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      4 <title>Time zone and daylight saving time data</title>
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     13 <body>
     14 <h1>Time zone and daylight saving time data</h1>
     15 <p>
     16 <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
     17 <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
     18 rules are controlled by individual
     19 governments. They are sometimes changed with little notice, and their
     20 histories and planned futures are often recorded only fitfully. Here
     21 is a summary of attempts to organize and record relevant data in this
     22 area.
     23 </p>
     24   <nav>
     25     <ul>
     26       <li>The <code><abbr>tz</abbr></code> database product and process
     27 	<ul>
     28 	  <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
     29 	  <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
     30 	  <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
     31 	  <li><a href="#coordinating">Coordinating with governments and distributors</a></li>
     32 	  <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
     33 	</ul>
     34       </li>
     35       <li>Uses of the <code><abbr>tz</abbr></code> database
     36 	<ul>
     37 	  <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
     38 	  <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
     39 	  <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
     40 	  <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
     41 	  <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
     42 	</ul>
     43       </li>
     44       <li>Related data
     45 	<ul>
     46 	  <li><a href="#other-dbs">Other time zone databases</a></li>
     47 	  <li><a href="#maps">Maps</a></li>
     48 	  <li><a href="#boundaries">Time zone boundaries</a></li>
     49 	</ul>
     50       </li>
     51       <li>Timekeeping concepts
     52 	<ul>
     53 	  <li><a href="#civil">Civil time concepts and history</a></li>
     54 	  <li><a href="#national">National histories of legal time</a></li>
     55 	  <li><a href="#costs">Costs and benefits of time shifts</a></li>
     56 	  <li><a href="#precision">Precision timekeeping</a></li>
     57 	  <li><a href="#notation">Time notation</a></li>
     58 	</ul>
     59       </li>
     60       <li><a href="#see-also">See also</a></li>
     61     </ul>
     62   </nav>
     63 
     64 <section>
     65 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
     66 <p>
     67 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
     68 time zone database contains code and data
     69 that represent the history of local time
     70 for many representative locations around the globe.
     71 It is updated periodically to reflect changes made by political bodies
     72 to time zone boundaries and daylight saving rules.
     73 This database (known as <code><abbr>tz</abbr></code>,
     74 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
     75 is used by several implementations,
     76 including
     77 <a href="https://www.gnu.org/software/libc/">the
     78 <abbr title="GNUs Not Unix">GNU</abbr>
     79 C Library</a> (used in
     80 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
     81 <a href="https://www.android.com">Android</a>,
     82 <a href="https://www.freebsd.org">Free<abbr
     83 title="Berkeley Software Distribution">BSD</abbr></a>,
     84 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
     85 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
     86 <a href="https://www.chromium.org/chromium-os/">ChromiumOS</a>,
     87 <a href="https://cygwin.com">Cygwin</a>,
     88 <a href="https://mariadb.org">MariaDB</a>,
     89 <a href="https://musl.libc.org">musl libc</a>,
     90 <a href="https://www.mysql.com">MySQL</a>,
     91 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
     92 title="Web Operating System">webOS</abbr></a>,
     93 <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
     94 title="Advanced Interactive eXecutive">AIX</abbr></a>,
     95 <a href="https://www.apple.com/os/ios/"><abbr
     96 title="iPhone OS">iOS</abbr></a>,
     97 <a href="https://www.apple.com/os/macos/">macOS</a>,
     98 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
     99 <a href="https://vmssoftware.com">Open<abbr
    100 title="Virtual Memory System">VMS</abbr></a>,
    101 <a href="https://www.oracle.com/database/">Oracle Database</a>,
    102 <a href="https://www.oracle.com/solaris/solaris11/">Oracle Solaris</a>,
    103 and <a href="https://qnx.software/en">QNX</a>.</p>
    104 <p>
    105 Each main entry in the database represents a <dfn>timezone</dfn>
    106 for a set of civil-time clocks that have all agreed since 1970.
    107 Timezones are typically identified by continent or ocean and then by the
    108 name of the largest city within the region containing the clocks.
    109 For example, <code>America/New_York</code>
    110 represents most of the <abbr title="United States">US</abbr> eastern time zone;
    111 <code>America/Phoenix</code> represents most of Arizona, which
    112 uses mountain time without daylight saving time (<abbr>DST</abbr>);
    113 <code>America/Detroit</code> represents most of Michigan, which uses
    114 eastern time but with different <abbr>DST</abbr> rules in 1975;
    115 and other entries represent smaller regions like Starke County,
    116 Indiana, which switched from central to eastern time in 1991
    117 and switched back in 2006.
    118 To use the database on a <a
    119 href="https://en.wikipedia.org/wiki/POSIX"><abbr
    120 title="Portable Operating System Interface">POSIX</abbr>.1-2024</a>
    121 implementation set the <code><abbr>TZ</abbr></code>
    122 environment variable to the locations full name,
    123 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
    124 <p>
    125 Associated with each timezone is a history of offsets from
    126 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
    127 Time</a> (<abbr>UT</abbr>), which is <a
    128 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
    129 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
    130 for timestamps after 1960 this is more precisely <a
    131 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
    132 Universal Time</a> (<abbr>UTC</abbr>).
    133 The database also records when daylight saving time was in use,
    134 along with some time zone abbreviations such as <abbr>EST</abbr>
    135 for Eastern Standard Time in the <abbr>US</abbr>.</p>
    136 </section>
    137 
    138 <section>
    139 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
    140 <p>
    141 The following <a
    142 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
    143 the latest releases two
    144 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
    145 to a <abbr>GNU</abbr>/Linux or similar host.</p>
    146 <pre><code>mkdir tzdb
    147 cd tzdb
    148 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
    149 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
    150 <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 -
    151 gzip -dc tzdata-latest.tar.gz | tar -xf -
    152 </code></pre>
    153 <p>Alternatively, the following shell commands download the same
    154 release in a single-tarball format containing extra data
    155 useful for regression testing:</p>
    156 <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>
    157 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
    158 </code></pre>
    159 <p>These commands use convenience links to the latest release
    160 of the <code><abbr>tz</abbr></code> database hosted by the
    161 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
    162 of the <a href="https://www.iana.org">Internet Assigned Numbers
    163 Authority (IANA)</a>.
    164 Older releases are in files named
    165 <code>tzcode<var>V</var>.tar.gz</code>,
    166 <code>tzdata<var>V</var>.tar.gz</code>, and
    167 <code>tzdb-<var>V</var>.tar.lz</code>,
    168 where <code><var>V</var></code> is the version.
