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4 <title>Time zone and daylight saving time data</title>
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10 <body>
11 <h1>Time zone and daylight saving time data</h1>
12 <p>
13 <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
14 <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
15 rules are controlled by individual
16 governments. They are sometimes changed with little notice, and their
17 histories and planned futures are often recorded only fitfully. Here
18 is a summary of attempts to organize and record relevant data in this
19 area.
20 </p>
21 <h3>Outline</h3>
22 <nav>
23 <ul>
24 <li>The <code><abbr>tz</abbr></code> database product and process
25 <ul>
26 <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
27 <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
28 <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
29 <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
30 </ul>
31 </li>
32 <li>Uses of the <code><abbr>tz</abbr></code> database
33 <ul>
34 <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
35 <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
36 <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
37 <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
38 <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
39 </ul>
40 </li>
41 <li>Related data
42 <ul>
43 <li><a href="#other-dbs">Other time zone databases</a></li>
44 <li><a href="#maps">Maps</a></li>
45 <li><a href="#boundaries">Time zone boundaries</a></li>
46 </ul>
47 </li>
48 <li>Timekeeping concepts
49 <ul>
50 <li><a href="#civil">Civil time concepts and history</a></li>
51 <li><a href="#national">National histories of legal time</a></li>
52 <li><a href="#costs">Costs and benefits of time shifts</a></li>
53 <li><a href="#precision">Precision timekeeping</a></li>
54 <li><a href="#notation">Time notation</a></li>
55 </ul>
56 </li>
57 <li><a href="#see-also">See also</a></li>
58 </ul>
59 </nav>
60
61 <section>
62 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
63 <p>
64 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
65 time zone database contains code and data
66 that represent the history of local time
67 for many representative locations around the globe.
68 It is updated periodically to reflect changes made by political bodies
69 to time zone boundaries and daylight saving rules.
70 This database (known as <code><abbr>tz</abbr></code>,
71 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
72 is used by several implementations,
73 including
74 <a href="https://www.gnu.org/software/libc/">the
75 <abbr title="GNU's Not Unix">GNU</abbr>
76 C Library</a> (used in
77 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
78 <a href="https://www.android.com">Android</a>,
79 <a href="https://www.freebsd.org">Free<abbr
80 title="Berkeley Software Distribution">BSD</abbr></a>,
81 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
82 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
83 <a href="https://www.chromium.org/chromium-os/">Chromium OS</a>,
84 <a href="https://cygwin.com">Cygwin</a>,
85 <a href="https://mariadb.org">MariaDB</a>,
86 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
87 <a href="https://www.mysql.com">MySQL</a>,
88 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
89 title="Web Operating System">webOS</abbr></a>,
90 <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
91 title="Advanced Interactive eXecutive">AIX</abbr></a>,
92 <a href="https://www.apple.com/ios"><abbr
93 title="iPhone OS">iOS</abbr></a>,
94 <a href="https://www.apple.com/macos">macOS</a>,
95 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
96 <a href="https://www.vmssoftware.com">Open<abbr
97 title="Virtual Memory System">VMS</abbr></a>,
98 <a href="https://www.oracle.com/database/">Oracle Database</a>, and
99 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
100 <p>
101 Each main entry in the database represents a <dfn>timezone</dfn>
102 for a set of civil-time clocks that have all agreed since 1970.
103 Timezones are typically identified by continent or ocean and then by the
104 name of the largest city within the region containing the clocks.
105 For example, <code>America/New_York</code>
106 represents most of the <abbr title="United States">US</abbr> eastern time zone;
107 <code>America/Phoenix</code> represents most of Arizona, which
108 uses mountain time without daylight saving time (<abbr>DST</abbr>);
109 <code>America/Detroit</code> represents most of Michigan, which uses
110 eastern time but with different <abbr>DST</abbr> rules in 1975;
111 and other entries represent smaller regions like Starke County,
112 Indiana, which switched from central to eastern time in 1991
113 and switched back in 2006.
114 To use the database on an extended <a
115 href="https://en.wikipedia.org/wiki/POSIX"><abbr
116 title="Portable Operating System Interface">POSIX</abbr></a>
117 implementation set the <code><abbr>TZ</abbr></code>
118 environment variable to the location's full name,
119 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
120 <p>
121 Associated with each timezone is a history of offsets from
122 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
123 Time</a> (<abbr>UT</abbr>), which is <a
124 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
125 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
126 for timestamps after 1960 this is more precisely <a
127 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
128 Universal Time</a> (<abbr>UTC</abbr>).
129 The database also records when daylight saving time was in use,
130 along with some time zone abbreviations such as <abbr>EST</abbr>
131 for Eastern Standard Time in the <abbr>US</abbr>.</p>
132 </section>
133
134 <section>
135 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
136 <p>
137 The following <a
138 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
139 the latest release's two
140 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
141 to a <abbr>GNU</abbr>/Linux or similar host.</p>
142 <pre><code>mkdir tzdb
143 cd tzdb
144 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
145 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
146 <a href="https://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="https://www.gnu.org/software/tar/">tar</a> -xf -
147 gzip -dc tzdata-latest.tar.gz | tar -xf -
148 </code></pre>
149 <p>Alternatively, the following shell commands download the same
150 release in a single-tarball format containing extra data
151 useful for regression testing:</p>
152 <pre><code>wget <a href="https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz">https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz</a>
153 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
154 </code></pre>
155 <p>These commands use convenience links to the latest release
156 of the <code><abbr>tz</abbr></code> database hosted by the
157 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
158 of the <a href="https://www.iana.org">Internet Assigned Numbers
159 Authority (IANA)</a>.
160 Older releases are in files named
161 <code>tzcode<var>V</var>.tar.gz</code>,
162 <code>tzdata<var>V</var>.tar.gz</code>, and
163 <code>tzdb-<var>V</var>.tar.lz</code>,
164 where <code><var>V</var></code> is the version.
