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      4 <title>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>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="#coordinating">Coordinating with governments and distributors</a></li>
     30 	  <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
     31 	</ul>
     32       </li>
     33       <li>Uses of the <code><abbr>tz</abbr></code> database
     34 	<ul>
     35 	  <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
     36 	  <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
     37 	  <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
     38 	  <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
     39 	  <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
     40 	</ul>
     41       </li>
     42       <li>Related data
     43 	<ul>
     44 	  <li><a href="#other-dbs">Other time zone databases</a></li>
     45 	  <li><a href="#maps">Maps</a></li>
     46 	  <li><a href="#boundaries">Time zone boundaries</a></li>
     47 	</ul>
     48       </li>
     49       <li>Timekeeping concepts
     50 	<ul>
     51 	  <li><a href="#civil">Civil time concepts and history</a></li>
     52 	  <li><a href="#national">National histories of legal time</a></li>
     53 	  <li><a href="#costs">Costs and benefits of time shifts</a></li>
     54 	  <li><a href="#precision">Precision timekeeping</a></li>
     55 	  <li><a href="#notation">Time notation</a></li>
     56 	</ul>
     57       </li>
     58       <li><a href="#see-also">See also</a></li>
     59     </ul>
     60   </nav>
     61 
     62 <section>
     63 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
     64 <p>
     65 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
     66 time zone database contains code and data
     67 that represent the history of local time
     68 for many representative locations around the globe.
     69 It is updated periodically to reflect changes made by political bodies
     70 to time zone boundaries and daylight saving rules.
     71 This database (known as <code><abbr>tz</abbr></code>,
     72 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
     73 is used by several implementations,
     74 including
     75 <a href="https://www.gnu.org/software/libc/">the
     76 <abbr title="GNU's Not Unix">GNU</abbr>
     77 C Library</a> (used in
     78 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
     79 <a href="https://www.android.com">Android</a>,
     80 <a href="https://www.freebsd.org">Free<abbr
     81 title="Berkeley Software Distribution">BSD</abbr></a>,
     82 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
     83 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
     84 <a href="https://www.chromium.org/chromium-os/">ChromiumOS</a>,
     85 <a href="https://cygwin.com">Cygwin</a>,
     86 <a href="https://mariadb.org">MariaDB</a>,
     87 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
     88 <a href="https://musl.libc.org">musl libc</a>,
     89 <a href="https://www.mysql.com">MySQL</a>,
     90 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
     91 title="Web Operating System">webOS</abbr></a>,
     92 <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
     93 title="Advanced Interactive eXecutive">AIX</abbr></a>,
     94 <a href="https://www.apple.com/ios"><abbr
     95 title="iPhone OS">iOS</abbr></a>,
     96 <a href="https://www.apple.com/macos">macOS</a>,
     97 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
     98 <a href="https://www.vmssoftware.com">Open<abbr
     99 title="Virtual Memory System">VMS</abbr></a>,
    100 <a href="https://www.oracle.com/database/">Oracle Database</a>, and
    101 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
    102 <p>
    103 Each main entry in the database represents a <dfn>timezone</dfn>
    104 for a set of civil-time clocks that have all agreed since 1970.
    105 Timezones are typically identified by continent or ocean and then by the
    106 name of the largest city within the region containing the clocks.
    107 For example, <code>America/New_York</code>
    108 represents most of the <abbr title="United States">US</abbr> eastern time zone;
    109 <code>America/Phoenix</code> represents most of Arizona, which
    110 uses mountain time without daylight saving time (<abbr>DST</abbr>);
    111 <code>America/Detroit</code> represents most of Michigan, which uses
    112 eastern time but with different <abbr>DST</abbr> rules in 1975;
    113 and other entries represent smaller regions like Starke County,
    114 Indiana, which switched from central to eastern time in 1991
    115 and switched back in 2006.
    116 To use the database on a <a
    117 href="https://en.wikipedia.org/wiki/POSIX"><abbr
    118 title="Portable Operating System Interface">POSIX</abbr>.1-2024</a>
    119 implementation set the <code><abbr>TZ</abbr></code>
    120 environment variable to the location's full name,
    121 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
    122 <p>
    123 Associated with each timezone is a history of offsets from
    124 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
    125 Time</a> (<abbr>UT</abbr>), which is <a
    126 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
    127 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
    128 for timestamps after 1960 this is more precisely <a
    129 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
    130 Universal Time</a> (<abbr>UTC</abbr>).
    131 The database also records when daylight saving time was in use,
    132 along with some time zone abbreviations such as <abbr>EST</abbr>
    133 for Eastern Standard Time in the <abbr>US</abbr>.</p>
    134 </section>
    135 
    136 <section>
    137 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
    138 <p>
    139 The following <a
    140 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
    141 the latest release's two
    142 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
    143 to a <abbr>GNU</abbr>/Linux or similar host.</p>
    144 <pre><code>mkdir tzdb
    145 cd tzdb
    146 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
    147 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
    148 <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 -
    149 gzip -dc tzdata-latest.tar.gz | tar -xf -
    150 </code></pre>
    151 <p>Alternatively, the following shell commands download the same
    152 release in a single-tarball format containing extra data
    153 useful for regression testing:</p>
    154 <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>
    155 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
    156 </code></pre>
    157 <p>These commands use convenience links to the latest release
    158 of the <code><abbr>tz</abbr></code> database hosted by the
    159 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
    160 of the <a href="https://www.iana.org">Internet Assigned Numbers
    161 Authority (IANA)</a>.
    162 Older releases are in files named
    163 <code>tzcode<var>V</var>.tar.gz</code>,
    164 <code>tzdata<var>V</var>.tar.gz</code>, and
    165 <code>tzdb-<var>V</var>.tar.lz</code>,
    166 where <code><var>V</var></code> is the version.
    167 Since 1996, each version has been a four-digit year followed by
    168 lower-case letter (<samp>a</samp> through <samp>z</samp>,
    169 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
    170 through <samp>zzz</samp>, and so on).
    171 Since version 2022a, each release has been distributed in
    172 <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
    173 ustar interchange format</a>, compressed as described above;
    174 older releases use a nearly compatible format.
    175 Since version 2016h, each release has contained a text file named
    176 "<code>version</code>" whose first (and currently only) line is the version.
