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10 <body>
11 <h1>Time zone and daylight saving time data</h1>
12 <p>
13 <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
14 <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
15 rules are controlled by individual
16 governments. They are sometimes changed with little notice, and their
17 histories and planned futures are often recorded only fitfully. Here
18 is a summary of attempts to organize and record relevant data in this
19 area.
20 </p>
21 <h3>Outline</h3>
22 <nav>
23 <ul>
24 <li>The <code><abbr>tz</abbr></code> database product and process
25 <ul>
26 <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
27 <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
28 <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
29 <li><a href="#coordinating">Coordinating with governments and distributors</a></li>
30 <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
31 </ul>
32 </li>
33 <li>Uses of the <code><abbr>tz</abbr></code> database
34 <ul>
35 <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
36 <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
37 <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
38 <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
39 <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
40 </ul>
41 </li>
42 <li>Related data
43 <ul>
44 <li><a href="#other-dbs">Other time zone databases</a></li>
45 <li><a href="#maps">Maps</a></li>
46 <li><a href="#boundaries">Time zone boundaries</a></li>
47 </ul>
48 </li>
49 <li>Timekeeping concepts
50 <ul>
51 <li><a href="#civil">Civil time concepts and history</a></li>
52 <li><a href="#national">National histories of legal time</a></li>
53 <li><a href="#costs">Costs and benefits of time shifts</a></li>
54 <li><a href="#precision">Precision timekeeping</a></li>
55 <li><a href="#notation">Time notation</a></li>
56 </ul>
57 </li>
58 <li><a href="#see-also">See also</a></li>
59 </ul>
60 </nav>
61
62 <section>
63 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
64 <p>
65 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
66 time zone database contains code and data
67 that represent the history of local time
68 for many representative locations around the globe.
69 It is updated periodically to reflect changes made by political bodies
70 to time zone boundaries and daylight saving rules.
71 This database (known as <code><abbr>tz</abbr></code>,
72 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
73 is used by several implementations,
74 including
75 <a href="https://www.gnu.org/software/libc/">the
76 <abbr title="GNU's Not Unix">GNU</abbr>
77 C Library</a> (used in
78 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
79 <a href="https://www.android.com">Android</a>,
80 <a href="https://www.freebsd.org">Free<abbr
81 title="Berkeley Software Distribution">BSD</abbr></a>,
82 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
83 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
84 <a href="https://www.chromium.org/chromium-os/">ChromiumOS</a>,
85 <a href="https://cygwin.com">Cygwin</a>,
86 <a href="https://mariadb.org">MariaDB</a>,
87 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
88 <a href="https://musl.libc.org">musl libc</a>,
89 <a href="https://www.mysql.com">MySQL</a>,
90 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
91 title="Web Operating System">webOS</abbr></a>,
92 <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
93 title="Advanced Interactive eXecutive">AIX</abbr></a>,
94 <a href="https://www.apple.com/ios"><abbr
95 title="iPhone OS">iOS</abbr></a>,
96 <a href="https://www.apple.com/macos">macOS</a>,
97 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
98 <a href="https://www.vmssoftware.com">Open<abbr
99 title="Virtual Memory System">VMS</abbr></a>,
100 <a href="https://www.oracle.com/database/">Oracle Database</a>, and
101 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
102 <p>
103 Each main entry in the database represents a <dfn>timezone</dfn>
104 for a set of civil-time clocks that have all agreed since 1970.
105 Timezones are typically identified by continent or ocean and then by the
106 name of the largest city within the region containing the clocks.
107 For example, <code>America/New_York</code>
108 represents most of the <abbr title="United States">US</abbr> eastern time zone;
109 <code>America/Phoenix</code> represents most of Arizona, which
110 uses mountain time without daylight saving time (<abbr>DST</abbr>);
111 <code>America/Detroit</code> represents most of Michigan, which uses
112 eastern time but with different <abbr>DST</abbr> rules in 1975;
113 and other entries represent smaller regions like Starke County,
114 Indiana, which switched from central to eastern time in 1991
115 and switched back in 2006.
