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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="#precision">Precision timekeeping</a></li>
     53 	  <li><a href="#notation">Time notation</a></li>
     54 	  <li><a href="#see-also">See also</a></li>
     55 	</ul>
     56       </li>
     57     </ul>
     58   </nav>
     59 
     60 <section>
     61 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
     62 <p>
     63 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
     64 time zone database contains code and data
     65 that represent the history of local time
     66 for many representative locations around the globe.
     67 It is updated periodically to reflect changes made by political bodies
     68 to time zone boundaries and daylight saving rules.
     69 This database (known as <code><abbr>tz</abbr></code>,
     70 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
     71 is used by several implementations,
     72 including
     73 <a href="https://www.gnu.org/software/libc/">the
     74 <abbr title="GNU's Not Unix">GNU</abbr>
     75 C Library</a> (used in
     76 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
     77 <a href="https://www.android.com">Android</a>,
     78 <a href="https://www.freebsd.org">Free<abbr
     79 title="Berkeley Software Distribution">BSD</abbr></a>,
     80 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
     81 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
     82 <a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
     83 <a href="https://cygwin.com">Cygwin</a>,
     84 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
     85 <a href="https://www.mysql.com">MySQL</a>,
     86 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
     87 title="Web Operating System">webOS</abbr></a>,
     88 <a href="https://ibm.com/aix"><abbr
     89 title="Advanced Interactive eXecutive">AIX</abbr></a>,
     90 <a href="https://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</a>,
     91 <a href="https://www.apple.com/ios"><abbr
     92 title="iPhone OS">iOS</abbr></a>,
     93 <a href="https://www.apple.com/macos">macOS</a>,
     94 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
     95 <a href="https://www.vmssoftware.com">Open<abbr
     96 title="Virtual Memory System">VMS</abbr></a>,
     97 <a href="https://www.oracle.com/database/">Oracle Database</a>, and
     98 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
     99 <p>
    100 Each main entry in the database represents a <dfn>timezone</dfn>
    101 for a set of civil-time clocks that have all agreed since 1970.
    102 Timezones are typically identified by continent or ocean and then by the
    103 name of the largest city within the region containing the clocks.
    104 For example, <code>America/New_York</code>
    105 represents most of the <abbr title="United States">US</abbr> eastern time zone;
    106 <code>America/Phoenix</code> represents most of Arizona, which
    107 uses mountain time without daylight saving time (<abbr>DST</abbr>);
    108 <code>America/Detroit</code> represents most of Michigan, which uses
    109 eastern time but with different <abbr>DST</abbr> rules in 1975;
    110 and other entries represent smaller regions like Starke County,
    111 Indiana, which switched from central to eastern time in 1991
    112 and switched back in 2006.
    113 To use the database on an extended <a
    114 href="https://en.wikipedia.org/wiki/POSIX"><abbr
    115 title="Portable Operating System Interface">POSIX</abbr></a>
    116 implementation set the <code><abbr>TZ</abbr></code>
    117 environment variable to the location's full name,
    118 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
    119 <p>
    120 Associated with each timezone is a history of offsets from
    121 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
    122 Time</a> (<abbr>UT</abbr>), which is <a
    123 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
    124 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
    125 for timestamps after 1960 this is more precisely <a
    126 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
    127 Universal Time</a> (<abbr>UTC</abbr>).
    128 The database also records when daylight saving time was in use,
    129 along with some time zone abbreviations such as <abbr>EST</abbr>
    130 for Eastern Standard Time in the <abbr>US</abbr>.</p>
    131 </section>
    132 
    133 <section>
    134 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
    135 <p>
    136 The following <a
    137 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
    138 the latest release's two
    139 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
    140 to a <abbr>GNU</abbr>/Linux or similar host.</p>
    141 <pre><code>mkdir tzdb
    142 cd tzdb
    143 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
    144 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
    145 <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 -
    146 gzip -dc tzdata-latest.tar.gz | tar -xf -
    147 </code></pre>
    148 <p>Alternatively, the following shell commands download the same
    149 release in a single-tarball format containing extra data
    150 useful for regression testing:</p>
    151 <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>
    152 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
    153 </code></pre>
    154 <p>These commands use convenience links to the latest release
    155 of the <code><abbr>tz</abbr></code> database hosted by the
    156 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
    157 of the <a href="https://www.iana.org">Internet Assigned Numbers
    158 Authority (IANA)</a>.
    159 Older releases are in files named
    160 <code>tzcode<var>V</var>.tar.gz</code>,
    161 <code>tzdata<var>V</var>.tar.gz</code>, and
    162 <code>tzdb-<var>V</var>.tar.lz</code>,
    163 where <code><var>V</var></code> is the version.
    164 Since 1996, each version has been a four-digit year followed by
    165 lower-case letter (<samp>a</samp> through <samp>z</samp>,
    166 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
    167 through <samp>zzz</samp>, and so on).