    177 Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
    178 and are also available in an
    179 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
    180 title="File Transfer Protocol">FTP</abbr> directory</a> via a
    181 less secure protocol.</p>
    182 <p>Alternatively, a development repository of code and data can be
    183 retrieved from <a href="https://github.com">GitHub</a> via the shell
    184 command:</p>
    185 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
    186 </code></pre>
    187 <p>
    188 Since version 2012e, each release has been tagged in development repositories.
    189 Untagged commits are less well tested and probably contain
    190 more errors.</p>
    191 <p>
    192 After obtaining the code and data files, see the
    193 <code>README</code> file for what to do next.
    194 The code lets you compile the <code><abbr>tz</abbr></code> source files into
    195 machine-readable binary files, one for each location. The binary files
    196 are in a special format specified by
    197 <a href="https://www.rfc-editor.org/rfc/9636">The
    198 Time Zone Information Format (<abbr>TZif</abbr>)</a>
    199 (Internet <abbr title="Request For Comments">RFC</abbr> 9636).
    200 The code also lets
    201 you read a <abbr>TZif</abbr> file and interpret timestamps for that
    202 location.</p>
    203 </section>
    204 
    205 <section>
    206 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    207 <p>
    208 The <code><abbr>tz</abbr></code> code and data
    209 are by no means authoritative. If you find errors, please
    210 email changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    211 the time zone mailing list. See
    212 <a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing
    213 list's main page</a> to subscribe or to browse its archive of old messages.
    214 <a href="https://tzdata-meta.timtimeonline.com">Metadata for mailing list
    215 discussions</a> and corresponding data changes can be
    216 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
    217 </p>
    218 <p>
    219 Changes to the <code><abbr>tz</abbr></code> code and data are often
    220 propagated to clients via operating system updates, so
    221 client <code><abbr>tz</abbr></code> data can often be corrected by
    222 applying these updates. With GNU/Linux and similar systems, if your
    223 maintenance provider has not yet adopted the
    224 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
    225 the process by tailoring the generic instructions in
    226 the <code><abbr>tz</abbr> README</code> file and installing the latest
    227 data yourself. System-specific instructions for installing the
    228 latest <code><abbr>tz</abbr></code> data have also been published
    229 for <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates"><abbr>AIX</abbr></a>,
    230 <a
    231 href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
    232 <a
    233 href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
    234 title="International Components for Unicode">ICU</abbr></a>,
    235 <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
    236 JDK</a>,
    237 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    238 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    239 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
    240 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
    241 </p>
    242 <p>Since version 2013a,
    243 sources for the <code><abbr>tz</abbr></code> database have been
    244 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
    245 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
    246 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
    247 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
    248 title="linefeed">LF</abbr></a>,
    249 which can be modified by common text editors such
    250 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
    251 <a href="https://gedit-technology.github.io/apps/gedit/">gedit</a>, and
    252 <a href="https://www.vim.org">vim</a>.
    253 Specialized source-file editing can be done via the
    254 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
    255 zoneinfo</a> package for <a
    256 href="https://www.sublimetext.com">Sublime Text</a> and the <a
    257 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
    258 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
    259 Studio Code</a>.
    260 </p>
    261 <p>
    262 For further information about updates, please see
    263 <a href="https://www.rfc-editor.org/rfc/rfc6557">Procedures for
    264 Maintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557).
    265 More detail can be
    266 found in <a href="theory.html">Theory and pragmatics of the
    267 <code><abbr>tz</abbr></code> code and data</a>.
    268 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
    269 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
    270 </p>
    271 </section>
    272 
    273 <section>
    274 <h2 id="coordinating">Coordinating with governments and distributors</h2>
    275 <p>
    276 As discussed in
    277 "<a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How
    278 Time Zones Are Coordinated</a>", the time zone database relies on
    279 collaboration among governments, the time zone database volunteer
    280 community, and data distributors downstream.
    281 <p>
    282 If your government plans to change its time zone boundaries or
    283 daylight saving rules, please send email to <a
    284 href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a> well in advance,
    285 as this will lessen confusion and will coordinate updates to many cell phones,
    286 computers, and other devices around the world.
    287 In your email, please cite the legislation or regulation that specifies
    288 the change, so that it can be checked for details such as the exact times
    289 when clock transitions occur.
    290 It is OK if a rule change is planned to affect clocks
    291 far into the future, as a long-planned change can easily be reverted
    292 or otherwise altered with a year's notice before the change would have
    293 affected clocks.</p>
    294 <p>
    295 There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.
    296 However, typically a release occurs every few months.
    297 Many downstream timezone data distributors wait for
    298 a <code><abbr>tzdb</abbr></code> release before they produce an update
    299 to time zone behavior in consumer devices and software products.
    300 After a release, various parties must integrate, test,
    301 and roll out an update before <a
    302 href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.
    303 These updates can be expensive, for both the <a
    304 href="https://en.wikipedia.org/wiki/Quality_assurance">quality
    305 assurance</a> process and the overall cost of shipping and installing
    306 updates to each device's copy of <code><abbr>tzdb</abbr></code>.
    307 Updates may be batched with other updates and may take substantial
    308 time to reach end users after a release.
    309 Older devices may no longer be supported and thus may never be updated,
    310 which means they will continue to use out-of-date rules.</p>
    311 <p>
    312 For these reasons any rule change should be promulgated at least a
    313 year before it affects how clocks operate; otherwise, there is a good
    314 chance that many clocks will be wrong due to delays in propagating updates,
    315 and that residents will be confused or even actively resist the change.
    316 The shorter the notice, the more likely clock problems will arise; see "<a
    317 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
    318 the Timing of Time Zone Changes</a>" for examples.