165 Since 1996, each version has been a four-digit year followed by
166 lower-case letter (<samp>a</samp> through <samp>z</samp>,
167 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
168 through <samp>zzz</samp>, and so on).
169 Since version 2022a, each release has been distributed in
170 <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
171 ustar interchange format</a>, compressed as described above;
172 older releases use a nearly-compatible format.
173 Since version 2016h, each release has contained a text file named
174 "<samp>version</samp>" whose first (and currently only) line is the version.
175 Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
176 and are also available in an
177 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
178 title="File Transfer Protocol">FTP</abbr> directory</a> via a
179 less-secure protocol.</p>
180 <p>Alternatively, a development repository of code and data can be
181 retrieved from <a href="https://github.com">GitHub</a> via the shell
182 command:</p>
183 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
184 </code></pre>
185 <p>
186 Since version 2012e, each release has been tagged in development repositories.
187 Untagged commits are less well tested and probably contain
188 more errors.</p>
189 <p>
190 After obtaining the code and data files, see the
191 <code>README</code> file for what to do next.
192 The code lets you compile the <code><abbr>tz</abbr></code> source files into
193 machine-readable binary files, one for each location. The binary files
194 are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
195 specified by <a href="https://datatracker.ietf.org/doc/html/8536">Internet
196 <abbr>RFC</abbr> 8536</a>.
197 The code also lets
198 you read a <abbr>TZif</abbr> file and interpret timestamps for that
199 location.</p>
200 </section>
201
202 <section>
203 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
204 <p>
205 The <code><abbr>tz</abbr></code> code and data
206 are by no means authoritative. If you find errors, please
207 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
208 the time zone mailing list. You can also <a
209 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
210 and browse the <a
211 href="https://mm.icann.org/pipermail/tz/">archive of old
212 messages</a>.
213 <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
214 discussions</a> and corresponding data changes can be
215 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
216 </p>
217 <p>
218 If your government plans to change its time zone boundaries or
219 daylight saving rules, inform <code>tz (a] iana.org</code> well in
220 advance, as this will coordinate updates to many cell phones,
221 computers, and other devices around the world.
222 The change should be officially announced at least a year before it affects
223 how clocks operate; otherwise, there is a good chance that some
224 clocks will operate incorrectly after the change, due
225 to delays in propagating updates to software and data. The shorter
226 the notice, the more likely clock problems will arise; see "<a
227 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
228 the Timing of Time Zone Changes</a>" for examples.
229 The <code><abbr>tz</abbr></code> data can represent planned changes
230 far into the future, and a long-planned change can easily be reverted
231 or otherwise altered with a year's notice before the change would have
232 affected clocks.
233 </p>
234 <p>
235 Changes to the <code><abbr>tz</abbr></code> code and data are often
236 propagated to clients via operating system updates, so
237 client <code><abbr>tz</abbr></code> data can often be corrected by
238 applying these updates. With GNU/Linux and similar systems, if your
239 maintenance provider has not yet adopted the
240 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
241 the process by tailoring the generic instructions in
242 the <code><abbr>tz</abbr> README</code> file and installing the latest
243 data yourself. System-specific instructions for installing the
244 latest <code><abbr>tz</abbr></code> data have also been published
245 for <a href="https://developer.ibm.com/articles/au-aix-olson-time-zone/"><abbr>AIX</abbr></a>,
246 <a
247 href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
248 <a
249 href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
250 title="International Components for Unicode">ICU</abbr></a>,
251 <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
252 JDK</a>,
253 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
254 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
255 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
256 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
257 </p>
258 <p>Since version 2013a,
259 sources for the <code><abbr>tz</abbr></code> database have been
260 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
261 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
262 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
263 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
264 title="linefeed">LF</abbr></a>,
265 which can be modified by common text editors such
266 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
267 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
268 <a href="https://www.vim.org">vim</a>.
269 Specialized source-file editing can be done via the
270 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
271 zoneinfo</a> package for <a
272 href="https://www.sublimetext.com">Sublime Text</a> and the <a
273 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
274 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
275 Studio Code</a>.
276 </p>
277 <p>
278 For further information about updates, please see
279 <a href="https://datatracker.ietf.org/doc/html/rfc6557">Procedures for
280 Maintaining the Time Zone Database</a> (Internet <abbr
281 title="Request For Comments">RFC</abbr> 6557). More detail can be
282 found in <a href="theory.html">Theory and pragmatics of the
283 <code><abbr>tz</abbr></code> code and data</a>.