    168 Since version 2016h, each release has contained a text file named
    169 "<samp>version</samp>" whose first (and currently only) line is the version.
    170 The releases are also available in an
    171 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
    172 title="File Transfer Protocol">FTP</abbr> directory</a> via a
    173 less-secure protocol.</p>
    174 <p>Alternatively, a development repository of code and data can be
    175 retrieved from <a href="https://github.com">GitHub</a> via the shell
    176 command:</p>
    177 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
    178 </code></pre>
    179 <p>
    180 Since version 2012e, each release has been tagged in development repositories.
    181 Untagged commits are less well tested and probably contain
    182 more errors.</p>
    183 <p>
    184 After obtaining the code and data files, see the
    185 <code>README</code> file for what to do next.
    186 The code lets you compile the <code><abbr>tz</abbr></code> source files into
    187 machine-readable binary files, one for each location. The binary files
    188 are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
    189 specified by <a href="https://tools.ietf.org/html/8536">Internet
    190 <abbr>RFC</abbr> 8536</a>.
    191 The code also lets
    192 you read a <abbr>TZif</abbr> file and interpret timestamps for that
    193 location.</p>
    194 </section>
    195 
    196 <section>
    197 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    198 <p>
    199 The <code><abbr>tz</abbr></code> code and data
    200 are by no means authoritative. If you find errors, please
    201 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    202 the time zone mailing list. You can also <a
    203 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
    204 and browse the <a
    205 href="https://mm.icann.org/pipermail/tz/">archive of old
    206 messages</a>.
    207 <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
    208 discussions</a> and corresponding data changes can be
    209 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
    210 </p>
    211 <p>
    212 If your government plans to change its time zone boundaries or
    213 daylight saving rules, inform <code>tz (a] iana.org</code> well in
    214 advance, as this will coordinate updates to many cell phones,
    215 computers, and other devices around the world. With
    216 less than a year's notice there is a good chance that some
    217 computer-based clocks will operate incorrectly after the change, due
    218 to delays in propagating updates to software and data. The shorter
    219 the notice, the more likely clock problems will arise; see "<a
    220 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
    221 the Timing of Time Zone Changes</a>" for examples.
    222 </p>
    223 <p>
    224 Changes to the <code><abbr>tz</abbr></code> code and data are often
    225 propagated to clients via operating system updates, so
    226 client <code><abbr>tz</abbr></code> data can often be corrected by
    227 applying these updates. With GNU/Linux and similar systems, if your
    228 maintenance provider has not yet adopted the
    229 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
    230 the process by tailoring the generic instructions in
    231 the <code><abbr>tz</abbr> README</code> file and installing the latest
    232 data yourself. System-specific instructions for installing the
    233 latest <code><abbr>tz</abbr></code> data have also been published
    234 for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
    235 <a
    236 href="https://play.google.com/store/apps/details?id=com.google.android.timezone.data">Android</a>,
    237 <a
    238 href="http://userguide.icu-project.org/datetime/timezone"><abbr
    239 title="International Components for Unicode">ICU</abbr></a>,
    240 <a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr></a>
    241 and <a
    242 href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a>
    243 Java, <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    244 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    245 and <a
    246 href="https://nodatime.org/userguide/tzdb">Noda Time</a> (see below).
    247 </p>
    248 <p>Sources for the <code><abbr>tz</abbr></code> database are
    249 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
    250 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
    251 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
    252 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
    253 title="linefeed">LF</abbr></a>,
    254 which can be modified by common text editors such
    255 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
    256 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
    257 <a href="https://www.vim.org">vim</a>.
    258 Specialized source-file editing can be done via the
    259 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
    260 zoneinfo</a> package for <a
    261 href="https://www.sublimetext.com">Sublime Text</a> and the <a
    262 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
    263 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
    264 Studio Code</a>.
    265 </p>
    266 <p>
    267 For further information about updates, please see
    268 <a href="https://tools.ietf.org/html/rfc6557">Procedures for
    269 Maintaining the Time Zone Database</a> (Internet <abbr
    270 title="Request For Comments">RFC</abbr> 6557). More detail can be
    271 found in <a href="theory.html">Theory and pragmatics of the tz code and data</a>.