    319 </p>
    320 </section>
    321 
    322 <section>
    323 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
    324 <ul>
    325 <li>The article
    326 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
    327 an encyclopedic summary.</li>
    328 <li><a href="tz-how-to.html">How to Read the
    329 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
    330 database format.</li>
    331 <li><a
    332 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
    333 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
    334 database's style.</li>
    335 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
    336 managing time in the internet</a> analyzes the database longitudinally.</li>
    337 </ul>
    338 </section>
    339 
    340 <section>
    341 <h2 id="web">Web sites using recent versions of the
    342 <code><abbr>tz</abbr></code> database</h2>
    343 <p>
    344 These are listed roughly in ascending order of complexity and fanciness.
    345 </p>
    346 <ul>
    347 <li><a href="https://time.is">Time.is</a> shows locations'
    348 time and zones.</li>
    349 <li><a href="https://www.timejones.com">TimeJones.com</a>,
    350 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
    351 <a href="https://www.worldclock.com">The World Clock</a>
    352 are time zone converters.</li>
    353 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
    354 publishes <code><abbr>tzdb</abbr></code>-derived data in
    355 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
    356 title="comma-separated values">CSV</abbr></a> and
    357 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
    358 title="Structured Query Language">SQL</abbr></a> form.</li>
    359 <li><a
    360 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
    361 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
    362 <li><a
    363 href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
    364 Time in 1000 Places</a> uses descriptions of the values.</li>
    365 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock &ndash;
    366 Worldwide</a> lets you sort zone names and convert times.</li>
    367 <li><a href="https://24timezones.com">24TimeZones</a> has a world
    368 time map and a time converter.</li>
    369 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    370 calculates the current time difference between locations.</li>
    371 <li><a href="https://www.wx-now.com">Weather Now</a> and
    372 <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    373 </ul>
    374 </section>
    375 
    376 <section>
    377 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    378 <ul>
    379 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
    380 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
    381 Distribution Service (tzdist) working group</a> defined <a
    382 href="https://www.rfc-editor.org/rfc/rfc7808">TZDIST</a>
    383 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
    384 along with <a href="https://www.rfc-editor.org/rfc/rfc7809">CalDAV</a>
    385 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    386 transferring time zone data by reference.
    387 <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
    388 implementations</a> are available.
    389 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
    390 mailing list</a> discusses possible extensions.</li>
    391 <li>The <a href="https://www.rfc-editor.org/rfc/rfc5545">
    392 Internet Calendaring and Scheduling Core Object Specification
    393 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
    394 covers time zone
    395 data; see its VTIMEZONE calendar component.
    396 The iCalendar format requires specialized parsers and generators; a
    397 variant <a href="https://www.rfc-editor.org/rfc/rfc6321">xCal</a>
    398 (Internet <abbr>RFC</abbr> 6321) uses
    399 <a href="https://www.w3.org/XML/"><abbr
    400 title="Extensible Markup Language">XML</abbr></a> format, and a variant
    401 <a href="https://www.rfc-editor.org/rfc/rfc7265">jCal</a>
    402 (Internet <abbr>RFC</abbr> 7265)
    403 uses <a href="https://www.json.org/json-en.html"><abbr
    404 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    405 </ul>
    406 </section>
    407 
    408 <section>
    409 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    410 <p>Although some of these do not fully support
    411 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
    412 distributions you can generally work around compatibility problems by
    413 running the command <code>make rearguard_tarballs</code> and compiling
    414 from the resulting tarballs instead.</p>
    415 <ul>
    416 <li><a href="https://github.com/libical/vzic">Vzic</a> is a <a
    417 href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>
    418 program that compiles
    419 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    420 Vzic is freely
    421 available under the <a
    422 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    423 General Public License (<abbr
    424 title="General Public License">GPL</abbr>)</a>.</li>
    425 <li><a href="https://sourceforge.net/projects/tzical/">tziCal &ndash; tz
    426 database conversion utility</a> is like Vzic, except for the <a
    427 href="https://dotnet.microsoft.com">.NET framework</a>
    428 and with a <abbr>BSD</abbr>-style license.</li>
    429 <li><a
    430 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
    431 contains a script <code>parse_olson</code> that compiles
    432 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
    433 modules. It is part of the Perl <a
    434 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
    435 which is freely
    436 available under both the <abbr>GPL</abbr> and the Perl Artistic
    437 License. DateTime::TimeZone also contains a script
    438 <code>tests_from_zdump</code> that generates test cases for each clock
    439 transition in the <code><abbr>tz</abbr></code> database.</li>
    440 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
    441 Database Parser</a> is a
    442 <a href="https://en.wikipedia.org/wiki/C++">C++</a> parser and
    443 runtime library with a <a
    444 href="https://en.cppreference.com/w/cpp/chrono"><code>std::chrono</code> API</a>
    445 that is a standard part of C++.
    446 It is freely available under the
    447 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
    448 <li><a id="ICU" href="https://icu.unicode.org">International Components for
    449 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
    450 href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a>
    451 libraries for internationalization that
    452 has a compiler from <code><abbr>tz</abbr></code> source
    453 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
    454 (mentioned <a href="#CLDR">below</a>)
    455 into an <abbr>ICU</abbr>-specific format.
    456 <abbr>ICU</abbr> is freely available under a
    457 <abbr>BSD</abbr>-style license.</li>
    458 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
    459 the <a href="https://elixir-lang.org">Elixir</a> language downloads
    460 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
    461 title="Application Program Interface">API</abbr>s for use. It is
    462 freely available under the <abbr>MIT</abbr> license.</li>
    463 <li>Java-based compilers and libraries include:
    464 <ul>
    465 <li>The <a
    466 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
    467 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
    468 <a href="https://openjdk.org">OpenJDK</a> and
    469 <a href="https://jdk.java.net">Oracle JDK</a>.
    470 Although its source code is proprietary, its executable is available under the
    471 <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
    472 Timezone Updater License Agreement</a>.</li>
    473 <li>The <a
    474 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
    475 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
    476 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
    477 which is freely available under a <abbr>BSD</abbr>-style license.</li>
    478 <li><a href="https://www.joda.org/joda-time/">Joda-Time &ndash; Java date
    479 and time <abbr>API</abbr></a> contains a class
    480 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
    481 <code><abbr>tz</abbr></code> source into a binary format. It inspired
    482 Java 8 <code>java.time</code>, which its users should migrate to once
    483 they can assume Java 8 or later. It is available under the <a
    484 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
    485 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
    486 href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
    487 are alternatives to TZUpdater. IANA Updater's license is unclear;
    488 ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
    489 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
    490 time library for Android</a> and
    491 <a href="https://github.com/MenoData/Time4J">Time4J: Advanced date,
    492 time and interval library for Java</a> compile
    493 <code><abbr>tz</abbr></code> source into a binary format.
