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      4  1.13  christos <title>Time zone and daylight saving time data</title>
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     14  1.13  christos <h1>Time zone and daylight saving time data</h1>
     15   1.1  christos <p>
     16   1.1  christos <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
     17   1.1  christos <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
     18   1.1  christos rules are controlled by individual
     19   1.1  christos governments. They are sometimes changed with little notice, and their
     20   1.1  christos histories and planned futures are often recorded only fitfully. Here
     21   1.1  christos is a summary of attempts to organize and record relevant data in this
     22   1.1  christos area.
     23   1.1  christos </p>
     24   1.6  christos   <nav>
     25   1.6  christos     <ul>
     26   1.6  christos       <li>The <code><abbr>tz</abbr></code> database product and process
     27   1.6  christos 	<ul>
     28   1.6  christos 	  <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
     29   1.6  christos 	  <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
     30   1.6  christos 	  <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
     31  1.14  christos 	  <li><a href="#coordinating">Coordinating with governments and distributors</a></li>
     32   1.6  christos 	  <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
     33   1.6  christos 	</ul>
     34   1.6  christos       </li>
     35   1.6  christos       <li>Uses of the <code><abbr>tz</abbr></code> database
     36   1.6  christos 	<ul>
     37   1.6  christos 	  <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
     38   1.6  christos 	  <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
     39   1.6  christos 	  <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
     40   1.6  christos 	  <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
     41   1.6  christos 	  <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
     42   1.6  christos 	</ul>
     43   1.6  christos       </li>
     44   1.6  christos       <li>Related data
     45   1.6  christos 	<ul>
     46   1.6  christos 	  <li><a href="#other-dbs">Other time zone databases</a></li>
     47   1.6  christos 	  <li><a href="#maps">Maps</a></li>
     48   1.6  christos 	  <li><a href="#boundaries">Time zone boundaries</a></li>
     49   1.6  christos 	</ul>
     50   1.6  christos       </li>
     51   1.6  christos       <li>Timekeeping concepts
     52   1.6  christos 	<ul>
     53   1.6  christos 	  <li><a href="#civil">Civil time concepts and history</a></li>
     54   1.6  christos 	  <li><a href="#national">National histories of legal time</a></li>
     55   1.7  christos 	  <li><a href="#costs">Costs and benefits of time shifts</a></li>
     56   1.6  christos 	  <li><a href="#precision">Precision timekeeping</a></li>
     57   1.6  christos 	  <li><a href="#notation">Time notation</a></li>
     58   1.6  christos 	</ul>
     59   1.6  christos       </li>
     60  1.13  christos       <li><a href="#see-also">See also</a></li>
     61   1.6  christos     </ul>
     62   1.6  christos   </nav>
     63   1.6  christos 
     64   1.6  christos <section>
     65   1.1  christos <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
     66   1.1  christos <p>
     67   1.1  christos The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
     68   1.1  christos time zone database contains code and data
     69   1.1  christos that represent the history of local time
     70   1.1  christos for many representative locations around the globe.
     71   1.1  christos It is updated periodically to reflect changes made by political bodies
     72   1.1  christos to time zone boundaries and daylight saving rules.
     73   1.1  christos This database (known as <code><abbr>tz</abbr></code>,
     74   1.1  christos <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
     75   1.1  christos is used by several implementations,
     76   1.1  christos including
     77   1.1  christos <a href="https://www.gnu.org/software/libc/">the
     78  1.20  christos <abbr title="GNUs Not Unix">GNU</abbr>
     79   1.1  christos C Library</a> (used in
     80   1.1  christos <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
     81   1.1  christos <a href="https://www.android.com">Android</a>,
     82   1.1  christos <a href="https://www.freebsd.org">Free<abbr
     83   1.1  christos title="Berkeley Software Distribution">BSD</abbr></a>,
     84   1.1  christos <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
     85   1.1  christos <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
     86  1.17  christos <a href="https://www.chromium.org/chromium-os/">ChromiumOS</a>,
     87   1.1  christos <a href="https://cygwin.com">Cygwin</a>,
     88   1.7  christos <a href="https://mariadb.org">MariaDB</a>,
     89  1.17  christos <a href="https://musl.libc.org">musl libc</a>,
     90   1.1  christos <a href="https://www.mysql.com">MySQL</a>,
     91   1.1  christos <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
     92   1.1  christos title="Web Operating System">webOS</abbr></a>,
     93   1.7  christos <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
     94   1.1  christos title="Advanced Interactive eXecutive">AIX</abbr></a>,
     95  1.21  christos <a href="https://www.apple.com/os/ios/"><abbr
     96   1.1  christos title="iPhone OS">iOS</abbr></a>,
     97  1.21  christos <a href="https://www.apple.com/os/macos/">macOS</a>,
     98   1.1  christos <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
     99  1.21  christos <a href="https://vmssoftware.com">Open<abbr
    100   1.1  christos title="Virtual Memory System">VMS</abbr></a>,
    101  1.20  christos <a href="https://www.oracle.com/database/">Oracle Database</a>,
    102  1.21  christos <a href="https://www.oracle.com/solaris/solaris11/">Oracle Solaris</a>,
    103  1.21  christos and <a href="https://qnx.software/en">QNX</a>.</p>
    104   1.1  christos <p>
    105   1.2  christos Each main entry in the database represents a <dfn>timezone</dfn>
    106   1.2  christos for a set of civil-time clocks that have all agreed since 1970.
    107   1.2  christos Timezones are typically identified by continent or ocean and then by the
    108   1.2  christos name of the largest city within the region containing the clocks.
    109   1.1  christos For example, <code>America/New_York</code>
    110   1.1  christos represents most of the <abbr title="United States">US</abbr> eastern time zone;
    111   1.1  christos <code>America/Phoenix</code> represents most of Arizona, which
    112   1.2  christos uses mountain time without daylight saving time (<abbr>DST</abbr>);
    113   1.1  christos <code>America/Detroit</code> represents most of Michigan, which uses
    114   1.1  christos eastern time but with different <abbr>DST</abbr> rules in 1975;
    115   1.1  christos and other entries represent smaller regions like Starke County,
    116   1.1  christos Indiana, which switched from central to eastern time in 1991
    117   1.1  christos and switched back in 2006.
    118  1.17  christos To use the database on a <a
    119   1.1  christos href="https://en.wikipedia.org/wiki/POSIX"><abbr
    120  1.17  christos title="Portable Operating System Interface">POSIX</abbr>.1-2024</a>
    121   1.1  christos implementation set the <code><abbr>TZ</abbr></code>
    122  1.20  christos environment variable to the locations full name,
    123   1.1  christos e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
    124   1.1  christos <p>
    125   1.2  christos Associated with each timezone is a history of offsets from
    126   1.1  christos <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
    127   1.1  christos Time</a> (<abbr>UT</abbr>), which is <a
    128   1.1  christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
    129   1.1  christos Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
    130   1.2  christos for timestamps after 1960 this is more precisely <a
    131   1.1  christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
    132   1.1  christos Universal Time</a> (<abbr>UTC</abbr>).
    133   1.1  christos The database also records when daylight saving time was in use,
    134   1.1  christos along with some time zone abbreviations such as <abbr>EST</abbr>
    135   1.1  christos for Eastern Standard Time in the <abbr>US</abbr>.</p>
    136   1.6  christos </section>
    137   1.6  christos 
    138   1.6  christos <section>
    139   1.1  christos <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
    140   1.1  christos <p>
    141   1.1  christos The following <a
    142   1.1  christos href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
    143  1.20  christos the latest releases two
    144   1.1  christos <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
    145   1.1  christos to a <abbr>GNU</abbr>/Linux or similar host.</p>
    146   1.2  christos <pre><code>mkdir tzdb
    147   1.1  christos cd tzdb
    148   1.1  christos <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
    149   1.1  christos wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
    150   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 -
    151   1.1  christos gzip -dc tzdata-latest.tar.gz | tar -xf -
    152   1.1  christos </code></pre>
    153   1.1  christos <p>Alternatively, the following shell commands download the same
    154   1.1  christos release in a single-tarball format containing extra data
    155   1.1  christos useful for regression testing:</p>
    156   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>
    157   1.1  christos <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
    158   1.1  christos </code></pre>
    159   1.1  christos <p>These commands use convenience links to the latest release
    160   1.1  christos of the <code><abbr>tz</abbr></code> database hosted by the
    161   1.1  christos <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
    162   1.1  christos of the <a href="https://www.iana.org">Internet Assigned Numbers
    163   1.1  christos Authority (IANA)</a>.
    164   1.1  christos Older releases are in files named
    165   1.1  christos <code>tzcode<var>V</var>.tar.gz</code>,
    166   1.1  christos <code>tzdata<var>V</var>.tar.gz</code>, and
    167   1.1  christos <code>tzdb-<var>V</var>.tar.lz</code>,
    168   1.1  christos where <code><var>V</var></code> is the version.