    169 Since 1996, each version has been a four-digit year followed by
    170 lower-case letter (<samp>a</samp> through <samp>z</samp>,
    171 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
    172 through <samp>zzz</samp>, and so on).
    173 Since version 2022a, each release has been distributed in
    174 <a href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/pax.html#tag_20_94_13_06">POSIX
    175 ustar interchange format</a>, compressed as described above;
    176 older releases use a nearly compatible format.
    177 Since version 2016h, each release has contained a text file named
    178 <code>version</code> whose first (and currently only) line is the version.
    179 <a href="https://ftp.iana.org/tz/releases/">Older archived releases are
    180 available</a> via
    181 <a href="https://en.wikipedia.org/wiki/HTTPS"><abbr
    182 title="Hypertext Transfer Protocol Secure">HTTPS</abbr></a>,
    183 <a href="https://en.wikipedia.org/wiki/Rsync"><abbr
    184 title="remote sync">rsync</abbr></a>, and
    185 <a href="https://en.wikipedia.org/wiki/FTP"><abbr
    186 title="File Transfer Protocol">FTP</abbr></a>.
    187 <p>Alternatively, a development repository of code and data can be
    188 retrieved from <a href="https://github.com">GitHub</a> via the shell
    189 command:</p>
    190 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
    191 </code></pre>
    192 <p>
    193 Since version 2012e, each release has been tagged in development repositories.
    194 Untagged commits are less well tested and probably contain
    195 more errors.</p>
    196 <p>
    197 After obtaining the code and data files, see the
    198 <code>README</code> file for what to do next.
    199 The code lets you compile the <code><abbr>tz</abbr></code> source files into
    200 machine-readable binary files, one for each location. The binary files
    201 are in a special format specified by
    202 <a href="https://www.rfc-editor.org/rfc/rfc9636">The
    203 Time Zone Information Format (<abbr>TZif</abbr>)</a>
    204 (Internet <abbr title="Request For Comments">RFC</abbr> 9636).
    205 The code also lets
    206 you read a <abbr>TZif</abbr> file and interpret timestamps for that
    207 location.</p>
    208 </section>
    209 
    210 <section>
    211 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
    212 <p>
    213 The <code><abbr>tz</abbr></code> code and data
    214 are by no means authoritative. If you find errors, please
    215 email changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
    216 the time zone mailing list.
    217 The mailing list and its archives are public,
    218 so please do not send confidential information.
    219 See
    220 <a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing
    221 lists main page</a> to subscribe or to browse its archive of old messages.
    222 <a href="https://tzdata-meta.timtimeonline.com">Metadata for mailing list
    223 discussions</a> and corresponding data changes can be
    224 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
    225 </p>
    226 <p>
    227 Changes to the <code><abbr>tz</abbr></code> code and data are often
    228 propagated to clients via operating system updates, so
    229 client <code><abbr>tz</abbr></code> data can often be corrected by
    230 applying these updates. With GNU/Linux and similar systems, if your
    231 maintenance provider has not yet adopted the
    232 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
    233 the process by tailoring the generic instructions in
    234 the <code><abbr>tz</abbr> README</code> file and installing the latest
    235 data yourself. System-specific instructions for installing the
    236 latest <code><abbr>tz</abbr></code> data have also been published
    237 for <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates"><abbr>AIX</abbr></a>,
    238 <a
    239 href="https://source.android.com/docs/core/permissions/timezone-rules">Android</a>,
    240 <a
    241 href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
    242 title="International Components for Unicode">ICU</abbr></a>,
    243 <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
    244 JDK</a>,
    245 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
    246 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
    247 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
    248 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
    249 </p>
    250 <p>Since version 2013a,
    251 sources for the <code><abbr>tz</abbr></code> database have been
    252 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
    253 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
    254 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
    255 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
    256 title="linefeed">LF</abbr></a>,
    257 which can be modified by common text editors such
    258 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
    259 <a href="https://gedit-text-editor.org">gedit</a>, and
    260 <a href="https://www.vim.org">vim</a>.
    261 Specialized source-file editing can be done via the
    262 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
    263 zoneinfo</a> package for <a
    264 href="https://www.sublimetext.com">Sublime Text</a> and the <a
    265 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
    266 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
    267 Studio Code</a>.
    268 </p>
    269 <p>
    270 For further information about updates, please see
    271 <a href="https://www.rfc-editor.org/rfc/rfc6557">Procedures for
    272 Maintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557).
    273 More detail can be
    274 found in <a href="theory.html">Theory and pragmatics of the
    275 <code><abbr>tz</abbr></code> code and data</a>.
    276 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
    277 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
    278 </p>
    279 </section>
    280 
    281 <section>
    282 <h2 id="coordinating">Coordinating with governments and distributors</h2>
    283 <p>
    284 As discussed in
    285 <a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How
    286 Time Zones Are Coordinated</a>, the time zone database relies on
    287 collaboration among governments, the time zone database volunteer
    288 community, and data distributors downstream.
    289 <p>
    290 If your government plans to change its time zone boundaries or
    291 daylight saving rules, please send email as described in
    292 <a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a>.
    293 Do this well in advance,
    294 as this will lessen confusion and will coordinate updates to many cell phones,
    295 computers, and other devices around the world.
    296 In your email, please cite the legislation or regulation that specifies
    297 the change, so that it can be checked for details such as the exact times
    298 when clock transitions occur.