    494 Time4A is available under the Apache License and Time4J is
    495 available under the <a
    496 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
    497 General Public License (<abbr title="Lesser General Public
    498 License">LGPL</abbr>)</a>.</li>
    499 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
    500 Java-based libraries.</li>
    501 </ul>
    502 <li><a href="https://nodatime.org">Noda Time &ndash; Date and
    503 time <abbr>API</abbr> for .NET</a>
    504 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
    505 It is freely available under the Apache License.</li>
    506 <li>Many modern
    507 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
    508 runtimes support <code><abbr>tz</abbr></code> natively via the
    509 <code>timeZone</code> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><code>Intl.DateTimeFormat</code></a>.
    510 This can be used as-is or with most of the following libraries,
    511 many of which also support runtimes lacking the <code>timeZone</code> option.
    512 <ul>
    513 <li>The <a
    514 href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code>
    515 timezone polyfill</a>
    516 is freely available under a <abbr>BSD</abbr>-style license.</li>
    517 <li>The <a href="https://date-fns.org">date-fns</a>
    518 library manipulates timezone-aware timestamps in browsers and
    519 in <a href="https://nodejs.org/en/">Node.js</a>.
    520 It is freely available under the <abbr>MIT</abbr> license.</li>
    521 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
    522 minimalist replacement for the date and time API of
    523 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
    524 manipulation library.
    525 It is freely available under the <abbr>MIT</abbr> license.</li>
    526 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
    527 timezone support for the <code>Intl</code> API.
    528 It is freely available under the <abbr>MIT</abbr> license.</li>
    529 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
    530 Moment.js plugin.
    531 It is freely available under the <abbr>MIT</abbr> license.</li>
    532 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
    533 JavaScript library that supports date arithmetic that is time zone
    534 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
    535 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
    536 supports live <code><abbr>tzdb</abbr></code> updates,
    537 astronomical and atomic time, a command-line interface,
    538 and full <a
    539 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
    540 Its companion <a
    541 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
    542 can generate <abbr>TZif</abbr> and other files, and a companion website
    543 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
    544 convert timestamps, view transition histories, and download code and data.
    545 It is freely available under the <abbr>MIT</abbr> license.</li>
    546 </ul>
    547 The proposed <a
    548 href="https://github.com/tc39/proposal-temporal"><code>Temporal</code>
    549 objects</a> let programs access an abstract view of
    550 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
    551 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
    552 problematic <code>Date</code> objects</a> when working with dates and times.
    553 <li><a href="https://github.com/JuliaTime">JuliaTime</a> contains a
    554 compiler from <code><abbr>tz</abbr></code> source into
    555 <a href="https://julialang.org">Julia</a>. It is freely available
    556 under the <abbr>MIT</abbr> license.</li>
    557 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> &ndash;
    558 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
    559 title="Free Pascal Compiler">FPC</abbr></a>
    560 compiles from <code><abbr>tz</abbr></code> source into
    561 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
    562 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
    563 and <a
    564 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
    565 It is freely available under a <abbr>BSD</abbr>-style license.</li>
    566 <li><a href="https://pythonhosted.org/pytz/">pytz &ndash; World Timezone
    567 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
    568 <a href="https://www.python.org">Python</a>.
    569 It is freely available under a <abbr>BSD</abbr>-style license.
    570 In code that can assume Python 3.6 or later it is largely superseded; see <a
    571 href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
    572 The Fastest Footgun in the West</a>.</li>
    573 <li><a href="https://tzinfo.github.io">TZInfo &ndash;
    574 Ruby Timezone Library</a>
    575 compiles <code><abbr>tz</abbr></code> source into
    576 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
    577 It is freely available under the <abbr>MIT</abbr> license.</li>
    578 <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
    579 Library</a> is
    580 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
    581 library that compiles <code><abbr>tz</abbr></code> source into a time
    582 zone repository whose format
    583 is either proprietary or an <abbr>XML</abbr>-encoded
    584 representation.</li>
    585 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
    586 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
    587 source into text files, along with a runtime that can read those
    588 files. Tcl is freely available under a <abbr>BSD</abbr>-style
    589 license.</li>
    590 </ul>
    591 </section>
    592 
    593 <section>
    594 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
    595 <ul>
    596 <li>The <a
    597 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
    598 Library</a>
    599 has an independent, thread-safe implementation of
    600 a <abbr>TZif</abbr> file reader.
    601 This library is freely available under the LGPL
    602 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
    603 <li><a href="https://www.gnome.org">GNOME</a>'s
    604 <a href="https://developer.gnome.org/glib/">GLib</a> has
    605 a <abbr>TZif</abbr> file reader written in C that
    606 creates a <code>GTimeZone</code> object representing sets
    607 of <abbr>UT</abbr> offsets.
    608 It is freely available under the <abbr>LGPL</abbr>.</li>
    609 <li>The
    610 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
    611 <code>baltzo::TimeZoneUtil</code> component contains a C++
    612 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
    613 the Apache License.</li>
    614 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
    615 library that translates between <abbr>UT</abbr> and civil time and
    616 can read <abbr>TZif</abbr> files. It is freely available under the Apache
    617 License.</li>
    618 <li>The
    619 <a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code>
    620 package</a> contains Python code
    621 to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map
    622 <code><abbr>tz</abbr></code> settings to proleptic TZ approximations.
    623 For example, it maps <code>"Africa/Cairo"</code>
    624 to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>,
    625 an approximation valid for Cairo timestamps from 2023 on.
    626 This can help porting to platforms that support only proleptic TZ.
    627 The package is freely available under the MIT license.</li>
    628 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    629 library that reads <abbr>TZif</abbr> files and converts
    630 timestamps from one time zone or format to another.
    631 It is used by <a href="https://secure.php.net"><abbr
    632 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    633 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    634 and <a href="https://www.mongodb.com">MongoDB</a>.
    635 It is freely available under the <abbr>MIT</abbr> license.</li>
    636 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    637 <abbr>TZif</abbr> file reader.</li>
    638 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
    639 DateTime::TimeZone::Tzfile</a>
    640 is a <abbr>TZif</abbr> file reader written in Perl.