    169   1.1  christos Since 1996, each version has been a four-digit year followed by
    170   1.1  christos lower-case letter (<samp>a</samp> through <samp>z</samp>,
    171   1.1  christos then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
    172   1.1  christos through <samp>zzz</samp>, and so on).
    173  1.13  christos Since version 2022a, each release has been distributed in
    174  1.20  christos <a href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/pax.html#tag_20_94_13_06">POSIX
    175  1.10  christos ustar interchange format</a>, compressed as described above;
    176  1.14  christos older releases use a nearly compatible format.
    177   1.1  christos Since version 2016h, each release has contained a text file named
    178  1.20  christos <code>version</code> whose first (and currently only) line is the version.
    179  1.20  christos <a href="https://ftp.iana.org/tz/releases/">Older archived releases are
    180  1.20  christos available</a> via
    181  1.20  christos <a href="https://en.wikipedia.org/wiki/HTTPS"><abbr
    182  1.20  christos title="Hypertext Transfer Protocol Secure">HTTPS</abbr></a>,
    183  1.20  christos <a href="https://en.wikipedia.org/wiki/Rsync"><abbr
    184  1.20  christos title="remote sync">rsync</abbr></a>, and
    185  1.20  christos <a href="https://en.wikipedia.org/wiki/FTP"><abbr
    186  1.20  christos title="File Transfer Protocol">FTP</abbr></a>.
    187  1.21  christos <p>Alternatively, a development repository of code and data can be
    188   1.1  christos retrieved from <a href="https://github.com">GitHub</a> via the shell
    189   1.1  christos command:</p>
    190   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>
    191   1.1  christos </code></pre>
    192   1.1  christos <p>
    193   1.1  christos Since version 2012e, each release has been tagged in development repositories.
    194   1.1  christos Untagged commits are less well tested and probably contain
    195   1.1  christos more errors.</p>
    196   1.1  christos <p>
    197   1.1  christos After obtaining the code and data files, see the
    198   1.1  christos <code>README</code> file for what to do next.
    199   1.1  christos The code lets you compile the <code><abbr>tz</abbr></code> source files into
    200   1.2  christos machine-readable binary files, one for each location. The binary files
    201  1.17  christos are in a special format specified by
    202  1.20  christos <a href="https://www.rfc-editor.org/rfc/rfc9636">The
    203  1.17  christos Time Zone Information Format (<abbr>TZif</abbr>)</a>
    204  1.18  christos (Internet <abbr title="Request For Comments">RFC</abbr> 9636).
    205   1.2  christos The code also lets
    206   1.2  christos you read a <abbr>TZif</abbr> file and interpret timestamps for that
    207   1.1  christos location.</p>
    208   1.6  christos </section>
    209   1.6  christos 
    210   1.6  christos <section>
    211   1.1  christos <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    212   1.1  christos <p>
    213   1.1  christos The <code><abbr>tz</abbr></code> code and data
    214   1.1  christos are by no means authoritative. If you find errors, please
    215  1.17  christos email changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    216  1.20  christos the time zone mailing list.
    217  1.20  christos The mailing list and its archives are public,
    218  1.20  christos so please do not send confidential information.
    219  1.20  christos See
    220  1.17  christos <a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing
    221  1.20  christos lists main page</a> to subscribe or to browse its archive of old messages.
    222  1.17  christos <a href="https://tzdata-meta.timtimeonline.com">Metadata for mailing list
    223   1.5  christos discussions</a> and corresponding data changes can be
    224   1.5  christos generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
    225   1.5  christos </p>
    226   1.1  christos <p>
    227   1.1  christos Changes to the <code><abbr>tz</abbr></code> code and data are often
    228   1.1  christos propagated to clients via operating system updates, so
    229   1.1  christos client <code><abbr>tz</abbr></code> data can often be corrected by
    230   1.1  christos applying these updates. With GNU/Linux and similar systems, if your
    231   1.1  christos maintenance provider has not yet adopted the
    232   1.1  christos latest <code><abbr>tz</abbr></code> data, you can often short-circuit
    233   1.1  christos the process by tailoring the generic instructions in
    234   1.1  christos the <code><abbr>tz</abbr> README</code> file and installing the latest
    235   1.1  christos data yourself. System-specific instructions for installing the
    236   1.1  christos latest <code><abbr>tz</abbr></code> data have also been published
    237  1.17  christos for <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates"><abbr>AIX</abbr></a>,
    238   1.1  christos <a
    239  1.21  christos href="https://source.android.com/docs/core/permissions/timezone-rules">Android</a>,
    240   1.1  christos <a
    241  1.11  christos href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
    242   1.1  christos title="International Components for Unicode">ICU</abbr></a>,
    243  1.11  christos <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
    244   1.9  christos JDK</a>,
    245   1.9  christos <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    246   1.1  christos href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    247   1.9  christos <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
    248  1.11  christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
    249   1.1  christos </p>
    250  1.12  christos <p>Since version 2013a,
    251  1.12  christos sources for the <code><abbr>tz</abbr></code> database have been
    252   1.1  christos <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
    253   1.1  christos title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
    254   1.1  christos <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
    255   1.1  christos with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
    256   1.1  christos title="linefeed">LF</abbr></a>,
    257   1.1  christos which can be modified by common text editors such
    258   1.1  christos as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
    259  1.20  christos <a href="https://gedit-text-editor.org">gedit</a>, and
    260   1.1  christos <a href="https://www.vim.org">vim</a>.
    261   1.1  christos Specialized source-file editing can be done via the
    262   1.1  christos <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
    263   1.1  christos zoneinfo</a> package for <a
    264   1.1  christos href="https://www.sublimetext.com">Sublime Text</a> and the <a
    265   1.1  christos href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
    266   1.1  christos zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
    267   1.1  christos Studio Code</a>.
    268   1.1  christos </p>
    269   1.1  christos <p>
    270   1.1  christos For further information about updates, please see
    271  1.18  christos <a href="https://www.rfc-editor.org/rfc/rfc6557">Procedures for
    272  1.17  christos Maintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557).
    273  1.17  christos More detail can be
    274   1.7  christos found in <a href="theory.html">Theory and pragmatics of the
    275   1.7  christos <code><abbr>tz</abbr></code> code and data</a>.
    276   1.2  christos <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
    277   1.2  christos displays changes between recent <code><abbr>tzdb</abbr></code> versions.
    278   1.1  christos </p>
    279   1.6  christos </section>
    280   1.6  christos 
    281   1.6  christos <section>
    282  1.14  christos <h2 id="coordinating">Coordinating with governments and distributors</h2>
    283  1.14  christos <p>
    284  1.14  christos As discussed in
    285  1.20  christos <a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How
    286  1.20  christos Time Zones Are Coordinated</a>, the time zone database relies on
    287  1.14  christos collaboration among governments, the time zone database volunteer
    288  1.14  christos community, and data distributors downstream.
    289  1.14  christos <p>
    290  1.14  christos If your government plans to change its time zone boundaries or
    291  1.20  christos daylight saving rules, please send email as described in
    292  1.21  christos <a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a>.
    293  1.20  christos Do this well in advance,
    294  1.14  christos as this will lessen confusion and will coordinate updates to many cell phones,
    295  1.14  christos computers, and other devices around the world.
    296  1.14  christos In your email, please cite the legislation or regulation that specifies
    297  1.14  christos the change, so that it can be checked for details such as the exact times
    298  1.14  christos when clock transitions occur.
    299  1.14  christos It is OK if a rule change is planned to affect clocks
    300  1.14  christos far into the future, as a long-planned change can easily be reverted
    301  1.20  christos or otherwise altered with a years notice before the change would have
    302  1.14  christos affected clocks.</p>
    303  1.14  christos <p>
    304  1.14  christos There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.
    305  1.14  christos However, typically a release occurs every few months.
    306  1.14  christos Many downstream timezone data distributors wait for
    307  1.14  christos a <code><abbr>tzdb</abbr></code> release before they produce an update
    308  1.14  christos to time zone behavior in consumer devices and software products.
    309  1.14  christos After a release, various parties must integrate, test,
    310  1.14  christos and roll out an update before <a
    311  1.14  christos href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.
    312  1.14  christos These updates can be expensive, for both the <a
    313  1.14  christos href="https://en.wikipedia.org/wiki/Quality_assurance">quality
    314  1.14  christos assurance</a> process and the overall cost of shipping and installing
    315  1.20  christos updates to each devices copy of <code><abbr>tzdb</abbr></code>.
    316  1.14  christos Updates may be batched with other updates and may take substantial
    317  1.14  christos time to reach end users after a release.
    318  1.14  christos Older devices may no longer be supported and thus may never be updated,
    319  1.14  christos which means they will continue to use out-of-date rules.</p>
    320  1.14  christos <p>
    321  1.14  christos For these reasons any rule change should be promulgated at least a
    322  1.14  christos year before it affects how clocks operate; otherwise, there is a good
    323  1.14  christos chance that many clocks will be wrong due to delays in propagating updates,
    324  1.14  christos and that residents will be confused or even actively resist the change.
