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      3 <head>
      4 <title>Sources for time zone and daylight saving time data</title>
      5 <meta charset="UTF-8">
      6 <style>
      7 pre {margin-left: 2em; white-space: pre-wrap;}
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     10 <body>
     11 <h1>Sources for time zone and daylight saving time data</h1>
     12 <p>
     13 <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
     14 <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
     15 rules are controlled by individual
     16 governments. They are sometimes changed with little notice, and their
     17 histories and planned futures are often recorded only fitfully. Here
     18 is a summary of attempts to organize and record relevant data in this
     19 area.
     20 </p>
     21   <h3>Outline</h3>
     22   <nav>
     23     <ul>
     24       <li>The <code><abbr>tz</abbr></code> database product and process
     25 	<ul>
     26 	  <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
     27 	  <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
     28 	  <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
     29 	  <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
     30 	</ul>
     31       </li>
     32       <li>Uses of the <code><abbr>tz</abbr></code> database
     33 	<ul>
     34 	  <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
     35 	  <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
     36 	  <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
     37 	  <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
     38 	  <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
     39 	</ul>
     40       </li>
     41       <li>Related data
     42 	<ul>
     43 	  <li><a href="#other-dbs">Other time zone databases</a></li>
     44 	  <li><a href="#maps">Maps</a></li>
     45 	  <li><a href="#boundaries">Time zone boundaries</a></li>
     46 	</ul>
     47       </li>
     48       <li>Timekeeping concepts
     49 	<ul>
     50 	  <li><a href="#civil">Civil time concepts and history</a></li>
     51 	  <li><a href="#national">National histories of legal time</a></li>
     52 	  <li><a href="#costs">Costs and benefits of time shifts</a></li>
     53 	  <li><a href="#precision">Precision timekeeping</a></li>
     54 	  <li><a href="#notation">Time notation</a></li>
     55 	  <li><a href="#see-also">See also</a></li>
     56 	</ul>
     57       </li>
     58     </ul>
     59   </nav>
     60 
     61 <section>
     62 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
     63 <p>
     64 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
     65 time zone database contains code and data
     66 that represent the history of local time
     67 for many representative locations around the globe.
     68 It is updated periodically to reflect changes made by political bodies
     69 to time zone boundaries and daylight saving rules.
     70 This database (known as <code><abbr>tz</abbr></code>,
     71 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
     72 is used by several implementations,
     73 including
     74 <a href="https://www.gnu.org/software/libc/">the
     75 <abbr title="GNU's Not Unix">GNU</abbr>
     76 C Library</a> (used in
     77 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
     78 <a href="https://www.android.com">Android</a>,
     79 <a href="https://www.freebsd.org">Free<abbr
     80 title="Berkeley Software Distribution">BSD</abbr></a>,
     81 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
     82 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
     83 <a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
     84 <a href="https://cygwin.com">Cygwin</a>,
     85 <a href="https://mariadb.org">MariaDB</a>,
     86 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
     87 <a href="https://www.mysql.com">MySQL</a>,
     88 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
     89 title="Web Operating System">webOS</abbr></a>,
     90 <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
     91 title="Advanced Interactive eXecutive">AIX</abbr></a>,
     92 <a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>,
     93 <a href="https://www.apple.com/ios"><abbr
     94 title="iPhone OS">iOS</abbr></a>,
     95 <a href="https://www.apple.com/macos">macOS</a>,
     96 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
     97 <a href="https://www.vmssoftware.com">Open<abbr
     98 title="Virtual Memory System">VMS</abbr></a>,
     99 <a href="https://www.oracle.com/database/">Oracle Database</a>, and
    100 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
    101 <p>
    102 Each main entry in the database represents a <dfn>timezone</dfn>
    103 for a set of civil-time clocks that have all agreed since 1970.
    104 Timezones are typically identified by continent or ocean and then by the
    105 name of the largest city within the region containing the clocks.
    106 For example, <code>America/New_York</code>
    107 represents most of the <abbr title="United States">US</abbr> eastern time zone;
    108 <code>America/Phoenix</code> represents most of Arizona, which
    109 uses mountain time without daylight saving time (<abbr>DST</abbr>);
    110 <code>America/Detroit</code> represents most of Michigan, which uses
    111 eastern time but with different <abbr>DST</abbr> rules in 1975;
    112 and other entries represent smaller regions like Starke County,
    113 Indiana, which switched from central to eastern time in 1991
    114 and switched back in 2006.
    115 To use the database on an extended <a
    116 href="https://en.wikipedia.org/wiki/POSIX"><abbr
    117 title="Portable Operating System Interface">POSIX</abbr></a>
    118 implementation set the <code><abbr>TZ</abbr></code>
    119 environment variable to the location's full name,
    120 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
    121 <p>
    122 Associated with each timezone is a history of offsets from
    123 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
    124 Time</a> (<abbr>UT</abbr>), which is <a
    125 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
    126 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
    127 for timestamps after 1960 this is more precisely <a
    128 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
    129 Universal Time</a> (<abbr>UTC</abbr>).
    130 The database also records when daylight saving time was in use,
    131 along with some time zone abbreviations such as <abbr>EST</abbr>
    132 for Eastern Standard Time in the <abbr>US</abbr>.</p>
    133 </section>
    134 
    135 <section>
    136 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
    137 <p>
    138 The following <a
    139 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
    140 the latest release's two
    141 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
    142 to a <abbr>GNU</abbr>/Linux or similar host.</p>
    143 <pre><code>mkdir tzdb
    144 cd tzdb
    145 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
    146 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
    147 <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 gzip -dc tzdata-latest.tar.gz | tar -xf -
    149 </code></pre>
    150 <p>Alternatively, the following shell commands download the same
    151 release in a single-tarball format containing extra data
    152 useful for regression testing:</p>
    153 <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 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
    155 </code></pre>
    156 <p>These commands use convenience links to the latest release
    157 of the <code><abbr>tz</abbr></code> database hosted by the
    158 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
    159 of the <a href="https://www.iana.org">Internet Assigned Numbers
    160 Authority (IANA)</a>.
    161 Older releases are in files named
    162 <code>tzcode<var>V</var>.tar.gz</code>,
    163 <code>tzdata<var>V</var>.tar.gz</code>, and
    164 <code>tzdb-<var>V</var>.tar.lz</code>,
    165 where <code><var>V</var></code> is the version.