    641 It is freely available under the same terms as Perl
    642 (dual <abbr>GPL</abbr> and Artistic license).</li>
    643 <li>Python has a <a id="python-zoneinfo"
    644 href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
    645 class</a> that reads <abbr>TZif</abbr> data and creates objects
    646 that represent <code><abbr>tzdb</abbr></code> timezones.
    647 Python is freely available under the
    648 <a href="https://docs.python.org/3/license.html">Python Software Foundation
    649 License</a>.
    650 A companion <a id="pypi-tzdata" href="https://pypi.org">PyPI</a> module
    651 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
    652 supplies TZif data if the underlying system data cannot be found;
    653 it is freely available under the Apache License.</li>
    654 <li>The
    655 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    656 library contains a Python tool that
    657 converts <abbr>TZif</abbr> data into
    658 <abbr>JSON</abbr>-format data suitable for use
    659 in its JavaScript library for time zone conversion. Dates before 1970
    660 are not supported.</li>
    661 <li>The <a
    662 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    663 package contains <a href="https://www.haskell.org">Haskell</a> code that
    664 parses and uses <abbr>TZif</abbr> data. It is freely
    665 available under a <abbr>BSD</abbr>-style license.</li>
    666 </ul>
    667 </section>
    668 
    669 <section>
    670 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    671 <ul>
    672 <li><a href="https://foxclocks.org">FoxClocks</a>
    673 is an extension for <a href="https://www.google.com/chrome/">Google
    674 Chrome</a>, <a
    675 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
    676 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
    677 It displays multiple clocks in the application window, and has a mapping
    678 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
    679 It is freely available under the <abbr>GPL</abbr>.</li>
    680 <li><a href="https://golang.org">Go programming language</a>
    681 implementations contain a copy of a 32-bit subset of a recent
    682 <code><abbr>tz</abbr></code> database in a
    683 Go-specific format.</li>
    684 <li>Microsoft Windows 8.1
    685 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    686 data (mentioned <a href="#CLDR">below</a>) used by the
    687 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    688 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    689 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    690 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
    691 <a id="System.TimeZoneInfo"
    692 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
    693 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    694 the older, proprietary method of Microsoft Windows 2000 and later,
    695 which stores time zone data in the
    696 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    697 <a
    698 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone &rarr;
    699 Tzid table</a> or <a
    700 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    701 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    702 to <code><abbr>tz</abbr></code> names.
    703 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
    704 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
    705 <li><a
    706 href="https://www.oracle.com/java/index.html">Oracle
    707 Java</a> contains a copy of a subset of a recent
    708 <code><abbr>tz</abbr></code> database in a
    709 Java-specific format.</li>
    710 </ul>
    711 </section>
    712 
    713 <section>
    714 <h2 id="other-dbs">Other time zone databases</h2>
    715 <ul>
    716 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    717 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
    718 time zone history atlases
    719 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
    720 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
    721 Although these extensive atlases
    722 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
    723 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
    724 they are unreliable as Shanks appears to have
    725 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    726 sources and do not indicate which entries are guesswork.</li>
    727 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    728 its own <code>tztab</code>(4) format.</li>
    729 <li>Microsoft Windows has proprietary data mentioned
    730 <a href="#System.TimeZoneInfo">above</a>.</li>
    731 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    732 is another time zone database.</li>
    733 <li>The <a
    734 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
    735 Schedules Information Manual</a> of the
    736 International Air Transport Association
    737 gives current time zone rules for airports served by commercial aviation.</li>
    738 </ul>
    739 </section>
    740 
    741 <section>
    742 <h2 id="maps">Maps</h2>
    743 <ul>
    744 <li>The <a
    745 href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
    746 and Regional Maps section</a> of <em>The World Factbook</em>, published by the
    747 <a href="https://www.cia.gov">US Central Intelligence
    748 Agency (<abbr
    749 title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
    750 zone map; the
    751 <a
    752 href="https://legacy.lib.utexas.edu/maps/world.html">Perry&ndash;Casta&ntilde;eda
    753 Library Map Collection</a>
    754 of the University of Texas at Austin has copies of
    755 recent editions.
    756 The pictorial quality is good,
    757 but the maps do not indicate daylight saving time,
    758 and parts of the data are a few years out of date.</li>
    759 <li><a href="https://www.worldtimezone.com">World Time Zone Map
    760 with current time</a>
    761 has several fancy time zone maps; it covers Russia particularly well.
    762 The maps' pictorial quality is not quite as good as the
    763 <abbr>CIA</abbr>'s
    764 but the maps are more up to date.</li>
    765 <li><a
    766 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    767 much is time wrong around the world?</a> maps the difference between
    768 mean solar and standard time, highlighting areas such as western China
    769 where the two differ greatly. It's a bit out of date, unfortunately.</li>
    770 </ul>
    771 </section>
    772 
    773 <section>
    774 <h2 id="boundaries">Time zone boundaries</h2>
    775 <p>Geographical boundaries between timezones are available
    776 from several <a
    777 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
    778 geolocation</a>
    779 services and other sources.</p>
    780 <ul>
    781 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    782 Boundary Builder</a> extracts
    783 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    784 boundaries of <code><abbr>tzdb</abbr></code> timezones.
    785 Its code is freely available under the <abbr>MIT</abbr> license, and
    786 its data entries are freely available under the
    787 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    788 Open Database License</a>. The borders appear to be quite accurate.