    299 It is OK if a rule change is planned to affect clocks
    300 far into the future, as a long-planned change can easily be reverted
    301 or otherwise altered with a years notice before the change would have
    302 affected clocks.</p>
    303 <p>
    304 There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.
    305 However, typically a release occurs every few months.
    306 Many downstream timezone data distributors wait for
    307 a <code><abbr>tzdb</abbr></code> release before they produce an update
    308 to time zone behavior in consumer devices and software products.
    309 After a release, various parties must integrate, test,
    310 and roll out an update before <a
    311 href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.
    312 These updates can be expensive, for both the <a
    313 href="https://en.wikipedia.org/wiki/Quality_assurance">quality
    314 assurance</a> process and the overall cost of shipping and installing
    315 updates to each devices copy of <code><abbr>tzdb</abbr></code>.
    316 Updates may be batched with other updates and may take substantial
    317 time to reach end users after a release.
    318 Older devices may no longer be supported and thus may never be updated,
    319 which means they will continue to use out-of-date rules.</p>
    320 <p>
    321 For these reasons any rule change should be promulgated at least a
    322 year before it affects how clocks operate; otherwise, there is a good
    323 chance that many clocks will be wrong due to delays in propagating updates,
    324 and that residents will be confused or even actively resist the change.
    325 The shorter the notice, the more likely clock problems will arise; see <a
    326 href="https://codeofmatt.com/on-the-timing-of-time-zone-changes/">On
    327 the Timing of Time Zone Changes</a> for examples.
    328 </p>
    329 </section>
    330 
    331 <section>
    332 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
    333 <ul>
    334 <li>The article
    335 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
    336 an encyclopedic summary.</li>
    337 <li><a href="tz-how-to.html">How to Read the
    338 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
    339 database format.</li>
    340 <li><a
    341 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
    342 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
    343 databases style.</li>
    344 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
    345 managing time in the internet</a> analyzes the database longitudinally.</li>
    346 </ul>
    347 </section>
    348 
    349 <section>
    350 <h2 id="web">Web sites using recent versions of the
    351 <code><abbr>tz</abbr></code> database</h2>
    352 <p>
    353 These are listed roughly in ascending order of complexity and fanciness.
    354 </p>
    355 <ul>
    356 <li><a href="https://time.is">Time.is</a> shows locations
    357 time and zones.</li>
    358 <li><a href="https://www.timejones.com">TimeJones.com</a>,
    359 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
    360 <a href="https://www.worldclock.com">The World Clock</a>
    361 are time zone converters.</li>
    362 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
    363 publishes <code><abbr>tzdb</abbr></code>-derived data in
    364 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
    365 title="comma-separated values">CSV</abbr></a> and
    366 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
    367 title="Structured Query Language">SQL</abbr></a> form.</li>
    368 <li><a
    369 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
    370 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
    371 <li><a
    372 href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
    373 Time in 1000 Places</a> uses descriptions of the values.</li>
    374 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock 
    375 Worldwide</a> lets you sort zone names and convert times.</li>
    376 <li><a href="https://24timezones.com">24TimeZones</a> has a world
    377 time map and a time converter.</li>
    378 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
    379 calculates the current time difference between locations.</li>
    380 <li><a href="https://www.wx-now.com">Weather Now</a> and
    381 <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
    382 </ul>
    383 </section>
    384 
    385 <section>
    386 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
    387 <ul>
    388 <li><a href="https://www.rfc-editor.org/rfc/rfc7808">Time Zone
    389 Data Distribution Service</a> (TZDIST, Internet <abbr>RFC</abbr> 7808)
    390 is associated with
    391 <a href="https://www.rfc-editor.org/rfc/rfc7809">CalDAV</a>
    392 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
    393 transferring time zone data by reference.
    394 <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
    395 implementations</a> are available.</li>
    396 <li>The <a href="https://www.rfc-editor.org/rfc/rfc5545">iCalendar format</a>
    397 (Internet <abbr>RFC</abbr> 5445)
    398 covers time zone
    399 data; see its VTIMEZONE calendar component.
    400 The iCalendar format requires specialized parsers and generators; a
    401 variant <a href="https://www.rfc-editor.org/rfc/rfc6321">xCal</a>
    402 (Internet <abbr>RFC</abbr> 6321) uses
    403 <a href="https://www.w3.org/XML/"><abbr
    404 title="Extensible Markup Language">XML</abbr></a> format, and a variant
    405 <a href="https://www.rfc-editor.org/rfc/rfc7265">jCal</a>
    406 (Internet <abbr>RFC</abbr> 7265)
    407 uses <a href="https://www.json.org/json-en.html"><abbr
    408 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
    409 </ul>
    410 </section>
    411 
    412 <section>
    413 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
    414 <p>Although some of these do not fully support
    415 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
    416 distributions you can generally work around compatibility problems by
    417 running the command <code>make rearguard_tarballs</code> and compiling
    418 from the resulting tarballs instead.</p>
    419 <ul>
    420 <li><a href="https://github.com/libical/vzic">Vzic</a> is a <a
    421 href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>
    422 program that compiles
    423 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
    424 Vzic is freely
    425 available under the <a
    426 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
    427 General Public License (<abbr
    428 title="General Public License">GPL</abbr>)</a>.</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/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/technical-resources/articles/java/jf14-date-time.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  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/components/ziupdater-time-zone-tool/">ZIUpdater</a>
    487 are alternatives to TZUpdater. IANA Updaters 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  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/">JavaScripts
    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> 
    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  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 
    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://www.tcl-lang.org">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://docs.gtk.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 <a href="https://go.dev">Go programming language</a>
    619 has a <abbr>TZif</abbr> file reader <a
    620 href="https://pkg.go.dev/time#LoadLocationFromTZData"><code>LoadLocationFromTZData</code></a>.</li>
    621 <li>The
    622 <a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code>
    623 package</a> contains Python code
    624 to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map
    625 <code><abbr>tz</abbr></code> settings to proleptic TZ approximations.
    626 For example, it maps <code>"Africa/Cairo"</code>
    627 to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>,
    628 an approximation valid for Cairo timestamps from 2023 on.
