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      4 <title>Time zone and daylight saving time data</title>
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      6 <style>
      7 pre {margin-left: 2em; white-space: pre-wrap;}
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     10 <body>
     11 <h1>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 	</ul>
     56       </li>
     57       <li><a href="#see-also">See also</a></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://www.apple.com/ios"><abbr
     93 title="iPhone OS">iOS</abbr></a>,
     94 <a href="https://www.apple.com/macos">macOS</a>,
     95 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
     96 <a href="https://www.vmssoftware.com">Open<abbr
     97 title="Virtual Memory System">VMS</abbr></a>,
     98 <a href="https://www.oracle.com/database/">Oracle Database</a>, and
     99 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
    100 <p>
    101 Each main entry in the database represents a <dfn>timezone</dfn>
    102 for a set of civil-time clocks that have all agreed since 1970.
    103 Timezones are typically identified by continent or ocean and then by the
    104 name of the largest city within the region containing the clocks.
    105 For example, <code>America/New_York</code>
    106 represents most of the <abbr title="United States">US</abbr> eastern time zone;
    107 <code>America/Phoenix</code> represents most of Arizona, which
    108 uses mountain time without daylight saving time (<abbr>DST</abbr>);
    109 <code>America/Detroit</code> represents most of Michigan, which uses
    110 eastern time but with different <abbr>DST</abbr> rules in 1975;
    111 and other entries represent smaller regions like Starke County,
    112 Indiana, which switched from central to eastern time in 1991
    113 and switched back in 2006.
    114 To use the database on an extended <a
    115 href="https://en.wikipedia.org/wiki/POSIX"><abbr
    116 title="Portable Operating System Interface">POSIX</abbr></a>
    117 implementation set the <code><abbr>TZ</abbr></code>
    118 environment variable to the location's full name,
    119 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
    120 <p>
    121 Associated with each timezone is a history of offsets from
    122 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
    123 Time</a> (<abbr>UT</abbr>), which is <a
    124 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
    125 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
    126 for timestamps after 1960 this is more precisely <a
    127 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
    128 Universal Time</a> (<abbr>UTC</abbr>).
    129 The database also records when daylight saving time was in use,
    130 along with some time zone abbreviations such as <abbr>EST</abbr>
    131 for Eastern Standard Time in the <abbr>US</abbr>.</p>
    132 </section>
    133 
    134 <section>
    135 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
    136 <p>
    137 The following <a
    138 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
    139 the latest release's two
    140 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
    141 to a <abbr>GNU</abbr>/Linux or similar host.</p>
    142 <pre><code>mkdir tzdb
    143 cd tzdb
    144 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
    145 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
    146 <a href="https://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="https://www.gnu.org/software/tar/">tar</a> -xf -
    147 gzip -dc tzdata-latest.tar.gz | tar -xf -
    148 </code></pre>
    149 <p>Alternatively, the following shell commands download the same
    150 release in a single-tarball format containing extra data
    151 useful for regression testing:</p>
    152 <pre><code>wget <a href="https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz">https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz</a>
    153 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
    154 </code></pre>
    155 <p>These commands use convenience links to the latest release
    156 of the <code><abbr>tz</abbr></code> database hosted by the
    157 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
    158 of the <a href="https://www.iana.org">Internet Assigned Numbers
    159 Authority (IANA)</a>.
    160 Older releases are in files named
    161 <code>tzcode<var>V</var>.tar.gz</code>,
    162 <code>tzdata<var>V</var>.tar.gz</code>, and
    163 <code>tzdb-<var>V</var>.tar.lz</code>,
    164 where <code><var>V</var></code> is the version.
    165 Since 1996, each version has been a four-digit year followed by
    166 lower-case letter (<samp>a</samp> through <samp>z</samp>,
    167 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
    168 through <samp>zzz</samp>, and so on).
    169 Since version 2022a, each release has been distributed in
    170 <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
    171 ustar interchange format</a>, compressed as described above;
    172 older releases use a nearly-compatible format.
    173 Since version 2016h, each release has contained a text file named
    174 "<samp>version</samp>" whose first (and currently only) line is the version.
    175 Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
    176 and are also available in an
    177 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
    178 title="File Transfer Protocol">FTP</abbr> directory</a> via a
    179 less-secure protocol.</p>
    180 <p>Alternatively, a development repository of code and data can be
    181 retrieved from <a href="https://github.com">GitHub</a> via the shell
    182 command:</p>
    183 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
    184 </code></pre>
    185 <p>
    186 Since version 2012e, each release has been tagged in development repositories.
    187 Untagged commits are less well tested and probably contain
    188 more errors.</p>
    189 <p>
    190 After obtaining the code and data files, see the
    191 <code>README</code> file for what to do next.
    192 The code lets you compile the <code><abbr>tz</abbr></code> source files into
    193 machine-readable binary files, one for each location. The binary files
    194 are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
    195 specified by <a href="https://datatracker.ietf.org/doc/html/8536">Internet
    196 <abbr>RFC</abbr> 8536</a>.
