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      3  1.1  christos <head>
      4  1.1  christos <title>Sources for time zone and daylight saving time data</title>
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     10  1.1  christos <body>
     11  1.1  christos <h1>Sources for time zone and daylight saving time data</h1>
     12  1.1  christos <p>
     13  1.1  christos <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
     14  1.1  christos <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
     15  1.1  christos rules are controlled by individual
     16  1.1  christos governments. They are sometimes changed with little notice, and their
     17  1.1  christos histories and planned futures are often recorded only fitfully. Here
     18  1.1  christos is a summary of attempts to organize and record relevant data in this
     19  1.1  christos area.
     20  1.1  christos </p>
     21  1.6  christos   <h3>Outline</h3>
     22  1.6  christos   <nav>
     23  1.6  christos     <ul>
     24  1.6  christos       <li>The <code><abbr>tz</abbr></code> database product and process
     25  1.6  christos 	<ul>
     26  1.6  christos 	  <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
     27  1.6  christos 	  <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
     28  1.6  christos 	  <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
     29  1.6  christos 	  <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
     30  1.6  christos 	</ul>
     31  1.6  christos       </li>
     32  1.6  christos       <li>Uses of the <code><abbr>tz</abbr></code> database
     33  1.6  christos 	<ul>
     34  1.6  christos 	  <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
     35  1.6  christos 	  <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
     36  1.6  christos 	  <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
     37  1.6  christos 	  <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
     38  1.6  christos 	  <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
     39  1.6  christos 	</ul>
     40  1.6  christos       </li>
     41  1.6  christos       <li>Related data
     42  1.6  christos 	<ul>
     43  1.6  christos 	  <li><a href="#other-dbs">Other time zone databases</a></li>
     44  1.6  christos 	  <li><a href="#maps">Maps</a></li>
     45  1.6  christos 	  <li><a href="#boundaries">Time zone boundaries</a></li>
     46  1.6  christos 	</ul>
     47  1.6  christos       </li>
     48  1.6  christos       <li>Timekeeping concepts
     49  1.6  christos 	<ul>
     50  1.6  christos 	  <li><a href="#civil">Civil time concepts and history</a></li>
     51  1.6  christos 	  <li><a href="#national">National histories of legal time</a></li>
     52  1.7  christos 	  <li><a href="#costs">Costs and benefits of time shifts</a></li>
     53  1.6  christos 	  <li><a href="#precision">Precision timekeeping</a></li>
     54  1.6  christos 	  <li><a href="#notation">Time notation</a></li>
     55  1.6  christos 	  <li><a href="#see-also">See also</a></li>
     56  1.6  christos 	</ul>
     57  1.6  christos       </li>
     58  1.6  christos     </ul>
     59  1.6  christos   </nav>
     60  1.6  christos 
     61  1.6  christos <section>
     62  1.1  christos <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
     63  1.1  christos <p>
     64  1.1  christos The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
     65  1.1  christos time zone database contains code and data
     66  1.1  christos that represent the history of local time
     67  1.1  christos for many representative locations around the globe.
     68  1.1  christos It is updated periodically to reflect changes made by political bodies
     69  1.1  christos to time zone boundaries and daylight saving rules.
     70  1.1  christos This database (known as <code><abbr>tz</abbr></code>,
     71  1.1  christos <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
     72  1.1  christos is used by several implementations,
     73  1.1  christos including
     74  1.1  christos <a href="https://www.gnu.org/software/libc/">the
     75  1.1  christos <abbr title="GNU's Not Unix">GNU</abbr>
     76  1.1  christos C Library</a> (used in
     77  1.1  christos <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
     78  1.1  christos <a href="https://www.android.com">Android</a>,
     79  1.1  christos <a href="https://www.freebsd.org">Free<abbr
     80  1.1  christos title="Berkeley Software Distribution">BSD</abbr></a>,
     81  1.1  christos <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
     82  1.1  christos <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
     83  1.1  christos <a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
     84  1.1  christos <a href="https://cygwin.com">Cygwin</a>,
     85  1.7  christos <a href="https://mariadb.org">MariaDB</a>,
     86  1.1  christos <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
     87  1.1  christos <a href="https://www.mysql.com">MySQL</a>,
     88  1.1  christos <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
     89  1.1  christos title="Web Operating System">webOS</abbr></a>,
     90  1.7  christos <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
     91  1.1  christos title="Advanced Interactive eXecutive">AIX</abbr></a>,
     92  1.1  christos <a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>,
     93  1.6  christos <a href="https://www.apple.com/ios"><abbr
     94  1.1  christos title="iPhone OS">iOS</abbr></a>,
     95  1.6  christos <a href="https://www.apple.com/macos">macOS</a>,
     96  1.1  christos <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
     97  1.6  christos <a href="https://www.vmssoftware.com">Open<abbr
     98  1.1  christos title="Virtual Memory System">VMS</abbr></a>,
     99  1.6  christos <a href="https://www.oracle.com/database/">Oracle Database</a>, and
    100  1.1  christos <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
    101  1.1  christos <p>
    102  1.2  christos Each main entry in the database represents a <dfn>timezone</dfn>
    103  1.2  christos for a set of civil-time clocks that have all agreed since 1970.
    104  1.2  christos Timezones are typically identified by continent or ocean and then by the
    105  1.2  christos name of the largest city within the region containing the clocks.
    106  1.1  christos For example, <code>America/New_York</code>
    107  1.1  christos represents most of the <abbr title="United States">US</abbr> eastern time zone;
    108  1.1  christos <code>America/Phoenix</code> represents most of Arizona, which
    109  1.2  christos uses mountain time without daylight saving time (<abbr>DST</abbr>);
    110  1.1  christos <code>America/Detroit</code> represents most of Michigan, which uses
    111  1.1  christos eastern time but with different <abbr>DST</abbr> rules in 1975;
    112  1.1  christos and other entries represent smaller regions like Starke County,
    113  1.1  christos Indiana, which switched from central to eastern time in 1991
    114  1.1  christos and switched back in 2006.
    115  1.1  christos To use the database on an extended <a
    116  1.1  christos href="https://en.wikipedia.org/wiki/POSIX"><abbr
    117  1.1  christos title="Portable Operating System Interface">POSIX</abbr></a>
    118  1.1  christos implementation set the <code><abbr>TZ</abbr></code>
    119  1.1  christos environment variable to the location's full name,
    120  1.1  christos e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
    121  1.1  christos <p>
    122  1.2  christos Associated with each timezone is a history of offsets from
    123  1.1  christos <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
    124  1.1  christos Time</a> (<abbr>UT</abbr>), which is <a
    125  1.1  christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
    126  1.1  christos Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
    127  1.2  christos for timestamps after 1960 this is more precisely <a
    128  1.1  christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
    129  1.1  christos Universal Time</a> (<abbr>UTC</abbr>).