    272 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
    273 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
    274 </p>
    275 </section>
    276 
    277 <section>
    278 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
    279 <ul>
    280 <li>The article
    281 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
    282 an encyclopedic summary.</li>
    283 <li><a href="tz-how-to.html">How to Read the
    284 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
    285 database format.</li>
    286 <li><a
    287 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
    288 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
    289 database's style.</li>
    290 </ul>
    291 </section>
    292 
    293 <section>
    294 <h2 id="web">Web sites using recent versions of the
    295 <code><abbr>tz</abbr></code> database</h2>
    296 <p>
    297 These are listed roughly in ascending order of complexity and fanciness.
    298 </p>
    299 <ul>
    300 <li><a href="https://time.is">Time.is</a> shows locations'
    301 time and zones.</li>
    302 <li><a href="https://www.timejones.com">TimeJones.com</a>,
    303 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
    304 <a href="https://worldclock.com">The World Clock</a>
    305 are time zone converters.</li>
    306 <li><a
    307 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
    308 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
    309 <li><a
    310 href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
    311 Time in 1000 Places</a> uses descriptions of the values.</li>
    312 <li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
    313 uses a pulldown menu.</li>
    314 <li><a href="http://home.kpn.nl/vanadovv/time/TZworld.html">Complete
    315 timezone information for all countries</a> displays tables of DST rules.
    316 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
    317 Worldwide</a> lets you sort zone names and convert times.</li>
    318 <li><a href="https://24timezones.com">24TimeZones</a> has a world
    319 time map and a time converter.</li>
    320 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    321 calculates the current time difference between locations.</li>
    322 <li><a href="http://www.wx-now.com">Weather Now</a> and
    323 <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    324 </ul>
    325 </section>
    326 
    327 <section>
    328 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    329 <ul>
    330 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
    331 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
    332 Distribution Service (tzdist) working group</a> defined <a
    333 href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
    334 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
    335 along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
    336 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    337 transferring time zone data by reference.
    338 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
    339 mailing list</a> discussed the Internet draft <a
    340 id="TZDIST-Geolocate"
    341 href="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate">TZDIST
    342 Geolocate Extension</a> (now expired), which let a client determine its timezone
    343 from its geographic location using a <a
    344 href="https://tools.ietf.org/html/rfc5870">'geo' URI</a>.</li>
    345 <li>The <a href="https://tools.ietf.org/html/rfc5545">
    346 Internet Calendaring and Scheduling Core Object Specification
    347 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
    348 covers time zone
    349 data; see its VTIMEZONE calendar component.
    350 The iCalendar format requires specialized parsers and generators; a
    351 variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
    352 (Internet <abbr>RFC</abbr> 6321) uses
    353 <a href="https://www.w3.org/XML/"><abbr
    354 title="Extensible Markup Language">XML</abbr></a> format, and a variant
    355 <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
    356 (Internet <abbr>RFC</abbr> 7265)
    357 uses <a href="https://www.json.org"><abbr
    358 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    359 </ul>
    360 </section>
    361 
    362 <section>
    363 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    364 <ul>
    365 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
    366 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
    367 program that compiles
    368 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    369 Vzic is freely
    370 available under the <a
    371 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    372 General Public License (<abbr
    373 title="General Public License">GPL</abbr>)</a>.</li>
    374 <li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
    375 database conversion utility</a> is like Vzic, except for the <a
    376 href="https://dotnet.microsoft.com">.NET framework</a>
    377 and with a <abbr>BSD</abbr>-style license.</li>
    378 <li><a
    379 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
    380 contains a script <code>parse_olson</code> that compiles
    381 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
    382 modules. It is part of the Perl <a
    383 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
    384 which is freely
    385 available under both the <abbr>GPL</abbr> and the Perl Artistic
    386 License. DateTime::TimeZone also contains a script
    387 <code>tests_from_zdump</code> that generates test cases for each clock
    388 transition in the <code><abbr>tz</abbr></code> database.</li>
    389 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
    390 Database Parser</a> is a
    391 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
    392 runtime library that is <a
    393 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0355r4.html">moving
    394 forward</a> for inclusion in
    395 <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
    396 the next iteration of the C++ standard.
    397 It is freely available under the
    398 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
    399 <li><a id="ICU" href="http://site.icu-project.org">International Components for
    400 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
    401 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
    402 libraries for internationalization that
    403 has a compiler from <code><abbr>tz</abbr></code> source
    404 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
    405 (mentioned <a href="#CLDR">below</a>)
    406 into an <abbr>ICU</abbr>-specific format.
    407 <abbr>ICU</abbr> is freely available under a
    408 <abbr>BSD</abbr>-style license.</li>
    409 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
    410 the <a href="https://elixir-lang.org">Elixir</a> language downloads
    411 and compiles tz source and exposes <abbr
    412 title="Application Program Interface">API</abbr>s for use. It is
    413 freely available under the <abbr>MIT</abbr> license.</li>
    414 <li>Java-based compilers and libraries include:
    415 <ul>
    416 <li>The <a
    417 href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
    418 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
    419 Oracle Java.</li>
    420 <li>The <a
    421 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
    422 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
    423 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
    424 which is freely available under a <abbr>BSD</abbr>-style license.</li>
    425 <li><a href="https://www.joda.org/joda-time/">Joda-Time &ndash; Java date
    426 and time <abbr>API</abbr></a> contains a class
    427 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
    428 <code><abbr>tz</abbr></code> source into a binary format. It inspired
    429 Java 8 <code>java.time</code>, which its users should migrate to once
    430 they can assume Java 8 or later. It is available under the <a
    431 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
    432 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
    433 time library for Android</a> and
    434 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
    435 time and interval library for Java</a> compile
    436 <code><abbr>tz</abbr></code> source into a binary format.