    197 The code also lets
    198 you read a <abbr>TZif</abbr> file and interpret timestamps for that
    199 location.</p>
    200 </section>
    201 
    202 <section>
    203 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    204 <p>
    205 The <code><abbr>tz</abbr></code> code and data
    206 are by no means authoritative. If you find errors, please
    207 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    208 the time zone mailing list. You can also <a
    209 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
    210 and browse the <a
    211 href="https://mm.icann.org/pipermail/tz/">archive of old
    212 messages</a>.
    213 <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
    214 discussions</a> and corresponding data changes can be
    215 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
    216 </p>
    217 <p>
    218 If your government plans to change its time zone boundaries or
    219 daylight saving rules, inform <code>tz (a] iana.org</code> well in
    220 advance, as this will coordinate updates to many cell phones,
    221 computers, and other devices around the world.
    222 The change should be officially announced at least a year before it affects
    223 how clocks operate; otherwise, there is a good chance that some
    224 clocks will operate incorrectly after the change, due
    225 to delays in propagating updates to software and data. The shorter
    226 the notice, the more likely clock problems will arise; see "<a
    227 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
    228 the Timing of Time Zone Changes</a>" for examples.
    229 The <code><abbr>tz</abbr></code> data can represent planned changes
    230 far into the future, and a long-planned change can easily be reverted
    231 or otherwise altered with a year's notice before the change would have
    232 affected clocks.
    233 </p>
    234 <p>
    235 Changes to the <code><abbr>tz</abbr></code> code and data are often
    236 propagated to clients via operating system updates, so
    237 client <code><abbr>tz</abbr></code> data can often be corrected by
    238 applying these updates. With GNU/Linux and similar systems, if your
    239 maintenance provider has not yet adopted the
    240 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
    241 the process by tailoring the generic instructions in
    242 the <code><abbr>tz</abbr> README</code> file and installing the latest
    243 data yourself. System-specific instructions for installing the
    244 latest <code><abbr>tz</abbr></code> data have also been published
    245 for <a href="https://developer.ibm.com/articles/au-aix-olson-time-zone/"><abbr>AIX</abbr></a>,
    246 <a
    247 href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
    248 <a
    249 href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
    250 title="International Components for Unicode">ICU</abbr></a>,
    251 <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
    252 JDK</a>,
    253 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    254 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    255 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
    256 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
    257 </p>
    258 <p>Since version 2013a,
    259 sources for the <code><abbr>tz</abbr></code> database have been
    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://datatracker.ietf.org/doc/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="https://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://home.kpn.nl/vanadovv/time/TZworld.html">Complete
    333 timezone information for all countries</a>
    334 displays tables of <abbr>DST</abbr> rules.
    335 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
    336 Worldwide</a> lets you sort zone names and convert times.</li>
    337 <li><a href="https://24timezones.com">24TimeZones</a> has a world
    338 time map and a time converter.</li>
    339 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    340 calculates the current time difference between locations.</li>
    341 <li><a href="https://www.wx-now.com">Weather Now</a> and
    342 <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    343 </ul>
    344 </section>
    345 
    346 <section>
    347 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    348 <ul>
    349 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
    350 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
    351 Distribution Service (tzdist) working group</a> defined <a
    352 href="https://datatracker.ietf.org/doc/html/rfc7808">TZDIST</a>
    353 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
    354 along with <a href="https://datatracker.ietf.org/doc/html/rfc7809">CalDAV</a>
    355 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    356 transferring time zone data by reference.
    357 <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
    358 implementations</a> are available.
    359 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
    360 mailing list</a> discusses possible extensions.</li>
    361 <li>The <a href="https://datatracker.ietf.org/doc/html/rfc5545">
    362 Internet Calendaring and Scheduling Core Object Specification
    363 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
    364 covers time zone
    365 data; see its VTIMEZONE calendar component.
    366 The iCalendar format requires specialized parsers and generators; a
    367 variant <a href="https://datatracker.ietf.org/doc/html/rfc6321">xCal</a>
    368 (Internet <abbr>RFC</abbr> 6321) uses
    369 <a href="https://www.w3.org/XML/"><abbr
    370 title="Extensible Markup Language">XML</abbr></a> format, and a variant
    371 <a href="https://datatracker.ietf.org/doc/html/rfc7265">jCal</a>
    372 (Internet <abbr>RFC</abbr> 7265)
    373 uses <a href="https://www.json.org"><abbr
    374 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    375 </ul>
    376 </section>
    377 
    378 <section>
    379 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    380 <p>Although some of these do not fully support
    381 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
    382 distributions you can generally work around compatibility problems by
    383 running the command <code>make rearguard_tarballs</code> and compiling
    384 from the resulting tarballs instead.</p>
    385 <ul>
    386 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
    387 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
    388 program that compiles
    389 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    390 Vzic is freely
    391 available under the <a
    392 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    393 General Public License (<abbr
    394 title="General Public License">GPL</abbr>)</a>.</li>
    395 <li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
    396 database conversion utility</a> is like Vzic, except for the <a
    397 href="https://dotnet.microsoft.com">.NET framework</a>
    398 and with a <abbr>BSD</abbr>-style license.</li>
    399 <li><a
    400 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
    401 contains a script <code>parse_olson</code> that compiles
    402 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
    403 modules. It is part of the Perl <a
    404 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
    405 which is freely
    406 available under both the <abbr>GPL</abbr> and the Perl Artistic
    407 License. DateTime::TimeZone also contains a script
    408 <code>tests_from_zdump</code> that generates test cases for each clock
    409 transition in the <code><abbr>tz</abbr></code> database.</li>
    410 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
    411 Database Parser</a> is a
    412 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
    413 runtime library with <a
    414 href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
    415 adopted by
    416 <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
    417 the current iteration of the C++ standard.