    325  1.20  christos The shorter the notice, the more likely clock problems will arise; see <a
    326  1.21  christos href="https://codeofmatt.com/on-the-timing-of-time-zone-changes/">On
    327  1.20  christos the Timing of Time Zone Changes</a> for examples.
    328  1.14  christos </p>
    329  1.14  christos </section>
    330  1.14  christos 
    331  1.14  christos <section>
    332   1.1  christos <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
    333   1.1  christos <ul>
    334   1.1  christos <li>The article
    335   1.1  christos <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
    336   1.1  christos an encyclopedic summary.</li>
    337   1.1  christos <li><a href="tz-how-to.html">How to Read the
    338   1.1  christos tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
    339   1.1  christos database format.</li>
    340   1.1  christos <li><a
    341   1.1  christos href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
    342   1.1  christos literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
    343  1.20  christos databases style.</li>
    344  1.10  christos <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
    345  1.10  christos managing time in the internet</a> analyzes the database longitudinally.</li>
    346   1.1  christos </ul>
    347   1.6  christos </section>
    348   1.6  christos 
    349   1.6  christos <section>
    350   1.1  christos <h2 id="web">Web sites using recent versions of the
    351   1.1  christos <code><abbr>tz</abbr></code> database</h2>
    352   1.1  christos <p>
    353   1.1  christos These are listed roughly in ascending order of complexity and fanciness.
    354   1.1  christos </p>
    355   1.1  christos <ul>
    356  1.20  christos <li><a href="https://time.is">Time.is</a> shows locations
    357   1.1  christos time and zones.</li>
    358   1.1  christos <li><a href="https://www.timejones.com">TimeJones.com</a>,
    359   1.1  christos <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
    360   1.7  christos <a href="https://www.worldclock.com">The World Clock</a>
    361   1.1  christos are time zone converters.</li>
    362   1.8  christos <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
    363   1.8  christos publishes <code><abbr>tzdb</abbr></code>-derived data in
    364   1.8  christos <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
    365   1.8  christos title="comma-separated values">CSV</abbr></a> and
    366   1.8  christos in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
    367   1.8  christos title="Structured Query Language">SQL</abbr></a> form.</li>
    368   1.1  christos <li><a
    369   1.6  christos href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
    370   1.1  christos lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
    371   1.1  christos <li><a
    372  1.13  christos href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
    373   1.1  christos Time in 1000 Places</a> uses descriptions of the values.</li>
    374  1.20  christos <li><a href="https://www.timeanddate.com/worldclock/">The World Clock 
    375   1.1  christos Worldwide</a> lets you sort zone names and convert times.</li>
    376   1.1  christos <li><a href="https://24timezones.com">24TimeZones</a> has a world
    377   1.1  christos time map and a time converter.</li>
    378   1.1  christos <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    379   1.1  christos calculates the current time difference between locations.</li>
    380  1.13  christos <li><a href="https://www.wx-now.com">Weather Now</a> and
    381  1.13  christos <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    382   1.1  christos </ul>
    383   1.6  christos </section>
    384   1.6  christos 
    385   1.6  christos <section>
    386   1.1  christos <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    387   1.1  christos <ul>
    388  1.20  christos <li><a href="https://www.rfc-editor.org/rfc/rfc7808">Time Zone
    389  1.20  christos Data Distribution Service</a> (TZDIST, Internet <abbr>RFC</abbr> 7808)
    390  1.20  christos is associated with
    391  1.20  christos <a href="https://www.rfc-editor.org/rfc/rfc7809">CalDAV</a>
    392   1.1  christos (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    393   1.2  christos transferring time zone data by reference.
    394   1.8  christos <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
    395  1.20  christos implementations</a> are available.</li>
    396  1.20  christos <li>The <a href="https://www.rfc-editor.org/rfc/rfc5545">iCalendar format</a>
    397  1.20  christos (Internet <abbr>RFC</abbr> 5445)
    398   1.1  christos covers time zone
    399   1.1  christos data; see its VTIMEZONE calendar component.
    400   1.1  christos The iCalendar format requires specialized parsers and generators; a
    401  1.18  christos variant <a href="https://www.rfc-editor.org/rfc/rfc6321">xCal</a>
    402   1.1  christos (Internet <abbr>RFC</abbr> 6321) uses
    403   1.1  christos <a href="https://www.w3.org/XML/"><abbr
    404   1.1  christos title="Extensible Markup Language">XML</abbr></a> format, and a variant
    405  1.18  christos <a href="https://www.rfc-editor.org/rfc/rfc7265">jCal</a>
    406   1.1  christos (Internet <abbr>RFC</abbr> 7265)
    407  1.17  christos uses <a href="https://www.json.org/json-en.html"><abbr
    408   1.1  christos title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    409   1.1  christos </ul>
    410   1.6  christos </section>
    411   1.6  christos 
    412   1.6  christos <section>
    413   1.1  christos <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    414   1.7  christos <p>Although some of these do not fully support
    415   1.7  christos <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
    416   1.7  christos distributions you can generally work around compatibility problems by
    417   1.7  christos running the command <code>make rearguard_tarballs</code> and compiling
    418   1.7  christos from the resulting tarballs instead.</p>
    419   1.1  christos <ul>
    420  1.17  christos <li><a href="https://github.com/libical/vzic">Vzic</a> is a <a
    421  1.14  christos href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>
    422   1.1  christos program that compiles
    423   1.1  christos <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    424   1.1  christos Vzic is freely
    425   1.1  christos available under the <a
    426   1.1  christos href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    427   1.1  christos General Public License (<abbr
    428   1.1  christos title="General Public License">GPL</abbr>)</a>.</li>
    429   1.1  christos <li><a
    430   1.4  christos href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
    431   1.1  christos contains a script <code>parse_olson</code> that compiles
    432   1.1  christos <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
    433   1.1  christos modules. It is part of the Perl <a
    434   1.4  christos href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
    435   1.4  christos which is freely
    436   1.1  christos available under both the <abbr>GPL</abbr> and the Perl Artistic
    437   1.1  christos License. DateTime::TimeZone also contains a script
    438   1.1  christos <code>tests_from_zdump</code> that generates test cases for each clock
    439   1.1  christos transition in the <code><abbr>tz</abbr></code> database.</li>
    440   1.1  christos <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
    441   1.1  christos Database Parser</a> is a
    442  1.14  christos <a href="https://en.wikipedia.org/wiki/C++">C++</a> parser and
    443  1.17  christos runtime library with a <a
    444  1.21  christos href="https://en.cppreference.com/w/cpp/chrono.html"><code>std::chrono</code> API</a>
    445  1.17  christos that is a standard part of C++.
    446   1.1  christos It is freely available under the
    447   1.1  christos <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
    448  1.13  christos <li><a id="ICU" href="https://icu.unicode.org">International Components for
    449   1.1  christos Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
    450  1.14  christos href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a>
    451   1.1  christos libraries for internationalization that
    452   1.1  christos has a compiler from <code><abbr>tz</abbr></code> source
    453   1.1  christos and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
    454   1.1  christos (mentioned <a href="#CLDR">below</a>)
    455   1.1  christos into an <abbr>ICU</abbr>-specific format.
    456   1.1  christos <abbr>ICU</abbr> is freely available under a
    457   1.1  christos <abbr>BSD</abbr>-style license.</li>
    458   1.1  christos <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
    459   1.1  christos the <a href="https://elixir-lang.org">Elixir</a> language downloads
    460   1.7  christos and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
    461   1.1  christos title="Application Program Interface">API</abbr>s for use. It is
    462   1.1  christos freely available under the <abbr>MIT</abbr> license.</li>
    463   1.1  christos <li>Java-based compilers and libraries include:
    464   1.1  christos <ul>
    465   1.1  christos <li>The <a
    466   1.9  christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
    467   1.1  christos tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
    468  1.17  christos <a href="https://openjdk.org">OpenJDK</a> and
    469  1.17  christos <a href="https://jdk.java.net">Oracle JDK</a>.
    470   1.9  christos Although its source code is proprietary, its executable is available under the
    471   1.9  christos <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
    472   1.9  christos Timezone Updater License Agreement</a>.</li>
    473   1.1  christos <li>The <a
    474  1.21  christos href="https://www.oracle.com/technical-resources/articles/java/jf14-date-time.html">Java
    475   1.6  christos SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
    476   1.4  christos href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
    477   1.1  christos which is freely available under a <abbr>BSD</abbr>-style license.</li>
    478  1.20  christos <li><a href="https://www.joda.org/joda-time/">Joda-Time  Java date
    479   1.1  christos and time <abbr>API</abbr></a> contains a class
    480   1.1  christos <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
    481   1.1  christos <code><abbr>tz</abbr></code> source into a binary format. It inspired
    482   1.1  christos Java 8 <code>java.time</code>, which its users should migrate to once
    483   1.1  christos they can assume Java 8 or later. It is available under the <a
    484   1.1  christos href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
    485   1.9  christos <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
    486  1.21  christos href="https://www.azul.com/products/components/ziupdater-time-zone-tool/">ZIUpdater</a>
    487  1.20  christos are alternatives to TZUpdater. IANA Updaters license is unclear;
    488   1.9  christos ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
    489   1.4  christos <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
    490   1.4  christos time library for Android</a> and
    491  1.17  christos <a href="https://github.com/MenoData/Time4J">Time4J: Advanced date,
    492   1.4  christos time and interval library for Java</a> compile
    493   1.4  christos <code><abbr>tz</abbr></code> source into a binary format.