    437 Time4A is available under the Apache License and Time4J is
    438 available under the <a
    439 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
    440 General Public License (<abbr title="Lesser General Public
    441 License">LGPL</abbr>)</a>.</li>
    442 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
    443 Java-based libraries.</li>
    444 </ul>
    445 <li><a href="https://nodatime.org">Noda Time &ndash; Date and
    446 time <abbr>API</abbr> for .NET</a>
    447 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
    448 It is freely available under the Apache License.</li>
    449 <li><a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>-based
    450 compilers and libraries include:
    451 <ul>
    452 <li><a
    453 href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
    454 compiles time zone data into a compact form designed for
    455 JavaScript. It is freely available under a combination of
    456 the <abbr>MIT</abbr> license and the Apache License.</li>
    457 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    458 plugin for the <a href="https://momentjs.com">Moment.js</a> date
    459 manipulation library. It is freely available under the <abbr>MIT</abbr>
    460 license.</li>
    461 <li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
    462 <abbr>API</abbr> is upward compatible with standard JavaScript
    463 Dates. It is freely available under the Apache License.</li>
    464 </ul>
    465 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
    466 compiler from <code><abbr>tz</abbr></code> source into
    467 <a href="https://julialang.org/">Julia</a>. It is freely available
    468 under the <abbr>MIT</abbr> license.</li>
    469 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> &ndash;
    470 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
    471 title="Free Pascal Compiler">FPC</abbr></a>
    472 compiles from <code><abbr>tz</abbr></code> source into
    473 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
    474 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
    475 and <a
    476 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
    477 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    478 <li><a href="http://pytz.sourceforge.net">pytz &ndash; World Timezone
    479 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    480 <a href="https://www.python.org">Python</a>.
    481 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    482 <li><a href="https://tzinfo.github.io">TZInfo &ndash;
    483 Ruby Timezone Library</a>
    484 compiles <code><abbr>tz</abbr></code> source into
    485 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    486 It is freely available under the <abbr>MIT</abbr> license.</li>
    487 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
    488 Library</a> is
    489 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    490 library that compiles <code><abbr>tz</abbr></code> source into a time
    491 zone repository whose format
    492 is either proprietary or an <abbr>XML</abbr>-encoded
    493 representation.</li>
    494 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
    495 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    496 source into text files, along with a runtime that can read those
    497 files. Tcl is freely available under a <abbr>BSD</abbr>-style
    498 license.</li>
    499 </ul>
    500 </section>
    501 
    502 <section>
    503 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
    504 <ul>
    505 <li>The <a
    506 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
    507 Library</a>
    508 has an independent, thread-safe implementation of
    509 a <abbr>TZif</abbr> file reader.
    510 This library is freely available under the LGPL
    511 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    512 <li><a href="https://www.gnome.org">GNOME</a>'s
    513 <a href="https://developer.gnome.org/glib/">GLib</a> has
    514 a <abbr>TZif</abbr> file reader written in C that
    515 creates a <code>GTimeZone</code> object representing sets
    516 of <abbr>UT</abbr> offsets.
    517 It is freely available under the <abbr>LGPL</abbr>.</li>
    518 <li>The
    519 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
    520 <code>baltzo::TimeZoneUtil</code> component contains a C++
    521 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
    522 the Apache License.</li>
    523 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    524 library that translates between <abbr>UT</abbr> and civil time and
    525 can read <abbr>TZif</abbr> files. It is freely available under the Apache
    526 License.</li>
    527 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
    528 is a <abbr>TZif</abbr> file reader written in Java.
    529 It is freely available under the <abbr>LGPL</abbr>.</li>
    530 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    531 library that reads <abbr>TZif</abbr> files and converts
    532 timestamps from one time zone or format to another.
    533 It is used by <a href="https://secure.php.net"><abbr
    534 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    535 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    536 and <a href="https://www.mongodb.com">MongoDB</a>.
    537 It is freely available under the <abbr>MIT</abbr> license.</li>
    538 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    539 JavaScript library that supports date arithmetic that is time zone
    540 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    541 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    542 <abbr>TZif</abbr> file reader.</li>
    543 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
    544 DateTime::TimeZone::Tzfile</a>
    545 is a <abbr>TZif</abbr> file reader written in Perl.