284 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
285 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
286 </p>
287 </section>
288
289 <section>
290 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
291 <ul>
292 <li>The article
293 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
294 an encyclopedic summary.</li>
295 <li><a href="tz-how-to.html">How to Read the
296 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
297 database format.</li>
298 <li><a
299 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
300 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
301 database's style.</li>
302 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
303 managing time in the internet</a> analyzes the database longitudinally.</li>
304 </ul>
305 </section>
306
307 <section>
308 <h2 id="web">Web sites using recent versions of the
309 <code><abbr>tz</abbr></code> database</h2>
310 <p>
311 These are listed roughly in ascending order of complexity and fanciness.
312 </p>
313 <ul>
314 <li><a href="https://time.is">Time.is</a> shows locations'
315 time and zones.</li>
316 <li><a href="https://www.timejones.com">TimeJones.com</a>,
317 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
318 <a href="https://www.worldclock.com">The World Clock</a>
319 are time zone converters.</li>
320 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
321 publishes <code><abbr>tzdb</abbr></code>-derived data in
322 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
323 title="comma-separated values">CSV</abbr></a> and
324 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
325 title="Structured Query Language">SQL</abbr></a> form.</li>
326 <li><a
327 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
328 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
329 <li><a
330 href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
331 Time in 1000 Places</a> uses descriptions of the values.</li>
332 <li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
333 timezone information for all countries</a>
334 displays tables of <abbr>DST</abbr> rules.
335 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
336 Worldwide</a> lets you sort zone names and convert times.</li>
337 <li><a href="https://24timezones.com">24TimeZones</a> has a world
338 time map and a time converter.</li>
339 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
340 calculates the current time difference between locations.</li>
341 <li><a href="https://www.wx-now.com">Weather Now</a> and
342 <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
343 </ul>
344 </section>
345
346 <section>
347 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
348 <ul>
349 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
350 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
351 Distribution Service (tzdist) working group</a> defined <a
352 href="https://datatracker.ietf.org/doc/html/rfc7808">TZDIST</a>
353 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
354 along with <a href="https://datatracker.ietf.org/doc/html/rfc7809">CalDAV</a>
355 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
356 transferring time zone data by reference.
357 <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
358 implementations</a> are available.
359 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
360 mailing list</a> discusses possible extensions.</li>
361 <li>The <a href="https://datatracker.ietf.org/doc/html/rfc5545">
362 Internet Calendaring and Scheduling Core Object Specification
363 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
364 covers time zone
365 data; see its VTIMEZONE calendar component.
366 The iCalendar format requires specialized parsers and generators; a
367 variant <a href="https://datatracker.ietf.org/doc/html/rfc6321">xCal</a>
368 (Internet <abbr>RFC</abbr> 6321) uses
369 <a href="https://www.w3.org/XML/"><abbr
370 title="Extensible Markup Language">XML</abbr></a> format, and a variant
371 <a href="https://datatracker.ietf.org/doc/html/rfc7265">jCal</a>
372 (Internet <abbr>RFC</abbr> 7265)
373 uses <a href="https://www.json.org"><abbr
374 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
375 </ul>
376 </section>
377
378 <section>
379 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
380 <p>Although some of these do not fully support
381 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
382 distributions you can generally work around compatibility problems by
383 running the command <code>make rearguard_tarballs</code> and compiling
384 from the resulting tarballs instead.</p>
385 <ul>
386 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
387 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
388 program that compiles
389 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
390 Vzic is freely
391 available under the <a
392 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
393 General Public License (<abbr
394 title="General Public License">GPL</abbr>)</a>.</li>
395 <li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz
396 database conversion utility</a> is like Vzic, except for the <a
397 href="https://dotnet.microsoft.com">.NET framework</a>
398 and with a <abbr>BSD</abbr>-style license.</li>
399 <li><a
400 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
401 contains a script <code>parse_olson</code> that compiles
402 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
403 modules. It is part of the Perl <a
404 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
405 which is freely
406 available under both the <abbr>GPL</abbr> and the Perl Artistic
407 License. DateTime::TimeZone also contains a script
408 <code>tests_from_zdump</code> that generates test cases for each clock
409 transition in the <code><abbr>tz</abbr></code> database.</li>
410 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
411 Database Parser</a> is a
412 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
413 runtime library with <a
414 href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
415 adopted by
416 <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
417 the current iteration of the C++ standard.
418 It is freely available under the
419 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
420 <li><a id="ICU" href="https://icu.unicode.org">International Components for
421 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
422 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
423 libraries for internationalization that
424 has a compiler from <code><abbr>tz</abbr></code> source
425 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
426 (mentioned <a href="#CLDR">below</a>)
427 into an <abbr>ICU</abbr>-specific format.
428 <abbr>ICU</abbr> is freely available under a
429 <abbr>BSD</abbr>-style license.</li>
430 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
431 the <a href="https://elixir-lang.org">Elixir</a> language downloads
432 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
433 title="Application Program Interface">API</abbr>s for use. It is
434 freely available under the <abbr>MIT</abbr> license.</li>
435 <li>Java-based compilers and libraries include:
436 <ul>
437 <li>The <a
438 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
439 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
440 <a href="https://openjdk.java.net/">OpenJDK</a> and
441 <a href="https://jdk.java.net/">Oracle JDK</a>.