    629 This can help porting to platforms that support only proleptic TZ.
    630 The package is freely available under the MIT license.</li>
    631 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
    632 library that reads <abbr>TZif</abbr> files and converts
    633 timestamps from one time zone or format to another.
    634 It is used by <a href="https://www.php.net"><abbr
    635 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
    636 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
    637 and <a href="https://www.mongodb.com">MongoDB</a>.
    638 It is freely available under the <abbr>MIT</abbr> license.</li>
    639 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
    640 <abbr>TZif</abbr> file reader.</li>
    641 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
    642 DateTime::TimeZone::Tzfile</a>
    643 is a <abbr>TZif</abbr> file reader written in Perl.
    644 It is freely available under the same terms as Perl
    645 (dual <abbr>GPL</abbr> and Artistic license).</li>
    646 <li>Python has a <a id="python-zoneinfo"
    647 href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
    648 class</a> that reads <abbr>TZif</abbr> data and creates objects
    649 that represent <code><abbr>tzdb</abbr></code> timezones.
    650 Python is freely available under the
    651 <a href="https://docs.python.org/3/license.html">Python Software Foundation
    652 License</a>.
    653 A companion <a id="pypi-tzdata" href="https://pypi.org">PyPI</a> module
    654 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
    655 supplies TZif data if the underlying system data cannot be found;
    656 it is freely available under the Apache License.</li>
    657 <li>The
    658 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
    659 library contains a Python tool that
    660 converts <abbr>TZif</abbr> data into
    661 <abbr>JSON</abbr>-format data suitable for use
    662 in its JavaScript library for time zone conversion. Dates before 1970
    663 are not supported.</li>
    664 <li>The <a
    665 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
    666 package contains <a href="https://www.haskell.org">Haskell</a> code that
    667 parses and uses <abbr>TZif</abbr> data. It is freely
    668 available under a <abbr>BSD</abbr>-style license.</li>
    669 </ul>
    670 </section>
    671 
    672 <section>
    673 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
    674 <ul>
    675 <li><a href="https://foxclocks.org">FoxClocks</a>
    676 is an extension for <a href="https://www.google.com/chrome/">Google
    677 Chrome</a>, <a
    678 href="https://www.firefox.com/en-US/">Firefox</a> and <a
    679 href="https://www.thunderbird.net/en-US/">Thunderbird</a>.
    680 It displays multiple clocks in the application window, and has a mapping
    681 interface to <a href="https://earth.google.com/web/">Google Earth</a>.
    682 It is freely available under the <abbr>GPL</abbr>.</li>
    683 <li>Microsoft Windows 8.1
    684 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
    685 data (mentioned <a href="#CLDR">below</a>) used by the
    686 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
    687 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
    688 <a href="https://learn.microsoft.com/en-us/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
    689 <a href="https://learn.microsoft.com/en-us/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
    690 <a id="System.TimeZoneInfo"
    691 href="https://learn.microsoft.com/en-us/archive/blogs/bclteam/exploring-windows-time-zones-with-system-timezoneinfo-josh-free">Exploring
    692 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
    693 the older, proprietary method of Microsoft Windows 2000 and later,
    694 which stores time zone data in the
    695 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
    696 <a
    697 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone
    698  Tzid table</a> or <a
    699 href="https://github.com/unicode-org/cldr/blob/main/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
    700 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
    701 to <code><abbr>tz</abbr></code> names.
    702 These mappings can be performed programmatically via the
    703 <a href="https://github.com/mattjohnsonpint/TimeZoneConverter">TimeZoneConverter</a>
    704 .NET library,
    705 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
    706 <li><a
    707 href="https://www.oracle.com/java/">Oracle
    708 Java</a> contains a copy of a subset of a recent
    709 <code><abbr>tz</abbr></code> database in a
    710 Java-specific format.</li>
    711 </ul>
    712 </section>
    713 
    714 <section>
    715 <h2 id="other-dbs">Other time zone databases</h2>
    716 <ul>
    717 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
    718 is Astrodiensts Web version of Shanks and Pottengers out-of-print
    719 time zone history atlases
    720 <a href="https://search.worldcat.org/title/468828649">for the US</a> and
    721 <a href="https://search.worldcat.org/title/76950459">for the world</a>.
    722 Although these extensive atlases
    723 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
    724 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
    725 they are unreliable as Shanks appears to have
    726 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
    727 sources and do not indicate which entries are guesswork.</li>
    728 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
    729 its own <code>tztab</code>(4) format.</li>
    730 <li>Microsoft Windows has proprietary data mentioned
    731 <a href="#System.TimeZoneInfo">above</a>.</li>
    732 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
    733 is another time zone database.</li>
    734 <li>The <a
    735 href="https://www.iata.org/en/publications/manuals/standard-schedules-information">Standard
    736 Schedules Information Manual</a> of the
    737 International Air Transport Association
    738 gives current time zone rules for airports served by commercial aviation.</li>
    739 </ul>
    740 </section>
    741 
    742 <section>
    743 <h2 id="maps">Maps</h2>
    744 <ul>
    745 <li><a href="https://www.worldtimezone.com">World Time Zone Map
    746 with current time</a>
    747 has several fancy time zone maps; it covers Russia particularly well.</li>
    748 <li><a
    749 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
    750 much is time wrong around the world?</a> maps the difference between
    751 mean solar and standard time, highlighting areas such as western China
    752 where the two differ greatly. Its a bit out of date, unfortunately.</li>
    753 <li>The
    754 <a
    755 href="https://maps.lib.utexas.edu/maps/world.html">PerryCastaeda
    756 Library Map Collection</a> of the University of Texas at Austin has
    757 copies of old maps taken from <a
    758 href="https://en.wikipedia.org/wiki/The_World_Factbook"><em>The
    759 World Factbook</em></a>, formerly published by the
    760 <a href="https://www.cia.gov">US Central Intelligence Agency</a>.