116 To use the database on a <a
117 href="https://en.wikipedia.org/wiki/POSIX"><abbr
118 title="Portable Operating System Interface">POSIX</abbr>.1-2024</a>
119 implementation set the <code><abbr>TZ</abbr></code>
120 environment variable to the location's full name,
121 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
122 <p>
123 Associated with each timezone is a history of offsets from
124 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
125 Time</a> (<abbr>UT</abbr>), which is <a
126 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
127 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
128 for timestamps after 1960 this is more precisely <a
129 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
130 Universal Time</a> (<abbr>UTC</abbr>).
131 The database also records when daylight saving time was in use,
132 along with some time zone abbreviations such as <abbr>EST</abbr>
133 for Eastern Standard Time in the <abbr>US</abbr>.</p>
134 </section>
135
136 <section>
137 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
138 <p>
139 The following <a
140 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
141 the latest release's two
142 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
143 to a <abbr>GNU</abbr>/Linux or similar host.</p>
144 <pre><code>mkdir tzdb
145 cd tzdb
146 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
147 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
148 <a href="https://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="https://www.gnu.org/software/tar/">tar</a> -xf -
149 gzip -dc tzdata-latest.tar.gz | tar -xf -
150 </code></pre>
151 <p>Alternatively, the following shell commands download the same
152 release in a single-tarball format containing extra data
153 useful for regression testing:</p>
154 <pre><code>wget <a href="https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz">https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz</a>
155 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
156 </code></pre>
157 <p>These commands use convenience links to the latest release
158 of the <code><abbr>tz</abbr></code> database hosted by the
159 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
160 of the <a href="https://www.iana.org">Internet Assigned Numbers
161 Authority (IANA)</a>.
162 Older releases are in files named
163 <code>tzcode<var>V</var>.tar.gz</code>,
164 <code>tzdata<var>V</var>.tar.gz</code>, and
165 <code>tzdb-<var>V</var>.tar.lz</code>,
166 where <code><var>V</var></code> is the version.
167 Since 1996, each version has been a four-digit year followed by
168 lower-case letter (<samp>a</samp> through <samp>z</samp>,
169 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
170 through <samp>zzz</samp>, and so on).
171 Since version 2022a, each release has been distributed in
172 <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
173 ustar interchange format</a>, compressed as described above;
174 older releases use a nearly compatible format.
175 Since version 2016h, each release has contained a text file named
176 "<code>version</code>" whose first (and currently only) line is the version.
177 Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
178 and are also available in an
179 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
180 title="File Transfer Protocol">FTP</abbr> directory</a> via a
181 less secure protocol.</p>
182 <p>Alternatively, a development repository of code and data can be
183 retrieved from <a href="https://github.com">GitHub</a> via the shell
184 command:</p>
185 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
186 </code></pre>
187 <p>
188 Since version 2012e, each release has been tagged in development repositories.
189 Untagged commits are less well tested and probably contain
190 more errors.</p>
191 <p>
192 After obtaining the code and data files, see the
193 <code>README</code> file for what to do next.
194 The code lets you compile the <code><abbr>tz</abbr></code> source files into
195 machine-readable binary files, one for each location. The binary files
196 are in a special format specified by
197 <a href="https://datatracker.ietf.org/doc/html/8536">The
198 Time Zone Information Format (<abbr>TZif</abbr>)</a>
199 (Internet <abbr title="Request For Comments">RFC</abbr> 8536).
200 The code also lets
201 you read a <abbr>TZif</abbr> file and interpret timestamps for that
202 location.</p>
203 </section>
204
205 <section>
206 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
207 <p>
208 The <code><abbr>tz</abbr></code> code and data
209 are by no means authoritative. If you find errors, please
210 email changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
211 the time zone mailing list. See
212 <a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing
213 list's main page</a> to subscribe or to browse its archive of old messages.
214 <a href="https://tzdata-meta.timtimeonline.com">Metadata for mailing list
215 discussions</a> and corresponding data changes can be
216 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
217 </p>
218 <p>
219 Changes to the <code><abbr>tz</abbr></code> code and data are often
220 propagated to clients via operating system updates, so
221 client <code><abbr>tz</abbr></code> data can often be corrected by
222 applying these updates. With GNU/Linux and similar systems, if your
223 maintenance provider has not yet adopted the
224 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
225 the process by tailoring the generic instructions in
226 the <code><abbr>tz</abbr> README</code> file and installing the latest
227 data yourself. System-specific instructions for installing the
228 latest <code><abbr>tz</abbr></code> data have also been published
229 for <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates"><abbr>AIX</abbr></a>,
230 <a
231 href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
232 <a
233 href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
234 title="International Components for Unicode">ICU</abbr></a>,
235 <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
236 JDK</a>,
237 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
238 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
239 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
240 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
241 </p>
242 <p>Since version 2013a,
243 sources for the <code><abbr>tz</abbr></code> database have been
244 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
245 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
246 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
247 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
248 title="linefeed">LF</abbr></a>,
249 which can be modified by common text editors such
250 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
251 <a href="https://gedit-technology.github.io/apps/gedit/">gedit</a>, and
252 <a href="https://www.vim.org">vim</a>.
253 Specialized source-file editing can be done via the
254 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
255 zoneinfo</a> package for <a
256 href="https://www.sublimetext.com">Sublime Text</a> and the <a
257 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
258 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
259 Studio Code</a>.
260 </p>
261 <p>
262 For further information about updates, please see
263 <a href="https://datatracker.ietf.org/doc/html/rfc6557">Procedures for
264 Maintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557).
265 More detail can be
266 found in <a href="theory.html">Theory and pragmatics of the
267 <code><abbr>tz</abbr></code> code and data</a>.
268 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
269 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
270 </p>
271 </section>
272
273 <section>
274 <h2 id="coordinating">Coordinating with governments and distributors</h2>
275 <p>
276 As discussed in
277 "<a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How
278 Time Zones Are Coordinated</a>", the time zone database relies on
279 collaboration among governments, the time zone database volunteer
280 community, and data distributors downstream.