    494   1.4  christos Time4A is available under the Apache License and Time4J is
    495   1.1  christos available under the <a
    496   1.1  christos href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
    497   1.1  christos General Public License (<abbr title="Lesser General Public
    498   1.1  christos License">LGPL</abbr>)</a>.</li>
    499   1.1  christos <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
    500   1.1  christos Java-based libraries.</li>
    501   1.1  christos </ul>
    502  1.20  christos <li><a href="https://nodatime.org">Noda Time  Date and
    503   1.1  christos time <abbr>API</abbr> for .NET</a>
    504   1.4  christos is like Joda-Time and Time4J, but for the .NET framework instead of Java.
    505   1.4  christos It is freely available under the Apache License.</li>
    506   1.8  christos <li>Many modern
    507   1.8  christos <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
    508   1.8  christos runtimes support <code><abbr>tz</abbr></code> natively via the
    509  1.16  christos <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>.
    510  1.10  christos This can be used as-is or with most of the following libraries,
    511  1.16  christos many of which also support runtimes lacking the <code>timeZone</code> option.
    512   1.1  christos <ul>
    513   1.8  christos <li>The <a
    514  1.16  christos href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code>
    515   1.8  christos timezone polyfill</a>
    516   1.8  christos is freely available under a <abbr>BSD</abbr>-style license.</li>
    517  1.17  christos <li>The <a href="https://date-fns.org">date-fns</a>
    518  1.10  christos library manipulates timezone-aware timestamps in browsers and
    519  1.10  christos in <a href="https://nodejs.org/en/">Node.js</a>.
    520  1.10  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    521  1.10  christos <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
    522  1.10  christos minimalist replacement for the date and time API of
    523  1.10  christos the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
    524  1.10  christos manipulation library.
    525  1.10  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    526  1.10  christos <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
    527  1.16  christos timezone support for the <code>Intl</code> API.
    528  1.10  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    529   1.1  christos <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    530  1.10  christos Moment.js plugin.
    531  1.10  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    532   1.8  christos <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    533   1.8  christos JavaScript library that supports date arithmetic that is time zone
    534   1.8  christos aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    535  1.10  christos <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
    536  1.10  christos supports live <code><abbr>tzdb</abbr></code> updates,
    537  1.10  christos astronomical and atomic time, a command-line interface,
    538  1.10  christos and full <a
    539  1.10  christos href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
    540  1.10  christos Its companion <a
    541  1.10  christos href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
    542  1.10  christos can generate <abbr>TZif</abbr> and other files, and a companion website
    543  1.10  christos <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
    544  1.10  christos convert timestamps, view transition histories, and download code and data.
    545  1.10  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    546   1.1  christos </ul>
    547  1.10  christos The proposed <a
    548  1.16  christos href="https://github.com/tc39/proposal-temporal"><code>Temporal</code>
    549  1.10  christos objects</a> let programs access an abstract view of
    550  1.10  christos <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
    551  1.20  christos href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScripts
    552  1.16  christos problematic <code>Date</code> objects</a> when working with dates and times.
    553  1.17  christos <li><a href="https://github.com/JuliaTime">JuliaTime</a> contains a
    554   1.1  christos compiler from <code><abbr>tz</abbr></code> source into
    555  1.17  christos <a href="https://julialang.org">Julia</a>. It is freely available
    556   1.1  christos under the <abbr>MIT</abbr> license.</li>
    557  1.20  christos <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> 
    558   1.6  christos <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
    559   1.6  christos title="Free Pascal Compiler">FPC</abbr></a>
    560   1.2  christos compiles from <code><abbr>tz</abbr></code> source into
    561   1.2  christos <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
    562   1.2  christos as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
    563   1.2  christos and <a
    564   1.2  christos href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
    565   1.2  christos It is freely available under a <abbr>BSD</abbr>-style license.</li>
    566  1.20  christos <li><a href="https://pythonhosted.org/pytz/">pytz  World Timezone
    567   1.1  christos Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    568   1.1  christos <a href="https://www.python.org">Python</a>.
    569   1.9  christos It is freely available under a <abbr>BSD</abbr>-style license.
    570  1.11  christos In code that can assume Python 3.6 or later it is largely superseded; see <a
    571  1.11  christos href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
    572  1.11  christos The Fastest Footgun in the West</a>.</li>
    573  1.20  christos <li><a href="https://tzinfo.github.io">TZInfo 
    574   1.1  christos Ruby Timezone Library</a>
    575   1.1  christos compiles <code><abbr>tz</abbr></code> source into
    576   1.1  christos <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    577   1.1  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    578  1.13  christos <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
    579   1.1  christos Library</a> is
    580   1.1  christos a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    581   1.1  christos library that compiles <code><abbr>tz</abbr></code> source into a time
    582   1.1  christos zone repository whose format
    583   1.1  christos is either proprietary or an <abbr>XML</abbr>-encoded
    584   1.1  christos representation.</li>
    585  1.21  christos <li><a id="Tcl" href="https://www.tcl-lang.org">Tcl</a>
    586   1.1  christos contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    587   1.1  christos source into text files, along with a runtime that can read those
    588   1.1  christos files. Tcl is freely available under a <abbr>BSD</abbr>-style
    589   1.1  christos license.</li>
    590   1.1  christos </ul>
    591   1.6  christos </section>
    592   1.6  christos 
    593   1.6  christos <section>
    594   1.2  christos <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
    595   1.1  christos <ul>
    596   1.1  christos <li>The <a
    597   1.1  christos href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
    598   1.1  christos Library</a>
    599   1.1  christos has an independent, thread-safe implementation of
    600   1.2  christos a <abbr>TZif</abbr> file reader.
    601   1.1  christos This library is freely available under the LGPL
    602   1.1  christos and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    603  1.20  christos <li><a href="https://www.gnome.org">GNOME</a>s
    604  1.21  christos <a href="https://docs.gtk.org/glib/">GLib</a> has
    605   1.2  christos a <abbr>TZif</abbr> file reader written in C that
    606   1.1  christos creates a <code>GTimeZone</code> object representing sets
    607   1.1  christos of <abbr>UT</abbr> offsets.
    608   1.1  christos It is freely available under the <abbr>LGPL</abbr>.</li>
    609   1.1  christos <li>The
    610  1.20  christos <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>s
    611   1.1  christos <code>baltzo::TimeZoneUtil</code> component contains a C++
    612   1.2  christos implementation of a <abbr>TZif</abbr> file reader. It is freely available under
    613   1.1  christos the Apache License.</li>
    614   1.1  christos <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    615   1.1  christos library that translates between <abbr>UT</abbr> and civil time and
    616   1.2  christos can read <abbr>TZif</abbr> files. It is freely available under the Apache
    617   1.1  christos License.</li>
    618  1.21  christos <li>The <a href="https://go.dev">Go programming language</a>
    619  1.20  christos has a <abbr>TZif</abbr> file reader <a
    620  1.20  christos href="https://pkg.go.dev/time#LoadLocationFromTZData"><code>LoadLocationFromTZData</code></a>.</li>
    621  1.16  christos <li>The
    622  1.16  christos <a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code>
    623  1.16  christos package</a> contains Python code
    624  1.16  christos to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map
    625  1.17  christos <code><abbr>tz</abbr></code> settings to proleptic TZ approximations.
    626  1.16  christos For example, it maps <code>"Africa/Cairo"</code>
    627  1.16  christos to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>,
    628  1.16  christos an approximation valid for Cairo timestamps from 2023 on.
    629  1.17  christos This can help porting to platforms that support only proleptic TZ.
    630  1.16  christos The package is freely available under the MIT license.</li>
    631   1.1  christos <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    632   1.2  christos library that reads <abbr>TZif</abbr> files and converts
    633   1.2  christos timestamps from one time zone or format to another.
    634  1.21  christos It is used by <a href="https://www.php.net"><abbr
    635   1.1  christos title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    636   1.1  christos <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    637   1.1  christos and <a href="https://www.mongodb.com">MongoDB</a>.
    638   1.1  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    639   1.1  christos <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    640   1.2  christos <abbr>TZif</abbr> file reader.</li>
    641   1.4  christos <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
    642   1.1  christos DateTime::TimeZone::Tzfile</a>
    643   1.2  christos is a <abbr>TZif</abbr> file reader written in Perl.