    418 It is freely available under the
    419 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
    420 <li><a id="ICU" href="https://icu.unicode.org">International Components for
    421 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
    422 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
    423 libraries for internationalization that
    424 has a compiler from <code><abbr>tz</abbr></code> source
    425 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
    426 (mentioned <a href="#CLDR">below</a>)
    427 into an <abbr>ICU</abbr>-specific format.
    428 <abbr>ICU</abbr> is freely available under a
    429 <abbr>BSD</abbr>-style license.</li>
    430 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
    431 the <a href="https://elixir-lang.org">Elixir</a> language downloads
    432 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
    433 title="Application Program Interface">API</abbr>s for use. It is
    434 freely available under the <abbr>MIT</abbr> license.</li>
    435 <li>Java-based compilers and libraries include:
    436 <ul>
    437 <li>The <a
    438 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
    439 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
    440 <a href="https://openjdk.java.net/">OpenJDK</a> and
    441 <a href="https://jdk.java.net/">Oracle JDK</a>.
    442 Although its source code is proprietary, its executable is available under the
    443 <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
    444 Timezone Updater License Agreement</a>.</li>
    445 <li>The <a
    446 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
    447 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
    448 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
    449 which is freely available under a <abbr>BSD</abbr>-style license.</li>
    450 <li><a href="https://www.joda.org/joda-time/">Joda-Time &ndash; Java date
    451 and time <abbr>API</abbr></a> contains a class
    452 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
    453 <code><abbr>tz</abbr></code> source into a binary format. It inspired
    454 Java 8 <code>java.time</code>, which its users should migrate to once
    455 they can assume Java 8 or later. It is available under the <a
    456 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
    457 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
    458 href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
    459 are alternatives to TZUpdater. IANA Updater's license is unclear;
    460 ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
    461 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
    462 time library for Android</a> and
    463 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
    464 time and interval library for Java</a> compile
    465 <code><abbr>tz</abbr></code> source into a binary format.
    466 Time4A is available under the Apache License and Time4J is
    467 available under the <a
    468 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
    469 General Public License (<abbr title="Lesser General Public
    470 License">LGPL</abbr>)</a>.</li>
    471 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
    472 Java-based libraries.</li>
    473 </ul>
    474 <li><a href="https://nodatime.org">Noda Time &ndash; Date and
    475 time <abbr>API</abbr> for .NET</a>
    476 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
    477 It is freely available under the Apache License.</li>
    478 <li>Many modern
    479 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
    480 runtimes support <code><abbr>tz</abbr></code> natively via the
    481 <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>.
    482 This can be used as-is or with most of the following libraries,
    483 many of which also support runtimes lacking the <samp>timeZone</samp> option.
    484 <ul>
    485 <li>The <a
    486 href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
    487 timezone polyfill</a>
    488 is freely available under a <abbr>BSD</abbr>-style license.</li>
    489 <li>The <a href="https://date-fns.org/">date-fns</a>
    490 library manipulates timezone-aware timestamps in browsers and
    491 in <a href="https://nodejs.org/en/">Node.js</a>.
    492 It is freely available under the <abbr>MIT</abbr> license.</li>
    493 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
    494 minimalist replacement for the date and time API of
    495 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
    496 manipulation library.
    497 It is freely available under the <abbr>MIT</abbr> license.</li>
    498 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
    499 timezone support for the <samp>Intl</samp> API.
    500 It is freely available under the <abbr>MIT</abbr> license.</li>
    501 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    502 Moment.js plugin.
    503 It is freely available under the <abbr>MIT</abbr> license.</li>
    504 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    505 JavaScript library that supports date arithmetic that is time zone
    506 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    507 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
    508 supports live <code><abbr>tzdb</abbr></code> updates,
    509 astronomical and atomic time, a command-line interface,
    510 and full <a
    511 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
    512 Its companion <a
    513 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
    514 can generate <abbr>TZif</abbr> and other files, and a companion website
    515 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
    516 convert timestamps, view transition histories, and download code and data.
    517 It is freely available under the <abbr>MIT</abbr> license.</li>
    518 </ul>
    519 The proposed <a
    520 href="https://github.com/tc39/proposal-temporal"><samp>Temporal</samp>
    521 objects</a> let programs access an abstract view of
    522 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
    523 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
    524 problematic <samp>Date</samp> objects</a> when working with dates and times.