    130  1.1  christos The database also records when daylight saving time was in use,
    131  1.1  christos along with some time zone abbreviations such as <abbr>EST</abbr>
    132  1.1  christos for Eastern Standard Time in the <abbr>US</abbr>.</p>
    133  1.6  christos </section>
    134  1.6  christos 
    135  1.6  christos <section>
    136  1.1  christos <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
    137  1.1  christos <p>
    138  1.1  christos The following <a
    139  1.1  christos href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
    140  1.1  christos the latest release's two
    141  1.1  christos <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
    142  1.1  christos to a <abbr>GNU</abbr>/Linux or similar host.</p>
    143  1.2  christos <pre><code>mkdir tzdb
    144  1.1  christos cd tzdb
    145  1.1  christos <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
    146  1.1  christos wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
    147  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 -
    148  1.1  christos gzip -dc tzdata-latest.tar.gz | tar -xf -
    149  1.1  christos </code></pre>
    150  1.1  christos <p>Alternatively, the following shell commands download the same
    151  1.1  christos release in a single-tarball format containing extra data
    152  1.1  christos useful for regression testing:</p>
    153  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>
    154  1.1  christos <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
    155  1.1  christos </code></pre>
    156  1.1  christos <p>These commands use convenience links to the latest release
    157  1.1  christos of the <code><abbr>tz</abbr></code> database hosted by the
    158  1.1  christos <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
    159  1.1  christos of the <a href="https://www.iana.org">Internet Assigned Numbers
    160  1.1  christos Authority (IANA)</a>.
    161  1.1  christos Older releases are in files named
    162  1.1  christos <code>tzcode<var>V</var>.tar.gz</code>,
    163  1.1  christos <code>tzdata<var>V</var>.tar.gz</code>, and
    164  1.1  christos <code>tzdb-<var>V</var>.tar.lz</code>,
    165  1.1  christos where <code><var>V</var></code> is the version.
    166  1.1  christos Since 1996, each version has been a four-digit year followed by
    167  1.1  christos lower-case letter (<samp>a</samp> through <samp>z</samp>,
    168  1.1  christos then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
    169  1.1  christos through <samp>zzz</samp>, and so on).
    170  1.1  christos Since version 2016h, each release has contained a text file named
    171  1.1  christos "<samp>version</samp>" whose first (and currently only) line is the version.
    172  1.7  christos Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
    173  1.7  christos and are also available in an
    174  1.1  christos <a href="ftp://ftp.iana.org/tz/releases/"><abbr
    175  1.1  christos title="File Transfer Protocol">FTP</abbr> directory</a> via a
    176  1.1  christos less-secure protocol.</p>
    177  1.1  christos <p>Alternatively, a development repository of code and data can be
    178  1.1  christos retrieved from <a href="https://github.com">GitHub</a> via the shell
    179  1.1  christos command:</p>
    180  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>
    181  1.1  christos </code></pre>
    182  1.1  christos <p>
    183  1.1  christos Since version 2012e, each release has been tagged in development repositories.
    184  1.1  christos Untagged commits are less well tested and probably contain
    185  1.1  christos more errors.</p>
    186  1.1  christos <p>
    187  1.1  christos After obtaining the code and data files, see the
    188  1.1  christos <code>README</code> file for what to do next.
    189  1.1  christos The code lets you compile the <code><abbr>tz</abbr></code> source files into
    190  1.2  christos machine-readable binary files, one for each location. The binary files
    191  1.5  christos are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
    192  1.5  christos specified by <a href="https://tools.ietf.org/html/8536">Internet
    193  1.5  christos <abbr>RFC</abbr> 8536</a>.
    194  1.2  christos The code also lets
    195  1.2  christos you read a <abbr>TZif</abbr> file and interpret timestamps for that
    196  1.1  christos location.</p>
    197  1.6  christos </section>
    198  1.6  christos 
    199  1.6  christos <section>
    200  1.1  christos <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    201  1.1  christos <p>
    202  1.1  christos The <code><abbr>tz</abbr></code> code and data
    203  1.1  christos are by no means authoritative. If you find errors, please
    204  1.1  christos send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    205  1.1  christos the time zone mailing list. You can also <a
    206  1.1  christos href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
    207  1.1  christos and browse the <a
    208  1.1  christos href="https://mm.icann.org/pipermail/tz/">archive of old
    209  1.5  christos messages</a>.
    210  1.5  christos <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
    211  1.5  christos discussions</a> and corresponding data changes can be
    212  1.5  christos generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
    213  1.5  christos </p>
    214  1.1  christos <p>
    215  1.1  christos If your government plans to change its time zone boundaries or
    216  1.1  christos daylight saving rules, inform <code>tz (a] iana.org</code> well in
    217  1.1  christos advance, as this will coordinate updates to many cell phones,
    218  1.1  christos computers, and other devices around the world. With
    219  1.1  christos less than a year's notice there is a good chance that some
    220  1.1  christos computer-based clocks will operate incorrectly after the change, due
    221  1.1  christos to delays in propagating updates to software and data. The shorter
    222  1.1  christos the notice, the more likely clock problems will arise; see "<a
    223  1.1  christos href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
    224  1.1  christos the Timing of Time Zone Changes</a>" for examples.
    225  1.1  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  1.1  christos for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
    238  1.1  christos <a
    239  1.1  christos href="https://play.google.com/store/apps/details?id=com.google.android.timezone.data">Android</a>,
    240  1.1  christos <a
    241  1.1  christos href="http://userguide.icu-project.org/datetime/timezone"><abbr
    242  1.1  christos title="International Components for Unicode">ICU</abbr></a>,
    243  1.1  christos <a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr></a>
    244  1.1  christos and <a
    245  1.4  christos href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a>
    246  1.4  christos Java, <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    247  1.1  christos href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    248  1.1  christos and <a
    249  1.1  christos href="https://nodatime.org/userguide/tzdb">Noda Time</a> (see below).
    250  1.1  christos </p>
    251  1.1  christos <p>Sources for the <code><abbr>tz</abbr></code> database are
    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  1.1  christos <a href="https://wiki.gnome.org/Apps/Gedit">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  1.1  christos <a href="https://tools.ietf.org/html/rfc6557">Procedures for
    272  1.1  christos Maintaining the Time Zone Database</a> (Internet <abbr
    273  1.1  christos title="Request For Comments">RFC</abbr> 6557). 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  1.1  christos <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
    283  1.1  christos <ul>
    284  1.1  christos <li>The article
    285  1.1  christos <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
    286  1.1  christos an encyclopedic summary.</li>
    287  1.1  christos <li><a href="tz-how-to.html">How to Read the
    288  1.1  christos tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
    289  1.1  christos database format.</li>
    290  1.1  christos <li><a
    291  1.1  christos href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
    292  1.1  christos literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
    293  1.1  christos database's style.</li>
    294  1.1  christos </ul>
    295  1.6  christos </section>
    296  1.6  christos 
    297  1.6  christos <section>
    298  1.1  christos <h2 id="web">Web sites using recent versions of the
    299  1.1  christos <code><abbr>tz</abbr></code> database</h2>
    300  1.1  christos <p>
    301  1.1  christos These are listed roughly in ascending order of complexity and fanciness.
    302  1.1  christos </p>
    303  1.1  christos <ul>
    304  1.1  christos <li><a href="https://time.is">Time.is</a> shows locations'
    305  1.1  christos time and zones.</li>
    306  1.1  christos <li><a href="https://www.timejones.com">TimeJones.com</a>,
    307  1.1  christos <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
    308  1.7  christos <a href="https://www.worldclock.com">The World Clock</a>
    309  1.1  christos are time zone converters.</li>
    310  1.1  christos <li><a
    311  1.6  christos href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
    312  1.1  christos lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
    313  1.1  christos <li><a
    314  1.1  christos href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
    315  1.1  christos Time in 1000 Places</a> uses descriptions of the values.</li>
    316  1.6  christos <li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
    317  1.1  christos uses a pulldown menu.</li>
    318  1.7  christos <li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
    319  1.1  christos timezone information for all countries</a> displays tables of DST rules.