    546 It is freely available under the same terms as Perl
    547 (dual <abbr>GPL</abbr> and Artistic license).</li>
    548 <li>The
    549 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    550 library contains a Python tool that
    551 converts <abbr>TZif</abbr> data into
    552 <abbr>JSON</abbr>-format data suitable for use
    553 in its JavaScript library for time zone conversion. Dates before 1970
    554 are not supported.</li>
    555 <li>The <a
    556 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    557 package contains <a href="https://www.haskell.org">Haskell</a> code that
    558 parses and uses <abbr>TZif</abbr> data. It is freely
    559 available under a <abbr>BSD</abbr>-style license.</li>
    560 </ul>
    561 </section>
    562 
    563 <section>
    564 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    565 <ul>
    566 <li><a href="https://foxclocks.org">FoxClocks</a>
    567 is an extension for <a href="https://www.google.com/chrome/">Google
    568 Chrome</a> and for <a
    569 href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
    570 Toolkit</a> applications like <a
    571 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
    572 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
    573 It displays multiple clocks in the application window, and has a mapping
    574 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
    575 It is freely available under the <abbr>GPL</abbr>.</li>
    576 <li><a href="https://golang.org">Go programming language</a>
    577 implementations contain a copy of a 32-bit subset of a recent
    578 <code><abbr>tz</abbr></code> database in a
    579 Go-specific format.</li>
    580 <li><a
    581 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
    582 clock (intclock)</a> is a clock that displays multiple time zones on
    583 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
    584 under the <abbr>GPL</abbr>.</li>
    585 <li>Microsoft Windows 8.1
    586 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    587 data (mentioned <a href="#CLDR">below</a>) used by the
    588 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    589 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    590 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    591 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
    592 <a id="System.TimeZoneInfo"
    593 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
    594 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    595 the older, proprietary method of Microsoft Windows 2000 and later,
    596 which stores time zone data in the
    597 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    598 <a
    599 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
    600 Tzid table</a> or <a
    601 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    602 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    603 to <code><abbr>tz</abbr></code> names.
    604 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
    605 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
    606 <li><a
    607 href="https://www.oracle.com/java/index.html">Oracle
    608 Java</a> contains a copy of a subset of a recent
    609 <code><abbr>tz</abbr></code> database in a
    610 Java-specific format.</li>
    611 <li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone
    612 Master</a> is a Microsoft Windows clock program that can automatically
    613 download, compile and use <code><abbr>tz</abbr></code> releases.
    614 The Basic version is free.</li>
    615 <li><a
    616 href="http://veladg.com/velaterra.html">VelaTerra</a> is
    617 a macOS program. Its developers
    618 <a href="http://veladg.com/tzoffer.html">offer free
    619 licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
    620 </ul>
    621 </section>
    622 
    623 <section>
    624 <h2 id="other-dbs">Other time zone databases</h2>
    625 <ul>
    626 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    627 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
    628 time zone history atlases
    629 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
    630 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>,
    631 now published in <a
    632 href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a>
    633 form by <a href="https://astrocom.com">ACS-Starcrafts</a>.
    634 Although these extensive atlases
    635 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
    636 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
    637 they are unreliable as Shanks appears to have
    638 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    639 sources and do not indicate which entries are guesswork.</li>
    640 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    641 its own <code>tztab</code>(4) format.</li>
    642 <li>Microsoft Windows has proprietary data mentioned
    643 <a href="#System.TimeZoneInfo">above</a>.</li>
    644 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    645 is another time zone database.</li>
    646 <li>The <a
    647 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
    648 Schedules Information Manual</a> of the
    649 International Air Transport Association
    650 gives current time zone rules for airports served by commercial aviation.</li>
    651 </ul>
    652 </section>
    653 
    654 <section>
    655 <h2 id="maps">Maps</h2>
    656 <ul>
    657 <li>The <a href="https://www.cia.gov/index.html">United States Central
    658 Intelligence Agency (<abbr
    659 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
    660 href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
    661 zone map</a>; the
    662 <a
    663 href="https://legacy.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
    664 Library Map Collection</a>
    665 of the University of Texas at Austin has copies of
    666 recent editions.
    667 The pictorial quality is good,
    668 but the maps do not indicate daylight saving time,
    669 and parts of the data are a few years out of date.</li>
    670 <li><a href="https://www.worldtimezone.com">World Time Zone Map
    671 with current time</a>
    672 has several fancy time zone maps; it covers Russia particularly well.
    673 The maps' pictorial quality is not quite as good as the
    674 <abbr>CIA</abbr>'s
    675 but the maps are more up to date.</li>
    676 <li><a
    677 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    678 much is time wrong around the world?</a> maps the difference between
    679 mean solar and standard time, highlighting areas such as western China
    680 where the two differ greatly. It's a bit out of date, unfortunately.</li>
    681 </ul>
    682 </section>
    683 
    684 <section>
    685 <h2 id="boundaries">Time zone boundaries</h2>
    686 <p>Geographical boundaries between timezones are available
    687 from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a>
    688 services and other sources.</p>
    689 <ul>
    690 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    691 Boundary Builder</a> extracts
    692 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    693 boundaries of <code><abbr>tzdb</abbr></code> timezones.