    166 Since 1996, each version has been a four-digit year followed by
    167 lower-case letter (<samp>a</samp> through <samp>z</samp>,
    168 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
    169 through <samp>zzz</samp>, and so on).
    170 Since version 1999g, each release has been distributed in
    171 <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
    172 ustar interchange format</a>, compressed as described above;
    173 older releases use a nearly-compatible format.
    174 Since version 2016h, each release has contained a text file named
    175 "<samp>version</samp>" whose first (and currently only) line is the version.
    176 Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
    177 and are also available in an
    178 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
    179 title="File Transfer Protocol">FTP</abbr> directory</a> via a
    180 less-secure protocol.</p>
    181 <p>Alternatively, a development repository of code and data can be
    182 retrieved from <a href="https://github.com">GitHub</a> via the shell
    183 command:</p>
    184 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
    185 </code></pre>
    186 <p>
    187 Since version 2012e, each release has been tagged in development repositories.
    188 Untagged commits are less well tested and probably contain
    189 more errors.</p>
    190 <p>
    191 After obtaining the code and data files, see the
    192 <code>README</code> file for what to do next.
    193 The code lets you compile the <code><abbr>tz</abbr></code> source files into
    194 machine-readable binary files, one for each location. The binary files
    195 are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
    196 specified by <a href="https://tools.ietf.org/html/8536">Internet
    197 <abbr>RFC</abbr> 8536</a>.
    198 The code also lets
    199 you read a <abbr>TZif</abbr> file and interpret timestamps for that
    200 location.</p>
    201 </section>
    202 
    203 <section>
    204 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    205 <p>
    206 The <code><abbr>tz</abbr></code> code and data
    207 are by no means authoritative. If you find errors, please
    208 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    209 the time zone mailing list. You can also <a
    210 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
    211 and browse the <a
    212 href="https://mm.icann.org/pipermail/tz/">archive of old
    213 messages</a>.
    214 <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
    215 discussions</a> and corresponding data changes can be
    216 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
    217 </p>
    218 <p>
    219 If your government plans to change its time zone boundaries or
    220 daylight saving rules, inform <code>tz (a] iana.org</code> well in
    221 advance, as this will coordinate updates to many cell phones,
    222 computers, and other devices around the world.
    223 The change should be officially announced at least a year before it affects
    224 how clocks operate; otherwise, there is a good chance that some
    225 clocks will operate incorrectly after the change, due
    226 to delays in propagating updates to software and data. The shorter
    227 the notice, the more likely clock problems will arise; see "<a
    228 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
    229 the Timing of Time Zone Changes</a>" for examples.
    230 The <code><abbr>tz</abbr></code> data can represent planned changes
    231 far into the future, and a long-planned change can easily be reverted
    232 or otherwise altered with a year's notice before the change would have
    233 affected clocks.
    234 </p>
    235 <p>
    236 Changes to the <code><abbr>tz</abbr></code> code and data are often
    237 propagated to clients via operating system updates, so
    238 client <code><abbr>tz</abbr></code> data can often be corrected by
    239 applying these updates. With GNU/Linux and similar systems, if your
    240 maintenance provider has not yet adopted the
    241 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
    242 the process by tailoring the generic instructions in
    243 the <code><abbr>tz</abbr> README</code> file and installing the latest
    244 data yourself. System-specific instructions for installing the
    245 latest <code><abbr>tz</abbr></code> data have also been published
    246 for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
    247 <a
    248 href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
    249 <a
    250 href="http://userguide.icu-project.org/datetime/timezone"><abbr
    251 title="International Components for Unicode">ICU</abbr></a>,
    252 <a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr>
    253 JDK</a>,
    254 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    255 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    256 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
    257 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html#usage">OpenJDK/Oracle JDK</a>.
    258 </p>
    259 <p>Sources for the <code><abbr>tz</abbr></code> database are
    260 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
    261 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
    262 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
    263 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
    264 title="linefeed">LF</abbr></a>,
    265 which can be modified by common text editors such
    266 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
    267 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
    268 <a href="https://www.vim.org">vim</a>.
    269 Specialized source-file editing can be done via the
    270 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
    271 zoneinfo</a> package for <a
    272 href="https://www.sublimetext.com">Sublime Text</a> and the <a
    273 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
    274 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
    275 Studio Code</a>.
    276 </p>
    277 <p>
    278 For further information about updates, please see
    279 <a href="https://tools.ietf.org/html/rfc6557">Procedures for
    280 Maintaining the Time Zone Database</a> (Internet <abbr
    281 title="Request For Comments">RFC</abbr> 6557). More detail can be
    282 found in <a href="theory.html">Theory and pragmatics of the
    283 <code><abbr>tz</abbr></code> code and data</a>.