    789 Its main web page lists more than twenty libraries
    790 for looking up a timezone name from a GPS coordinate.</li>
    791 <li>Free access via a network API, if you register a key, is provided by
    792 the <a
    793 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
    794 Timezone web service</a>, the <a
    795 href="https://developers.google.com/maps/documentation/timezone/intro">Google
    796 Maps Time Zone API</a>, and
    797 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
    798 Commercial network API access is provided
    799 by <a href="https://askgeo.com">AskGeo</a>
    800 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    801 </li>
    802 <li>"<a
    803 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
    804 to get a time zone from a location using latitude and longitude
    805 coordinates?</a>" discusses other geolocation possibilities.</li>
    806 <li><a href="http://statoids.com/statoids.html">Administrative
    807 Divisions of Countries ("Statoids")</a> lists
    808 political subdivision data related to time zones.</li>
    809 <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
    810 &ndash; GIS and Database Tools</a> includes a Manifold-format map of
    811 world time zone boundaries circa 2007, distributed under the
    812 <abbr>GPL</abbr>.</li>
    813 <li>A ship within the <a
    814 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    815 waters</a> of any nation uses that nation's time. In international
    816 waters, time zone boundaries are meridians 15&deg; apart, except that
    817 <abbr>UT</abbr>&minus;12 and <abbr>UT</abbr>+12 are each 7.5&deg;
    818 wide and are separated by
    819 the 180&deg; meridian (not by the International Date Line, which is
    820 for land and territorial waters only). A captain can change ship's
    821 clocks any time after entering a new time zone; midnight changes are
    822 common.</li>
    823 </ul>
    824 </section>
    825 
    826 <section>
    827 <h2 id="civil">Civil time concepts and history</h2>
    828 <ul>
    829 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    830 Walk through Time</a>
    831 surveys the evolution of timekeeping.</li>
    832 <li>The history of daylight saving time is surveyed in <a
    833 href="http://www.webexhibits.org/daylightsaving/">About Daylight
    834 Saving Time &ndash; History, rationale, laws &amp; dates</a> and summarized in
    835 <a href="http://seizethedaylight.com/dst/">A Brief
    836 History of Daylight Saving Time</a>.</li>
    837 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
    838 Lords</a> discusses how authoritarians manipulate civil time.</li>
    839 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
    840 contains guidelines and best practices for software applications that
    841 deal with civil time.</li>
    842 <li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of
    843 the International Date Line</a> tells the story of the most important
    844 time zone boundary.</li>
    845 <li><a href="http://statoids.com/tconcept.html">Basic Time
    846 Zone Concepts</a> discusses terminological issues behind time zones.</li>
    847 </ul>
    848 </section>
    849 
    850 <section>
    851 <h2 id="national">National histories of legal time</h2>
    852 <dl>
    853 <dt>Australia</dt>
    854 <dd>The Parliamentary Library commissioned a <a
    855 href="https://parlinfo.aph.gov.au/parlInfo/download/library/prspub/359V6/upload_binary/359v60.pdf">research
    856 paper on daylight saving time in Australia</a>.
    857 The Bureau of Meteorology publishes a list of <a
    858 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    859 Dates of Daylight Savings Time within Australia</a>.</dd>
    860 <dt>Belgium</dt>
    861 <dd>The Royal Observatory of Belgium maintains a table of time in
    862 Belgium (in
    863 <a href="https://robinfo.oma.be/nl/astro-info/tijd/"
    864 hreflang="nl">Dutch</a> and <a
    865 href="https://robinfo.oma.be/fr/astro-info/heure/"
    866 hreflang="fr">French</a>).</dd>
    867 <dt>Brazil</dt>
    868 <dd>The Time Service Department of the National Observatory
    869 records <a href="http://pcdsh01.on.br/DecHV.html"
    870 hreflang="pt-BR">Brazil's daylight saving time decrees (in
    871 Portuguese)</a>.</dd>
    872 <dt>Canada</dt>
    873 <dd>National Research Council Canada publishes current
    874 and some older information about <a
    875 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
    876 zones and daylight saving time</a>.</dd>
    877 <dt>Chile</dt>
    878 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    879 <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
    880 Chile's official time (in Spanish)</a>.</dd>
    881 <dt>China</dt>
    882 <dd>The Hong Kong Observatory maintains a
    883 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
    884  summer time in Hong Kong</a>,
    885 and Macau's Meteorological and Geophysical Bureau maintains a <a
    886 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
    887 history for Macau</a>.
    888 Unfortunately the latter is incomplete and has errors.</dd>
    889 <dt>Czech Republic</dt>
    890 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    891 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    892 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
    893 <dt>Germany</dt>
    894 <dd>The National Institute for Science and Technology maintains the <a
    895 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    896 of Legal Time in Germany</a>.</dd>
    897 <dt>Israel</dt>
    898 <dd><a
    899 href="https://tz.cs.huji.ac.il">Israel Timezone Files</a>
    900 lists official time-change announcements and laws since 1940,
    901 almost all in Hebrew.</dd>
    902 <dt>Malaysia</dt>
    903 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    904 <dt>Mexico</dt>
    905 <dd>The Investigation and Analysis Service of the Mexican Library of
    906 Congress has published a <a
    907 href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    908 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    909 <dt>Netherlands</dt>
    910 <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    911 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    912 covers the history of local time in the Netherlands from ancient times.</dd>
    913 <dt>New Zealand</dt>
    914 <dd>The Department of Internal Affairs maintains a brief <a
    915 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
    916 Daylight Saving</a>.</dd>
    917 <dt>Palestine</dt>
    918 <dd>The Ministry of Telecom and IT publishes a <a
    919 href="https://mtit.pna.ps/home/TimeZone"
    920 hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
    921 <dt>Portugal</dt>
    922 <dd>The Lisbon Astronomical Observatory publishes a
    923 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
    924 legal time (in Portuguese)</a>.</dd>
    925 <dt>Singapore</dt>
    926 <dd><a id="Singapore"
    927 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
    928 is Singapore in the "Wrong" Time Zone?</a> details the
    929 history of legal time in Singapore and Malaysia.</dd>
    930 <dt>United Kingdom</dt>
    931 <dd><a
    932 href="https://www.polyomino.org.uk/british-time/">History of
    933 legal time in Britain</a> discusses in detail the country
    934 with perhaps the best-documented history of clock adjustments.</dd>
    935 <dt>United States</dt>
    936 <dd>The Department of Transportation's <a
    937 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    938 Time Zone Proceedings</a> lists changes to
    939 official written time zone boundaries, and its <a
    940 href="https://geodata.bts.gov/datasets/usdot::time-zones/about">Time
    941 Zones dataset</a> maps current boundaries.
    942 These boundaries are only for standard time, so the current map puts
    943 all of Arizona in one time zone even though part of Arizona
    944 observes <abbr>DST</abbr> and part does not.</dd>
    945 <dt>Uruguay</dt>
    946 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    947 Navy (SOHMA) publishes an annual <a
    948 href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
    949 (in Spanish)</a>.</dd>
    950 </dl>
    951 </section>
    952 
    953 <section>
    954 <h2 id="costs">Costs and benefits of time shifts</h2>
    955 <p>Various sources argue for and against daylight saving time and time
    956 zone shifts, and many scientific studies have been conducted. This
    957 section summarizes reviews and position statements based on
    958 scientific literature in the area.</p>
    959 <ul>
    960 <li>In 2022 the American Medical Association issued a
    961 <a href="https://www.ama-assn.org/press-center/press-releases/ama-calls-permanent-standard-time">statement
    962 supporting permanent standard time</a> on health grounds.</li>
    963 <li>Carey RN, Sarma KM.