    644   1.1  christos It is freely available under the same terms as Perl
    645   1.1  christos (dual <abbr>GPL</abbr> and Artistic license).</li>
    646   1.9  christos <li>Python has a <a id="python-zoneinfo"
    647  1.11  christos href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
    648   1.8  christos class</a> that reads <abbr>TZif</abbr> data and creates objects
    649   1.8  christos that represent <code><abbr>tzdb</abbr></code> timezones.
    650   1.8  christos Python is freely available under the
    651  1.11  christos <a href="https://docs.python.org/3/license.html">Python Software Foundation
    652   1.8  christos License</a>.
    653  1.17  christos A companion <a id="pypi-tzdata" href="https://pypi.org">PyPI</a> module
    654   1.8  christos <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
    655   1.8  christos supplies TZif data if the underlying system data cannot be found;
    656   1.8  christos it is freely available under the Apache License.</li>
    657   1.1  christos <li>The
    658   1.1  christos public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    659   1.1  christos library contains a Python tool that
    660   1.2  christos converts <abbr>TZif</abbr> data into
    661   1.1  christos <abbr>JSON</abbr>-format data suitable for use
    662   1.1  christos in its JavaScript library for time zone conversion. Dates before 1970
    663   1.1  christos are not supported.</li>
    664   1.1  christos <li>The <a
    665   1.1  christos href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    666   1.1  christos package contains <a href="https://www.haskell.org">Haskell</a> code that
    667   1.2  christos parses and uses <abbr>TZif</abbr> data. It is freely
    668   1.1  christos available under a <abbr>BSD</abbr>-style license.</li>
    669   1.1  christos </ul>
    670   1.6  christos </section>
    671   1.6  christos 
    672   1.6  christos <section>
    673   1.1  christos <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    674   1.1  christos <ul>
    675   1.1  christos <li><a href="https://foxclocks.org">FoxClocks</a>
    676   1.1  christos is an extension for <a href="https://www.google.com/chrome/">Google
    677  1.16  christos Chrome</a>, <a
    678  1.21  christos href="https://www.firefox.com/en-US/">Firefox</a> and <a
    679  1.21  christos href="https://www.thunderbird.net/en-US/">Thunderbird</a>.
    680   1.1  christos It displays multiple clocks in the application window, and has a mapping
    681  1.21  christos interface to <a href="https://earth.google.com/web/">Google Earth</a>.
    682   1.1  christos It is freely available under the <abbr>GPL</abbr>.</li>
    683   1.1  christos <li>Microsoft Windows 8.1
    684   1.1  christos and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    685   1.3  christos data (mentioned <a href="#CLDR">below</a>) used by the
    686   1.3  christos <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    687   1.3  christos <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    688  1.21  christos <a href="https://learn.microsoft.com/en-us/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    689  1.21  christos <a href="https://learn.microsoft.com/en-us/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
    690   1.1  christos <a id="System.TimeZoneInfo"
    691  1.21  christos href="https://learn.microsoft.com/en-us/archive/blogs/bclteam/exploring-windows-time-zones-with-system-timezoneinfo-josh-free">Exploring
    692   1.1  christos Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    693   1.1  christos the older, proprietary method of Microsoft Windows 2000 and later,
    694   1.1  christos which stores time zone data in the
    695   1.1  christos <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    696   1.1  christos <a
    697  1.20  christos href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone
    698  1.20  christos  Tzid table</a> or <a
    699  1.21  christos href="https://github.com/unicode-org/cldr/blob/main/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    700   1.1  christos file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    701   1.1  christos to <code><abbr>tz</abbr></code> names.
    702  1.21  christos These mappings can be performed programmatically via the
    703  1.21  christos <a href="https://github.com/mattjohnsonpint/TimeZoneConverter">TimeZoneConverter</a>
    704  1.21  christos .NET library,
    705   1.3  christos or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
    706   1.1  christos <li><a
    707  1.21  christos href="https://www.oracle.com/java/">Oracle
    708   1.1  christos Java</a> contains a copy of a subset of a recent
    709   1.1  christos <code><abbr>tz</abbr></code> database in a
    710   1.1  christos Java-specific format.</li>
    711   1.1  christos </ul>
    712   1.6  christos </section>
    713   1.6  christos 
    714   1.6  christos <section>
    715   1.1  christos <h2 id="other-dbs">Other time zone databases</h2>
    716   1.1  christos <ul>
    717   1.1  christos <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    718  1.20  christos is Astrodiensts Web version of Shanks and Pottengers out-of-print
    719   1.4  christos time zone history atlases
    720  1.21  christos <a href="https://search.worldcat.org/title/468828649">for the US</a> and
    721  1.21  christos <a href="https://search.worldcat.org/title/76950459">for the world</a>.
    722   1.4  christos Although these extensive atlases
    723   1.4  christos <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
    724   1.4  christos sources for much of the older <code><abbr>tz</abbr></code> data</a>,
    725   1.4  christos they are unreliable as Shanks appears to have
    726   1.1  christos guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    727   1.1  christos sources and do not indicate which entries are guesswork.</li>
    728   1.1  christos <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    729   1.1  christos its own <code>tztab</code>(4) format.</li>
    730   1.1  christos <li>Microsoft Windows has proprietary data mentioned
    731   1.1  christos <a href="#System.TimeZoneInfo">above</a>.</li>
    732   1.1  christos <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    733   1.1  christos is another time zone database.</li>
    734   1.1  christos <li>The <a
    735  1.21  christos href="https://www.iata.org/en/publications/manuals/standard-schedules-information">Standard
    736   1.1  christos Schedules Information Manual</a> of the
    737   1.1  christos International Air Transport Association
    738   1.1  christos gives current time zone rules for airports served by commercial aviation.</li>
    739   1.1  christos </ul>
    740   1.6  christos </section>
    741   1.6  christos 
    742   1.6  christos <section>
    743   1.1  christos <h2 id="maps">Maps</h2>
    744   1.1  christos <ul>
    745   1.6  christos <li><a href="https://www.worldtimezone.com">World Time Zone Map
    746   1.6  christos with current time</a>
    747  1.21  christos has several fancy time zone maps; it covers Russia particularly well.</li>
    748   1.1  christos <li><a
    749   1.1  christos href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    750   1.1  christos much is time wrong around the world?</a> maps the difference between
    751   1.1  christos mean solar and standard time, highlighting areas such as western China
    752  1.20  christos where the two differ greatly. Its a bit out of date, unfortunately.</li>
    753  1.21  christos <li>The
    754  1.21  christos <a
    755  1.21  christos href="https://maps.lib.utexas.edu/maps/world.html">PerryCastaeda
    756  1.21  christos Library Map Collection</a> of the University of Texas at Austin has
    757  1.21  christos copies of old maps taken from <a
    758  1.21  christos href="https://en.wikipedia.org/wiki/The_World_Factbook"><em>The
    759  1.21  christos World Factbook</em></a>, formerly published by the
    760  1.21  christos <a href="https://www.cia.gov">US Central Intelligence Agency</a>.
    761  1.21  christos Although the maps pictorial quality is good,
    762  1.21  christos the maps do not indicate daylight saving time.</li>
    763   1.1  christos </ul>
    764   1.6  christos </section>
    765   1.6  christos 
    766   1.6  christos <section>
    767   1.1  christos <h2 id="boundaries">Time zone boundaries</h2>
    768   1.2  christos <p>Geographical boundaries between timezones are available
    769   1.9  christos from several <a
    770   1.9  christos href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
    771   1.9  christos geolocation</a>
    772   1.1  christos services and other sources.</p>
    773   1.1  christos <ul>
    774   1.1  christos <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    775   1.1  christos Boundary Builder</a> extracts
    776   1.1  christos <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    777   1.2  christos boundaries of <code><abbr>tzdb</abbr></code> timezones.
    778   1.1  christos Its code is freely available under the <abbr>MIT</abbr> license, and
    779   1.1  christos its data entries are freely available under the
    780   1.1  christos <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    781  1.16  christos Open Database License</a>. The borders appear to be quite accurate.