    284 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
    285 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
    286 </p>
    287 </section>
    288 
    289 <section>
    290 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
    291 <ul>
    292 <li>The article
    293 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
    294 an encyclopedic summary.</li>
    295 <li><a href="tz-how-to.html">How to Read the
    296 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
    297 database format.</li>
    298 <li><a
    299 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
    300 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
    301 database's style.</li>
    302 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
    303 managing time in the internet</a> analyzes the database longitudinally.</li>
    304 </ul>
    305 </section>
    306 
    307 <section>
    308 <h2 id="web">Web sites using recent versions of the
    309 <code><abbr>tz</abbr></code> database</h2>
    310 <p>
    311 These are listed roughly in ascending order of complexity and fanciness.
    312 </p>
    313 <ul>
    314 <li><a href="https://time.is">Time.is</a> shows locations'
    315 time and zones.</li>
    316 <li><a href="https://www.timejones.com">TimeJones.com</a>,
    317 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
    318 <a href="https://www.worldclock.com">The World Clock</a>
    319 are time zone converters.</li>
    320 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
    321 publishes <code><abbr>tzdb</abbr></code>-derived data in
    322 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
    323 title="comma-separated values">CSV</abbr></a> and
    324 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
    325 title="Structured Query Language">SQL</abbr></a> form.</li>
    326 <li><a
    327 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
    328 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
    329 <li><a
    330 href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
    331 Time in 1000 Places</a> uses descriptions of the values.</li>
    332 <li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
    333 uses a pulldown menu.</li>
    334 <li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
    335 timezone information for all countries</a>
    336 displays tables of <abbr>DST</abbr> rules.
    337 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
    338 Worldwide</a> lets you sort zone names and convert times.</li>
    339 <li><a href="https://24timezones.com">24TimeZones</a> has a world
    340 time map and a time converter.</li>
    341 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    342 calculates the current time difference between locations.</li>
    343 <li><a href="http://www.wx-now.com">Weather Now</a> and
    344 <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    345 </ul>
    346 </section>
    347 
    348 <section>
    349 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    350 <ul>
    351 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
    352 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
    353 Distribution Service (tzdist) working group</a> defined <a
    354 href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
    355 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
    356 along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
    357 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    358 transferring time zone data by reference.
    359 <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
    360 implementations</a> are available.
    361 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
    362 mailing list</a> discusses possible extensions.</li>
    363 <li>The <a href="https://tools.ietf.org/html/rfc5545">
    364 Internet Calendaring and Scheduling Core Object Specification
    365 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
    366 covers time zone
    367 data; see its VTIMEZONE calendar component.
    368 The iCalendar format requires specialized parsers and generators; a
    369 variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
    370 (Internet <abbr>RFC</abbr> 6321) uses
    371 <a href="https://www.w3.org/XML/"><abbr
    372 title="Extensible Markup Language">XML</abbr></a> format, and a variant
    373 <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
    374 (Internet <abbr>RFC</abbr> 7265)
    375 uses <a href="https://www.json.org"><abbr
    376 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    377 </ul>
    378 </section>
    379 
    380 <section>
    381 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    382 <p>Although some of these do not fully support
    383 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
    384 distributions you can generally work around compatibility problems by
    385 running the command <code>make rearguard_tarballs</code> and compiling
    386 from the resulting tarballs instead.</p>
    387 <ul>
    388 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
    389 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
    390 program that compiles
    391 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    392 Vzic is freely
    393 available under the <a
    394 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    395 General Public License (<abbr
    396 title="General Public License">GPL</abbr>)</a>.</li>
    397 <li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
    398 database conversion utility</a> is like Vzic, except for the <a
    399 href="https://dotnet.microsoft.com">.NET framework</a>
    400 and with a <abbr>BSD</abbr>-style license.</li>
    401 <li><a
    402 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
    403 contains a script <code>parse_olson</code> that compiles
    404 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
    405 modules. It is part of the Perl <a
    406 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
    407 which is freely
    408 available under both the <abbr>GPL</abbr> and the Perl Artistic
    409 License. DateTime::TimeZone also contains a script
    410 <code>tests_from_zdump</code> that generates test cases for each clock
    411 transition in the <code><abbr>tz</abbr></code> database.</li>
    412 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
    413 Database Parser</a> is a
    414 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
    415 runtime library with <a
    416 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
    417 adopted by
    418 <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
    419 the current iteration of the C++ standard.
    420 It is freely available under the
    421 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
    422 <li><a id="ICU" href="http://site.icu-project.org">International Components for
    423 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
    424 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
    425 libraries for internationalization that
    426 has a compiler from <code><abbr>tz</abbr></code> source
    427 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
    428 (mentioned <a href="#CLDR">below</a>)
    429 into an <abbr>ICU</abbr>-specific format.
    430 <abbr>ICU</abbr> is freely available under a
    431 <abbr>BSD</abbr>-style license.</li>
    432 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
    433 the <a href="https://elixir-lang.org">Elixir</a> language downloads
    434 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
    435 title="Application Program Interface">API</abbr>s for use. It is
    436 freely available under the <abbr>MIT</abbr> license.</li>
    437 <li>Java-based compilers and libraries include:
    438 <ul>
    439 <li>The <a
    440 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
    441 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
    442 <a href="https://openjdk.java.net/">OpenJDK</a> and
    443 <a href="https://jdk.java.net/">Oracle JDK</a>.