    320  1.1  christos <li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
    321  1.1  christos Worldwide</a> lets you sort zone names and convert times.</li>
    322  1.1  christos <li><a href="https://24timezones.com">24TimeZones</a> has a world
    323  1.1  christos time map and a time converter.</li>
    324  1.1  christos <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    325  1.1  christos calculates the current time difference between locations.</li>
    326  1.1  christos <li><a href="http://www.wx-now.com">Weather Now</a> and
    327  1.1  christos <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    328  1.1  christos </ul>
    329  1.6  christos </section>
    330  1.6  christos 
    331  1.6  christos <section>
    332  1.1  christos <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    333  1.1  christos <ul>
    334  1.1  christos <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
    335  1.1  christos <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
    336  1.1  christos Distribution Service (tzdist) working group</a> defined <a
    337  1.1  christos href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
    338  1.1  christos (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
    339  1.1  christos along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
    340  1.1  christos (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    341  1.2  christos transferring time zone data by reference.
    342  1.2  christos The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
    343  1.6  christos mailing list</a> discussed the Internet draft <a
    344  1.1  christos id="TZDIST-Geolocate"
    345  1.2  christos href="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate">TZDIST
    346  1.6  christos Geolocate Extension</a> (now expired), which let a client determine its timezone
    347  1.1  christos from its geographic location using a <a
    348  1.5  christos href="https://tools.ietf.org/html/rfc5870">'geo' URI</a>.</li>
    349  1.1  christos <li>The <a href="https://tools.ietf.org/html/rfc5545">
    350  1.1  christos Internet Calendaring and Scheduling Core Object Specification
    351  1.1  christos (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
    352  1.1  christos covers time zone
    353  1.1  christos data; see its VTIMEZONE calendar component.
    354  1.1  christos The iCalendar format requires specialized parsers and generators; a
    355  1.1  christos variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
    356  1.1  christos (Internet <abbr>RFC</abbr> 6321) uses
    357  1.1  christos <a href="https://www.w3.org/XML/"><abbr
    358  1.1  christos title="Extensible Markup Language">XML</abbr></a> format, and a variant
    359  1.1  christos <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
    360  1.1  christos (Internet <abbr>RFC</abbr> 7265)
    361  1.1  christos uses <a href="https://www.json.org"><abbr
    362  1.1  christos title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    363  1.1  christos </ul>
    364  1.6  christos </section>
    365  1.6  christos 
    366  1.6  christos <section>
    367  1.1  christos <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    368  1.7  christos <p>Although some of these do not fully support
    369  1.7  christos <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
    370  1.7  christos distributions you can generally work around compatibility problems by
    371  1.7  christos running the command <code>make rearguard_tarballs</code> and compiling
    372  1.7  christos from the resulting tarballs instead.</p>
    373  1.1  christos <ul>
    374  1.1  christos <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
    375  1.1  christos href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
    376  1.1  christos program that compiles
    377  1.1  christos <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    378  1.1  christos Vzic is freely
    379  1.1  christos available under the <a
    380  1.1  christos href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    381  1.1  christos General Public License (<abbr
    382  1.1  christos title="General Public License">GPL</abbr>)</a>.</li>
    383  1.1  christos <li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
    384  1.1  christos database conversion utility</a> is like Vzic, except for the <a
    385  1.6  christos href="https://dotnet.microsoft.com">.NET framework</a>
    386  1.1  christos and with a <abbr>BSD</abbr>-style license.</li>
    387  1.1  christos <li><a
    388  1.4  christos href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
    389  1.1  christos contains a script <code>parse_olson</code> that compiles
    390  1.1  christos <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
    391  1.1  christos modules. It is part of the Perl <a
    392  1.4  christos href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
    393  1.4  christos which is freely
    394  1.1  christos available under both the <abbr>GPL</abbr> and the Perl Artistic
    395  1.1  christos License. DateTime::TimeZone also contains a script
    396  1.1  christos <code>tests_from_zdump</code> that generates test cases for each clock
    397  1.1  christos transition in the <code><abbr>tz</abbr></code> database.</li>
    398  1.1  christos <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
    399  1.1  christos Database Parser</a> is a
    400  1.1  christos <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
    401  1.7  christos runtime library with API <a
    402  1.7  christos href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">adopted
    403  1.7  christos into the draft standard</a> for
    404  1.6  christos <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
    405  1.6  christos the next iteration of the C++ standard.
    406  1.1  christos It is freely available under the
    407  1.1  christos <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
    408  1.1  christos <li><a id="ICU" href="http://site.icu-project.org">International Components for
    409  1.1  christos Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
    410  1.1  christos href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
    411  1.1  christos libraries for internationalization that
    412  1.1  christos has a compiler from <code><abbr>tz</abbr></code> source
    413  1.1  christos and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
    414  1.1  christos (mentioned <a href="#CLDR">below</a>)
    415  1.1  christos into an <abbr>ICU</abbr>-specific format.
    416  1.1  christos <abbr>ICU</abbr> is freely available under a
    417  1.1  christos <abbr>BSD</abbr>-style license.</li>
    418  1.1  christos <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
    419  1.1  christos the <a href="https://elixir-lang.org">Elixir</a> language downloads
    420  1.7  christos and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
    421  1.1  christos title="Application Program Interface">API</abbr>s for use. It is
    422  1.1  christos freely available under the <abbr>MIT</abbr> license.</li>
    423  1.1  christos <li>Java-based compilers and libraries include:
    424  1.1  christos <ul>
    425  1.1  christos <li>The <a
    426  1.4  christos href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
    427  1.1  christos tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
    428  1.1  christos Oracle Java.</li>
    429  1.1  christos <li>The <a
    430  1.4  christos href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
    431  1.6  christos SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
    432  1.4  christos href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
    433  1.1  christos which is freely available under a <abbr>BSD</abbr>-style license.</li>
    434  1.4  christos <li><a href="https://www.joda.org/joda-time/">Joda-Time &ndash; Java date
    435  1.1  christos and time <abbr>API</abbr></a> contains a class
    436  1.1  christos <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
    437  1.1  christos <code><abbr>tz</abbr></code> source into a binary format. It inspired
    438  1.1  christos Java 8 <code>java.time</code>, which its users should migrate to once
    439  1.1  christos they can assume Java 8 or later. It is available under the <a
    440  1.1  christos href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
    441  1.7  christos <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a>,
    442  1.7  christos <a href="https://github.com/akashche/tzdbgen">tzdbgen</a>, and <a
    443  1.7  christos href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
    444  1.7  christos are other alternatives to TZUpdater. IANA Updater's license is unclear;
    445  1.7  christos the others are licensed under the <abbr>GPL</abbr>.</li>
    446  1.4  christos <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
    447  1.4  christos time library for Android</a> and
    448  1.4  christos <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
    449  1.4  christos time and interval library for Java</a> compile
    450  1.4  christos <code><abbr>tz</abbr></code> source into a binary format.