    761 Although the maps pictorial quality is good,
    762 the maps do not indicate daylight saving time.</li>
    763 </ul>
    764 </section>
    765 
    766 <section>
    767 <h2 id="boundaries">Time zone boundaries</h2>
    768 <p>Geographical boundaries between timezones are available
    769 from several <a
    770 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
    771 geolocation</a>
    772 services and other sources.</p>
    773 <ul>
    774 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
    775 Boundary Builder</a> extracts
    776 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
    777 boundaries of <code><abbr>tzdb</abbr></code> timezones.
    778 Its code is freely available under the <abbr>MIT</abbr> license, and
    779 its data entries are freely available under the
    780 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
    781 Open Database License</a>. The borders appear to be quite accurate.
    782 Its main web page lists more than twenty libraries
    783 for looking up a timezone name from a GPS coordinate.</li>
    784 <li>Free access via a network API, if you register a key, is provided by
    785 the <a
    786 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
    787 Timezone web service</a>, the <a
    788 href="https://developers.google.com/maps/documentation/timezone/overview">Google
    789 Maps Time Zone API</a>, and
    790 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
    791 Commercial network API access is provided
    792 by <a href="https://askgeo.com">AskGeo</a>
    793 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
    794 </li>
    795 <li><a
    796 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
    797 to get a time zone from a location using latitude and longitude
    798 coordinates?</a> discusses other geolocation possibilities.</li>
    799 <li><a href="https://statoids.com/statoids.html">Administrative
    800 Divisions of Countries (Statoids)</a> lists
    801 political subdivision data related to time zones.</li>
    802 <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
    803  GIS and Database Tools</a> includes a Manifold-format map of
    804 world time zone boundaries circa 2007, distributed under the
    805 <abbr>GPL</abbr>.</li>
    806 <li>A ship within the <a
    807 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
    808 waters</a> of any nation uses that nations time. In international
    809 waters, time zone boundaries are meridians 15 apart, except that
    810 <abbr>UT</abbr>12 and <abbr>UT</abbr>+12 are each 7.5
    811 wide and are separated by
    812 the 180 meridian (not by the International Date Line, which is
    813 for land and territorial waters only). A captain can change ships
    814 clocks any time after entering a new time zone; midnight changes are
    815 common.</li>
    816 </ul>
    817 </section>
    818 
    819 <section>
    820 <h2 id="civil">Civil time concepts and history</h2>
    821 <ul>
    822 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
    823 Walk through Time</a>
    824 surveys the evolution of timekeeping.</li>
    825 <li>The history of daylight saving time is surveyed in <a
    826 href="https://www.webexhibits.org/daylightsaving/">About Daylight
    827 Saving Time  History, rationale, laws &amp; dates</a> and summarized in
    828 <a href="http://seizethedaylight.com/dst/">A Brief
    829 History of Daylight Saving Time</a>.</li>
    830 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
    831 Lords</a> discusses how authoritarians manipulate civil time.</li>
    832 <li><a href="https://www.w3.org/TR/timezone/">Working with Time
    833 and Time Zones</a>
    834 contains guidelines and best practices for software applications that
    835 deal with civil time.</li>
    836 <li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of
    837 the International Date Line</a> tells the story of the most important
    838 time zone boundary.</li>
    839 <li><a href="https://statoids.com/tconcept.html">Basic Time
    840 Zone Concepts</a> discusses terminological issues behind time zones.</li>
    841 </ul>
    842 </section>
    843 
    844 <section>
    845 <h2 id="national">National histories of legal time</h2>
    846 <dl>
    847 <dt>Australia</dt>
    848 <dd>The Bureau of Meteorology publishes a list of <a
    849 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
    850 Dates of Daylight Savings Time within Australia</a>.</dd>
    851 <dt>Belgium</dt>
    852 <dd>The Royal Observatory of Belgium maintains a table of time in
    853 Belgium (in
    854 <a href="https://robinfo.oma.be/nl/astro-info/tijd/"
    855 hreflang="nl">Dutch</a> and <a
    856 href="https://robinfo.oma.be/fr/astro-info/heure/"
    857 hreflang="fr">French</a>).</dd>
    858 <dt>Brazil</dt>
    859 <dd>The Time Service Department of the National Observatory
    860 records <a href="http://pcdsh01.on.br/DecHV.html"
    861 hreflang="pt-BR">Brazils daylight saving time decrees (in
    862 Portuguese)</a>.</dd>
    863 <dt>Canada</dt>
    864 <dd>National Research Council Canada publishes current
    865 and some older information about <a
    866 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
    867 zones and daylight saving time</a>.</dd>
    868 <dt>Chile</dt>
    869 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
    870 <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
    871 Chiles official time (in Spanish)</a>.</dd>
    872 <dt>China</dt>
    873 <dd>The Hong Kong Observatory maintains a
    874 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
    875  summer time in Hong Kong</a>,
    876 and Macaus Meteorological and Geophysical Bureau maintains a <a
    877 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
    878 history for Macau</a>.