    444 Although its source code is proprietary, its executable is available under the
    445 <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
    446 Timezone Updater License Agreement</a>.</li>
    447 <li>The <a
    448 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
    449 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
    450 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
    451 which is freely available under a <abbr>BSD</abbr>-style license.</li>
    452 <li><a href="https://www.joda.org/joda-time/">Joda-Time &ndash; Java date
    453 and time <abbr>API</abbr></a> contains a class
    454 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
    455 <code><abbr>tz</abbr></code> source into a binary format. It inspired
    456 Java 8 <code>java.time</code>, which its users should migrate to once
    457 they can assume Java 8 or later. It is available under the <a
    458 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
    459 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
    460 href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
    461 are alternatives to TZUpdater. IANA Updater's license is unclear;
    462 ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
    463 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
    464 time library for Android</a> and
    465 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
    466 time and interval library for Java</a> compile
    467 <code><abbr>tz</abbr></code> source into a binary format.
    468 Time4A is available under the Apache License and Time4J is
    469 available under the <a
    470 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
    471 General Public License (<abbr title="Lesser General Public
    472 License">LGPL</abbr>)</a>.</li>
    473 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
    474 Java-based libraries.</li>
    475 </ul>
    476 <li><a href="https://nodatime.org">Noda Time &ndash; Date and
    477 time <abbr>API</abbr> for .NET</a>
    478 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
    479 It is freely available under the Apache License.</li>
    480 <li>Many modern
    481 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
    482 runtimes support <code><abbr>tz</abbr></code> natively via the
    483 <samp>timeZone</samp> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><samp>Intl.DateTimeFormat</samp></a>.
    484 This can be used as-is or with most of the following libraries,
    485 many of which also support runtimes lacking the <samp>timeZone</samp> option.
    486 <ul>
    487 <li>The <a
    488 href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
    489 timezone polyfill</a>
    490 is freely available under a <abbr>BSD</abbr>-style license.</li>
    491 <li><a
    492 href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
    493 compiles time zone data into a compact form designed for
    494 JavaScript. It is freely available under a combination of
    495 the <abbr>MIT</abbr> license and the Apache License.</li>
    496 <li>The <a href="https://date-fns.org/">date-fns</a>
    497 library manipulates timezone-aware timestamps in browsers and
    498 in <a href="https://nodejs.org/en/">Node.js</a>.
    499 It is freely available under the <abbr>MIT</abbr> license.</li>
    500 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
    501 minimalist replacement for the date and time API of
    502 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
    503 manipulation library.
    504 It is freely available under the <abbr>MIT</abbr> license.</li>
    505 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
    506 timezone support for the <samp>Intl</samp> API.
    507 It is freely available under the <abbr>MIT</abbr> license.</li>
    508 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    509 Moment.js plugin.
    510 It is freely available under the <abbr>MIT</abbr> license.</li>
    511 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    512 JavaScript library that supports date arithmetic that is time zone
    513 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    514 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
    515 supports live <code><abbr>tzdb</abbr></code> updates,
    516 astronomical and atomic time, a command-line interface,
    517 and full <a
    518 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
    519 Its companion <a
    520 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
    521 can generate <abbr>TZif</abbr> and other files, and a companion website
    522 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
    523 convert timestamps, view transition histories, and download code and data.
    524 It is freely available under the <abbr>MIT</abbr> license.</li>
    525 </ul>
    526 The proposed <a
    527 href="https://github.com/tc39/proposal-temporal"><samp>Temporal</samp>
    528 objects</a> let programs access an abstract view of
    529 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
    530 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
    531 problematic <samp>Date</samp> objects</a> when working with dates and times.
    532 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
    533 compiler from <code><abbr>tz</abbr></code> source into
    534 <a href="https://julialang.org/">Julia</a>. It is freely available
    535 under the <abbr>MIT</abbr> license.</li>
    536 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> &ndash;
    537 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
    538 title="Free Pascal Compiler">FPC</abbr></a>
    539 compiles from <code><abbr>tz</abbr></code> source into
    540 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
    541 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
    542 and <a
    543 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
    544 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    545 <li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
    546 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    547 <a href="https://www.python.org">Python</a>.
    548 It is freely available under a <abbr>BSD</abbr>-style license.
    549 In code that can assume Python 3.9 or later it is superseded by
    550 <a href="#python-zoneinfo"><code>zoneinfo.ZoneInfo</code></a>
    551 and the <a href="#pypi-tzdata"><code>tzdata</code> Python package</a>.</li>
    552 <li><a href="https://tzinfo.github.io">TZInfo &ndash;
    553 Ruby Timezone Library</a>
    554 compiles <code><abbr>tz</abbr></code> source into
    555 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    556 It is freely available under the <abbr>MIT</abbr> license.</li>
    557 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
    558 Library</a> is
    559 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    560 library that compiles <code><abbr>tz</abbr></code> source into a time
    561 zone repository whose format
    562 is either proprietary or an <abbr>XML</abbr>-encoded
    563 representation.</li>
    564 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
    565 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    566 source into text files, along with a runtime that can read those
    567 files. Tcl is freely available under a <abbr>BSD</abbr>-style
    568 license.</li>
    569 </ul>
    570 </section>
    571 
    572 <section>
    573 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
    574 <ul>
    575 <li>The <a
    576 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
    577 Library</a>
    578 has an independent, thread-safe implementation of
    579 a <abbr>TZif</abbr> file reader.
    580 This library is freely available under the LGPL
    581 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    582 <li><a href="https://www.gnome.org">GNOME</a>'s
    583 <a href="https://developer.gnome.org/glib/">GLib</a> has
    584 a <abbr>TZif</abbr> file reader written in C that
    585 creates a <code>GTimeZone</code> object representing sets
    586 of <abbr>UT</abbr> offsets.