    782  1.16  christos Its main web page lists more than twenty libraries
    783  1.16  christos for looking up a timezone name from a GPS coordinate.</li>
    784   1.1  christos <li>Free access via a network API, if you register a key, is provided by
    785   1.4  christos the <a
    786   1.4  christos href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
    787   1.4  christos Timezone web service</a>, the <a
    788  1.21  christos href="https://developers.google.com/maps/documentation/timezone/overview">Google
    789   1.4  christos Maps Time Zone API</a>, and
    790   1.8  christos the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
    791   1.1  christos Commercial network API access is provided
    792   1.1  christos by <a href="https://askgeo.com">AskGeo</a>
    793   1.1  christos and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    794   1.1  christos </li>
    795  1.20  christos <li><a
    796   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
    797   1.1  christos to get a time zone from a location using latitude and longitude
    798  1.20  christos coordinates?</a> discusses other geolocation possibilities.</li>
    799  1.21  christos <li><a href="https://statoids.com/statoids.html">Administrative
    800  1.20  christos Divisions of Countries (Statoids)</a> lists
    801   1.1  christos political subdivision data related to time zones.</li>
    802  1.13  christos <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
    803  1.20  christos  GIS and Database Tools</a> includes a Manifold-format map of
    804  1.16  christos world time zone boundaries circa 2007, distributed under the
    805   1.1  christos <abbr>GPL</abbr>.</li>
    806   1.1  christos <li>A ship within the <a
    807   1.1  christos href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    808  1.20  christos waters</a> of any nation uses that nations time. In international
    809  1.20  christos waters, time zone boundaries are meridians 15 apart, except that
    810  1.20  christos <abbr>UT</abbr>12 and <abbr>UT</abbr>+12 are each 7.5
    811   1.1  christos wide and are separated by
    812  1.20  christos the 180 meridian (not by the International Date Line, which is
    813  1.20  christos for land and territorial waters only). A captain can change ships
    814   1.1  christos clocks any time after entering a new time zone; midnight changes are
    815   1.1  christos common.</li>
    816   1.1  christos </ul>
    817   1.6  christos </section>
    818   1.6  christos 
    819   1.6  christos <section>
    820   1.1  christos <h2 id="civil">Civil time concepts and history</h2>
    821   1.1  christos <ul>
    822   1.1  christos <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    823   1.1  christos Walk through Time</a>
    824   1.1  christos surveys the evolution of timekeeping.</li>
    825   1.4  christos <li>The history of daylight saving time is surveyed in <a
    826  1.21  christos href="https://www.webexhibits.org/daylightsaving/">About Daylight
    827  1.20  christos Saving Time  History, rationale, laws &amp; dates</a> and summarized in
    828   1.4  christos <a href="http://seizethedaylight.com/dst/">A Brief
    829   1.4  christos History of Daylight Saving Time</a>.</li>
    830   1.4  christos <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
    831   1.4  christos Lords</a> discusses how authoritarians manipulate civil time.</li>
    832  1.20  christos <li><a href="https://www.w3.org/TR/timezone/">Working with Time
    833  1.20  christos and Time Zones</a>
    834   1.1  christos contains guidelines and best practices for software applications that
    835   1.1  christos deal with civil time.</li>
    836  1.16  christos <li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of
    837   1.1  christos the International Date Line</a> tells the story of the most important
    838   1.1  christos time zone boundary.</li>
    839  1.21  christos <li><a href="https://statoids.com/tconcept.html">Basic Time
    840   1.1  christos Zone Concepts</a> discusses terminological issues behind time zones.</li>
    841   1.1  christos </ul>
    842   1.6  christos </section>
    843   1.6  christos 
    844   1.6  christos <section>
    845   1.1  christos <h2 id="national">National histories of legal time</h2>
    846   1.1  christos <dl>
    847   1.1  christos <dt>Australia</dt>
    848  1.20  christos <dd>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  1.16  christos <a href="https://robinfo.oma.be/nl/astro-info/tijd/"
    855   1.7  christos hreflang="nl">Dutch</a> and <a
    856  1.16  christos href="https://robinfo.oma.be/fr/astro-info/heure/"
    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  1.20  christos hreflang="pt-BR">Brazils 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  1.20  christos Chiles 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  1.20  christos and Macaus 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  1.21  christos href="https://www.ptb.de/cms/en/ptb/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  1.17  christos <dd><a
    890  1.17  christos href="https://tz.cs.huji.ac.il">Israel Timezone Files</a>
    891  1.17  christos lists official time-change announcements and laws since 1940,
    892  1.17  christos almost all in Hebrew.</dd>
    893   1.2  christos <dt>Malaysia</dt>
    894   1.2  christos <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    895   1.1  christos <dt>Mexico</dt>
    896   1.1  christos <dd>The Investigation and Analysis Service of the Mexican Library of
    897   1.1  christos Congress has published a <a
    898  1.13  christos href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    899   1.1  christos hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    900   1.1  christos <dt>Netherlands</dt>
    901  1.16  christos <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    902   1.1  christos hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    903   1.1  christos covers the history of local time in the Netherlands from ancient times.</dd>
    904   1.1  christos <dt>New Zealand</dt>
    905   1.1  christos <dd>The Department of Internal Affairs maintains a brief <a
    906  1.21  christos href="https://www.govt.nz/browse/recreation-and-the-environment/daylight-saving/history-of-daylight-saving-in-nz/">History
    907  1.21  christos of Daylight Saving in NZ</a>.</dd>
    908  1.14  christos <dt>Palestine</dt>
    909  1.20  christos <dd>The Ministry of Telecom and Digital Economy publishes a <a
    910  1.20  christos href="https://mtde.gov.ps/home/TimeZone"
    911  1.14  christos hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
    912  1.10  christos <dt>Portugal</dt>
    913  1.10  christos <dd>The Lisbon Astronomical Observatory publishes a
    914  1.10  christos <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
    915  1.10  christos legal time (in Portuguese)</a>.</dd>
    916   1.1  christos <dt>Singapore</dt>
    917   1.1  christos <dd><a id="Singapore"
    918   1.7  christos href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
    919  1.20  christos is Singapore in the Wrong Time Zone?</a> details the
    920   1.1  christos history of legal time in Singapore and Malaysia.</dd>
    921   1.1  christos <dt>United Kingdom</dt>
    922   1.1  christos <dd><a
    923   1.1  christos href="https://www.polyomino.org.uk/british-time/">History of
    924   1.1  christos legal time in Britain</a> discusses in detail the country
    925   1.6  christos with perhaps the best-documented history of clock adjustments.</dd>
    926   1.1  christos <dt>United States</dt>
    927  1.20  christos <dd>The Department of Transportations <a
    928   1.1  christos href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    929  1.18  christos Time Zone Proceedings</a> lists changes to
    930  1.18  christos official written time zone boundaries, and its <a
    931  1.18  christos href="https://geodata.bts.gov/datasets/usdot::time-zones/about">Time
    932  1.18  christos Zones dataset</a> maps current boundaries.
    933  1.18  christos These boundaries are only for standard time, so the current map puts
    934  1.18  christos all of Arizona in one time zone even though part of Arizona
    935  1.18  christos observes <abbr>DST</abbr> and part does not.</dd>
    936   1.1  christos <dt>Uruguay</dt>
    937   1.1  christos <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    938   1.1  christos Navy (SOHMA) publishes an annual <a
    939  1.13  christos href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
    940   1.1  christos (in Spanish)</a>.</dd>
    941   1.1  christos </dl>
    942   1.6  christos </section>
    943   1.6  christos 
    944   1.6  christos <section>
    945   1.7  christos <h2 id="costs">Costs and benefits of time shifts</h2>
    946   1.7  christos <p>Various sources argue for and against daylight saving time and time
    947   1.7  christos zone shifts, and many scientific studies have been conducted. This
    948   1.8  christos section summarizes reviews and position statements based on
    949   1.8  christos scientific literature in the area.</p>
    950   1.7  christos <ul>
    951   1.7  christos <li>Carey RN, Sarma KM.
    952   1.7  christos <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
    953   1.7  christos daylight saving time on road traffic collision risk: a systematic
    954   1.7  christos review</a>.
    955   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>.
    956   1.7  christos This reviews research literature and concludes that the evidence
    957   1.7  christos neither supports nor refutes road safety benefits from
    958   1.7  christos shifts in time zones.</li>
    959   1.7  christos <li>Havranek T, Herman D, Irsova D.
    960  1.20  christos Does daylight saving save electricity? A meta-analysis.
    961  1.20  christos <em>Energy J.</em> 2018;39(2):3561.
    962   1.7  christos doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
    963  1.20  christos This analyzes research literature and concludes, Electricity savings
    964   1.7  christos are larger for countries farther away from the equator, while
    965  1.20  christos subtropical regions consume more electricity because of <abbr>DST</abbr>.</li>
    966  1.20  christos <li>Neumann P, von Blanckenburg K. <a
    967  1.20  christos href="https://journals.sagepub.com/doi/full/10.1177/0961463X241310562">What
    968  1.20  christos time will it be? A comprehensive literature review on daylight saving time</a>.
    969  1.21  christos <em>Time Soc</em>. 2025;34(4):684745.
    970  1.20  christos doi:<a href="https://doi.org/10.1177/0961463X241310562">10.1177/0961463X241310562</a>.
    971  1.21  christos This reviews <abbr>DST</abbr>s effects on electricity, health, crime, road
    972  1.21  christos safety, and the economy, focusing on research since 2010, and concludes that
    973  1.21  christos year-round standard time is preferable overall.</li>
    974  1.21  christos <li>Romigi A, Franco V, Scoditti E <em>et al</em>.
    975  1.21  christos The effects of daylight saving time and clock time transitions on sleep and
    976  1.21  christos sleepiness: a systematic review. <em>Sleep Med Rev.</em> 2025;84:102161. doi:<a
    977  1.21  christos href="https://doi.org/10.1016/j.smrv.2025.102161">10.1016/j.smrv.2025.102161</a>.
