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
   1018   1.1  christos code for converting among time scales like
   1019   1.1  christos <abbr title="International Atomic Time">TAI</abbr>,
   1020   1.1  christos <abbr>TDB</abbr>, <abbr>TDT</abbr> and
   1021   1.1  christos <abbr>UTC</abbr>.</li>
   1022   1.1  christos <li><a
   1023   1.1  christos href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
   1024   1.1  christos &ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
   1025   1.1  christos diverse local time
   1026   1.1  christos scales used by each landed mission on Mars.</li>
   1027   1.1  christos <li><a href="http://leapsecond.com">LeapSecond.com</a> is
   1028   1.1  christos dedicated not only to leap seconds but to precise time and frequency
   1029   1.1  christos in general. It covers the state of the art in amateur timekeeping, and
   1030   1.1  christos how the art has progressed over the past few decades.</li>
   1031   1.7  christos <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
   1032   1.7  christos href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
   1033   1.7  christos and time-signal emissions</a>, International Telecommunication Union &ndash;
   1034   1.7  christos Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
   1035   1.1  christos <li><a
   1036   1.1  christos href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
   1037   1.1  christos title="International Earth Rotation and Reference Systems Service">IERS</abbr>
   1038   1.1  christos Bulletins</a> contains official publications of the International
   1039   1.1  christos Earth Rotation and Reference Systems Service, which decides when leap
   1040   1.4  christos seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
   1041   1.9  christos via an optional "<code>right</code>" configuration where a computer's internal
   1042   1.9  christos <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
   1043   1.9  christos as opposed to the default "<code>posix</code>" configuration
   1044   1.9  christos where the internal clock ignores leap seconds.
   1045   1.9  christos The two configurations agree for timestamps starting with 1972-01-01 00:00:00
   1046   1.9  christos <abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge for
   1047   1.9  christos timestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,
   1048   1.9  christos which corresponds to the first leap second
   1049   1.9  christos 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
   1050   1.9  christos and to
   1051   1.9  christos 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
   1052   1.9  christos In practice the two configurations also agree for timestamps before
   1053   1.9  christos 1972 even though the historical situation is messy, partly because
   1054   1.9  christos neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
   1055   1.9  christos is well-defined for sufficiently-old timestamps.</li>
   1056   1.1  christos <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
   1057   1.1  christos discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
   1058   1.1  christos leap second so that they disagree with <abbr>UTC</abbr> by at most a
   1059   1.1  christos half second, even though every <abbr>POSIX</abbr> minute has exactly
   1060   1.4  christos sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
   1061   1.1  christos "<code>posix</code>" configuration, is <a
   1062   1.1  christos href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
   1063   1.1  christos the <abbr>NTP</abbr> reference implementation, and is used by major
   1064   1.7  christos cloud service providers. However, according to
   1065   1.7  christos <a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">&sect;3.7.1 of
   1066   1.7  christos Network Time Protocol Best Current Practices</a>
   1067   1.7  christos (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
   1068   1.9  christos applications requiring accurate <abbr>UTC</abbr> or civil time,
   1069   1.7  christos and is intended for use only in single, well-controlled environments.</li>
   1070   1.1  christos <li>The <a
   1071   1.1  christos href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
   1072   1.1  christos Second Discussion List</a> covers <a
   1073   1.1  christos href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
   1074   1.1  christos and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
   1075   1.1  christos discussed further in
   1076   1.1  christos <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
   1077   1.1  christos leap second: its history and possible future</a>.