    587 It is freely available under the <abbr>LGPL</abbr>.</li>
    588 <li>The
    589 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
    590 <code>baltzo::TimeZoneUtil</code> component contains a C++
    591 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
    592 the Apache License.</li>
    593 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    594 library that translates between <abbr>UT</abbr> and civil time and
    595 can read <abbr>TZif</abbr> files. It is freely available under the Apache
    596 License.</li>
    597 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    598 library that reads <abbr>TZif</abbr> files and converts
    599 timestamps from one time zone or format to another.
    600 It is used by <a href="https://secure.php.net"><abbr
    601 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    602 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    603 and <a href="https://www.mongodb.com">MongoDB</a>.
    604 It is freely available under the <abbr>MIT</abbr> license.</li>
    605 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    606 <abbr>TZif</abbr> file reader.</li>
    607 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
    608 DateTime::TimeZone::Tzfile</a>
    609 is a <abbr>TZif</abbr> file reader written in Perl.
    610 It is freely available under the same terms as Perl
    611 (dual <abbr>GPL</abbr> and Artistic license).</li>
    612 <li>Python has a <a id="python-zoneinfo"
    613 href="https://docs.python.org/3.9/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
    614 class</a> that reads <abbr>TZif</abbr> data and creates objects
    615 that represent <code><abbr>tzdb</abbr></code> timezones.
    616 Python is freely available under the
    617 <a href="https://docs.python.org/3.9/license.html">Python Software Foundation
    618 License</a>.
    619 A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
    620 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
    621 supplies TZif data if the underlying system data cannot be found;
    622 it is freely available under the Apache License.</li>
    623 <li>The
    624 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    625 library contains a Python tool that
    626 converts <abbr>TZif</abbr> data into
    627 <abbr>JSON</abbr>-format data suitable for use
    628 in its JavaScript library for time zone conversion. Dates before 1970
    629 are not supported.</li>
    630 <li>The <a
    631 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    632 package contains <a href="https://www.haskell.org">Haskell</a> code that
    633 parses and uses <abbr>TZif</abbr> data. It is freely
    634 available under a <abbr>BSD</abbr>-style license.</li>
    635 </ul>
    636 </section>
    637 
    638 <section>
    639 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    640 <ul>
    641 <li><a href="https://foxclocks.org">FoxClocks</a>
    642 is an extension for <a href="https://www.google.com/chrome/">Google
    643 Chrome</a> and for <a
    644 href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
    645 Toolkit</a> applications like <a
    646 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
    647 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
    648 It displays multiple clocks in the application window, and has a mapping
    649 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
    650 It is freely available under the <abbr>GPL</abbr>.</li>
    651 <li><a href="https://golang.org">Go programming language</a>
    652 implementations contain a copy of a 32-bit subset of a recent
    653 <code><abbr>tz</abbr></code> database in a
    654 Go-specific format.</li>
    655 <li>Microsoft Windows 8.1
    656 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    657 data (mentioned <a href="#CLDR">below</a>) used by the
    658 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    659 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    660 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    661 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
    662 <a id="System.TimeZoneInfo"
    663 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
    664 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    665 the older, proprietary method of Microsoft Windows 2000 and later,
    666 which stores time zone data in the
    667 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    668 <a
    669 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
    670 Tzid table</a> or <a
    671 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    672 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    673 to <code><abbr>tz</abbr></code> names.
    674 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
    675 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
    676 <li><a
    677 href="https://www.oracle.com/java/index.html">Oracle
    678 Java</a> contains a copy of a subset of a recent
    679 <code><abbr>tz</abbr></code> database in a
    680 Java-specific format.</li>
    681 </ul>
    682 </section>
    683 
    684 <section>
    685 <h2 id="other-dbs">Other time zone databases</h2>
    686 <ul>
    687 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    688 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
    689 time zone history atlases
    690 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
    691 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
    692 Although these extensive atlases
    693 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
    694 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
    695 they are unreliable as Shanks appears to have
    696 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    697 sources and do not indicate which entries are guesswork.</li>
    698 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    699 its own <code>tztab</code>(4) format.</li>
    700 <li>Microsoft Windows has proprietary data mentioned
    701 <a href="#System.TimeZoneInfo">above</a>.</li>
    702 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    703 is another time zone database.</li>
    704 <li>The <a
    705 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
    706 Schedules Information Manual</a> of the
    707 International Air Transport Association
    708 gives current time zone rules for airports served by commercial aviation.</li>
    709 </ul>
    710 </section>
    711 
    712 <section>
    713 <h2 id="maps">Maps</h2>
    714 <ul>
    715 <li>The <a
    716 href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
    717 and Regional Maps section</a> of <em>The World Factbook</em>, published by the
    718 <a href="https://www.cia.gov">US Central Intelligence
    719 Agency (<abbr
    720 title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
    721 zone map; the
    722 <a
    723 href="https://legacy.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
    724 Library Map Collection</a>
    725 of the University of Texas at Austin has copies of
    726 recent editions.
    727 The pictorial quality is good,
    728 but the maps do not indicate daylight saving time,
    729 and parts of the data are a few years out of date.</li>
    730 <li><a href="https://www.worldtimezone.com">World Time Zone Map
    731 with current time</a>
    732 has several fancy time zone maps; it covers Russia particularly well.
    733 The maps' pictorial quality is not quite as good as the
    734 <abbr>CIA</abbr>'s
    735 but the maps are more up to date.</li>
    736 <li><a
    737 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    738 much is time wrong around the world?</a> maps the difference between
    739 mean solar and standard time, highlighting areas such as western China
    740 where the two differ greatly. It's a bit out of date, unfortunately.</li>
    741 </ul>
    742 </section>
    743 
    744 <section>
    745 <h2 id="boundaries">Time zone boundaries</h2>
    746 <p>Geographical boundaries between timezones are available
    747 from several <a
    748 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
    749 geolocation</a>
    750 services and other sources.</p>
    751 <ul>
    752 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    753 Boundary Builder</a> extracts
    754 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    755 boundaries of <code><abbr>tzdb</abbr></code> timezones.