    451  1.4  christos Time4A is available under the Apache License and Time4J is
    452  1.1  christos available under the <a
    453  1.1  christos href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
    454  1.1  christos General Public License (<abbr title="Lesser General Public
    455  1.1  christos License">LGPL</abbr>)</a>.</li>
    456  1.1  christos <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
    457  1.1  christos Java-based libraries.</li>
    458  1.1  christos </ul>
    459  1.1  christos <li><a href="https://nodatime.org">Noda Time &ndash; Date and
    460  1.1  christos time <abbr>API</abbr> for .NET</a>
    461  1.4  christos is like Joda-Time and Time4J, but for the .NET framework instead of Java.
    462  1.4  christos It is freely available under the Apache License.</li>
    463  1.1  christos <li><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based
    464  1.1  christos compilers and libraries include:
    465  1.1  christos <ul>
    466  1.1  christos <li><a
    467  1.1  christos href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
    468  1.1  christos compiles time zone data into a compact form designed for
    469  1.1  christos JavaScript. It is freely available under a combination of
    470  1.1  christos the <abbr>MIT</abbr> license and the Apache License.</li>
    471  1.1  christos <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    472  1.1  christos plugin for the <a href="https://momentjs.com">Moment.js</a> date
    473  1.1  christos manipulation library. It is freely available under the <abbr>MIT</abbr>
    474  1.1  christos license.</li>
    475  1.1  christos <li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
    476  1.1  christos <abbr>API</abbr> is upward compatible with standard JavaScript
    477  1.1  christos Dates. It is freely available under the Apache License.</li>
    478  1.1  christos </ul>
    479  1.1  christos <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
    480  1.1  christos compiler from <code><abbr>tz</abbr></code> source into
    481  1.1  christos <a href="https://julialang.org/">Julia</a>. It is freely available
    482  1.1  christos under the <abbr>MIT</abbr> license.</li>
    483  1.6  christos <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> &ndash;
    484  1.6  christos <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
    485  1.6  christos title="Free Pascal Compiler">FPC</abbr></a>
    486  1.2  christos compiles from <code><abbr>tz</abbr></code> source into
    487  1.2  christos <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
    488  1.2  christos as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
    489  1.2  christos and <a
    490  1.2  christos href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
    491  1.2  christos It is freely available under a <abbr>BSD</abbr>-style license.</li>
    492  1.1  christos <li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
    493  1.1  christos Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    494  1.1  christos <a href="https://www.python.org">Python</a>.
    495  1.1  christos It is freely available under a <abbr>BSD</abbr>-style license.</li>
    496  1.1  christos <li><a href="https://tzinfo.github.io">TZInfo &ndash;
    497  1.1  christos Ruby Timezone Library</a>
    498  1.1  christos compiles <code><abbr>tz</abbr></code> source into
    499  1.1  christos <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    500  1.1  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    501  1.1  christos <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
    502  1.1  christos Library</a> is
    503  1.1  christos a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    504  1.1  christos library that compiles <code><abbr>tz</abbr></code> source into a time
    505  1.1  christos zone repository whose format
    506  1.1  christos is either proprietary or an <abbr>XML</abbr>-encoded
    507  1.1  christos representation.</li>
    508  1.1  christos <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
    509  1.1  christos contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    510  1.1  christos source into text files, along with a runtime that can read those
    511  1.1  christos files. Tcl is freely available under a <abbr>BSD</abbr>-style
    512  1.1  christos license.</li>
    513  1.1  christos </ul>
    514  1.6  christos </section>
    515  1.6  christos 
    516  1.6  christos <section>
    517  1.2  christos <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
    518  1.1  christos <ul>
    519  1.1  christos <li>The <a
    520  1.1  christos href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
    521  1.1  christos Library</a>
    522  1.1  christos has an independent, thread-safe implementation of
    523  1.2  christos a <abbr>TZif</abbr> file reader.
    524  1.1  christos This library is freely available under the LGPL
    525  1.1  christos and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    526  1.1  christos <li><a href="https://www.gnome.org">GNOME</a>'s
    527  1.1  christos <a href="https://developer.gnome.org/glib/">GLib</a> has
    528  1.2  christos a <abbr>TZif</abbr> file reader written in C that
    529  1.1  christos creates a <code>GTimeZone</code> object representing sets
    530  1.1  christos of <abbr>UT</abbr> offsets.
    531  1.1  christos It is freely available under the <abbr>LGPL</abbr>.</li>
    532  1.1  christos <li>The
    533  1.1  christos <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
    534  1.1  christos <code>baltzo::TimeZoneUtil</code> component contains a C++
    535  1.2  christos implementation of a <abbr>TZif</abbr> file reader. It is freely available under
    536  1.1  christos the Apache License.</li>
    537  1.1  christos <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    538  1.1  christos library that translates between <abbr>UT</abbr> and civil time and
    539  1.2  christos can read <abbr>TZif</abbr> files. It is freely available under the Apache
    540  1.1  christos License.</li>
    541  1.1  christos <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
    542  1.2  christos is a <abbr>TZif</abbr> file reader written in Java.
    543  1.1  christos It is freely available under the <abbr>LGPL</abbr>.</li>
    544  1.1  christos <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    545  1.2  christos library that reads <abbr>TZif</abbr> files and converts
    546  1.2  christos timestamps from one time zone or format to another.
    547  1.1  christos It is used by <a href="https://secure.php.net"><abbr
    548  1.1  christos title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    549  1.1  christos <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    550  1.1  christos and <a href="https://www.mongodb.com">MongoDB</a>.
    551  1.1  christos It is freely available under the <abbr>MIT</abbr> license.</li>
    552  1.1  christos <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    553  1.1  christos JavaScript library that supports date arithmetic that is time zone
    554  1.1  christos aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    555  1.1  christos <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    556  1.2  christos <abbr>TZif</abbr> file reader.</li>
    557  1.4  christos <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
    558  1.1  christos DateTime::TimeZone::Tzfile</a>
    559  1.2  christos is a <abbr>TZif</abbr> file reader written in Perl.
    560  1.1  christos It is freely available under the same terms as Perl
    561  1.1  christos (dual <abbr>GPL</abbr> and Artistic license).</li>
    562  1.1  christos <li>The
    563  1.1  christos public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    564  1.1  christos library contains a Python tool that
    565  1.2  christos converts <abbr>TZif</abbr> data into
    566  1.1  christos <abbr>JSON</abbr>-format data suitable for use
    567  1.1  christos in its JavaScript library for time zone conversion. Dates before 1970
    568  1.1  christos are not supported.</li>
    569  1.1  christos <li>The <a
    570  1.1  christos href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    571  1.1  christos package contains <a href="https://www.haskell.org">Haskell</a> code that
    572  1.2  christos parses and uses <abbr>TZif</abbr> data. It is freely
    573  1.1  christos available under a <abbr>BSD</abbr>-style license.</li>
    574  1.1  christos </ul>
    575  1.6  christos </section>
    576  1.6  christos 
    577  1.6  christos <section>
    578  1.1  christos <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    579  1.1  christos <ul>
    580  1.1  christos <li><a href="https://foxclocks.org">FoxClocks</a>
    581  1.1  christos is an extension for <a href="https://www.google.com/chrome/">Google
    582  1.1  christos Chrome</a> and for <a
    583  1.1  christos href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
    584  1.1  christos Toolkit</a> applications like <a
    585  1.1  christos href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
    586  1.1  christos href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
    587  1.1  christos It displays multiple clocks in the application window, and has a mapping
    588  1.1  christos interface to <a href="https://www.google.com/earth/">Google Earth</a>.