    525 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
    526 compiler from <code><abbr>tz</abbr></code> source into
    527 <a href="https://julialang.org/">Julia</a>. It is freely available
    528 under the <abbr>MIT</abbr> license.</li>
    529 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> &ndash;
    530 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
    531 title="Free Pascal Compiler">FPC</abbr></a>
    532 compiles from <code><abbr>tz</abbr></code> source into
    533 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
    534 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
    535 and <a
    536 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
    537 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    538 <li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
    539 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    540 <a href="https://www.python.org">Python</a>.
    541 It is freely available under a <abbr>BSD</abbr>-style license.
    542 In code that can assume Python 3.6 or later it is largely superseded; see <a
    543 href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
    544 The Fastest Footgun in the West</a>.</li>
    545 <li><a href="https://tzinfo.github.io">TZInfo &ndash;
    546 Ruby Timezone Library</a>
    547 compiles <code><abbr>tz</abbr></code> source into
    548 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    549 It is freely available under the <abbr>MIT</abbr> license.</li>
    550 <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
    551 Library</a> is
    552 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    553 library that compiles <code><abbr>tz</abbr></code> source into a time
    554 zone repository whose format
    555 is either proprietary or an <abbr>XML</abbr>-encoded
    556 representation.</li>
    557 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
    558 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    559 source into text files, along with a runtime that can read those
    560 files. Tcl is freely available under a <abbr>BSD</abbr>-style
    561 license.</li>
    562 </ul>
    563 </section>
    564 
    565 <section>
    566 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
    567 <ul>
    568 <li>The <a
    569 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
    570 Library</a>
    571 has an independent, thread-safe implementation of
    572 a <abbr>TZif</abbr> file reader.
    573 This library is freely available under the LGPL
    574 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    575 <li><a href="https://www.gnome.org">GNOME</a>'s
    576 <a href="https://developer.gnome.org/glib/">GLib</a> has
    577 a <abbr>TZif</abbr> file reader written in C that
    578 creates a <code>GTimeZone</code> object representing sets
    579 of <abbr>UT</abbr> offsets.
    580 It is freely available under the <abbr>LGPL</abbr>.</li>
    581 <li>The
    582 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
    583 <code>baltzo::TimeZoneUtil</code> component contains a C++
    584 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
    585 the Apache License.</li>
    586 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    587 library that translates between <abbr>UT</abbr> and civil time and
    588 can read <abbr>TZif</abbr> files. It is freely available under the Apache
    589 License.</li>
    590 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    591 library that reads <abbr>TZif</abbr> files and converts
    592 timestamps from one time zone or format to another.
    593 It is used by <a href="https://secure.php.net"><abbr
    594 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    595 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    596 and <a href="https://www.mongodb.com">MongoDB</a>.
    597 It is freely available under the <abbr>MIT</abbr> license.</li>
    598 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    599 <abbr>TZif</abbr> file reader.</li>
    600 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
    601 DateTime::TimeZone::Tzfile</a>
    602 is a <abbr>TZif</abbr> file reader written in Perl.
    603 It is freely available under the same terms as Perl
    604 (dual <abbr>GPL</abbr> and Artistic license).</li>
    605 <li>Python has a <a id="python-zoneinfo"
    606 href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
    607 class</a> that reads <abbr>TZif</abbr> data and creates objects
    608 that represent <code><abbr>tzdb</abbr></code> timezones.
    609 Python is freely available under the
    610 <a href="https://docs.python.org/3/license.html">Python Software Foundation
    611 License</a>.
    612 A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
    613 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
    614 supplies TZif data if the underlying system data cannot be found;
    615 it is freely available under the Apache License.</li>
    616 <li>The
    617 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    618 library contains a Python tool that
    619 converts <abbr>TZif</abbr> data into
    620 <abbr>JSON</abbr>-format data suitable for use
    621 in its JavaScript library for time zone conversion. Dates before 1970
    622 are not supported.</li>
    623 <li>The <a
    624 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    625 package contains <a href="https://www.haskell.org">Haskell</a> code that
    626 parses and uses <abbr>TZif</abbr> data. It is freely
    627 available under a <abbr>BSD</abbr>-style license.</li>
    628 </ul>
    629 </section>
    630 
    631 <section>
    632 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    633 <ul>
    634 <li><a href="https://foxclocks.org">FoxClocks</a>
    635 is an extension for <a href="https://www.google.com/chrome/">Google
    636 Chrome</a> and for <a
    637 href="https://wiki.mozilla.org/Modules/Toolkit">Mozilla
    638 Toolkit</a> applications like <a
    639 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
    640 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
    641 It displays multiple clocks in the application window, and has a mapping
    642 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
    643 It is freely available under the <abbr>GPL</abbr>.</li>
    644 <li><a href="https://golang.org">Go programming language</a>
    645 implementations contain a copy of a 32-bit subset of a recent
    646 <code><abbr>tz</abbr></code> database in a
    647 Go-specific format.</li>
    648 <li>Microsoft Windows 8.1
    649 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    650 data (mentioned <a href="#CLDR">below</a>) used by the
    651 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    652 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    653 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    654 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
    655 <a id="System.TimeZoneInfo"
    656 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
    657 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    658 the older, proprietary method of Microsoft Windows 2000 and later,
    659 which stores time zone data in the
    660 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    661 <a
    662 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
    663 Tzid table</a> or <a
    664 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    665 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    666 to <code><abbr>tz</abbr></code> names.