    756 Its code is freely available under the <abbr>MIT</abbr> license, and
    757 its data entries are freely available under the
    758 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    759 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
    760 <li>Programmatic interfaces that map geographical coordinates via tz_world to
    761 <code><abbr>tzdb</abbr></code> timezones include:
    762 <ul>
    763 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
    764 written in <a
    765 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
    766 and is freely available under the <abbr>MIT</abbr> license.</li>
    767 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
    768 is written in Go and is freely available under the Apache License.</li>
    769 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
    770 in both Java and
    771 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
    772 form, is freely available under the MIT license.</li>
    773 <li>For Node.js,
    774 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
    775 is freely available under the MIT license, and
    776 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
    777 is in the public domain.</li>
    778 <li>The <a
    779 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
    780 library for Python is freely available under the MIT license.
    781 <li>The <a
    782 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
    783 library for Ruby is freely available under the MIT license.</li>
    784 </ul></li>
    785 <li>Free access via a network API, if you register a key, is provided by
    786 the <a
    787 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
    788 Timezone web service</a>, the <a
    789 href="https://developers.google.com/maps/documentation/timezone/intro">Google
    790 Maps Time Zone API</a>, and
    791 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
    792 Commercial network API access is provided
    793 by <a href="https://askgeo.com">AskGeo</a>
    794 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    795 </li>
    796 <li>"<a
    797 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
    798 to get a time zone from a location using latitude and longitude
    799 coordinates?</a>" discusses other geolocation possibilities.</li>
    800 <li><a href="http://statoids.com/statoids.html">Administrative
    801 Divisions of Countries ("Statoids")</a> lists
    802 political subdivision data related to time zones.</li>
    803 <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
    804 zone boundaries for multizone countries</a> summarizes legal
    805 boundaries between time zones within countries.</li>
    806 <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
    807 &ndash; GIS and Database Tools</a> includes a Manifold-format map of
    808 world time zone boundaries distributed under the
    809 <abbr>GPL</abbr>.</li>
    810 <li>A ship within the <a
    811 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    812 waters</a> of any nation uses that nation's time. In international
    813 waters, time zone boundaries are meridians 15&deg; apart, except that
    814 <abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
    815 wide and are separated by
    816 the 180&deg; meridian (not by the International Date Line, which is
    817 for land and territorial waters only). A captain can change ship's
    818 clocks any time after entering a new time zone; midnight changes are
    819 common.</li>
    820 </ul>
    821 </section>
    822 
    823 <section>
    824 <h2 id="civil">Civil time concepts and history</h2>
    825 <ul>
    826 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    827 Walk through Time</a>
    828 surveys the evolution of timekeeping.</li>
    829 <li>The history of daylight saving time is surveyed in <a
    830 href="http://www.webexhibits.org/daylightsaving/">About Daylight
    831 Saving Time &ndash; History, rationale, laws &amp; dates</a> and summarized in
    832 <a href="http://seizethedaylight.com/dst/">A Brief
    833 History of Daylight Saving Time</a>.</li>
    834 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
    835 Lords</a> discusses how authoritarians manipulate civil time.</li>
    836 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
    837 contains guidelines and best practices for software applications that
    838 deal with civil time.</li>
    839 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
    840 the International Date Line</a> tells the story of the most important
    841 time zone boundary.</li>
    842 <li><a href="http://statoids.com/tconcept.html">Basic Time
    843 Zone Concepts</a> discusses terminological issues behind time zones.</li>
    844 </ul>
    845 </section>
    846 
    847 <section>
    848 <h2 id="national">National histories of legal time</h2>
    849 <dl>
    850 <dt>Australia</dt>
    851 <dd>The Parliamentary Library commissioned a <a
    852 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
    853 paper on daylight saving time in Australia</a>.
    854 The Bureau of Meteorology publishes a list of <a
    855 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    856 Dates of Daylight Savings Time within Australia</a>.</dd>
    857 <dt>Belgium</dt>
    858 <dd>The Royal Observatory of Belgium maintains a table of time in
    859 Belgium (in
    860 <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
    861 hreflang="nl">Dutch</a> and <a
    862 href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
    863 hreflang="fr">French</a>).</dd>
    864 <dt>Brazil</dt>
    865 <dd>The Time Service Department of the National Observatory
    866 records <a href="http://pcdsh01.on.br/DecHV.html"
    867 hreflang="pt-BR">Brazil's daylight saving time decrees (in
    868 Portuguese)</a>.</dd>
    869 <dt>Canada</dt>
    870 <dd>National Research Council Canada publishes current
    871 and some older information about <a
    872 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
    873 zones and daylight saving time</a>.</dd>
    874 <dt>Chile</dt>
    875 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    876 <a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
    877 Chile's official time (in Spanish)</a>.</dd>
    878 <dt>China</dt>
    879 <dd>The Hong Kong Observatory maintains a
    880 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
    881  summer time in Hong Kong</a>,
    882 and Macau's Meteorological and Geophysical Bureau maintains a <a
    883 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
    884 history for Macau</a>.