    589  1.1  christos It is freely available under the <abbr>GPL</abbr>.</li>
    590  1.1  christos <li><a href="https://golang.org">Go programming language</a>
    591  1.1  christos implementations contain a copy of a 32-bit subset of a recent
    592  1.1  christos <code><abbr>tz</abbr></code> database in a
    593  1.1  christos Go-specific format.</li>
    594  1.1  christos <li><a
    595  1.1  christos href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
    596  1.1  christos clock (intclock)</a> is a clock that displays multiple time zones on
    597  1.1  christos <abbr>GNU</abbr>/Linux and similar systems. It is freely available
    598  1.1  christos under the <abbr>GPL</abbr>.</li>
    599  1.1  christos <li>Microsoft Windows 8.1
    600  1.1  christos and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    601  1.3  christos data (mentioned <a href="#CLDR">below</a>) used by the
    602  1.3  christos <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    603  1.3  christos <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    604  1.3  christos <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    605  1.3  christos <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
    606  1.1  christos <a id="System.TimeZoneInfo"
    607  1.1  christos href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
    608  1.1  christos Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    609  1.1  christos the older, proprietary method of Microsoft Windows 2000 and later,
    610  1.1  christos which stores time zone data in the
    611  1.1  christos <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    612  1.1  christos <a
    613  1.1  christos href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
    614  1.1  christos Tzid table</a> or <a
    615  1.6  christos href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    616  1.1  christos file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    617  1.1  christos to <code><abbr>tz</abbr></code> names.
    618  1.3  christos These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
    619  1.3  christos or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
    620  1.1  christos <li><a
    621  1.1  christos href="https://www.oracle.com/java/index.html">Oracle
    622  1.1  christos Java</a> contains a copy of a subset of a recent
    623  1.1  christos <code><abbr>tz</abbr></code> database in a
    624  1.1  christos Java-specific format.</li>
    625  1.2  christos <li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone
    626  1.1  christos Master</a> is a Microsoft Windows clock program that can automatically
    627  1.4  christos download, compile and use <code><abbr>tz</abbr></code> releases.
    628  1.4  christos The Basic version is free.</li>
    629  1.1  christos <li><a
    630  1.1  christos href="http://veladg.com/velaterra.html">VelaTerra</a> is
    631  1.1  christos a macOS program. Its developers
    632  1.1  christos <a href="http://veladg.com/tzoffer.html">offer free
    633  1.1  christos licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
    634  1.1  christos </ul>
    635  1.6  christos </section>
    636  1.6  christos 
    637  1.6  christos <section>
    638  1.1  christos <h2 id="other-dbs">Other time zone databases</h2>
    639  1.1  christos <ul>
    640  1.1  christos <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    641  1.4  christos is Astrodienst's Web version of Shanks and Pottenger's out-of-print
    642  1.4  christos time zone history atlases
    643  1.4  christos <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
    644  1.4  christos <a href="https://www.worldcat.org/oclc/76950459">for the world</a>,
    645  1.4  christos now published in <a
    646  1.1  christos href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a>
    647  1.1  christos form by <a href="https://astrocom.com">ACS-Starcrafts</a>.
    648  1.4  christos Although these extensive atlases
    649  1.4  christos <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
    650  1.4  christos sources for much of the older <code><abbr>tz</abbr></code> data</a>,
    651  1.4  christos they are unreliable as Shanks appears to have
    652  1.1  christos guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    653  1.1  christos sources and do not indicate which entries are guesswork.</li>
    654  1.1  christos <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    655  1.1  christos its own <code>tztab</code>(4) format.</li>
    656  1.1  christos <li>Microsoft Windows has proprietary data mentioned
    657  1.1  christos <a href="#System.TimeZoneInfo">above</a>.</li>
    658  1.1  christos <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    659  1.1  christos is another time zone database.</li>
    660  1.1  christos <li>The <a
    661  1.1  christos href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
    662  1.1  christos Schedules Information Manual</a> of the
    663  1.1  christos International Air Transport Association
    664  1.1  christos gives current time zone rules for airports served by commercial aviation.</li>
    665  1.1  christos </ul>
    666  1.6  christos </section>
    667  1.6  christos 
    668  1.6  christos <section>
    669  1.1  christos <h2 id="maps">Maps</h2>
    670  1.1  christos <ul>
    671  1.1  christos <li>The <a href="https://www.cia.gov/index.html">United States Central
    672  1.1  christos Intelligence Agency (<abbr
    673  1.1  christos title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
    674  1.1  christos href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
    675  1.1  christos zone map</a>; the
    676  1.1  christos <a
    677  1.6  christos href="https://legacy.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
    678  1.1  christos Library Map Collection</a>
    679  1.1  christos of the University of Texas at Austin has copies of
    680  1.1  christos recent editions.
    681  1.1  christos The pictorial quality is good,
    682  1.1  christos but the maps do not indicate daylight saving time,
    683  1.1  christos and parts of the data are a few years out of date.</li>
    684  1.6  christos <li><a href="https://www.worldtimezone.com">World Time Zone Map
    685  1.6  christos with current time</a>
    686  1.1  christos has several fancy time zone maps; it covers Russia particularly well.
    687  1.1  christos The maps' pictorial quality is not quite as good as the
    688  1.1  christos <abbr>CIA</abbr>'s
    689  1.1  christos but the maps are more up to date.</li>
    690  1.1  christos <li><a
    691  1.1  christos href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    692  1.1  christos much is time wrong around the world?</a> maps the difference between
    693  1.1  christos mean solar and standard time, highlighting areas such as western China
    694  1.1  christos where the two differ greatly. It's a bit out of date, unfortunately.</li>
    695  1.1  christos </ul>
    696  1.6  christos </section>
    697  1.6  christos 
    698  1.6  christos <section>
    699  1.1  christos <h2 id="boundaries">Time zone boundaries</h2>
    700  1.2  christos <p>Geographical boundaries between timezones are available
    701  1.1  christos from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a>
    702  1.1  christos services and other sources.</p>
    703  1.1  christos <ul>
    704  1.1  christos <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    705  1.1  christos Boundary Builder</a> extracts
    706  1.1  christos <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    707  1.2  christos boundaries of <code><abbr>tzdb</abbr></code> timezones.
    708  1.1  christos Its code is freely available under the <abbr>MIT</abbr> license, and
    709  1.1  christos its data entries are freely available under the
    710  1.1  christos <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    711  1.1  christos Open Database License</a>. The maps' borders appear to be quite accurate.</li>
    712  1.1  christos <li>Programmatic interfaces that map geographical coordinates via tz_world to
    713  1.2  christos <code><abbr>tzdb</abbr></code> timezones include:
    714  1.1  christos <ul>
    715  1.1  christos <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
    716  1.1  christos written in <a
    717  1.1  christos href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
    718  1.1  christos and is freely available under the <abbr>MIT</abbr> license.</li>
    719  1.1  christos <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
    720  1.1  christos is written in Go and is freely available under the Apache License.</li>
    721  1.1  christos <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
    722  1.1  christos in both Java and
    723  1.1  christos <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
    724  1.1  christos form, is freely available under the MIT license.</li>
    725  1.1  christos <li>For <a href="https://nodejs.org/en/">Node.js</a>,
    726  1.1  christos the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
    727  1.1  christos is freely available under the MIT license, and
    728  1.1  christos the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
    729  1.1  christos is in the public domain.</li>
    730  1.1  christos <li>The <a
    731  1.1  christos href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
    732  1.1  christos library for Python is freely available under the MIT license.