    667 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
    668 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
    669 <li><a
    670 href="https://www.oracle.com/java/index.html">Oracle
    671 Java</a> contains a copy of a subset of a recent
    672 <code><abbr>tz</abbr></code> database in a
    673 Java-specific format.</li>
    674 </ul>
    675 </section>
    676 
    677 <section>
    678 <h2 id="other-dbs">Other time zone databases</h2>
    679 <ul>
    680 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    681 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
    682 time zone history atlases
    683 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
    684 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
    685 Although these extensive atlases
    686 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
    687 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
    688 they are unreliable as Shanks appears to have
    689 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    690 sources and do not indicate which entries are guesswork.</li>
    691 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    692 its own <code>tztab</code>(4) format.</li>
    693 <li>Microsoft Windows has proprietary data mentioned
    694 <a href="#System.TimeZoneInfo">above</a>.</li>
    695 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    696 is another time zone database.</li>
    697 <li>The <a
    698 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
    699 Schedules Information Manual</a> of the
    700 International Air Transport Association
    701 gives current time zone rules for airports served by commercial aviation.</li>
    702 </ul>
    703 </section>
    704 
    705 <section>
    706 <h2 id="maps">Maps</h2>
    707 <ul>
    708 <li>The <a
    709 href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
    710 and Regional Maps section</a> of <em>The World Factbook</em>, published by the
    711 <a href="https://www.cia.gov">US Central Intelligence
    712 Agency (<abbr
    713 title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
    714 zone map; the
    715 <a
    716 href="https://legacy.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
    717 Library Map Collection</a>
    718 of the University of Texas at Austin has copies of
    719 recent editions.
    720 The pictorial quality is good,
    721 but the maps do not indicate daylight saving time,
    722 and parts of the data are a few years out of date.</li>
    723 <li><a href="https://www.worldtimezone.com">World Time Zone Map
    724 with current time</a>
    725 has several fancy time zone maps; it covers Russia particularly well.
    726 The maps' pictorial quality is not quite as good as the
    727 <abbr>CIA</abbr>'s
    728 but the maps are more up to date.</li>
    729 <li><a
    730 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    731 much is time wrong around the world?</a> maps the difference between
    732 mean solar and standard time, highlighting areas such as western China
    733 where the two differ greatly. It's a bit out of date, unfortunately.</li>
    734 </ul>
    735 </section>
    736 
    737 <section>
    738 <h2 id="boundaries">Time zone boundaries</h2>
    739 <p>Geographical boundaries between timezones are available
    740 from several <a
    741 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
    742 geolocation</a>
    743 services and other sources.</p>
    744 <ul>
    745 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    746 Boundary Builder</a> extracts
    747 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    748 boundaries of <code><abbr>tzdb</abbr></code> timezones.
    749 Its code is freely available under the <abbr>MIT</abbr> license, and
    750 its data entries are freely available under the
    751 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    752 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
    753 <li>Programmatic interfaces that map geographical coordinates via tz_world to
    754 <code><abbr>tzdb</abbr></code> timezones include:
    755 <ul>
    756 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
    757 written in <a
    758 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
    759 and is freely available under the <abbr>MIT</abbr> license.</li>
    760 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
    761 is written in Go and is freely available under the Apache License.</li>
    762 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
    763 in both Java and
    764 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
    765 form, is freely available under the MIT license.</li>
    766 <li>For Node.js,
    767 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
    768 is freely available under the MIT license, and
    769 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
    770 is in the public domain.</li>
    771 <li>The <a
    772 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
    773 library for Python is freely available under the MIT license.