    885 Unfortunately the latter is incomplete and has errors.</dd>
    886 <dt>Czech Republic</dt>
    887 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    888 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    889 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
    890 <dt>Germany</dt>
    891 <dd>The National Institute for Science and Technology maintains the <a
    892 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    893 of Legal Time in Germany</a>.</dd>
    894 <dt>Israel</dt>
    895 <dd>The Interior Ministry periodically issues <a
    896 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
    897 hreflang="he">announcements (in Hebrew)</a>.</dd>
    898 <dt>Malaysia</dt>
    899 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    900 <dt>Mexico</dt>
    901 <dd>The Investigation and Analysis Service of the Mexican Library of
    902 Congress has published a <a
    903 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    904 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    905 <dt>Netherlands</dt>
    906 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    907 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    908 covers the history of local time in the Netherlands from ancient times.</dd>
    909 <dt>New Zealand</dt>
    910 <dd>The Department of Internal Affairs maintains a brief <a
    911 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
    912 Daylight Saving</a>.</dd>
    913 <dt>Portugal</dt>
    914 <dd>The Lisbon Astronomical Observatory publishes a
    915 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
    916 legal time (in Portuguese)</a>.</dd>
    917 <dt>Singapore</dt>
    918 <dd><a id="Singapore"
    919 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
    920 is Singapore in the "Wrong" Time Zone?</a> details the
    921 history of legal time in Singapore and Malaysia.</dd>
    922 <dt>United Kingdom</dt>
    923 <dd><a
    924 href="https://www.polyomino.org.uk/british-time/">History of
    925 legal time in Britain</a> discusses in detail the country
    926 with perhaps the best-documented history of clock adjustments.</dd>
    927 <dt>United States</dt>
    928 <dd>The Department of Transportation's <a
    929 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    930 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
    931 <dt>Uruguay</dt>
    932 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    933 Navy (SOHMA) publishes an annual <a
    934 href="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
    935 (in Spanish)</a>.</dd>
    936 </dl>
    937 </section>
    938 
    939 <section>
    940 <h2 id="costs">Costs and benefits of time shifts</h2>
    941 <p>Various sources argue for and against daylight saving time and time
    942 zone shifts, and many scientific studies have been conducted. This
    943 section summarizes reviews and position statements based on
    944 scientific literature in the area.</p>
    945 <ul>
    946 <li>Carey RN, Sarma KM.
    947 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
    948 daylight saving time on road traffic collision risk: a systematic
    949 review</a>.
    950 <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>.
    951 This reviews research literature and concludes that the evidence
    952 neither supports nor refutes road safety benefits from
    953 shifts in time zones.</li>
    954 <li>Havranek T, Herman D, Irsova D.
    955 <a href="https://www.econstor.eu/handle/10419/174191">Does daylight
    956 saving save electricity? A meta-analysis</a>. <em>Energy J.</em>
    957 2018;39(2).
    958 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
    959 This analyzes research literature and concludes, "Electricity savings
    960 are larger for countries farther away from the equator, while
    961 subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
    962 <li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
    963 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
    964 an American Academy of Sleep Medicine position statement</a>.
    965 <em>J Clin Sleep Med.</em>
    966 2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
    967 This argues for permanent standard time due to health risks of both
    968 <abbr>DST</abbr> transitions and permanent <abbr>DST</abbr>.</li>
    969 <li>Roenneberg T, Winnebeck EC, Klerman EB.
    970 <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
    971 saving time and artificial time zones &ndash; a battle between
    972 biological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
    973 doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
    974 This reviews evidence about the health effects of <abbr>DST</abbr>
    975 and concludes,
    976 "In summary, the scientific literature strongly argues against the
    977 switching between <abbr>DST</abbr> and Standard Time and even more so against
    978 adopting <abbr>DST</abbr> permanently."</li>
    979 </ul>
    980 </section>
    981 
    982 <section>
    983 <h2 id="precision">Precision timekeeping</h2>
    984 <ul>
    985 <li><a
    986 href="http://leapsecond.com/hpan/an1289.pdf">The
    987 Science of Timekeeping</a> is a thorough introduction
    988 to the theory and practice of precision timekeeping.</li>
    989 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
    990 Time 2016</a> contains several freely-readable papers.</li>
    991 <li><a href="http://www.ntp.org"><abbr
    992 title="Network Time Protocol">NTP</abbr>: The Network
    993 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
    994 discusses how to synchronize clocks of
    995 Internet hosts.</li>
    996 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
    997 family of software algorithms can achieve accuracy to a few tens of
    998 nanoseconds in scalable server farms without special hardware.</li>
    999 <li>The <a
   1000 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
   1001 Time Protocol</a> (<abbr
   1002 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
   1003 can achieve submicrosecond clock accuracy on a local area network
   1004 with special-purpose hardware.</li>
   1005 <li><a
   1006 href="https://tools.ietf.org/html/rfc4833">Timezone
   1007 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
   1008 (Internet <abbr>RFC</abbr> 4833)
   1009 specifies a <a
   1010 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
   1011 option for a server to configure
   1012 a client's time zone and daylight saving settings automatically.</li>
   1013 <li><a
   1014 href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
   1015 Times</a> explains more abstruse astronomical time scales like
   1016 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
   1017 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
   1018 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
   1019 <a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
   1020 Scales</a> goes into more detail, particularly for historical variants.</li>
   1021 <li>The <a href="https://www.iau.org"><abbr
   1022 title="International Astronomical Union">IAU</abbr></a>'s <a
   1023 href="http://www.iausofa.org"><abbr
   1024 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
   1025 collection contains C and <a
   1026 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
   1027 code for converting among time scales like
   1028 <abbr title="International Atomic Time">TAI</abbr>,
   1029 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
   1030 <abbr>UTC</abbr>.</li>
   1031 <li><a
   1032 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
   1033 &ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
   1034 diverse local time
   1035 scales used by each landed mission on Mars.</li>
   1036 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
   1037 dedicated not only to leap seconds but to precise time and frequency
   1038 in general. It covers the state of the art in amateur timekeeping, and
   1039 how the art has progressed over the past few decades.</li>
   1040 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
   1041 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
   1042 and time-signal emissions</a>, International Telecommunication Union &ndash;
   1043 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
   1044 <li><a
   1045 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
   1046 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
   1047 Bulletins</a> contains official publications of the International
   1048 Earth Rotation and Reference Systems Service, which decides when leap
   1049 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
   1050 via an optional "<code>right</code>" configuration where a computer's internal
   1051 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
   1052 as opposed to the default "<code>posix</code>" configuration
   1053 where the internal clock ignores leap seconds.