442 Although its source code is proprietary, its executable is available under the
443 <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
444 Timezone Updater License Agreement</a>.</li>
445 <li>The <a
446 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
447 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
448 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
449 which is freely available under a <abbr>BSD</abbr>-style license.</li>
450 <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
451 and time <abbr>API</abbr></a> contains a class
452 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
453 <code><abbr>tz</abbr></code> source into a binary format. It inspired
454 Java 8 <code>java.time</code>, which its users should migrate to once
455 they can assume Java 8 or later. It is available under the <a
456 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
457 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
458 href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
459 are alternatives to TZUpdater. IANA Updater's license is unclear;
460 ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
461 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
462 time library for Android</a> and
463 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
464 time and interval library for Java</a> compile
465 <code><abbr>tz</abbr></code> source into a binary format.
466 Time4A is available under the Apache License and Time4J is
467 available under the <a
468 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
469 General Public License (<abbr title="Lesser General Public
470 License">LGPL</abbr>)</a>.</li>
471 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
472 Java-based libraries.</li>
473 </ul>
474 <li><a href="https://nodatime.org">Noda Time – Date and
475 time <abbr>API</abbr> for .NET</a>
476 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
477 It is freely available under the Apache License.</li>
478 <li>Many modern
479 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
480 runtimes support <code><abbr>tz</abbr></code> natively via the
481 <samp>timeZone</samp> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><samp>Intl.DateTimeFormat</samp></a>.
482 This can be used as-is or with most of the following libraries,
483 many of which also support runtimes lacking the <samp>timeZone</samp> option.
484 <ul>
485 <li>The <a
486 href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
487 timezone polyfill</a>
488 is freely available under a <abbr>BSD</abbr>-style license.</li>
489 <li>The <a href="https://date-fns.org/">date-fns</a>
490 library manipulates timezone-aware timestamps in browsers and
491 in <a href="https://nodejs.org/en/">Node.js</a>.
492 It is freely available under the <abbr>MIT</abbr> license.</li>
493 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
494 minimalist replacement for the date and time API of
495 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
496 manipulation library.
497 It is freely available under the <abbr>MIT</abbr> license.</li>
498 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
499 timezone support for the <samp>Intl</samp> API.
500 It is freely available under the <abbr>MIT</abbr> license.</li>
501 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
502 Moment.js plugin.
503 It is freely available under the <abbr>MIT</abbr> license.</li>
504 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
505 JavaScript library that supports date arithmetic that is time zone
506 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
507 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
508 supports live <code><abbr>tzdb</abbr></code> updates,
509 astronomical and atomic time, a command-line interface,
510 and full <a
511 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
512 Its companion <a
513 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
514 can generate <abbr>TZif</abbr> and other files, and a companion website
515 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
516 convert timestamps, view transition histories, and download code and data.
517 It is freely available under the <abbr>MIT</abbr> license.</li>
518 </ul>
519 The proposed <a
520 href="https://github.com/tc39/proposal-temporal"><samp>Temporal</samp>
521 objects</a> let programs access an abstract view of
522 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
523 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
524 problematic <samp>Date</samp> objects</a> when working with dates and times.
525 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
526 compiler from <code><abbr>tz</abbr></code> source into
527 <a href="https://julialang.org/">Julia</a>. It is freely available
528 under the <abbr>MIT</abbr> license.</li>
529 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
530 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
531 title="Free Pascal Compiler">FPC</abbr></a>
532 compiles from <code><abbr>tz</abbr></code> source into
533 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
534 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
535 and <a
536 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
537 It is freely available under a <abbr>BSD</abbr>-style license.</li>
538 <li><a href="http://pytz.sourceforge.net">pytz – World Timezone
539 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
540 <a href="https://www.python.org">Python</a>.
541 It is freely available under a <abbr>BSD</abbr>-style license.
542 In code that can assume Python 3.6 or later it is largely superseded; see <a
543 href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
544 The Fastest Footgun in the West</a>.</li>
545 <li><a href="https://tzinfo.github.io">TZInfo –
546 Ruby Timezone Library</a>
547 compiles <code><abbr>tz</abbr></code> source into
548 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
549 It is freely available under the <abbr>MIT</abbr> license.</li>
550 <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
551 Library</a> is
552 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
553 library that compiles <code><abbr>tz</abbr></code> source into a time
554 zone repository whose format
555 is either proprietary or an <abbr>XML</abbr>-encoded
556 representation.</li>
557 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
558 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
559 source into text files, along with a runtime that can read those
560 files. Tcl is freely available under a <abbr>BSD</abbr>-style
561 license.</li>
562 </ul>
563 </section>
564
565 <section>
566 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
567 <ul>
568 <li>The <a
569 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
570 Library</a>
571 has an independent, thread-safe implementation of
572 a <abbr>TZif</abbr> file reader.
573 This library is freely available under the LGPL
574 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
575 <li><a href="https://www.gnome.org">GNOME</a>'s
576 <a href="https://developer.gnome.org/glib/">GLib</a> has
577 a <abbr>TZif</abbr> file reader written in C that
578 creates a <code>GTimeZone</code> object representing sets
579 of <abbr>UT</abbr> offsets.
580 It is freely available under the <abbr>LGPL</abbr>.</li>
581 <li>The
582 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
583 <code>baltzo::TimeZoneUtil</code> component contains a C++
584 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
585 the Apache License.</li>
586 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
587 library that translates between <abbr>UT</abbr> and civil time and
588 can read <abbr>TZif</abbr> files. It is freely available under the Apache
589 License.</li>
590 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
591 library that reads <abbr>TZif</abbr> files and converts
592 timestamps from one time zone or format to another.