    964 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
    965 daylight saving time on road traffic collision risk: a systematic
    966 review</a>.
    967 <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>.
    968 This reviews research literature and concludes that the evidence
    969 neither supports nor refutes road safety benefits from
    970 shifts in time zones.</li>
    971 <li>Havranek T, Herman D, Irsova D.
    972 <a href="https://www.iaee.org/en/publications/ejarticle.aspx?id=3051">Does
    973 daylight saving save electricity? A meta-analysis</a>.
    974 <em>Energy J.</em> 2018;39(2):35&ndash;61.
    975 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
    976 This analyzes research literature and concludes, "Electricity savings
    977 are larger for countries farther away from the equator, while
    978 subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
    979 <li>Malow BA. <a
    980 href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time
    981 to abolish the clock change and adopt permanent
    982 standard time in the United States:
    983 a Sleep Research Society position statement</a>.
    984 <em>Sleep.</em> 2022;45(12):zsac236.
    985 doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
    986 After reviewing the scientific literature, the Sleep Research Society
    987 advocates permanent standard time due to its health benefits.
    988 <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
    989 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
    990 is the optimal choice for health and safety:
    991 an American Academy of Sleep Medicine position statement</a>.
    992 <em>J Clin Sleep Med.</em> 2024;20(1):121&ndash;125.
    993 doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
    994 The AASM argues for permanent standard time due to health and safety risks
    995 and economic costs of both <abbr>DST</abbr> transitions and
    996 permanent <abbr>DST</abbr>.</li>
    997 <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
    998 <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why
    999 should we abolish Daylight Saving Time?</a>
   1000 <em>J Biol Rhythms.</em> 2019;34(3):227&ndash;230.
   1001 doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
   1002 The Society for Research on Biological Rhythms
   1003 opposes <abbr>DST</abbr> changes and permanent <abbr>DST</abbr>,
   1004 and advocates that governments adopt
   1005 "permanent Standard Time for the health and safety of their citizens".</li>
   1006 </ul>
   1007 </section>
   1008 
   1009 <section>
   1010 <h2 id="precision">Precision timekeeping</h2>
   1011 <ul>
   1012 <li><a
   1013 href="http://leapsecond.com/hpan/an1289.pdf">The
   1014 Science of Timekeeping</a> is a thorough introduction
   1015 to the theory and practice of precision timekeeping.</li>
   1016 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
   1017 Time 2016</a> contains several freely readable papers.</li>
   1018 <li><a href="https://www.ntp.org"><abbr
   1019 title="Network Time Protocol">NTP</abbr>: The Network
   1020 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
   1021 discusses how to synchronize clocks of
   1022 Internet hosts.</li>
   1023 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
   1024 family of software algorithms can achieve accuracy to a few tens of
   1025 nanoseconds in scalable server farms without special hardware.</li>
   1026 <li>The <a
   1027 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
   1028 Time Protocol</a> (<abbr
   1029 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
   1030 can achieve submicrosecond clock accuracy on a local area network
   1031 with special-purpose hardware.</li>
   1032 <li><a
   1033 href="https://www.rfc-editor.org/rfc/rfc4833">Timezone
   1034 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
   1035 (Internet <abbr>RFC</abbr> 4833)
   1036 specifies a <a
   1037 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
   1038 option for a server to configure
   1039 a client's time zone and daylight saving settings automatically.</li>
   1040 <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
   1041 Scales</a> describes astronomical time scales like
   1042 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
   1043 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
   1044 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
   1045 <li>The <a href="https://www.iau.org"><abbr
   1046 title="International Astronomical Union">IAU</abbr></a>'s <a
   1047 href="https://www.iausofa.org"><abbr
   1048 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
   1049 collection contains C and <a
   1050 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
   1051 code for converting among time scales like
   1052 <abbr title="International Atomic Time">TAI</abbr>,
   1053 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
   1054 <abbr>UTC</abbr>. It is freely available under the
   1055 <a href="https://www.iausofa.org/tandc.html">SOFA license</a>.</li>
   1056 <li><a
   1057 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
   1058 &ndash; Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
   1059 diverse local time
   1060 scales used by each landed mission on Mars.</li>
   1061 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
   1062 dedicated not only to leap seconds but to precise time and frequency
   1063 in general. It covers the state of the art in amateur timekeeping, and
   1064 how the art has progressed over the past few decades.</li>
   1065 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
   1066 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
   1067 and time-signal emissions</a>, International Telecommunication Union &ndash;
   1068 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
   1069 <li><a
   1070 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
   1071 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
   1072 Bulletins</a> contains official publications of the International
   1073 Earth Rotation and Reference Systems Service, which decides when leap
   1074 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
   1075 via an optional "<code>right</code>" configuration where a computer's internal
   1076 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
   1077 as opposed to the default "<code>posix</code>" configuration
   1078 where the internal clock ignores leap seconds.
   1079 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
   1080 <abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge for
   1081 timestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,
   1082 which corresponds to the first leap second
   1083 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
   1084 and to
   1085 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
   1086 In practice the two configurations also agree for timestamps before
   1087 1972 even though the historical situation is messy, partly because
   1088 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
   1089 is well-defined for sufficiently old timestamps.</li>
   1090 <li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The
   1091 <abbr>NTP</abbr> Leap Second File</a> covers the text file
   1092 <code>leap-seconds.list</code>, which lists the currently known leap seconds.
   1093 The <abbr>IERS</abbr> maintains this file, and a copy is distributed by
   1094 <code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like
   1095 <a href="https://www.ntp.org">classic
   1096 <code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a>
   1097 and <a href="https://ntpsec.org">NTPsec</a>.