    774 <li>The <a
    775 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
    776 library for Ruby is freely available under the MIT license.</li>
    777 </ul></li>
    778 <li>Free access via a network API, if you register a key, is provided by
    779 the <a
    780 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
    781 Timezone web service</a>, the <a
    782 href="https://developers.google.com/maps/documentation/timezone/intro">Google
    783 Maps Time Zone API</a>, and
    784 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
    785 Commercial network API access is provided
    786 by <a href="https://askgeo.com">AskGeo</a>
    787 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    788 </li>
    789 <li>"<a
    790 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
    791 to get a time zone from a location using latitude and longitude
    792 coordinates?</a>" discusses other geolocation possibilities.</li>
    793 <li><a href="http://statoids.com/statoids.html">Administrative
    794 Divisions of Countries ("Statoids")</a> lists
    795 political subdivision data related to time zones.</li>
    796 <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
    797 zone boundaries for multizone countries</a> summarizes legal
    798 boundaries between time zones within countries.</li>
    799 <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
    800 &ndash; GIS and Database Tools</a> includes a Manifold-format map of
    801 world time zone boundaries distributed under the
    802 <abbr>GPL</abbr>.</li>
    803 <li>A ship within the <a
    804 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    805 waters</a> of any nation uses that nation's time. In international
    806 waters, time zone boundaries are meridians 15&deg; apart, except that
    807 <abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
    808 wide and are separated by
    809 the 180&deg; meridian (not by the International Date Line, which is
    810 for land and territorial waters only). A captain can change ship's
    811 clocks any time after entering a new time zone; midnight changes are
    812 common.</li>
    813 </ul>
    814 </section>
    815 
    816 <section>
    817 <h2 id="civil">Civil time concepts and history</h2>
    818 <ul>
    819 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    820 Walk through Time</a>
    821 surveys the evolution of timekeeping.</li>
    822 <li>The history of daylight saving time is surveyed in <a
    823 href="http://www.webexhibits.org/daylightsaving/">About Daylight
    824 Saving Time &ndash; History, rationale, laws &amp; dates</a> and summarized in
    825 <a href="http://seizethedaylight.com/dst/">A Brief
    826 History of Daylight Saving Time</a>.</li>
    827 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
    828 Lords</a> discusses how authoritarians manipulate civil time.</li>
    829 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
    830 contains guidelines and best practices for software applications that
    831 deal with civil time.</li>
    832 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
    833 the International Date Line</a> tells the story of the most important
    834 time zone boundary.</li>
    835 <li><a href="http://statoids.com/tconcept.html">Basic Time
    836 Zone Concepts</a> discusses terminological issues behind time zones.</li>
    837 </ul>
    838 </section>
    839 
    840 <section>
    841 <h2 id="national">National histories of legal time</h2>
    842 <dl>
    843 <dt>Australia</dt>
    844 <dd>The Parliamentary Library commissioned a <a
    845 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
    846 paper on daylight saving time in Australia</a>.
    847 The Bureau of Meteorology publishes a list of <a
    848 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    849 Dates of Daylight Savings Time within Australia</a>.</dd>
    850 <dt>Belgium</dt>
    851 <dd>The Royal Observatory of Belgium maintains a table of time in
    852 Belgium (in
    853 <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
    854 hreflang="nl">Dutch</a> and <a
    855 href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
    856 hreflang="fr">French</a>).</dd>
    857 <dt>Brazil</dt>
    858 <dd>The Time Service Department of the National Observatory
    859 records <a href="http://pcdsh01.on.br/DecHV.html"
    860 hreflang="pt-BR">Brazil's daylight saving time decrees (in
    861 Portuguese)</a>.</dd>
    862 <dt>Canada</dt>
    863 <dd>National Research Council Canada publishes current
    864 and some older information about <a
    865 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
    866 zones and daylight saving time</a>.</dd>
    867 <dt>Chile</dt>
    868 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    869 <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
    870 Chile's official time (in Spanish)</a>.</dd>
    871 <dt>China</dt>
    872 <dd>The Hong Kong Observatory maintains a
    873 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
    874  summer time in Hong Kong</a>,
    875 and Macau's Meteorological and Geophysical Bureau maintains a <a
    876 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
    877 history for Macau</a>.
    878 Unfortunately the latter is incomplete and has errors.</dd>
    879 <dt>Czech Republic</dt>
    880 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    881 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    882 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
    883 <dt>Germany</dt>
    884 <dd>The National Institute for Science and Technology maintains the <a
    885 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    886 of Legal Time in Germany</a>.</dd>
    887 <dt>Israel</dt>
    888 <dd>The Interior Ministry periodically issues <a
    889 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
    890 hreflang="he">announcements (in Hebrew)</a>.</dd>
    891 <dt>Malaysia</dt>
    892 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    893 <dt>Mexico</dt>
    894 <dd>The Investigation and Analysis Service of the Mexican Library of
    895 Congress has published a <a
    896 href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    897 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    898 <dt>Netherlands</dt>
    899 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    900 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    901 covers the history of local time in the Netherlands from ancient times.</dd>
    902 <dt>New Zealand</dt>
    903 <dd>The Department of Internal Affairs maintains a brief <a
    904 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
    905 Daylight Saving</a>.</dd>
    906 <dt>Portugal</dt>
    907 <dd>The Lisbon Astronomical Observatory publishes a
    908 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
    909 legal time (in Portuguese)</a>.</dd>
    910 <dt>Singapore</dt>
    911 <dd><a id="Singapore"
    912 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
    913 is Singapore in the "Wrong" Time Zone?</a> details the
    914 history of legal time in Singapore and Malaysia.</dd>
    915 <dt>United Kingdom</dt>
    916 <dd><a
    917 href="https://www.polyomino.org.uk/british-time/">History of
    918 legal time in Britain</a> discusses in detail the country
    919 with perhaps the best-documented history of clock adjustments.</dd>
    920 <dt>United States</dt>
    921 <dd>The Department of Transportation's <a
    922 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    923 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
    924 <dt>Uruguay</dt>
    925 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    926 Navy (SOHMA) publishes an annual <a
    927 href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
    928 (in Spanish)</a>.</dd>
    929 </dl>
    930 </section>
    931 
    932 <section>
    933 <h2 id="costs">Costs and benefits of time shifts</h2>
    934 <p>Various sources argue for and against daylight saving time and time
    935 zone shifts, and many scientific studies have been conducted. This
    936 section summarizes reviews and position statements based on
    937 scientific literature in the area.</p>
    938 <ul>
    939 <li>Carey RN, Sarma KM.