593 It is used by <a href="https://secure.php.net"><abbr
594 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
595 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
596 and <a href="https://www.mongodb.com">MongoDB</a>.
597 It is freely available under the <abbr>MIT</abbr> license.</li>
598 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
599 <abbr>TZif</abbr> file reader.</li>
600 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
601 DateTime::TimeZone::Tzfile</a>
602 is a <abbr>TZif</abbr> file reader written in Perl.
603 It is freely available under the same terms as Perl
604 (dual <abbr>GPL</abbr> and Artistic license).</li>
605 <li>Python has a <a id="python-zoneinfo"
606 href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
607 class</a> that reads <abbr>TZif</abbr> data and creates objects
608 that represent <code><abbr>tzdb</abbr></code> timezones.
609 Python is freely available under the
610 <a href="https://docs.python.org/3/license.html">Python Software Foundation
611 License</a>.
612 A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
613 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
614 supplies TZif data if the underlying system data cannot be found;
615 it is freely available under the Apache License.</li>
616 <li>The
617 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
618 library contains a Python tool that
619 converts <abbr>TZif</abbr> data into
620 <abbr>JSON</abbr>-format data suitable for use
621 in its JavaScript library for time zone conversion. Dates before 1970
622 are not supported.</li>
623 <li>The <a
624 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
625 package contains <a href="https://www.haskell.org">Haskell</a> code that
626 parses and uses <abbr>TZif</abbr> data. It is freely
627 available under a <abbr>BSD</abbr>-style license.</li>
628 </ul>
629 </section>
630
631 <section>
632 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
633 <ul>
634 <li><a href="https://foxclocks.org">FoxClocks</a>
635 is an extension for <a href="https://www.google.com/chrome/">Google
636 Chrome</a> and for <a
637 href="https://wiki.mozilla.org/Modules/Toolkit">Mozilla
638 Toolkit</a> applications like <a
639 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
640 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
641 It displays multiple clocks in the application window, and has a mapping
642 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
643 It is freely available under the <abbr>GPL</abbr>.</li>
644 <li><a href="https://golang.org">Go programming language</a>
645 implementations contain a copy of a 32-bit subset of a recent
646 <code><abbr>tz</abbr></code> database in a
647 Go-specific format.</li>
648 <li>Microsoft Windows 8.1
649 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
650 data (mentioned <a href="#CLDR">below</a>) used by the
651 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
652 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
653 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
654 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
655 <a id="System.TimeZoneInfo"
656 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
657 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
658 the older, proprietary method of Microsoft Windows 2000 and later,
659 which stores time zone data in the
660 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
661 <a
662 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
663 Tzid table</a> or <a
664 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
665 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
666 to <code><abbr>tz</abbr></code> names.
667 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
668 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
669 <li><a
670 href="https://www.oracle.com/java/index.html">Oracle
671 Java</a> contains a copy of a subset of a recent
672 <code><abbr>tz</abbr></code> database in a
673 Java-specific format.</li>
674 </ul>
675 </section>
676
677 <section>
678 <h2 id="other-dbs">Other time zone databases</h2>
679 <ul>
680 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
681 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
682 time zone history atlases
683 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
684 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
685 Although these extensive atlases
686 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
687 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
688 they are unreliable as Shanks appears to have
689 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
690 sources and do not indicate which entries are guesswork.</li>
691 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
692 its own <code>tztab</code>(4) format.</li>
693 <li>Microsoft Windows has proprietary data mentioned
694 <a href="#System.TimeZoneInfo">above</a>.</li>
695 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
696 is another time zone database.</li>
697 <li>The <a
698 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
699 Schedules Information Manual</a> of the
700 International Air Transport Association
701 gives current time zone rules for airports served by commercial aviation.</li>
702 </ul>
703 </section>
704
705 <section>
706 <h2 id="maps">Maps</h2>
707 <ul>
708 <li>The <a
709 href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
710 and Regional Maps section</a> of <em>The World Factbook</em>, published by the
711 <a href="https://www.cia.gov">US Central Intelligence
712 Agency (<abbr
713 title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
714 zone map; the
715 <a
716 href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda
717 Library Map Collection</a>
718 of the University of Texas at Austin has copies of
719 recent editions.
720 The pictorial quality is good,
721 but the maps do not indicate daylight saving time,
722 and parts of the data are a few years out of date.</li>
723 <li><a href="https://www.worldtimezone.com">World Time Zone Map
724 with current time</a>
725 has several fancy time zone maps; it covers Russia particularly well.
726 The maps' pictorial quality is not quite as good as the
727 <abbr>CIA</abbr>'s
728 but the maps are more up to date.</li>
729 <li><a
730 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
731 much is time wrong around the world?</a> maps the difference between
732 mean solar and standard time, highlighting areas such as western China
733 where the two differ greatly. It's a bit out of date, unfortunately.</li>
734 </ul>
735 </section>
736
737 <section>
738 <h2 id="boundaries">Time zone boundaries</h2>
739 <p>Geographical boundaries between timezones are available
740 from several <a
741 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
742 geolocation</a>
743 services and other sources.</p>
744 <ul>
745 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
746 Boundary Builder</a> extracts
747 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
748 boundaries of <code><abbr>tzdb</abbr></code> timezones.