    694 Its code is freely available under the <abbr>MIT</abbr> license, and
    695 its data entries are freely available under the
    696 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    697 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
    698 <li>Programmatic interfaces that map geographical coordinates via tz_world to
    699 <code><abbr>tzdb</abbr></code> timezones include:
    700 <ul>
    701 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
    702 written in <a
    703 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
    704 and is freely available under the <abbr>MIT</abbr> license.</li>
    705 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
    706 is written in Go and is freely available under the Apache License.</li>
    707 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
    708 in both Java and
    709 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
    710 form, is freely available under the MIT license.</li>
    711 <li>For <a href="https://nodejs.org/en/">Node.js</a>,
    712 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
    713 is freely available under the MIT license, and
    714 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
    715 is in the public domain.</li>
    716 <li>The <a
    717 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
    718 library for Python is freely available under the MIT license.
    719 <li>The <a
    720 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
    721 library for Ruby is freely available under the MIT license.</li>
    722 </ul></li>
    723 <li>Free access via a network API, if you register a key, is provided by
    724 the <a
    725 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
    726 Timezone web service</a>, the <a
    727 href="https://developers.google.com/maps/documentation/timezone/intro">Google
    728 Maps Time Zone API</a>, and
    729 the <a href="https://timezonedb.com">Time Zone Database &amp; API</a>.
    730 Commercial network API access is provided
    731 by <a href="https://askgeo.com">AskGeo</a>
    732 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    733 </li>
    734 <li>"<a
    735 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
    736 to get a time zone from a location using latitude and longitude
    737 coordinates?</a>" discusses other geolocation possibilities.</li>
    738 <li><a href="http://statoids.com/statoids.html">Administrative
    739 Divisions of Countries ("Statoids")</a> lists
    740 political subdivision data related to time zones.</li>
    741 <li><a href="http://home.kpn.nl/vanadovv/time/Multizones.html">Time
    742 zone boundaries for multizone countries</a> summarizes legal
    743 boundaries between time zones within countries.</li>
    744 <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
    745 &ndash; GIS and Database Tools</a> includes a Manifold-format map of
    746 world time zone boundaries distributed under the
    747 <abbr>GPL</abbr>.</li>
    748 <li>A ship within the <a
    749 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    750 waters</a> of any nation uses that nation's time. In international
    751 waters, time zone boundaries are meridians 15&deg; apart, except that
    752 <abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
    753 wide and are separated by
    754 the 180&deg; meridian (not by the International Date Line, which is
    755 for land and territorial waters only). A captain can change ship's
    756 clocks any time after entering a new time zone; midnight changes are
    757 common.</li>
    758 </ul>
    759 </section>
    760 
    761 <section>
    762 <h2 id="civil">Civil time concepts and history</h2>
    763 <ul>
    764 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    765 Walk through Time</a>
    766 surveys the evolution of timekeeping.</li>
    767 <li>The history of daylight saving time is surveyed in <a
    768 href="http://www.webexhibits.org/daylightsaving/">About Daylight
    769 Saving Time &ndash; History, rationale, laws &amp; dates</a> and summarized in
    770 <a href="http://seizethedaylight.com/dst/">A Brief
    771 History of Daylight Saving Time</a>.</li>
    772 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
    773 Lords</a> discusses how authoritarians manipulate civil time.</li>
    774 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
    775 contains guidelines and best practices for software applications that
    776 deal with civil time.</li>
    777 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
    778 the International Date Line</a> tells the story of the most important
    779 time zone boundary.</li>
    780 <li><a href="http://statoids.com/tconcept.html">Basic Time
    781 Zone Concepts</a> discusses terminological issues behind time zones.</li>
    782 </ul>
    783 </section>
    784 
    785 <section>
    786 <h2 id="national">National histories of legal time</h2>
    787 <dl>
    788 <dt>Australia</dt>
    789 <dd>The Parliamentary Library commissioned a <a
    790 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
    791 paper on daylight saving time in Australia</a>.