   1098 The <code><abbr>tz</abbr></code> database also distributes leap second
   1099 information in a differently-formatted <code>leapseconds</code> text file,
   1100 as well as in the "<code>right</code>" configuration in binary form; for
   1101 example, <code>right/UTC</code> can be used
   1102 by <a href="https://chrony-project.org"><code>chrony</code></a>,
   1103 another <abbr>NTP</abbr> implementation.</li>
   1104 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
   1105 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
   1106 leap second so that they disagree with <abbr>UTC</abbr> by at most a
   1107 half second, even though every <abbr>POSIX</abbr> minute has exactly
   1108 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
   1109 "<code>posix</code>" configuration, is <a
   1110 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
   1111 the abovementioned <abbr>NTP</abbr> implementations, <a
   1112 href="https://github.com/google/unsmear">supports</a> conversion between
   1113 <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
   1114 cloud service providers. However, according to
   1115 <a href="https://www.rfc-editor.org/rfc/rfc8633#section-3.7.1">&sect;3.7.1 of
   1116 Network Time Protocol Best Current Practices</a>
   1117 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
   1118 applications requiring accurate <abbr>UTC</abbr> or civil time,
   1119 and is intended for use only in single, well-controlled environments.</li>
   1120 <li>The <a
   1121 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
   1122 Second Discussion List</a> covers <a
   1123 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
   1124 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
   1125 discussed further in
   1126 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
   1127 leap second: its history and possible future</a>.
   1128 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
   1129 might be redefined
   1130 without Leap Seconds</a> gives pointers on this
   1131 contentious issue.
   1132 The General Conference on Weights and Measures
   1133 <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>
   1134 to discontinue the use of leap seconds by 2035, and requested that no
   1135 discontinuous adjustments be made to UTC for at least a century.
   1136 The World Radiocommunication Conference <a
   1137 href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved
   1138 in 2023</a> to cooperate with this process.
   1139 <a href="https://www.preprints.org/manuscript/202406.0043/v1">A proposal
   1140 to change the leap-second adjustments to Coordinated Universal Time</a>
   1141 (doi:<a href="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a>)
   1142 would replace leap seconds with 13-second leap smears occurring once per
   1143 decade until 2100, with leap smears after that gradually increasing in size.
   1144 However, there is still no consensus on whether this is the best way
   1145 to replace leap seconds.
   1146 </li>
   1147 </ul>
   1148 </section>
   1149 
   1150 <section>
   1151 <h2 id="notation">Time notation</h2>
   1152 <ul>
   1153 <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
   1154 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
   1155 zone names, abbreviations, identifiers, and formats. For example, it
   1156 contains French translations for "Eastern European Summer Time",
   1157 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
   1158 "Bucharest". Its
   1159 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
   1160 charts</a> show these values for many locales. Data values are available in
   1161 both <abbr title="Locale Data Markup Language">LDML</abbr>
   1162 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
   1163 <li>
   1164 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
   1165 the international standard date and time notation</a> covers
   1166 <a
   1167 href="https://www.iso.org/standard/70907.html"><em><abbr
   1168 title="International Organization for Standardization">ISO</abbr>
   1169 8601-1:2019 &ndash; Date and time &ndash; Representations for information
   1170 interchange &ndash; Part 1: Basic rules</em></a>.</li>
   1171 <li>
   1172 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
   1173 Schema: Datatypes &ndash; dateTime</a> specifies a format inspired by
   1174 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
   1175 <li><a href="https://www.rfc-editor.org/rfc/rfc5322#section-3.3">&sect;3.3 of
   1176 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
   1177 specifies the time notation used in email and <a
   1178 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
   1179 headers.</li>
   1180 <li>
   1181 <a href="https://www.rfc-editor.org/rfc/rfc3339">Date and Time
   1182 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
   1183 specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.
   1184 An extension, <a href="https://www.rfc-editor.org/rfc/rfc9557">Date
   1185 and Time on the Internet: Timestamps with Additional Information</a>
   1186 (Internet <abbr>RFC</abbr> 9557) extends this profile
   1187 to let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp
   1188 via suffixes like "<code>[Asia/Tokyo]</code>".
   1189 <li>
   1190 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
   1191 Formats on the Web</a> surveys web- and Internet-oriented date and time
   1192 formats.</li>
   1193 <li>Alphabetic time zone abbreviations should not be used as unique
   1194 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
   1195 practice. For example, in English-speaking North America
   1196 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
   1197 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
   1198 and French-speaking North Americans prefer
   1199 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
   1200 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
   1201 database contains English abbreviations for many timestamps;
   1202 unfortunately some of these abbreviations were merely the database maintainers'
   1203 inventions, and these have been removed when possible.</li>
   1204 <li>Numeric time zone abbreviations typically count hours east of
   1205 <abbr>UT</abbr>, e.g., +09 for Japan and
   1206 &minus;10 for Hawaii. However, <abbr>POSIX</abbr> proleptic
   1207 <code><abbr>TZ</abbr></code> settings use the opposite convention.
   1208 For example, one might use <code><abbr>TZ</abbr>="<abbr
   1209 title="Japan Standard Time">JST</abbr>-9"</code> and
   1210 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
   1211 for Japan and Hawaii, respectively. If the
   1212 <code><abbr>tz</abbr></code> database is available, it is usually better to use
   1213 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
   1214 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
   1215 confusion, handle old timestamps better, and insulate you better from
   1216 any future changes to the rules. One should never set
   1217 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
   1218 <code>"GMT-9"</code>, though, since this would incorrectly imply that
   1219 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
   1220 is called "<abbr>GMT</abbr>".</li>
   1221 </ul>
   1222 </section>
   1223 
   1224 <section>
   1225 <h2 id="see-also">See also</h2>
   1226 <ul>
   1227 <li><a href="theory.html">Theory and pragmatics of the
   1228 <code><abbr>tz</abbr></code> code and data</a></li>
   1229 <li><a href="tz-art.html">Time and the Arts</a></li>
   1230 </ul>
   1231 </section>
   1232 
   1233 <footer>
   1234 <hr>
   1235 This web page is in the public domain, so clarified as of
   1236 2009-05-17 by Arthur David Olson.
   1237 <br>
   1238 Please send corrections to this web page to the
   1239 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
   1240 </footer>
   1241 </body>
   1242 </html>
   1243