    940 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
    941 daylight saving time on road traffic collision risk: a systematic
    942 review</a>.
    943 <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>.
    944 This reviews research literature and concludes that the evidence
    945 neither supports nor refutes road safety benefits from
    946 shifts in time zones.</li>
    947 <li>Havranek T, Herman D, Irsova D.
    948 <a href="https://www.iaee.org/en/publications/ejarticle.aspx?id=3051">Does
    949 daylight saving save electricity? A meta-analysis.</a>
    950 <em>Energy J.</em> 2018;39(2):35&ndash;61.
    951 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
    952 This analyzes research literature and concludes, "Electricity savings
    953 are larger for countries farther away from the equator, while
    954 subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
    955 <li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
    956 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
    957 an American Academy of Sleep Medicine position statement</a>.
    958 <em>J Clin Sleep Med.</em>
    959 2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
    960 This argues for permanent standard time due to health risks of both
    961 <abbr>DST</abbr> transitions and permanent <abbr>DST</abbr>.</li>
    962 <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
    963 <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7205184/">Why
    964 should we abolish Daylight Saving Time?</a>
    965 <em>J Biol Rhythms</em>. 2019;34(3):227&ndash;230.
    966 doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
    967 This position paper of the Society for Research on Biological Rhythms
    968 opposes DST changes and permanent DST, and advocates that governments adopt
    969 "permanent Standard Time for the health and safety of their citizens".</li>
    970 </ul>
    971 </section>
    972 
    973 <section>
    974 <h2 id="precision">Precision timekeeping</h2>
    975 <ul>
    976 <li><a
    977 href="http://leapsecond.com/hpan/an1289.pdf">The
    978 Science of Timekeeping</a> is a thorough introduction
    979 to the theory and practice of precision timekeeping.</li>
    980 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
    981 Time 2016</a> contains several freely-readable papers.</li>
    982 <li><a href="https://www.ntp.org"><abbr
    983 title="Network Time Protocol">NTP</abbr>: The Network
    984 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
    985 discusses how to synchronize clocks of
    986 Internet hosts.</li>
    987 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
    988 family of software algorithms can achieve accuracy to a few tens of
    989 nanoseconds in scalable server farms without special hardware.</li>
    990 <li>The <a
    991 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
    992 Time Protocol</a> (<abbr
    993 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
    994 can achieve submicrosecond clock accuracy on a local area network
    995 with special-purpose hardware.</li>
    996 <li><a
    997 href="https://datatracker.ietf.org/doc/html/rfc4833">Timezone
    998 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
    999 (Internet <abbr>RFC</abbr> 4833)
   1000 specifies a <a
   1001 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
   1002 option for a server to configure
   1003 a client's time zone and daylight saving settings automatically.</li>
   1004 <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
   1005 Scales</a> describes astronomical time scales like
   1006 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
   1007 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
   1008 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
   1009 <li>The <a href="https://www.iau.org"><abbr
   1010 title="International Astronomical Union">IAU</abbr></a>'s <a
   1011 href="https://www.iausofa.org"><abbr
   1012 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
   1013 collection contains C and <a
   1014 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
   1015 code for converting among time scales like
   1016 <abbr title="International Atomic Time">TAI</abbr>,
   1017 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
   1018 <abbr>UTC</abbr>. It is freely available under the
   1019 <a href="https://www.iausofa.org/tandc.html">SOFA license</a>.</li>
   1020 <li><a
   1021 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
   1022 &ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
   1023 diverse local time
   1024 scales used by each landed mission on Mars.</li>
   1025 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
   1026 dedicated not only to leap seconds but to precise time and frequency
   1027 in general. It covers the state of the art in amateur timekeeping, and
   1028 how the art has progressed over the past few decades.</li>
   1029 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
   1030 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
   1031 and time-signal emissions</a>, International Telecommunication Union &ndash;
   1032 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
   1033 <li><a
   1034 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
   1035 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
   1036 Bulletins</a> contains official publications of the International
   1037 Earth Rotation and Reference Systems Service, which decides when leap
   1038 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
   1039 via an optional "<code>right</code>" configuration where a computer's internal
   1040 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
   1041 as opposed to the default "<code>posix</code>" configuration
   1042 where the internal clock ignores leap seconds.