    792 The Bureau of Meteorology publishes a list of <a
    793 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    794 Dates of Daylight Savings Time within Australia</a>.</dd>
    795 <dt>Belgium</dt>
    796 <dd>The Royal Observatory of Belgium maintains a table of <a
    797 href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
    798 hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
    799 <dt>Brazil</dt>
    800 <dd>The Time Service Department of the National Observatory
    801 records <a href="http://pcdsh01.on.br/DecHV.html"
    802 hreflang="pt-BR">Brazil's daylight saving time decrees (in
    803 Portuguese)</a>.</dd>
    804 <dt>Canada</dt>
    805 <dd>National Research Council Canada publishes current
    806 and some older information about <a
    807 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
    808 zones and daylight saving time</a>.</dd>
    809 <dt>Chile</dt>
    810 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    811 <a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
    812 Chile's official time (in Spanish)</a>.</dd>
    813 <dt>China</dt>
    814 <dd>The Hong Kong Observatory maintains a
    815 <a href="https://www.hko.gov.hk/gts/time/Summertime.htm">history of
    816  summer time in Hong Kong</a>,
    817 and Macau's Meteorological and Geophysical Bureau maintains a <a
    818 href="http://www.smg.gov.mo/smg/geophysics/e_t_Summer%20Time.htm">similar
    819 history for Macau</a>.
    820 Unfortunately the latter is incomplete and has errors.</dd>
    821 <dt>Czech Republic</dt>
    822 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    823 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    824 summarizes and cites historical DST regulations.</dd>
    825 <dt>Germany</dt>
    826 <dd>The National Institute for Science and Technology maintains the <a
    827 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    828 of Legal Time in Germany</a>.</dd>
    829 <dt>Israel</dt>
    830 <dd>The Interior Ministry periodically issues <a
    831 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
    832 hreflang="he">announcements (in Hebrew)</a>.</dd>
    833 <dt>Italy</dt>
    834 <dd>The National Institute of Metrological Research publishes a
    835 <a href="http://oldsite.inrim.it/res/tf/ora_legale_i.shtml" hreflang="it">table
    836 of civil time (in Italian)</a>.</dd>
    837 <dt>Malaysia</dt>
    838 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    839 <dt>Mexico</dt>
    840 <dd>The Investigation and Analysis Service of the Mexican Library of
    841 Congress has published a <a
    842 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    843 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    844 <dt>Netherlands</dt>
    845 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    846 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    847 covers the history of local time in the Netherlands from ancient times.</dd>
    848 <dt>New Zealand</dt>
    849 <dd>The Department of Internal Affairs maintains a brief <a
    850 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
    851 Daylight Saving</a>.</dd>
    852 <dt>Singapore</dt>
    853 <dd><a id="Singapore"
    854 href="http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
    855 is Singapore in the "Wrong" Time Zone?</a> details the
    856 history of legal time in Singapore and Malaysia.</dd>
    857 <dt>United Kingdom</dt>
    858 <dd><a
    859 href="https://www.polyomino.org.uk/british-time/">History of
    860 legal time in Britain</a> discusses in detail the country
    861 with perhaps the best-documented history of clock adjustments.</dd>
    862 <dt>United States</dt>
    863 <dd>The Department of Transportation's <a
    864 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    865 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
    866 <dt>Uruguay</dt>
    867 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    868 Navy (SOHMA) publishes an annual <a
    869 href="http://www.armada.gub.uy/sohma/index.php/servicios/datos-astronomicos" hreflang="es">almanac
    870 (in Spanish)</a>.</dd>
    871 </dl>
    872 </section>
    873 
    874 <section>
    875 <h2 id="precision">Precision timekeeping</h2>
    876 <ul>
    877 <li><a
    878 href="http://leapsecond.com/hpan/an1289.pdf">The
    879 Science of Timekeeping</a> is a thorough introduction
    880 to the theory and practice of precision timekeeping.</li>
    881 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
    882 Time 2016</a> contains several freely-readable papers.</li>
    883 <li><a href="http://www.ntp.org"><abbr
    884 title="Network Time Protocol">NTP</abbr>: The Network
    885 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
    886 discusses how to synchronize clocks of
    887 Internet hosts.</li>
    888 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
    889 family of software algorithms can achieve accuracy to a few tens of
    890 nanoseconds in scalable server farms without special hardware.</li>
    891 <li>The <a
    892 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
    893 Time Protocol</a> (<abbr
    894 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
    895 can achieve submicrosecond clock accuracy on a local area network
    896 with special-purpose hardware.</li>
    897 <li><a
    898 href="https://tools.ietf.org/html/rfc4833">Timezone
    899 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
    900 (Internet <abbr>RFC</abbr> 4833)
    901 specifies a <a
    902 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
    903 option for a server to configure
    904 a client's time zone and daylight saving settings automatically.</li>
    905 <li><a
    906 href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
    907 Times</a> explains more abstruse astronomical time scales like
    908 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
    909 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
    910 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
    911 <a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
    912 Scales</a> goes into more detail, particularly for historical variants.</li>
    913 <li>The <a href="https://www.iau.org"><abbr
    914 title="International Astronomical Union">IAU</abbr></a>'s <a
    915 href="http://www.iausofa.org"><abbr
    916 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
    917 collection contains C and <a
    918 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
    919 code for converting among time scales like
    920 <abbr title="International Atomic Time">TAI</abbr>,
    921 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
    922 <abbr>UTC</abbr>.</li>
    923 <li><a
    924 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
    925 &ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
    926 diverse local time
    927 scales used by each landed mission on Mars.</li>
    928 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
    929 dedicated not only to leap seconds but to precise time and frequency
    930 in general. It covers the state of the art in amateur timekeeping, and
    931 how the art has progressed over the past few decades.</li>
    932 <li><a
    933 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
    934 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
    935 Bulletins</a> contains official publications of the International
    936 Earth Rotation and Reference Systems Service, which decides when leap
    937 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
    938 via an optional "<code>right</code>" configuration, as opposed to the
    939 default "<code>posix</code>" configuration.</li>
    940 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
    941 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
    942 leap second so that they disagree with <abbr>UTC</abbr> by at most a
    943 half second, even though every <abbr>POSIX</abbr> minute has exactly
    944 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
    945 "<code>posix</code>" configuration, is <a
    946 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
    947 the <abbr>NTP</abbr> reference implementation, and is used by major
    948 cloud service providers.</li>
    949 <li>The <a
    950 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
    951 Second Discussion List</a> covers <a
    952 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
    953 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
    954 discussed further in
    955 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
    956 leap second: its history and possible future</a>.