    978  1.21  christos This reviews <abbr>DST</abbr> and <abbr>DST</abbr> transitions,
    979  1.21  christos and concludes that they both harm sleep, health and behavior.</li>
    980  1.20  christos </ul>
    981  1.20  christos 
    982  1.20  christos <p>The following medical societies have taken positions on the
    983  1.20  christos advisability of clock shifts:</p>
    984  1.20  christos 
    985  1.20  christos <ul>
    986  1.20  christos <li>In 2022 the American Medical Association issued a
    987  1.21  christos <a href="https://www.ama-assn.org/press-center/ama-press-releases/ama-calls-permanent-standard-time">statement
    988  1.20  christos supporting permanent standard time</a> on health grounds.</li>
    989  1.20  christos <li>Crawford MR, Winnebeck EC, von Schantz M <em>et al</em>.
    990  1.20  christos <a href="https://onlinelibrary.wiley.com/doi/10.1111/jsr.14352">The
    991  1.20  christos British Sleep Society position statement on Daylight Saving Time in the UK</a>.
    992  1.20  christos <em>J Sleep Res.</em> 2025;34(3):e14352.
    993  1.20  christos doi:<a href="https://doi.org/10.1111/jsr.14352">10.1111/jsr.14352</a>.
    994  1.21  christos This recommends that the UK abolish <abbr>DST</abbr> for health reasons.</li>
    995  1.16  christos <li>Malow BA. <a
    996  1.16  christos href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time
    997  1.16  christos to abolish the clock change and adopt permanent
    998  1.16  christos standard time in the United States:
    999  1.16  christos a Sleep Research Society position statement</a>.
   1000  1.15  christos <em>Sleep.</em> 2022;45(12):zsac236.
   1001  1.15  christos doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
   1002  1.21  christos The Sleep Research Society
   1003  1.20  christos advocates permanent standard time due to its health benefits.</li>
   1004  1.15  christos <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
   1005  1.15  christos <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
   1006  1.15  christos is the optimal choice for health and safety:
   1007   1.8  christos an American Academy of Sleep Medicine position statement</a>.
   1008  1.20  christos <em>J Clin Sleep Med.</em> 2024;20(1):121125.
   1009  1.15  christos doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
   1010  1.15  christos The AASM argues for permanent standard time due to health and safety risks
   1011  1.15  christos and economic costs of both <abbr>DST</abbr> transitions and
   1012  1.15  christos permanent <abbr>DST</abbr>.</li>
   1013  1.13  christos <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
   1014  1.16  christos <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why
   1015  1.13  christos should we abolish Daylight Saving Time?</a>
   1016  1.20  christos <em>J Biol Rhythms.</em> 2019;34(3):227230.
   1017  1.13  christos doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
   1018  1.15  christos The Society for Research on Biological Rhythms
   1019  1.18  christos opposes <abbr>DST</abbr> changes and permanent <abbr>DST</abbr>,
   1020  1.18  christos and advocates that governments adopt
   1021  1.20  christos permanent Standard Time for the health and safety of their citizens.</li>
   1022   1.7  christos </ul>
   1023   1.7  christos </section>
   1024   1.7  christos 
   1025   1.7  christos <section>
   1026   1.1  christos <h2 id="precision">Precision timekeeping</h2>
   1027   1.1  christos <ul>
   1028   1.1  christos <li><a
   1029   1.1  christos href="http://leapsecond.com/hpan/an1289.pdf">The
   1030   1.1  christos Science of Timekeeping</a> is a thorough introduction
   1031   1.1  christos to the theory and practice of precision timekeeping.</li>
   1032   1.1  christos <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
   1033  1.14  christos Time 2016</a> contains several freely readable papers.</li>
   1034  1.13  christos <li><a href="https://www.ntp.org"><abbr
   1035   1.1  christos title="Network Time Protocol">NTP</abbr>: The Network
   1036   1.1  christos Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
   1037   1.1  christos discusses how to synchronize clocks of
   1038   1.1  christos Internet hosts.</li>
   1039   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>
   1040   1.2  christos family of software algorithms can achieve accuracy to a few tens of
   1041   1.2  christos nanoseconds in scalable server farms without special hardware.</li>
   1042   1.1  christos <li>The <a
   1043  1.21  christos href="https://www.nist.gov/el/intelligent-systems-division-73500/ieee-1588">Precision
   1044   1.1  christos Time Protocol</a> (<abbr
   1045   1.1  christos title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
   1046   1.2  christos can achieve submicrosecond clock accuracy on a local area network
   1047   1.2  christos with special-purpose hardware.</li>
   1048   1.1  christos <li><a
   1049  1.18  christos href="https://www.rfc-editor.org/rfc/rfc4833">Timezone
   1050   1.1  christos Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
   1051   1.1  christos (Internet <abbr>RFC</abbr> 4833)
   1052   1.1  christos specifies a <a
   1053   1.1  christos href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
   1054   1.1  christos option for a server to configure
   1055  1.20  christos a clients time zone and daylight saving settings automatically.</li>
   1056  1.11  christos <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
   1057  1.11  christos Scales</a> describes astronomical time scales like
   1058   1.1  christos <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
   1059   1.1  christos <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
   1060   1.1  christos <abbr title="Barycentric Dynamic Time">TDB</abbr>.
   1061   1.1  christos <li>The <a href="https://www.iau.org"><abbr
   1062  1.20  christos title="International Astronomical Union">IAU</abbr></a>s <a
   1063  1.13  christos href="https://www.iausofa.org"><abbr
   1064   1.1  christos title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
   1065   1.1  christos collection contains C and <a
   1066   1.1  christos href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
   1067   1.1  christos code for converting among time scales like
   1068   1.1  christos <abbr title="International Atomic Time">TAI</abbr>,
   1069   1.1  christos <abbr>TDB</abbr>, <abbr>TDT</abbr> and
   1070  1.13  christos <abbr>UTC</abbr>. It is freely available under the
   1071  1.21  christos <a href="https://www.iausofa.org/terms-and-conditions">SOFA license</a>.</li>
   1072   1.1  christos <li><a
   1073   1.1  christos href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
   1074  1.20  christos  Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
   1075   1.1  christos diverse local time
   1076   1.1  christos scales used by each landed mission on Mars.</li>
   1077   1.1  christos <li><a href="http://leapsecond.com">LeapSecond.com</a> is
   1078   1.1  christos dedicated not only to leap seconds but to precise time and frequency
   1079   1.1  christos in general. It covers the state of the art in amateur timekeeping, and
   1080   1.1  christos how the art has progressed over the past few decades.</li>
   1081   1.7  christos <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
   1082   1.7  christos href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
   1083  1.20  christos and time-signal emissions</a>, International Telecommunication Union 
   1084   1.7  christos Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
   1085   1.1  christos <li><a
   1086   1.1  christos href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
   1087   1.1  christos title="International Earth Rotation and Reference Systems Service">IERS</abbr>
   1088   1.1  christos Bulletins</a> contains official publications of the International
   1089   1.1  christos Earth Rotation and Reference Systems Service, which decides when leap
   1090  1.20  christos seconds occur.
   1091  1.20  christos The <code><abbr>tz</abbr></code> code and data support leap seconds
   1092  1.20  christos via an optional <code>"right"</code> configuration where a computers internal
   1093   1.9  christos <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
   1094  1.20  christos as opposed to the default <code>"posix"</code> configuration
   1095   1.9  christos where the internal clock ignores leap seconds.
   1096   1.9  christos The two configurations agree for timestamps starting with 1972-01-01 00:00:00
   1097   1.9  christos <abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge for
   1098   1.9  christos timestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,
   1099   1.9  christos which corresponds to the first leap second
   1100  1.20  christos 1972-06-30 23:59:60 <abbr>UTC</abbr> in the <code>"right"</code> configuration,
   1101   1.9  christos and to
   1102  1.20  christos 1972-07-01 00:00:00 <abbr>UTC</abbr> in the <code>"posix"</code> configuration.
   1103   1.9  christos In practice the two configurations also agree for timestamps before
   1104   1.9  christos 1972 even though the historical situation is messy, partly because
   1105   1.9  christos neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
   1106  1.14  christos is well-defined for sufficiently old timestamps.</li>
   1107  1.17  christos <li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The
   1108  1.17  christos <abbr>NTP</abbr> Leap Second File</a> covers the text file
   1109  1.17  christos <code>leap-seconds.list</code>, which lists the currently known leap seconds.
   1110  1.17  christos The <abbr>IERS</abbr> maintains this file, and a copy is distributed by
   1111  1.17  christos <code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like
   1112  1.17  christos <a href="https://www.ntp.org">classic
   1113  1.17  christos <code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a>
   1114  1.17  christos and <a href="https://ntpsec.org">NTPsec</a>.