749 Its code is freely available under the <abbr>MIT</abbr> license, and
750 its data entries are freely available under the
751 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
752 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
753 <li>Programmatic interfaces that map geographical coordinates via tz_world to
754 <code><abbr>tzdb</abbr></code> timezones include:
755 <ul>
756 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
757 written in <a
758 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
759 and is freely available under the <abbr>MIT</abbr> license.</li>
760 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
761 is written in Go and is freely available under the Apache License.</li>
762 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
763 in both Java and
764 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
765 form, is freely available under the MIT license.</li>
766 <li>For Node.js,
767 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
768 is freely available under the MIT license, and
769 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
770 is in the public domain.</li>
771 <li>The <a
772 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
773 library for Python is freely available under the MIT license.
774 <li>The <a
775 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
776 library for Ruby is freely available under the MIT license.</li>
777 </ul></li>
778 <li>Free access via a network API, if you register a key, is provided by
779 the <a
780 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
781 Timezone web service</a>, the <a
782 href="https://developers.google.com/maps/documentation/timezone/intro">Google
783 Maps Time Zone API</a>, and
784 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
785 Commercial network API access is provided
786 by <a href="https://askgeo.com">AskGeo</a>
787 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
788 </li>
789 <li>"<a
790 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
791 to get a time zone from a location using latitude and longitude
792 coordinates?</a>" discusses other geolocation possibilities.</li>
793 <li><a href="http://statoids.com/statoids.html">Administrative
794 Divisions of Countries ("Statoids")</a> lists
795 political subdivision data related to time zones.</li>
796 <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
797 zone boundaries for multizone countries</a> summarizes legal
798 boundaries between time zones within countries.</li>
799 <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
800 – GIS and Database Tools</a> includes a Manifold-format map of
801 world time zone boundaries distributed under the
802 <abbr>GPL</abbr>.</li>
803 <li>A ship within the <a
804 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
805 waters</a> of any nation uses that nation's time. In international
806 waters, time zone boundaries are meridians 15° apart, except that
807 <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
808 wide and are separated by
809 the 180° meridian (not by the International Date Line, which is
810 for land and territorial waters only). A captain can change ship's
811 clocks any time after entering a new time zone; midnight changes are
812 common.</li>
813 </ul>
814 </section>
815
816 <section>
817 <h2 id="civil">Civil time concepts and history</h2>
818 <ul>
819 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
820 Walk through Time</a>
821 surveys the evolution of timekeeping.</li>
822 <li>The history of daylight saving time is surveyed in <a
823 href="http://www.webexhibits.org/daylightsaving/">About Daylight
824 Saving Time – History, rationale, laws & dates</a> and summarized in
825 <a href="http://seizethedaylight.com/dst/">A Brief
826 History of Daylight Saving Time</a>.</li>
827 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
828 Lords</a> discusses how authoritarians manipulate civil time.</li>
829 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
830 contains guidelines and best practices for software applications that
831 deal with civil time.</li>
832 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
833 the International Date Line</a> tells the story of the most important
834 time zone boundary.</li>
835 <li><a href="http://statoids.com/tconcept.html">Basic Time
836 Zone Concepts</a> discusses terminological issues behind time zones.</li>
837 </ul>
838 </section>
839
840 <section>
841 <h2 id="national">National histories of legal time</h2>
842 <dl>
843 <dt>Australia</dt>
844 <dd>The Parliamentary Library commissioned a <a
845 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
846 paper on daylight saving time in Australia</a>.
847 The Bureau of Meteorology publishes a list of <a
848 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
849 Dates of Daylight Savings Time within Australia</a>.</dd>
850 <dt>Belgium</dt>
851 <dd>The Royal Observatory of Belgium maintains a table of time in
852 Belgium (in
853 <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
854 hreflang="nl">Dutch</a> and <a
855 href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
856 hreflang="fr">French</a>).</dd>
857 <dt>Brazil</dt>
858 <dd>The Time Service Department of the National Observatory
859 records <a href="http://pcdsh01.on.br/DecHV.html"
860 hreflang="pt-BR">Brazil's daylight saving time decrees (in
861 Portuguese)</a>.</dd>
862 <dt>Canada</dt>
863 <dd>National Research Council Canada publishes current
864 and some older information about <a
865 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
866 zones and daylight saving time</a>.</dd>
867 <dt>Chile</dt>
868 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
869 <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
870 Chile's official time (in Spanish)</a>.</dd>
871 <dt>China</dt>
872 <dd>The Hong Kong Observatory maintains a
873 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
874 summer time in Hong Kong</a>,
875 and Macau's Meteorological and Geophysical Bureau maintains a <a
876 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
877 history for Macau</a>.