    957 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
    958 might be redefined
    959 without Leap Seconds</a> gives pointers on this
    960 contentious issue, which was active until 2015 and could become active
    961 again.</li>
    962 </ul>
    963 </section>
    964 
    965 <section>
    966 <h2 id="notation">Time notation</h2>
    967 <ul>
    968 <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
    969 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
    970 zone names, abbreviations, identifiers, and formats. For example, it
    971 contains French translations for "Eastern European Summer Time",
    972 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
    973 "Bucharest". Its
    974 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
    975 charts</a> show these values for many locales. Data values are available in
    976 both <abbr title="Locale Data Markup Language">LDML</abbr>
    977 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
    978 <li>
    979 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
    980 the international standard date and time notation</a> covers
    981 <a
    982 href="https://www.iso.org/standard/70907.html"><em><abbr
    983 title="International Organization for Standardization">ISO</abbr>
    984 8601-1:2019 &ndash; Date and time &ndash; Representations for information
    985 interchange &ndash; Part 1: Basic rules</em></a>.</li>
    986 <li>
    987 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
    988 Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
    989 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
    990 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">&sect;3.3 of
    991 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
    992 specifies the time notation used in email and <a
    993 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
    994 headers.</li>
    995 <li>
    996 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
    997 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
    998 specifies an <abbr>ISO</abbr> 8601
    999 profile for use in new Internet
   1000 protocols.</li>
   1001 <li>
   1002 <a href="https://www.hackcraft.net/web/datetime/">Date &amp; Time
   1003 Formats on the Web</a> surveys web- and Internet-oriented date and time
   1004 formats.</li>
   1005 <li>Alphabetic time zone abbreviations should not be used as unique
   1006 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
   1007 practice. For example, in English-speaking North America
   1008 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
   1009 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
   1010 and French-speaking North Americans prefer
   1011 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
   1012 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
   1013 database contains English abbreviations for many timestamps;
   1014 unfortunately some of these abbreviations were merely the database maintainers'
   1015 inventions, and these have been removed when possible.</li>
   1016 <li>Numeric time zone abbreviations typically count hours east of
   1017 <abbr>UT</abbr>, e.g., +09 for Japan and
   1018 &minus;10 for Hawaii. However, the <abbr>POSIX</abbr>
   1019 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
   1020 For example, one might use <code><abbr>TZ</abbr>="<abbr
   1021 title="Japan Standard Time">JST</abbr>-9"</code> and
   1022 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
   1023 for Japan and Hawaii, respectively. If the
   1024 <code><abbr>tz</abbr></code> database is available, it is usually better to use
   1025 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
   1026 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
   1027 confusion, handle old timestamps better, and insulate you better from
   1028 any future changes to the rules. One should never set
   1029 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
   1030 <code>"GMT-9"</code>, though, since this would incorrectly imply that
   1031 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
   1032 is called "<abbr>GMT</abbr>".</li>
   1033 </ul>
   1034 </section>
   1035 
   1036 <section>
   1037 <h2 id="see-also">See also</h2>
   1038 <ul>
   1039 <li><a href="theory.html">Theory and pragmatics of the tz code and data</a></li>
   1040 <li><a href="tz-art.html">Time and the Arts</a></li>
   1041 </ul>
   1042 </section>
   1043 
   1044 <footer>
   1045 <hr>
   1046 This web page is in the public domain, so clarified as of
   1047 2009-05-17 by Arthur David Olson.
   1048 <br>
   1049 Please send corrections to this web page to the
   1050 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
   1051 </footer>
   1052 </body>
   1053 </html>
   1054