   1115  1.17  christos The <code><abbr>tz</abbr></code> database also distributes leap second
   1116  1.17  christos information in a differently-formatted <code>leapseconds</code> text file,
   1117  1.20  christos as well as in the <code>"right"</code> configuration in binary form; for
   1118  1.17  christos example, <code>right/UTC</code> can be used
   1119  1.17  christos by <a href="https://chrony-project.org"><code>chrony</code></a>,
   1120  1.17  christos another <abbr>NTP</abbr> implementation.</li>
   1121   1.1  christos <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
   1122   1.1  christos discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
   1123   1.1  christos leap second so that they disagree with <abbr>UTC</abbr> by at most a
   1124   1.1  christos half second, even though every <abbr>POSIX</abbr> minute has exactly
   1125   1.4  christos sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
   1126  1.20  christos <code>"posix"</code> configuration, is <a
   1127  1.21  christos href="https://gitlab.com/NTPsec/ntpsec/-/blob/master/docs/leapsmear.adoc">supported</a>
   1128  1.21  christos by the abovementioned <abbr>NTP</abbr> implementations, <a
   1129  1.12  christos href="https://github.com/google/unsmear">supports</a> conversion between
   1130  1.12  christos <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
   1131   1.7  christos cloud service providers. However, according to
   1132  1.20  christos <a href="https://www.rfc-editor.org/rfc/rfc8633#section-3.7.1">3.7.1 of
   1133   1.7  christos Network Time Protocol Best Current Practices</a>
   1134   1.7  christos (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
   1135   1.9  christos applications requiring accurate <abbr>UTC</abbr> or civil time,
   1136   1.7  christos and is intended for use only in single, well-controlled environments.</li>
   1137   1.1  christos <li>The <a
   1138   1.1  christos href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
   1139   1.1  christos Second Discussion List</a> covers <a
   1140  1.21  christos href="https://gge.ext.unb.ca/Resources/gpsworld.november99.pdf">McCarthy
   1141  1.20  christos and Klepczynskis 1999 proposal to discontinue leap seconds</a>,
   1142   1.1  christos discussed further in
   1143   1.1  christos <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
   1144   1.1  christos leap second: its history and possible future</a>.
   1145   1.1  christos <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
   1146   1.1  christos might be redefined
   1147   1.1  christos without Leap Seconds</a> gives pointers on this
   1148  1.14  christos contentious issue.
   1149  1.14  christos The General Conference on Weights and Measures
   1150  1.16  christos <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>
   1151  1.17  christos to discontinue the use of leap seconds by 2035, and requested that no
   1152  1.17  christos discontinuous adjustments be made to UTC for at least a century.
   1153  1.16  christos The World Radiocommunication Conference <a
   1154  1.16  christos href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved
   1155  1.20  christos in 2023</a> to cooperate with this process. One proposal to implement this
   1156  1.20  christos would replace leap seconds with seven 13-second leap smears occurring once per
   1157  1.17  christos decade until 2100, with leap smears after that gradually increasing in size.
   1158  1.20  christos See:
   1159  1.20  christos <ul>
   1160  1.21  christos <li>Levine J. <a href="https://www.preprints.org/manuscript/202406.0043">A
   1161  1.20  christos proposal to change the leap-second adjustments to
   1162  1.20  christos coordinated universal time</a>. <em>Metrologia.</em> 2024;61(5):055002. doi:<a
   1163  1.20  christos href="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a>.</li>
   1164  1.20  christos </ul>
   1165  1.17  christos However, there is still no consensus on whether this is the best way
   1166  1.17  christos to replace leap seconds.
   1167  1.14  christos </li>
   1168   1.1  christos </ul>
   1169   1.6  christos </section>
   1170   1.6  christos 
   1171   1.6  christos <section>
   1172   1.1  christos <h2 id="notation">Time notation</h2>
   1173   1.1  christos <ul>
   1174  1.13  christos <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
   1175   1.1  christos Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
   1176   1.1  christos zone names, abbreviations, identifiers, and formats. For example, it
   1177  1.20  christos contains French translations for Eastern European Summer Time,
   1178  1.20  christos <abbr title="Eastern European Summer Time">EEST</abbr>, and Bucharest. Its
   1179   1.1  christos <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
   1180   1.1  christos charts</a> show these values for many locales. Data values are available in
   1181   1.1  christos both <abbr title="Locale Data Markup Language">LDML</abbr>
   1182   1.1  christos (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
   1183   1.1  christos <li>
   1184   1.1  christos <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
   1185   1.6  christos the international standard date and time notation</a> covers
   1186   1.1  christos <a
   1187   1.6  christos href="https://www.iso.org/standard/70907.html"><em><abbr
   1188   1.6  christos title="International Organization for Standardization">ISO</abbr>
   1189  1.20  christos 8601-1:2019  Date and time  Representations for information
   1190  1.20  christos interchange  Part 1: Basic rules</em></a>.</li>
   1191   1.1  christos <li>
   1192   1.6  christos <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
   1193  1.20  christos Schema: Datatypes  dateTime</a> specifies a format inspired by
   1194   1.1  christos <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
   1195  1.20  christos <li><a href="https://www.rfc-editor.org/rfc/rfc5322#section-3.3">3.3 of
   1196   1.1  christos Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
   1197   1.1  christos specifies the time notation used in email and <a
   1198   1.1  christos href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
   1199   1.1  christos headers.</li>
   1200   1.1  christos <li>
   1201  1.18  christos <a href="https://www.rfc-editor.org/rfc/rfc3339">Date and Time
   1202   1.1  christos on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
   1203  1.17  christos specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.
   1204  1.18  christos An extension, <a href="https://www.rfc-editor.org/rfc/rfc9557">Date
   1205  1.17  christos and Time on the Internet: Timestamps with Additional Information</a>
   1206  1.17  christos (Internet <abbr>RFC</abbr> 9557) extends this profile
   1207  1.17  christos to let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp
   1208  1.20  christos via suffixes like <code>[Asia/Tokyo]</code>.
   1209   1.1  christos <li>
   1210   1.7  christos <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
   1211   1.1  christos Formats on the Web</a> surveys web- and Internet-oriented date and time
   1212   1.1  christos formats.</li>
   1213   1.1  christos <li>Alphabetic time zone abbreviations should not be used as unique
   1214   1.1  christos identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
   1215   1.1  christos practice. For example, in English-speaking North America
   1216  1.20  christos <abbr>CST</abbr> denotes 6 hours behind <abbr>UT</abbr>,
   1217   1.1  christos but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
   1218   1.1  christos and French-speaking North Americans prefer
   1219  1.20  christos <abbr title="Heure Normale du Centre">HNC</abbr> to
   1220  1.20  christos <abbr>CST</abbr>. The <code><abbr>tz</abbr></code>
   1221   1.2  christos database contains English abbreviations for many timestamps;
   1222  1.20  christos unfortunately some of these abbreviations were merely the database maintainers
   1223   1.1  christos inventions, and these have been removed when possible.</li>
   1224   1.1  christos <li>Numeric time zone abbreviations typically count hours east of
   1225   1.1  christos <abbr>UT</abbr>, e.g., +09 for Japan and
   1226  1.20  christos 10 for Hawaii. However, <abbr>POSIX</abbr> proleptic
   1227  1.17  christos <code><abbr>TZ</abbr></code> settings use the opposite convention.
   1228   1.1  christos For example, one might use <code><abbr>TZ</abbr>="<abbr
   1229   1.1  christos title="Japan Standard Time">JST</abbr>-9"</code> and
   1230   1.1  christos <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
   1231   1.1  christos for Japan and Hawaii, respectively. If the
   1232   1.1  christos <code><abbr>tz</abbr></code> database is available, it is usually better to use
   1233   1.1  christos settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
   1234   1.1  christos <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
   1235   1.2  christos confusion, handle old timestamps better, and insulate you better from
   1236   1.1  christos any future changes to the rules. One should never set
   1237   1.1  christos <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
   1238   1.1  christos <code>"GMT-9"</code>, though, since this would incorrectly imply that
   1239   1.1  christos local time is nine hours ahead of <abbr>UT</abbr> and the time zone
   1240  1.20  christos is called <abbr>GMT</abbr>.</li>
   1241   1.1  christos </ul>
   1242   1.6  christos </section>
   1243   1.6  christos 
   1244   1.6  christos <section>
   1245   1.1  christos <h2 id="see-also">See also</h2>
   1246   1.1  christos <ul>
   1247   1.7  christos <li><a href="theory.html">Theory and pragmatics of the
   1248   1.7  christos <code><abbr>tz</abbr></code> code and data</a></li>
   1249   1.1  christos <li><a href="tz-art.html">Time and the Arts</a></li>
   1250   1.1  christos </ul>
   1251   1.6  christos </section>
   1252   1.6  christos 
   1253   1.6  christos <footer>
   1254   1.1  christos <hr>
   1255   1.1  christos This web page is in the public domain, so clarified as of
   1256   1.1  christos 2009-05-17 by Arthur David Olson.
   1257   1.1  christos <br>
   1258   1.1  christos Please send corrections to this web page to the
   1259   1.1  christos <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
   1260  1.20  christos The mailing list and its archives are public,
   1261  1.20  christos so please do not send confidential information.
   1262   1.6  christos </footer>
   1263   1.1  christos </body>
   1264   1.1  christos </html>
   1265