   1078   1.1  christos <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
   1079   1.1  christos might be redefined
   1080   1.1  christos without Leap Seconds</a> gives pointers on this
   1081   1.1  christos contentious issue, which was active until 2015 and could become active
   1082   1.1  christos again.</li>
   1083   1.1  christos </ul>
   1084   1.6  christos </section>
   1085   1.6  christos 
   1086   1.6  christos <section>
   1087   1.1  christos <h2 id="notation">Time notation</h2>
   1088   1.1  christos <ul>
   1089   1.1  christos <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
   1090   1.1  christos Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
   1091   1.1  christos zone names, abbreviations, identifiers, and formats. For example, it
   1092   1.1  christos contains French translations for "Eastern European Summer Time",
   1093   1.1  christos "<abbr title="Eastern European Summer Time">EEST</abbr>", and
   1094   1.1  christos "Bucharest". Its
   1095   1.1  christos <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
   1096   1.1  christos charts</a> show these values for many locales. Data values are available in
   1097   1.1  christos both <abbr title="Locale Data Markup Language">LDML</abbr>
   1098   1.1  christos (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
   1099   1.1  christos <li>
   1100   1.1  christos <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
   1101   1.6  christos the international standard date and time notation</a> covers
   1102   1.1  christos <a
   1103   1.6  christos href="https://www.iso.org/standard/70907.html"><em><abbr
   1104   1.6  christos title="International Organization for Standardization">ISO</abbr>
   1105   1.6  christos 8601-1:2019 &ndash; Date and time &ndash; Representations for information
   1106   1.6  christos interchange &ndash; Part 1: Basic rules</em></a>.</li>
   1107   1.1  christos <li>
   1108   1.6  christos <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
   1109   1.1  christos Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
   1110   1.1  christos <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
   1111   1.1  christos <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">&sect;3.3 of
   1112   1.1  christos Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
   1113   1.1  christos specifies the time notation used in email and <a
   1114   1.1  christos href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
   1115   1.1  christos headers.</li>
   1116   1.1  christos <li>
   1117   1.1  christos <a href="https://tools.ietf.org/html/rfc3339">Date and Time
   1118   1.1  christos on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
   1119   1.1  christos specifies an <abbr>ISO</abbr> 8601
   1120   1.1  christos profile for use in new Internet
   1121   1.1  christos protocols.</li>
   1122   1.1  christos <li>
   1123   1.7  christos <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
   1124   1.1  christos Formats on the Web</a> surveys web- and Internet-oriented date and time
   1125   1.1  christos formats.</li>
   1126   1.1  christos <li>Alphabetic time zone abbreviations should not be used as unique
   1127   1.1  christos identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
   1128   1.1  christos practice. For example, in English-speaking North America
   1129   1.1  christos "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
   1130   1.1  christos but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
   1131   1.1  christos and French-speaking North Americans prefer
   1132   1.1  christos "<abbr title="Heure Normale du Centre">HNC</abbr>" to
   1133   1.1  christos "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
   1134   1.2  christos database contains English abbreviations for many timestamps;
   1135   1.1  christos unfortunately some of these abbreviations were merely the database maintainers'
   1136   1.1  christos inventions, and these have been removed when possible.</li>
   1137   1.1  christos <li>Numeric time zone abbreviations typically count hours east of
   1138   1.1  christos <abbr>UT</abbr>, e.g., +09 for Japan and
   1139   1.1  christos &minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
   1140   1.1  christos <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
   1141   1.1  christos For example, one might use <code><abbr>TZ</abbr>="<abbr
   1142   1.1  christos title="Japan Standard Time">JST</abbr>-9"</code> and
   1143   1.1  christos <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
   1144   1.1  christos for Japan and Hawaii, respectively. If the
   1145   1.1  christos <code><abbr>tz</abbr></code> database is available, it is usually better to use
   1146   1.1  christos settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
   1147   1.1  christos <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
   1148   1.2  christos confusion, handle old timestamps better, and insulate you better from
   1149   1.1  christos any future changes to the rules. One should never set
   1150   1.1  christos <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
   1151   1.1  christos <code>"GMT-9"</code>, though, since this would incorrectly imply that
   1152   1.1  christos local time is nine hours ahead of <abbr>UT</abbr> and the time zone
   1153   1.1  christos is called "<abbr>GMT</abbr>".</li>
   1154   1.1  christos </ul>
   1155   1.6  christos </section>
   1156   1.6  christos 
   1157   1.6  christos <section>
   1158   1.1  christos <h2 id="see-also">See also</h2>
   1159   1.1  christos <ul>
   1160   1.7  christos <li><a href="theory.html">Theory and pragmatics of the
   1161   1.7  christos <code><abbr>tz</abbr></code> code and data</a></li>
   1162   1.1  christos <li><a href="tz-art.html">Time and the Arts</a></li>
   1163   1.1  christos </ul>
   1164   1.6  christos </section>
   1165   1.6  christos 
   1166   1.6  christos <footer>
   1167   1.1  christos <hr>
   1168   1.1  christos This web page is in the public domain, so clarified as of
   1169   1.1  christos 2009-05-17 by Arthur David Olson.
   1170   1.1  christos <br>
   1171   1.1  christos Please send corrections to this web page to the
   1172   1.1  christos <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
   1173   1.6  christos </footer>
   1174   1.1  christos </body>
   1175   1.1  christos </html>
   1176