    733  1.1  christos <li>The <a
    734  1.1  christos href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
    735  1.1  christos library for Ruby is freely available under the MIT license.</li>
    736  1.1  christos </ul></li>
    737  1.1  christos <li>Free access via a network API, if you register a key, is provided by
    738  1.4  christos the <a
    739  1.4  christos href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
    740  1.4  christos Timezone web service</a>, the <a
    741  1.4  christos href="https://developers.google.com/maps/documentation/timezone/intro">Google
    742  1.4  christos Maps Time Zone API</a>, and
    743  1.1  christos the <a href="https://timezonedb.com">Time Zone Database &amp; API</a>.
    744  1.1  christos Commercial network API access is provided
    745  1.1  christos by <a href="https://askgeo.com">AskGeo</a>
    746  1.1  christos and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    747  1.1  christos </li>
    748  1.1  christos <li>"<a
    749  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
    750  1.1  christos to get a time zone from a location using latitude and longitude
    751  1.1  christos coordinates?</a>" discusses other geolocation possibilities.</li>
    752  1.1  christos <li><a href="http://statoids.com/statoids.html">Administrative
    753  1.1  christos Divisions of Countries ("Statoids")</a> lists
    754  1.1  christos political subdivision data related to time zones.</li>
    755  1.7  christos <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
    756  1.1  christos zone boundaries for multizone countries</a> summarizes legal
    757  1.1  christos boundaries between time zones within countries.</li>
    758  1.1  christos <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
    759  1.1  christos &ndash; GIS and Database Tools</a> includes a Manifold-format map of
    760  1.1  christos world time zone boundaries distributed under the
    761  1.1  christos <abbr>GPL</abbr>.</li>
    762  1.1  christos <li>A ship within the <a
    763  1.1  christos href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    764  1.1  christos waters</a> of any nation uses that nation's time. In international
    765  1.1  christos waters, time zone boundaries are meridians 15&deg; apart, except that
    766  1.1  christos <abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
    767  1.1  christos wide and are separated by
    768  1.1  christos the 180&deg; meridian (not by the International Date Line, which is
    769  1.1  christos for land and territorial waters only). A captain can change ship's
    770  1.1  christos clocks any time after entering a new time zone; midnight changes are
    771  1.1  christos common.</li>
    772  1.1  christos </ul>
    773  1.6  christos </section>
    774  1.6  christos 
    775  1.6  christos <section>
    776  1.1  christos <h2 id="civil">Civil time concepts and history</h2>
    777  1.1  christos <ul>
    778  1.1  christos <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    779  1.1  christos Walk through Time</a>
    780  1.1  christos surveys the evolution of timekeeping.</li>
    781  1.4  christos <li>The history of daylight saving time is surveyed in <a
    782  1.4  christos href="http://www.webexhibits.org/daylightsaving/">About Daylight
    783  1.4  christos Saving Time &ndash; History, rationale, laws &amp; dates</a> and summarized in
    784  1.4  christos <a href="http://seizethedaylight.com/dst/">A Brief
    785  1.4  christos History of Daylight Saving Time</a>.</li>
    786  1.4  christos <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
    787  1.4  christos Lords</a> discusses how authoritarians manipulate civil time.</li>
    788  1.1  christos <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
    789  1.1  christos contains guidelines and best practices for software applications that
    790  1.1  christos deal with civil time.</li>
    791  1.1  christos <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
    792  1.1  christos the International Date Line</a> tells the story of the most important
    793  1.1  christos time zone boundary.</li>
    794  1.1  christos <li><a href="http://statoids.com/tconcept.html">Basic Time
    795  1.1  christos Zone Concepts</a> discusses terminological issues behind time zones.</li>
    796  1.1  christos </ul>
    797  1.6  christos </section>
    798  1.6  christos 
    799  1.6  christos <section>
    800  1.1  christos <h2 id="national">National histories of legal time</h2>
    801  1.1  christos <dl>
    802  1.1  christos <dt>Australia</dt>
    803  1.6  christos <dd>The Parliamentary Library commissioned a <a
    804  1.1  christos href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
    805  1.1  christos paper on daylight saving time in Australia</a>.
    806  1.1  christos The Bureau of Meteorology publishes a list of <a
    807  1.1  christos href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    808  1.1  christos Dates of Daylight Savings Time within Australia</a>.</dd>
    809  1.1  christos <dt>Belgium</dt>
    810  1.7  christos <dd>The Royal Observatory of Belgium maintains a table of time in
    811  1.7  christos Belgium (in
    812  1.7  christos <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
    813  1.7  christos hreflang="nl">Dutch</a> and <a
    814  1.7  christos href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
    815  1.7  christos hreflang="fr">French</a>).</dd>
    816  1.1  christos <dt>Brazil</dt>
    817  1.1  christos <dd>The Time Service Department of the National Observatory
    818  1.1  christos records <a href="http://pcdsh01.on.br/DecHV.html"
    819  1.1  christos hreflang="pt-BR">Brazil's daylight saving time decrees (in
    820  1.1  christos Portuguese)</a>.</dd>
    821  1.1  christos <dt>Canada</dt>
    822  1.1  christos <dd>National Research Council Canada publishes current
    823  1.1  christos and some older information about <a
    824  1.6  christos href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
    825  1.6  christos zones and daylight saving time</a>.</dd>
    826  1.1  christos <dt>Chile</dt>
    827  1.1  christos <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    828  1.4  christos <a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
    829  1.1  christos Chile's official time (in Spanish)</a>.</dd>
    830  1.3  christos <dt>China</dt>
    831  1.3  christos <dd>The Hong Kong Observatory maintains a
    832  1.7  christos <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
    833  1.3  christos  summer time in Hong Kong</a>,
    834  1.3  christos and Macau's Meteorological and Geophysical Bureau maintains a <a
    835  1.7  christos href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
    836  1.3  christos history for Macau</a>.