    879 Unfortunately the latter is incomplete and has errors.</dd>
    880 <dt>Czech Republic</dt>
    881 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
    882 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
    883 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
    884 <dt>Germany</dt>
    885 <dd>The National Institute for Science and Technology maintains the <a
    886 href="https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
    887 of Legal Time in Germany</a>.</dd>
    888 <dt>Israel</dt>
    889 <dd><a
    890 href="https://tz.cs.huji.ac.il">Israel Timezone Files</a>
    891 lists official time-change announcements and laws since 1940,
    892 almost all in Hebrew.</dd>
    893 <dt>Malaysia</dt>
    894 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
    895 <dt>Mexico</dt>
    896 <dd>The Investigation and Analysis Service of the Mexican Library of
    897 Congress has published a <a
    898 href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
    899 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
    900 <dt>Netherlands</dt>
    901 <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    902 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
    903 covers the history of local time in the Netherlands from ancient times.</dd>
    904 <dt>New Zealand</dt>
    905 <dd>The Department of Internal Affairs maintains a brief <a
    906 href="https://www.govt.nz/browse/recreation-and-the-environment/daylight-saving/history-of-daylight-saving-in-nz/">History
    907 of Daylight Saving in NZ</a>.</dd>
    908 <dt>Palestine</dt>
    909 <dd>The Ministry of Telecom and Digital Economy publishes a <a
    910 href="https://mtde.gov.ps/home/TimeZone"
    911 hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
    912 <dt>Portugal</dt>
    913 <dd>The Lisbon Astronomical Observatory publishes a
    914 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
    915 legal time (in Portuguese)</a>.</dd>
    916 <dt>Singapore</dt>
    917 <dd><a id="Singapore"
    918 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
    919 is Singapore in the Wrong Time Zone?</a> details the
    920 history of legal time in Singapore and Malaysia.</dd>
    921 <dt>United Kingdom</dt>
    922 <dd><a
    923 href="https://www.polyomino.org.uk/british-time/">History of
    924 legal time in Britain</a> discusses in detail the country
    925 with perhaps the best-documented history of clock adjustments.</dd>
    926 <dt>United States</dt>
    927 <dd>The Department of Transportations <a
    928 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
    929 Time Zone Proceedings</a> lists changes to
    930 official written time zone boundaries, and its <a
    931 href="https://geodata.bts.gov/datasets/usdot::time-zones/about">Time
    932 Zones dataset</a> maps current boundaries.
    933 These boundaries are only for standard time, so the current map puts
    934 all of Arizona in one time zone even though part of Arizona
    935 observes <abbr>DST</abbr> and part does not.</dd>
    936 <dt>Uruguay</dt>
    937 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
    938 Navy (SOHMA) publishes an annual <a
    939 href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
    940 (in Spanish)</a>.</dd>
    941 </dl>
    942 </section>
    943 
    944 <section>
    945 <h2 id="costs">Costs and benefits of time shifts</h2>
    946 <p>Various sources argue for and against daylight saving time and time
    947 zone shifts, and many scientific studies have been conducted. This
    948 section summarizes reviews and position statements based on
    949 scientific literature in the area.</p>
    950 <ul>
    951 <li>Carey RN, Sarma KM.
    952 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
    953 daylight saving time on road traffic collision risk: a systematic
    954 review</a>.
    955 <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>.
    956 This reviews research literature and concludes that the evidence
    957 neither supports nor refutes road safety benefits from
    958 shifts in time zones.</li>
    959 <li>Havranek T, Herman D, Irsova D.
    960 Does daylight saving save electricity? A meta-analysis.
    961 <em>Energy J.</em> 2018;39(2):3561.
    962 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
    963 This analyzes research literature and concludes, Electricity savings
    964 are larger for countries farther away from the equator, while
    965 subtropical regions consume more electricity because of <abbr>DST</abbr>.</li>
    966 <li>Neumann P, von Blanckenburg K. <a
    967 href="https://journals.sagepub.com/doi/full/10.1177/0961463X241310562">What
    968 time will it be? A comprehensive literature review on daylight saving time</a>.
    969 <em>Time Soc</em>. 2025;34(4):684745.
    970 doi:<a href="https://doi.org/10.1177/0961463X241310562">10.1177/0961463X241310562</a>.
    971 This reviews <abbr>DST</abbr>s effects on electricity, health, crime, road
    972 safety, and the economy, focusing on research since 2010, and concludes that
    973 year-round standard time is preferable overall.</li>
    974 <li>Romigi A, Franco V, Scoditti E <em>et al</em>.
    975 The effects of daylight saving time and clock time transitions on sleep and
    976 sleepiness: a systematic review. <em>Sleep Med Rev.</em> 2025;84:102161. doi:<a
    977 href="https://doi.org/10.1016/j.smrv.2025.102161">10.1016/j.smrv.2025.102161</a>.
    978 This reviews <abbr>DST</abbr> and <abbr>DST</abbr> transitions,
    979 and concludes that they both harm sleep, health and behavior.</li>
    980 </ul>
    981 
    982 <p>The following medical societies have taken positions on the
    983 advisability of clock shifts:</p>
    984 
    985 <ul>
    986 <li>In 2022 the American Medical Association issued a
    987 <a href="https://www.ama-assn.org/press-center/ama-press-releases/ama-calls-permanent-standard-time">statement
    988 supporting permanent standard time</a> on health grounds.</li>
    989 <li>Crawford MR, Winnebeck EC, von Schantz M <em>et al</em>.
    990 <a href="https://onlinelibrary.wiley.com/doi/10.1111/jsr.14352">The
    991 British Sleep Society position statement on Daylight Saving Time in the UK</a>.
    992 <em>J Sleep Res.</em> 2025;34(3):e14352.
    993 doi:<a href="https://doi.org/10.1111/jsr.14352">10.1111/jsr.14352</a>.
    994 This recommends that the UK abolish <abbr>DST</abbr> for health reasons.</li>
    995 <li>Malow BA. <a
    996 href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time
    997 to abolish the clock change and adopt permanent
    998 standard time in the United States:
    999 a Sleep Research Society position statement</a>.
   1000 <em>Sleep.</em> 2022;45(12):zsac236.
   1001 doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
   1002 The Sleep Research Society
   1003 advocates permanent standard time due to its health benefits.</li>
   1004 <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
   1005 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
   1006 is the optimal choice for health and safety:
   1007 an American Academy of Sleep Medicine position statement</a>.
   1008 <em>J Clin Sleep Med.</em> 2024;20(1):121125.
   1009 doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
   1010 The AASM argues for permanent standard time due to health and safety risks
   1011 and economic costs of both <abbr>DST</abbr> transitions and
   1012 permanent <abbr>DST</abbr>.</li>
   1013 <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
   1014 <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why
   1015 should we abolish Daylight Saving Time?</a>
   1016 <em>J Biol Rhythms.</em> 2019;34(3):227230.