   1054 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
   1055 <abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge for
   1056 timestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,
   1057 which corresponds to the first leap second
   1058 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
   1059 and to
   1060 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
   1061 In practice the two configurations also agree for timestamps before
   1062 1972 even though the historical situation is messy, partly because
   1063 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
   1064 is well-defined for sufficiently-old timestamps.</li>
   1065 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
   1066 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
   1067 leap second so that they disagree with <abbr>UTC</abbr> by at most a
   1068 half second, even though every <abbr>POSIX</abbr> minute has exactly
   1069 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
   1070 "<code>posix</code>" configuration, is <a
   1071 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
   1072 the <abbr>NTP</abbr> reference implementation, and is used by major
   1073 cloud service providers. However, according to
   1074 <a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">&sect;3.7.1 of
   1075 Network Time Protocol Best Current Practices</a>
   1076 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
   1077 applications requiring accurate <abbr>UTC</abbr> or civil time,
   1078 and is intended for use only in single, well-controlled environments.</li>
   1079 <li>The <a
   1080 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
   1081 Second Discussion List</a> covers <a
   1082 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
   1083 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
   1084 discussed further in
   1085 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
   1086 leap second: its history and possible future</a>.
   1087 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
   1088 might be redefined
   1089 without Leap Seconds</a> gives pointers on this
   1090 contentious issue, which was active until 2015 and could become active
   1091 again.</li>
   1092 </ul>
   1093 </section>
   1094 
   1095 <section>
   1096 <h2 id="notation">Time notation</h2>
   1097 <ul>
   1098 <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
   1099 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
   1100 zone names, abbreviations, identifiers, and formats. For example, it
   1101 contains French translations for "Eastern European Summer Time",
   1102 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
   1103 "Bucharest". Its
   1104 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
   1105 charts</a> show these values for many locales. Data values are available in
   1106 both <abbr title="Locale Data Markup Language">LDML</abbr>
   1107 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
   1108 <li>
   1109 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
   1110 the international standard date and time notation</a> covers
   1111 <a
   1112 href="https://www.iso.org/standard/70907.html"><em><abbr
   1113 title="International Organization for Standardization">ISO</abbr>
   1114 8601-1:2019 &ndash; Date and time &ndash; Representations for information
   1115 interchange &ndash; Part 1: Basic rules</em></a>.</li>
   1116 <li>
   1117 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
   1118 Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
   1119 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
   1120 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">&sect;3.3 of
   1121 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
   1122 specifies the time notation used in email and <a
   1123 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
   1124 headers.</li>
   1125 <li>
   1126 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
   1127 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
   1128 specifies an <abbr>ISO</abbr> 8601
   1129 profile for use in new Internet
   1130 protocols.</li>
   1131 <li>
   1132 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
   1133 Formats on the Web</a> surveys web- and Internet-oriented date and time
   1134 formats.</li>
   1135 <li>Alphabetic time zone abbreviations should not be used as unique
   1136 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
   1137 practice. For example, in English-speaking North America
   1138 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
   1139 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
   1140 and French-speaking North Americans prefer
   1141 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
   1142 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
   1143 database contains English abbreviations for many timestamps;
   1144 unfortunately some of these abbreviations were merely the database maintainers'
   1145 inventions, and these have been removed when possible.</li>
   1146 <li>Numeric time zone abbreviations typically count hours east of
   1147 <abbr>UT</abbr>, e.g., +09 for Japan and
   1148 &minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
   1149 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
   1150 For example, one might use <code><abbr>TZ</abbr>="<abbr
   1151 title="Japan Standard Time">JST</abbr>-9"</code> and
   1152 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
   1153 for Japan and Hawaii, respectively. If the
   1154 <code><abbr>tz</abbr></code> database is available, it is usually better to use
   1155 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
   1156 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
   1157 confusion, handle old timestamps better, and insulate you better from
   1158 any future changes to the rules. One should never set
   1159 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
   1160 <code>"GMT-9"</code>, though, since this would incorrectly imply that
   1161 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
   1162 is called "<abbr>GMT</abbr>".</li>
   1163 </ul>
   1164 </section>
   1165 
   1166 <section>
   1167 <h2 id="see-also">See also</h2>
   1168 <ul>
   1169 <li><a href="theory.html">Theory and pragmatics of the
   1170 <code><abbr>tz</abbr></code> code and data</a></li>
   1171 <li><a href="tz-art.html">Time and the Arts</a></li>
   1172 </ul>
   1173 </section>
   1174 
   1175 <footer>
   1176 <hr>
   1177 This web page is in the public domain, so clarified as of
   1178 2009-05-17 by Arthur David Olson.
   1179 <br>
   1180 Please send corrections to this web page to the
   1181 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
   1182 </footer>
   1183 </body>
   1184 </html>
   1185