    837  1.3  christos Unfortunately the latter is incomplete and has errors.</dd>
    838  1.1  christos <dt>Czech Republic</dt>
    839  1.1  christos <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    840  1.1  christos hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    841  1.1  christos summarizes and cites historical DST regulations.</dd>
    842  1.1  christos <dt>Germany</dt>
    843  1.1  christos <dd>The National Institute for Science and Technology maintains the <a
    844  1.1  christos href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    845  1.1  christos of Legal Time in Germany</a>.</dd>
    846  1.1  christos <dt>Israel</dt>
    847  1.1  christos <dd>The Interior Ministry periodically issues <a
    848  1.1  christos href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
    849  1.1  christos hreflang="he">announcements (in Hebrew)</a>.</dd>
    850  1.1  christos <dt>Italy</dt>
    851  1.6  christos <dd>The National Institute of Metrological Research publishes a
    852  1.4  christos <a href="http://oldsite.inrim.it/res/tf/ora_legale_i.shtml" hreflang="it">table
    853  1.4  christos of civil time (in Italian)</a>.</dd>
    854  1.2  christos <dt>Malaysia</dt>
    855  1.2  christos <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    856  1.1  christos <dt>Mexico</dt>
    857  1.1  christos <dd>The Investigation and Analysis Service of the Mexican Library of
    858  1.1  christos Congress has published a <a
    859  1.1  christos href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    860  1.1  christos hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    861  1.1  christos <dt>Netherlands</dt>
    862  1.1  christos <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    863  1.1  christos hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    864  1.1  christos covers the history of local time in the Netherlands from ancient times.</dd>
    865  1.1  christos <dt>New Zealand</dt>
    866  1.1  christos <dd>The Department of Internal Affairs maintains a brief <a
    867  1.1  christos href="https://www.dia.govt.nz/Daylight-Saving-History">History of
    868  1.6  christos Daylight Saving</a>.</dd>
    869  1.1  christos <dt>Singapore</dt>
    870  1.1  christos <dd><a id="Singapore"
    871  1.7  christos href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
    872  1.1  christos is Singapore in the "Wrong" Time Zone?</a> details the
    873  1.1  christos history of legal time in Singapore and Malaysia.</dd>
    874  1.1  christos <dt>United Kingdom</dt>
    875  1.1  christos <dd><a
    876  1.1  christos href="https://www.polyomino.org.uk/british-time/">History of
    877  1.1  christos legal time in Britain</a> discusses in detail the country
    878  1.6  christos with perhaps the best-documented history of clock adjustments.</dd>
    879  1.1  christos <dt>United States</dt>
    880  1.1  christos <dd>The Department of Transportation's <a
    881  1.1  christos href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    882  1.1  christos Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
    883  1.1  christos <dt>Uruguay</dt>
    884  1.1  christos <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    885  1.1  christos Navy (SOHMA) publishes an annual <a
    886  1.7  christos href="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
    887  1.1  christos (in Spanish)</a>.</dd>
    888  1.1  christos </dl>
    889  1.6  christos </section>
    890  1.6  christos 
    891  1.6  christos <section>
    892  1.7  christos <h2 id="costs">Costs and benefits of time shifts</h2>
    893  1.7  christos <p>Various sources argue for and against daylight saving time and time
    894  1.7  christos zone shifts, and many scientific studies have been conducted. This
    895  1.7  christos section summarizes reviews of scientific literature in the area.</p>
    896  1.7  christos <ul>
    897  1.7  christos <li>Carey RN, Sarma KM.
    898  1.7  christos <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
    899  1.7  christos daylight saving time on road traffic collision risk: a systematic
    900  1.7  christos review</a>.
    901  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>.
    902  1.7  christos This reviews research literature and concludes that the evidence
    903  1.7  christos neither supports nor refutes road safety benefits from
    904  1.7  christos shifts in time zones.</li>
    905  1.7  christos <li>Havranek T, Herman D, Irsova D.
    906  1.7  christos <a href="https://www.econstor.eu/handle/10419/174191">Does daylight
    907  1.7  christos saving save electricity? A meta-analysis</a>. <em>Energy J.</em>
    908  1.7  christos 2018;39(2).
    909  1.7  christos doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
    910  1.7  christos This analyzes research literature and concludes, "Electricity savings
    911  1.7  christos are larger for countries farther away from the equator, while
    912  1.7  christos subtropical regions consume more electricity because of DST."</li>
    913  1.7  christos <li>Roenneberg T, Winnebeck EC, Klerman EB.
    914  1.7  christos <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
    915  1.7  christos saving time and artificial time zones &ndash; a battle between
    916  1.7  christos biological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
    917  1.7  christos doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
    918  1.7  christos This reviews evidence about the health effects of DST and concludes,
    919  1.7  christos "In summary, the scientific literature strongly argues against the
    920  1.7  christos switching between DST and Standard Time and even more so against
    921  1.7  christos adopting DST permanently."</li>
    922  1.7  christos </ul>
    923  1.7  christos </section>
    924  1.7  christos 
    925  1.7  christos <section>
    926  1.1  christos <h2 id="precision">Precision timekeeping</h2>
    927  1.1  christos <ul>
    928  1.1  christos <li><a
    929  1.1  christos href="http://leapsecond.com/hpan/an1289.pdf">The
    930  1.1  christos Science of Timekeeping</a> is a thorough introduction
    931  1.1  christos to the theory and practice of precision timekeeping.</li>
    932  1.1  christos <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
    933  1.1  christos Time 2016</a> contains several freely-readable papers.</li>
    934  1.1  christos <li><a href="http://www.ntp.org"><abbr
    935  1.1  christos title="Network Time Protocol">NTP</abbr>: The Network
    936  1.1  christos Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
    937  1.1  christos discusses how to synchronize clocks of
    938  1.1  christos Internet hosts.</li>
    939  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>
    940  1.2  christos family of software algorithms can achieve accuracy to a few tens of
    941  1.2  christos nanoseconds in scalable server farms without special hardware.</li>
    942  1.1  christos <li>The <a
    943  1.1  christos href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
    944  1.1  christos Time Protocol</a> (<abbr
    945  1.1  christos title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
    946  1.2  christos can achieve submicrosecond clock accuracy on a local area network
    947  1.2  christos with special-purpose hardware.</li>
    948  1.1  christos <li><a
    949  1.1  christos href="https://tools.ietf.org/html/rfc4833">Timezone
    950  1.1  christos Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
    951  1.1  christos (Internet <abbr>RFC</abbr> 4833)
    952  1.1  christos specifies a <a
    953  1.1  christos href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
    954  1.1  christos option for a server to configure
    955  1.1  christos a client's time zone and daylight saving settings automatically.</li>
    956  1.1  christos <li><a
    957  1.1  christos href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
    958  1.1  christos Times</a> explains more abstruse astronomical time scales like
    959  1.1  christos <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
    960  1.1  christos <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
    961  1.1  christos <abbr title="Barycentric Dynamic Time">TDB</abbr>.