   1017 doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
   1018 The Society for Research on Biological Rhythms
   1019 opposes <abbr>DST</abbr> changes and permanent <abbr>DST</abbr>,
   1020 and advocates that governments adopt
   1021 permanent Standard Time for the health and safety of their citizens.</li>
   1022 </ul>
   1023 </section>
   1024 
   1025 <section>
   1026 <h2 id="precision">Precision timekeeping</h2>
   1027 <ul>
   1028 <li><a
   1029 href="http://leapsecond.com/hpan/an1289.pdf">The
   1030 Science of Timekeeping</a> is a thorough introduction
   1031 to the theory and practice of precision timekeeping.</li>
   1032 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
   1033 Time 2016</a> contains several freely readable papers.</li>
   1034 <li><a href="https://www.ntp.org"><abbr
   1035 title="Network Time Protocol">NTP</abbr>: The Network
   1036 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
   1037 discusses how to synchronize clocks of
   1038 Internet hosts.</li>
   1039 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
   1040 family of software algorithms can achieve accuracy to a few tens of
   1041 nanoseconds in scalable server farms without special hardware.</li>
   1042 <li>The <a
   1043 href="https://www.nist.gov/el/intelligent-systems-division-73500/ieee-1588">Precision
   1044 Time Protocol</a> (<abbr
   1045 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
   1046 can achieve submicrosecond clock accuracy on a local area network
   1047 with special-purpose hardware.</li>
   1048 <li><a
   1049 href="https://www.rfc-editor.org/rfc/rfc4833">Timezone
   1050 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
   1051 (Internet <abbr>RFC</abbr> 4833)
   1052 specifies a <a
   1053 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
   1054 option for a server to configure
   1055 a clients time zone and daylight saving settings automatically.</li>
   1056 <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
   1057 Scales</a> describes astronomical time scales like
   1058 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
   1059 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
   1060 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
   1061 <li>The <a href="https://www.iau.org"><abbr
   1062 title="International Astronomical Union">IAU</abbr></a>s <a
   1063 href="https://www.iausofa.org"><abbr
   1064 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
   1065 collection contains C and <a
   1066 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
   1067 code for converting among time scales like
   1068 <abbr title="International Atomic Time">TAI</abbr>,
   1069 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
   1070 <abbr>UTC</abbr>. It is freely available under the
   1071 <a href="https://www.iausofa.org/terms-and-conditions">SOFA license</a>.</li>
   1072 <li><a
   1073 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
   1074  Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
   1075 diverse local time
   1076 scales used by each landed mission on Mars.</li>
   1077 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
   1078 dedicated not only to leap seconds but to precise time and frequency
   1079 in general. It covers the state of the art in amateur timekeeping, and
   1080 how the art has progressed over the past few decades.</li>
   1081 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
   1082 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
   1083 and time-signal emissions</a>, International Telecommunication Union 
   1084 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
   1085 <li><a
   1086 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
   1087 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
   1088 Bulletins</a> contains official publications of the International
   1089 Earth Rotation and Reference Systems Service, which decides when leap
   1090 seconds occur.
   1091 The <code><abbr>tz</abbr></code> code and data support leap seconds
   1092 via an optional <code>"right"</code> configuration where a computers internal
   1093 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
   1094 as opposed to the default <code>"posix"</code> configuration
   1095 where the internal clock ignores leap seconds.
   1096 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
   1097 <abbr>UTC</abbr> (<code>time_t</code> 63&thinsp;072&thinsp;000) and diverge for
   1098 timestamps starting with <code>time_t</code> 78&thinsp;796&thinsp;800,
   1099 which corresponds to the first leap second
   1100 1972-06-30 23:59:60 <abbr>UTC</abbr> in the <code>"right"</code> configuration,
   1101 and to
   1102 1972-07-01 00:00:00 <abbr>UTC</abbr> in the <code>"posix"</code> configuration.
   1103 In practice the two configurations also agree for timestamps before
   1104 1972 even though the historical situation is messy, partly because
   1105 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
   1106 is well-defined for sufficiently old timestamps.</li>
   1107 <li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The
   1108 <abbr>NTP</abbr> Leap Second File</a> covers the text file
   1109 <code>leap-seconds.list</code>, which lists the currently known leap seconds.
   1110 The <abbr>IERS</abbr> maintains this file, and a copy is distributed by
   1111 <code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like
   1112 <a href="https://www.ntp.org">classic
   1113 <code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a>
   1114 and <a href="https://ntpsec.org">NTPsec</a>.
   1115 The <code><abbr>tz</abbr></code> database also distributes leap second
   1116 information in a differently-formatted <code>leapseconds</code> text file,
   1117 as well as in the <code>"right"</code> configuration in binary form; for
   1118 example, <code>right/UTC</code> can be used
   1119 by <a href="https://chrony-project.org"><code>chrony</code></a>,
   1120 another <abbr>NTP</abbr> implementation.</li>
   1121 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
   1122 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
   1123 leap second so that they disagree with <abbr>UTC</abbr> by at most a
   1124 half second, even though every <abbr>POSIX</abbr> minute has exactly
   1125 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
   1126 <code>"posix"</code> configuration, is <a
   1127 href="https://gitlab.com/NTPsec/ntpsec/-/blob/master/docs/leapsmear.adoc">supported</a>
   1128 by the abovementioned <abbr>NTP</abbr> implementations, <a
   1129 href="https://github.com/google/unsmear">supports</a> conversion between
   1130 <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
   1131 cloud service providers. However, according to
   1132 <a href="https://www.rfc-editor.org/rfc/rfc8633#section-3.7.1">3.7.1 of
   1133 Network Time Protocol Best Current Practices</a>
   1134 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
   1135 applications requiring accurate <abbr>UTC</abbr> or civil time,
   1136 and is intended for use only in single, well-controlled environments.</li>
   1137 <li>The <a
   1138 href="https://groups.io/g/LEAPSECS">LEAPSECS List</a> covers <a
   1139 href="https://gge.ext.unb.ca/Resources/gpsworld.november99.pdf">McCarthy
   1140 and Klepczynskis 1999 proposal to discontinue leap seconds</a>,
   1141 discussed further in
   1142 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
   1143 leap second: its history and possible future</a>.
