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