   1043 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
   1044 <abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge for
   1045 timestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,
   1046 which corresponds to the first leap second
   1047 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
   1048 and to
   1049 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
   1050 In practice the two configurations also agree for timestamps before
   1051 1972 even though the historical situation is messy, partly because
   1052 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
   1053 is well-defined for sufficiently-old timestamps.</li>
   1054 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
   1055 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
   1056 leap second so that they disagree with <abbr>UTC</abbr> by at most a
   1057 half second, even though every <abbr>POSIX</abbr> minute has exactly
   1058 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
   1059 "<code>posix</code>" configuration, is <a
   1060 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
   1061 the <abbr>NTP</abbr> reference implementation, <a
   1062 href="https://github.com/google/unsmear">supports</a> conversion between
   1063 <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
   1064 cloud service providers. However, according to
   1065 <a href="https://datatracker.ietf.org/doc/html/rfc8633#section-3.7.1">&sect;3.7.1 of
   1066 Network Time Protocol Best Current Practices</a>
   1067 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
   1068 applications requiring accurate <abbr>UTC</abbr> or civil time,
   1069 and is intended for use only in single, well-controlled environments.</li>
   1070 <li>The <a
   1071 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
   1072 Second Discussion List</a> covers <a
   1073 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
   1074 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
   1075 discussed further in
   1076 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
   1077 leap second: its history and possible future</a>.
   1078 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
   1079 might be redefined
   1080 without Leap Seconds</a> gives pointers on this
   1081 contentious issue, which was active until 2015 and could become active
   1082 again.</li>
   1083 </ul>
   1084 </section>
   1085 
   1086 <section>
   1087 <h2 id="notation">Time notation</h2>
   1088 <ul>
   1089 <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
   1090 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
   1091 zone names, abbreviations, identifiers, and formats. For example, it
   1092 contains French translations for "Eastern European Summer Time",
   1093 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
   1094 "Bucharest". Its
   1095 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
   1096 charts</a> show these values for many locales. Data values are available in
   1097 both <abbr title="Locale Data Markup Language">LDML</abbr>
   1098 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
   1099 <li>
   1100 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
   1101 the international standard date and time notation</a> covers
   1102 <a
   1103 href="https://www.iso.org/standard/70907.html"><em><abbr
   1104 title="International Organization for Standardization">ISO</abbr>
   1105 8601-1:2019 &ndash; Date and time &ndash; Representations for information
   1106 interchange &ndash; Part 1: Basic rules</em></a>.</li>
   1107 <li>
   1108 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
   1109 Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
   1110 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
   1111 <li><a href="https://datatracker.ietf.org/doc/html/rfc5322#section-3.3">&sect;3.3 of
   1112 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
   1113 specifies the time notation used in email and <a
   1114 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
   1115 headers.</li>
   1116 <li>
   1117 <a href="https://datatracker.ietf.org/doc/html/rfc3339">Date and Time
   1118 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
   1119 specifies an <abbr>ISO</abbr> 8601
   1120 profile for use in new Internet
   1121 protocols.</li>
   1122 <li>
   1123 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
   1124 Formats on the Web</a> surveys web- and Internet-oriented date and time
   1125 formats.</li>
   1126 <li>Alphabetic time zone abbreviations should not be used as unique
   1127 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
   1128 practice. For example, in English-speaking North America
   1129 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
   1130 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
   1131 and French-speaking North Americans prefer
   1132 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
   1133 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
   1134 database contains English abbreviations for many timestamps;
   1135 unfortunately some of these abbreviations were merely the database maintainers'
   1136 inventions, and these have been removed when possible.</li>
   1137 <li>Numeric time zone abbreviations typically count hours east of
   1138 <abbr>UT</abbr>, e.g., +09 for Japan and
   1139 &minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
   1140 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
   1141 For example, one might use <code><abbr>TZ</abbr>="<abbr
   1142 title="Japan Standard Time">JST</abbr>-9"</code> and
   1143 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
   1144 for Japan and Hawaii, respectively. If the
   1145 <code><abbr>tz</abbr></code> database is available, it is usually better to use
   1146 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
   1147 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
   1148 confusion, handle old timestamps better, and insulate you better from
   1149 any future changes to the rules. One should never set
   1150 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
   1151 <code>"GMT-9"</code>, though, since this would incorrectly imply that
   1152 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
   1153 is called "<abbr>GMT</abbr>".</li>
   1154 </ul>
   1155 </section>
   1156 
   1157 <section>
   1158 <h2 id="see-also">See also</h2>
   1159 <ul>
   1160 <li><a href="theory.html">Theory and pragmatics of the
   1161 <code><abbr>tz</abbr></code> code and data</a></li>
   1162 <li><a href="tz-art.html">Time and the Arts</a></li>
   1163 </ul>
   1164 </section>
   1165 
   1166 <footer>
   1167 <hr>
   1168 This web page is in the public domain, so clarified as of
   1169 2009-05-17 by Arthur David Olson.
   1170 <br>
   1171 Please send corrections to this web page to the
   1172 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
   1173 </footer>
   1174 </body>
   1175 </html>
   1176