878 Unfortunately the latter is incomplete and has errors.</dd>
879 <dt>Czech Republic</dt>
880 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
881 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
882 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
883 <dt>Germany</dt>
884 <dd>The National Institute for Science and Technology maintains the <a
885 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
886 of Legal Time in Germany</a>.</dd>
887 <dt>Israel</dt>
888 <dd>The Interior Ministry periodically issues <a
889 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
890 hreflang="he">announcements (in Hebrew)</a>.</dd>
891 <dt>Malaysia</dt>
892 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
893 <dt>Mexico</dt>
894 <dd>The Investigation and Analysis Service of the Mexican Library of
895 Congress has published a <a
896 href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
897 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
898 <dt>Netherlands</dt>
899 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
900 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
901 covers the history of local time in the Netherlands from ancient times.</dd>
902 <dt>New Zealand</dt>
903 <dd>The Department of Internal Affairs maintains a brief <a
904 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
905 Daylight Saving</a>.</dd>
906 <dt>Portugal</dt>
907 <dd>The Lisbon Astronomical Observatory publishes a
908 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
909 legal time (in Portuguese)</a>.</dd>
910 <dt>Singapore</dt>
911 <dd><a id="Singapore"
912 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
913 is Singapore in the "Wrong" Time Zone?</a> details the
914 history of legal time in Singapore and Malaysia.</dd>
915 <dt>United Kingdom</dt>
916 <dd><a
917 href="https://www.polyomino.org.uk/british-time/">History of
918 legal time in Britain</a> discusses in detail the country
919 with perhaps the best-documented history of clock adjustments.</dd>
920 <dt>United States</dt>
921 <dd>The Department of Transportation's <a
922 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
923 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
924 <dt>Uruguay</dt>
925 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
926 Navy (SOHMA) publishes an annual <a
927 href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
928 (in Spanish)</a>.</dd>
929 </dl>
930 </section>
931
932 <section>
933 <h2 id="costs">Costs and benefits of time shifts</h2>
934 <p>Various sources argue for and against daylight saving time and time
935 zone shifts, and many scientific studies have been conducted. This
936 section summarizes reviews and position statements based on
937 scientific literature in the area.</p>
938 <ul>
939 <li>Carey RN, Sarma KM.
940 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
941 daylight saving time on road traffic collision risk: a systematic
942 review</a>.
943 <em>BMJ Open.</em> 2017;7(6):e014319. doi:<a href="https://doi.org/10.1136/bmjopen-2016-014319">10.1136/bmjopen-2016-014319</a>.
944 This reviews research literature and concludes that the evidence
945 neither supports nor refutes road safety benefits from
946 shifts in time zones.</li>
947 <li>Havranek T, Herman D, Irsova D.
948 <a href="https://www.iaee.org/en/publications/ejarticle.aspx?id=3051">Does
949 daylight saving save electricity? A meta-analysis.</a>
950 <em>Energy J.</em> 2018;39(2):35–61.
951 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
952 This analyzes research literature and concludes, "Electricity savings
953 are larger for countries farther away from the equator, while
954 subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
955 <li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
956 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
957 an American Academy of Sleep Medicine position statement</a>.
958 <em>J Clin Sleep Med.</em>
959 2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
960 This argues for permanent standard time due to health risks of both
961 <abbr>DST</abbr> transitions and permanent <abbr>DST</abbr>.</li>
962 <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
963 <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7205184/">Why
964 should we abolish Daylight Saving Time?</a>
965 <em>J Biol Rhythms</em>. 2019;34(3):227–230.
966 doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
967 This position paper of the Society for Research on Biological Rhythms
968 opposes DST changes and permanent DST, and advocates that governments adopt
969 "permanent Standard Time for the health and safety of their citizens".</li>
970 </ul>
971 </section>
972
973 <section>
974 <h2 id="precision">Precision timekeeping</h2>
975 <ul>
976 <li><a
977 href="http://leapsecond.com/hpan/an1289.pdf">The
978 Science of Timekeeping</a> is a thorough introduction
979 to the theory and practice of precision timekeeping.</li>
980 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
981 Time 2016</a> contains several freely-readable papers.</li>
982 <li><a href="https://www.ntp.org"><abbr
983 title="Network Time Protocol">NTP</abbr>: The Network
984 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
985 discusses how to synchronize clocks of
986 Internet hosts.</li>
987 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
988 family of software algorithms can achieve accuracy to a few tens of
989 nanoseconds in scalable server farms without special hardware.</li>
990 <li>The <a
991 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
992 Time Protocol</a> (<abbr
993 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
994 can achieve submicrosecond clock accuracy on a local area network
995 with special-purpose hardware.</li>
996 <li><a
997 href="https://datatracker.ietf.org/doc/html/rfc4833">Timezone
998 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
999 (Internet <abbr>RFC</abbr> 4833)
1000 specifies a <a
1001 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
1002 option for a server to configure
1003 a client's time zone and daylight saving settings automatically.</li>
1004 <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
1005 Scales</a> describes astronomical time scales like
1006 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
1007 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
1008 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
1009 <li>The <a href="https://www.iau.org"><abbr
1010 title="International Astronomical Union">IAU</abbr></a>'s <a
1011 href="https://www.iausofa.org"><abbr
1012 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
1013 collection contains C and <a
1014 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
1015 code for converting among time scales like
1016 <abbr title="International Atomic Time">TAI</abbr>,
1017 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
1018 <abbr>UTC</abbr>. It is freely available under the
1019 <a href="https://www.iausofa.org/tandc.html">SOFA license</a>.</li>
1020 <li><a
1021 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
1022 – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
1023 diverse local time
1024 scales used by each landed mission on Mars.</li>
1025 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
1026 dedicated not only to leap seconds but to precise time and frequency
1027 in general. It covers the state of the art in amateur timekeeping, and
1028 how the art has progressed over the past few decades.</li>
1029 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
1030 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
1031 and time-signal emissions</a>, International Telecommunication Union –
1032 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
1033 <li><a
1034 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
1035 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
1036 Bulletins</a> contains official publications of the International
1037 Earth Rotation and Reference Systems Service, which decides when leap
1038 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
1039 via an optional "<code>right</code>" configuration where a computer's internal
1040 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
1041 as opposed to the default "<code>posix</code>" configuration
1042 where the internal clock ignores leap seconds.