    962  1.1  christos <a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
    963  1.1  christos Scales</a> goes into more detail, particularly for historical variants.</li>
    964  1.1  christos <li>The <a href="https://www.iau.org"><abbr
    965  1.1  christos title="International Astronomical Union">IAU</abbr></a>'s <a
    966  1.1  christos href="http://www.iausofa.org"><abbr
    967  1.1  christos title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
    968  1.1  christos collection contains C and <a
    969  1.1  christos href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
    970  1.1  christos code for converting among time scales like
    971  1.1  christos <abbr title="International Atomic Time">TAI</abbr>,
    972  1.1  christos <abbr>TDB</abbr>, <abbr>TDT</abbr> and
    973  1.1  christos <abbr>UTC</abbr>.</li>
    974  1.1  christos <li><a
    975  1.1  christos href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
    976  1.1  christos &ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
    977  1.1  christos diverse local time
    978  1.1  christos scales used by each landed mission on Mars.</li>
    979  1.1  christos <li><a href="http://leapsecond.com">LeapSecond.com</a> is
    980  1.1  christos dedicated not only to leap seconds but to precise time and frequency
    981  1.1  christos in general. It covers the state of the art in amateur timekeeping, and
    982  1.1  christos how the art has progressed over the past few decades.</li>
    983  1.7  christos <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
    984  1.7  christos href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
    985  1.7  christos and time-signal emissions</a>, International Telecommunication Union &ndash;
    986  1.7  christos Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
    987  1.1  christos <li><a
    988  1.1  christos href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
    989  1.1  christos title="International Earth Rotation and Reference Systems Service">IERS</abbr>
    990  1.1  christos Bulletins</a> contains official publications of the International
    991  1.1  christos Earth Rotation and Reference Systems Service, which decides when leap
    992  1.4  christos seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
    993  1.1  christos via an optional "<code>right</code>" configuration, as opposed to the
    994  1.1  christos default "<code>posix</code>" configuration.</li>
    995  1.1  christos <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
    996  1.1  christos discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
    997  1.1  christos leap second so that they disagree with <abbr>UTC</abbr> by at most a
    998  1.1  christos half second, even though every <abbr>POSIX</abbr> minute has exactly
    999  1.4  christos sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
   1000  1.1  christos "<code>posix</code>" configuration, is <a
   1001  1.1  christos href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
   1002  1.1  christos the <abbr>NTP</abbr> reference implementation, and is used by major
   1003  1.7  christos cloud service providers. However, according to
   1004  1.7  christos <a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">&sect;3.7.1 of
   1005  1.7  christos Network Time Protocol Best Current Practices</a>
   1006  1.7  christos (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
   1007  1.7  christos applications requiring accurate UTC or civil time,
   1008  1.7  christos and is intended for use only in single, well-controlled environments.</li>
   1009  1.1  christos <li>The <a
   1010  1.1  christos href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
   1011  1.1  christos Second Discussion List</a> covers <a
   1012  1.1  christos href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
   1013  1.1  christos and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
   1014  1.1  christos discussed further in
   1015  1.1  christos <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
   1016  1.1  christos leap second: its history and possible future</a>.
   1017  1.1  christos <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
   1018  1.1  christos might be redefined
   1019  1.1  christos without Leap Seconds</a> gives pointers on this
   1020  1.1  christos contentious issue, which was active until 2015 and could become active
   1021  1.1  christos again.</li>
   1022  1.1  christos </ul>
   1023  1.6  christos </section>
   1024  1.6  christos 
   1025  1.6  christos <section>
   1026  1.1  christos <h2 id="notation">Time notation</h2>
   1027  1.1  christos <ul>
   1028  1.1  christos <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
   1029  1.1  christos Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
   1030  1.1  christos zone names, abbreviations, identifiers, and formats. For example, it
   1031  1.1  christos contains French translations for "Eastern European Summer Time",
   1032  1.1  christos "<abbr title="Eastern European Summer Time">EEST</abbr>", and
   1033  1.1  christos "Bucharest". Its
   1034  1.1  christos <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
   1035  1.1  christos charts</a> show these values for many locales. Data values are available in
   1036  1.1  christos both <abbr title="Locale Data Markup Language">LDML</abbr>
   1037  1.1  christos (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
   1038  1.1  christos <li>
   1039  1.1  christos <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
   1040  1.6  christos the international standard date and time notation</a> covers
   1041  1.1  christos <a
   1042  1.6  christos href="https://www.iso.org/standard/70907.html"><em><abbr
   1043  1.6  christos title="International Organization for Standardization">ISO</abbr>
   1044  1.6  christos 8601-1:2019 &ndash; Date and time &ndash; Representations for information
   1045  1.6  christos interchange &ndash; Part 1: Basic rules</em></a>.</li>
   1046  1.1  christos <li>
   1047  1.6  christos <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
   1048  1.1  christos Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
   1049  1.1  christos <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
   1050  1.1  christos <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">&sect;3.3 of
   1051  1.1  christos Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
   1052  1.1  christos specifies the time notation used in email and <a
   1053  1.1  christos href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
   1054  1.1  christos headers.</li>
   1055  1.1  christos <li>
   1056  1.1  christos <a href="https://tools.ietf.org/html/rfc3339">Date and Time
   1057  1.1  christos on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
   1058  1.1  christos specifies an <abbr>ISO</abbr> 8601
   1059  1.1  christos profile for use in new Internet
   1060  1.1  christos protocols.</li>
   1061  1.1  christos <li>
   1062  1.7  christos <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
   1063  1.1  christos Formats on the Web</a> surveys web- and Internet-oriented date and time
   1064  1.1  christos formats.</li>
   1065  1.1  christos <li>Alphabetic time zone abbreviations should not be used as unique
   1066  1.1  christos identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
   1067  1.1  christos practice. For example, in English-speaking North America
   1068  1.1  christos "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
   1069  1.1  christos but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
   1070  1.1  christos and French-speaking North Americans prefer
   1071  1.1  christos "<abbr title="Heure Normale du Centre">HNC</abbr>" to
   1072  1.1  christos "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
   1073  1.2  christos database contains English abbreviations for many timestamps;
   1074  1.1  christos unfortunately some of these abbreviations were merely the database maintainers'
   1075  1.1  christos inventions, and these have been removed when possible.</li>
   1076  1.1  christos <li>Numeric time zone abbreviations typically count hours east of
   1077  1.1  christos <abbr>UT</abbr>, e.g., +09 for Japan and
   1078  1.1  christos &minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
   1079  1.1  christos <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
   1080  1.1  christos For example, one might use <code><abbr>TZ</abbr>="<abbr
   1081  1.1  christos title="Japan Standard Time">JST</abbr>-9"</code> and
   1082  1.1  christos <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
   1083  1.1  christos for Japan and Hawaii, respectively. If the
   1084  1.1  christos <code><abbr>tz</abbr></code> database is available, it is usually better to use
   1085  1.1  christos settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
   1086  1.1  christos <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
   1087  1.2  christos confusion, handle old timestamps better, and insulate you better from
   1088  1.1  christos any future changes to the rules. One should never set
   1089  1.1  christos <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
   1090  1.1  christos <code>"GMT-9"</code>, though, since this would incorrectly imply that
   1091  1.1  christos local time is nine hours ahead of <abbr>UT</abbr> and the time zone
   1092  1.1  christos is called "<abbr>GMT</abbr>".</li>
   1093  1.1  christos </ul>
   1094  1.6  christos </section>
   1095  1.6  christos 
   1096  1.6  christos <section>
   1097  1.1  christos <h2 id="see-also">See also</h2>
   1098  1.1  christos <ul>
   1099  1.7  christos <li><a href="theory.html">Theory and pragmatics of the
   1100  1.7  christos <code><abbr>tz</abbr></code> code and data</a></li>
   1101  1.1  christos <li><a href="tz-art.html">Time and the Arts</a></li>
   1102  1.1  christos </ul>
   1103  1.6  christos </section>
   1104  1.6  christos 
   1105  1.6  christos <footer>
   1106  1.1  christos <hr>
   1107  1.1  christos This web page is in the public domain, so clarified as of
   1108  1.1  christos 2009-05-17 by Arthur David Olson.
   1109  1.1  christos <br>
   1110  1.1  christos Please send corrections to this web page to the
   1111  1.1  christos <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
   1112  1.6  christos </footer>
   1113  1.1  christos </body>
   1114  1.1  christos </html>
   1115