   1144 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
   1145 might be redefined
   1146 without Leap Seconds</a> gives pointers on this
   1147 contentious issue.
   1148 The General Conference on Weights and Measures (CGPM)
   1149 <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>
   1150 to discontinue the use of leap seconds by 2035, and requested that no
   1151 discontinuous adjustments be made to UTC for at least a century.
   1152 The World Radiocommunication Conference <a
   1153 href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved
   1154 in 2023</a> to cooperate with this process. A draft <a
   1155 href="https://www.bipm.org/documents/d/guest/cgpm-2026-draft-resolutions">Resolution
   1156 C to make continuous UTC effective on 2027-05-20</a>,
   1157 and thereby discontinue leap seconds,
   1158 has been scheduled for the 28th CGPM starting 2026-10-13 in Paris.
   1159 One proposal to implement this
   1160 would replace leap seconds with seven 13-second leap smears occurring once per
   1161 decade until 2100, with leap smears after that gradually increasing in size.
   1162 See:
   1163 <ul>
   1164 <li>Levine J. <a href="https://tf.nist.gov/general/pdf/3242.pdf">A
   1165 proposal to change the leap-second adjustments to
   1166 coordinated universal time</a>. <em>Metrologia.</em> 2024;61(5):055002. doi:<a
   1167 href="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a>
   1168 with followups in doi:<a
   1169 href="https://doi.org/10.1088/1681-7575/ade314">10.1088/1681-7575/ade314</a>
   1170 and doi:<a
   1171 href="https://doi.org/10.1088/1681-7575/ade315">10.1088/1681-7575/ade315</a>.</li>
   1172 </ul>
   1173 However, there is still no consensus on whether this is the best way
   1174 to replace leap seconds.
   1175 </li>
   1176 </ul>
   1177 </section>
   1178 
   1179 <section>
   1180 <h2 id="notation">Time notation</h2>
   1181 <ul>
   1182 <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
   1183 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
   1184 zone names, abbreviations, identifiers, and formats. For example, it
   1185 contains French translations for Eastern European Summer Time,
   1186 <abbr title="Eastern European Summer Time">EEST</abbr>, and Bucharest. Its
   1187 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
   1188 charts</a> show these values for many locales. Data values are available in
   1189 both <abbr title="Locale Data Markup Language">LDML</abbr>
   1190 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
   1191 <li>
   1192 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
   1193 the international standard date and time notation</a> covers
   1194 <a
   1195 href="https://www.iso.org/standard/70907.html"><em><abbr
   1196 title="International Organization for Standardization">ISO</abbr>
   1197 8601-1:2019  Date and time  Representations for information
   1198 interchange  Part 1: Basic rules</em></a>.</li>
   1199 <li>
   1200 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
   1201 Schema: Datatypes  dateTime</a> specifies a format inspired by
   1202 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
   1203 <li><a href="https://www.rfc-editor.org/rfc/rfc5322#section-3.3">3.3 of
   1204 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
   1205 specifies the time notation used in email and <a
   1206 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
   1207 headers.</li>
   1208 <li>
   1209 <a href="https://www.rfc-editor.org/rfc/rfc3339">Date and Time
   1210 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
   1211 specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.
   1212 An extension, <a href="https://www.rfc-editor.org/rfc/rfc9557">Date
   1213 and Time on the Internet: Timestamps with Additional Information</a>
   1214 (Internet <abbr>RFC</abbr> 9557) extends this profile
   1215 to let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp
   1216 via suffixes like <code>[Asia/Tokyo]</code>.
   1217 <li>
   1218 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date &amp; Time
   1219 Formats on the Web</a> surveys web- and Internet-oriented date and time
   1220 formats.</li>
   1221 <li>Alphabetic time zone abbreviations should not be used as unique
   1222 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
   1223 practice. For example, in English-speaking North America
   1224 <abbr>CST</abbr> denotes 6 hours behind <abbr>UT</abbr>,
   1225 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
   1226 and French-speaking North Americans prefer
   1227 <abbr title="Heure Normale du Centre">HNC</abbr> to
   1228 <abbr>CST</abbr>. The <code><abbr>tz</abbr></code>
   1229 database contains English abbreviations for many timestamps;
   1230 unfortunately some of these abbreviations were merely the database maintainers
   1231 inventions, and these have been removed when possible.</li>
   1232 <li>Numeric time zone abbreviations typically count hours east of
   1233 <abbr>UT</abbr>, e.g., +09 for Japan and
   1234 10 for Hawaii. However, <abbr>POSIX</abbr> proleptic
   1235 <code><abbr>TZ</abbr></code> settings use the opposite convention.
   1236 For example, one might use <code><abbr>TZ</abbr>="<abbr
   1237 title="Japan Standard Time">JST</abbr>-9"</code> and
   1238 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
   1239 for Japan and Hawaii, respectively. If the
   1240 <code><abbr>tz</abbr></code> database is available, it is usually better to use
   1241 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
   1242 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
   1243 confusion, handle old timestamps better, and insulate you better from
   1244 any future changes to the rules. One should never set
   1245 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
   1246 <code>"GMT-9"</code>, though, since this would incorrectly imply that
   1247 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
   1248 is called <abbr>GMT</abbr>.</li>
   1249 </ul>
   1250 </section>
   1251 
   1252 <section>
   1253 <h2 id="see-also">See also</h2>
   1254 <ul>
   1255 <li><a href="theory.html">Theory and pragmatics of the
   1256 <code><abbr>tz</abbr></code> code and data</a></li>
   1257 <li><a href="tz-art.html">Time and the Arts</a></li>
   1258 </ul>
   1259 </section>
   1260 
   1261 <footer>
   1262 <hr>
   1263 This web page is in the public domain, so clarified as of
   1264 2009-05-17 by Arthur David Olson.
   1265 <br>
   1266 Please send corrections to this web page to the
   1267 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
   1268 The mailing list and its archives are public,
   1269 so please do not send confidential information.
   1270 </footer>
   1271 </body>
   1272 </html>
   1273