1043 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
1044 <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for
1045 timestamps starting with <code>time_t</code> 78 796 800,
1046 which corresponds to the first leap second
1047 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
1048 and to
1049 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
1050 In practice the two configurations also agree for timestamps before
1051 1972 even though the historical situation is messy, partly because
1052 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
1053 is well-defined for sufficiently-old timestamps.</li>
1054 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1055 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1056 leap second so that they disagree with <abbr>UTC</abbr> by at most a
1057 half second, even though every <abbr>POSIX</abbr> minute has exactly
1058 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1059 "<code>posix</code>" configuration, is <a
1060 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
1061 the <abbr>NTP</abbr> reference implementation, <a
1062 href="https://github.com/google/unsmear">supports</a> conversion between
1063 <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
1064 cloud service providers. However, according to
1065 <a href="https://datatracker.ietf.org/doc/html/rfc8633#section-3.7.1">§3.7.1 of
1066 Network Time Protocol Best Current Practices</a>
1067 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1068 applications requiring accurate <abbr>UTC</abbr> or civil time,
1069 and is intended for use only in single, well-controlled environments.</li>
1070 <li>The <a
1071 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
1072 Second Discussion List</a> covers <a
1073 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
1074 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
1075 discussed further in
1076 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1077 leap second: its history and possible future</a>.
1078 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1079 might be redefined
1080 without Leap Seconds</a> gives pointers on this
1081 contentious issue, which was active until 2015 and could become active
1082 again.</li>
1083 </ul>
1084 </section>
1085
1086 <section>
1087 <h2 id="notation">Time notation</h2>
1088 <ul>
1089 <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
1090 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1091 zone names, abbreviations, identifiers, and formats. For example, it
1092 contains French translations for "Eastern European Summer Time",
1093 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
1094 "Bucharest". Its
1095 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1096 charts</a> show these values for many locales. Data values are available in
1097 both <abbr title="Locale Data Markup Language">LDML</abbr>
1098 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1099 <li>
1100 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1101 the international standard date and time notation</a> covers
1102 <a
1103 href="https://www.iso.org/standard/70907.html"><em><abbr
1104 title="International Organization for Standardization">ISO</abbr>
1105 8601-1:2019 – Date and time – Representations for information
1106 interchange – Part 1: Basic rules</em></a>.</li>
1107 <li>
1108 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1109 Schema: Datatypes – dateTime</a> specifies a format inspired by
1110 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1111 <li><a href="https://datatracker.ietf.org/doc/html/rfc5322#section-3.3">§3.3 of
1112 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1113 specifies the time notation used in email and <a
1114 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1115 headers.</li>
1116 <li>
1117 <a href="https://datatracker.ietf.org/doc/html/rfc3339">Date and Time
1118 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1119 specifies an <abbr>ISO</abbr> 8601
1120 profile for use in new Internet
1121 protocols.</li>
1122 <li>
1123 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1124 Formats on the Web</a> surveys web- and Internet-oriented date and time
1125 formats.</li>
1126 <li>Alphabetic time zone abbreviations should not be used as unique
1127 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1128 practice. For example, in English-speaking North America
1129 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
1130 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1131 and French-speaking North Americans prefer
1132 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
1133 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
1134 database contains English abbreviations for many timestamps;
1135 unfortunately some of these abbreviations were merely the database maintainers'
1136 inventions, and these have been removed when possible.</li>
1137 <li>Numeric time zone abbreviations typically count hours east of
1138 <abbr>UT</abbr>, e.g., +09 for Japan and
1139 −10 for Hawaii. However, the <abbr>POSIX</abbr>
1140 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
1141 For example, one might use <code><abbr>TZ</abbr>="<abbr
1142 title="Japan Standard Time">JST</abbr>-9"</code> and
1143 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1144 for Japan and Hawaii, respectively. If the
1145 <code><abbr>tz</abbr></code> database is available, it is usually better to use
1146 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1147 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1148 confusion, handle old timestamps better, and insulate you better from
1149 any future changes to the rules. One should never set
1150 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1151 <code>"GMT-9"</code>, though, since this would incorrectly imply that
1152 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1153 is called "<abbr>GMT</abbr>".</li>
1154 </ul>
1155 </section>
1156
1157 <section>
1158 <h2 id="see-also">See also</h2>
1159 <ul>
1160 <li><a href="theory.html">Theory and pragmatics of the
1161 <code><abbr>tz</abbr></code> code and data</a></li>
1162 <li><a href="tz-art.html">Time and the Arts</a></li>
1163 </ul>
1164 </section>
1165
1166 <footer>
1167 <hr>
1168 This web page is in the public domain, so clarified as of
1169 2009-05-17 by Arthur David Olson.
1170 <br>
1171 Please send corrections to this web page to the
1172 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1173 </footer>
1174 </body>
1175 </html>
1176