281 <p>
282 If your government plans to change its time zone boundaries or
283 daylight saving rules, please send email to <a
284 href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a> well in advance,
285 as this will lessen confusion and will coordinate updates to many cell phones,
286 computers, and other devices around the world.
287 In your email, please cite the legislation or regulation that specifies
288 the change, so that it can be checked for details such as the exact times
289 when clock transitions occur.
290 It is OK if a rule change is planned to affect clocks
291 far into the future, as a long-planned change can easily be reverted
292 or otherwise altered with a year's notice before the change would have
293 affected clocks.</p>
294 <p>
295 There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.
296 However, typically a release occurs every few months.
297 Many downstream timezone data distributors wait for
298 a <code><abbr>tzdb</abbr></code> release before they produce an update
299 to time zone behavior in consumer devices and software products.
300 After a release, various parties must integrate, test,
301 and roll out an update before <a
302 href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.
303 These updates can be expensive, for both the <a
304 href="https://en.wikipedia.org/wiki/Quality_assurance">quality
305 assurance</a> process and the overall cost of shipping and installing
306 updates to each device's copy of <code><abbr>tzdb</abbr></code>.
307 Updates may be batched with other updates and may take substantial
308 time to reach end users after a release.
309 Older devices may no longer be supported and thus may never be updated,
310 which means they will continue to use out-of-date rules.</p>
311 <p>
312 For these reasons any rule change should be promulgated at least a
313 year before it affects how clocks operate; otherwise, there is a good
314 chance that many clocks will be wrong due to delays in propagating updates,
315 and that residents will be confused or even actively resist the change.
316 The shorter the notice, the more likely clock problems will arise; see "<a
317 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
318 the Timing of Time Zone Changes</a>" for examples.
319 </p>
320 </section>
321
322 <section>
323 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
324 <ul>
325 <li>The article
326 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
327 an encyclopedic summary.</li>
328 <li><a href="tz-how-to.html">How to Read the
329 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
330 database format.</li>
331 <li><a
332 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
333 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
334 database's style.</li>
335 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
336 managing time in the internet</a> analyzes the database longitudinally.</li>
337 </ul>
338 </section>
339
340 <section>
341 <h2 id="web">Web sites using recent versions of the
342 <code><abbr>tz</abbr></code> database</h2>
343 <p>
344 These are listed roughly in ascending order of complexity and fanciness.
345 </p>
346 <ul>
347 <li><a href="https://time.is">Time.is</a> shows locations'
348 time and zones.</li>
349 <li><a href="https://www.timejones.com">TimeJones.com</a>,
350 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
351 <a href="https://www.worldclock.com">The World Clock</a>
352 are time zone converters.</li>
353 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
354 publishes <code><abbr>tzdb</abbr></code>-derived data in
355 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
356 title="comma-separated values">CSV</abbr></a> and
357 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
358 title="Structured Query Language">SQL</abbr></a> form.</li>
359 <li><a
360 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
361 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
362 <li><a
363 href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
364 Time in 1000 Places</a> uses descriptions of the values.</li>
365 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
366 Worldwide</a> lets you sort zone names and convert times.</li>
367 <li><a href="https://24timezones.com">24TimeZones</a> has a world
368 time map and a time converter.</li>
369 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
370 calculates the current time difference between locations.</li>
371 <li><a href="https://www.wx-now.com">Weather Now</a> and
372 <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
373 </ul>
374 </section>
375
376 <section>
377 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
378 <ul>
379 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
380 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
381 Distribution Service (tzdist) working group</a> defined <a
382 href="https://datatracker.ietf.org/doc/html/rfc7808">TZDIST</a>
383 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
384 along with <a href="https://datatracker.ietf.org/doc/html/rfc7809">CalDAV</a>
385 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
386 transferring time zone data by reference.
387 <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
388 implementations</a> are available.
389 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
390 mailing list</a> discusses possible extensions.</li>
391 <li>The <a href="https://datatracker.ietf.org/doc/html/rfc5545">
392 Internet Calendaring and Scheduling Core Object Specification
393 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
394 covers time zone
395 data; see its VTIMEZONE calendar component.
396 The iCalendar format requires specialized parsers and generators; a
397 variant <a href="https://datatracker.ietf.org/doc/html/rfc6321">xCal</a>
398 (Internet <abbr>RFC</abbr> 6321) uses
399 <a href="https://www.w3.org/XML/"><abbr
400 title="Extensible Markup Language">XML</abbr></a> format, and a variant
401 <a href="https://datatracker.ietf.org/doc/html/rfc7265">jCal</a>
402 (Internet <abbr>RFC</abbr> 7265)
403 uses <a href="https://www.json.org/json-en.html"><abbr
404 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
405 </ul>
406 </section>
407
408 <section>
409 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
410 <p>Although some of these do not fully support
411 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
412 distributions you can generally work around compatibility problems by
413 running the command <code>make rearguard_tarballs</code> and compiling
414 from the resulting tarballs instead.</p>
415 <ul>
416 <li><a href="https://github.com/libical/vzic">Vzic</a> is a <a
417 href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>
418 program that compiles
419 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
420 Vzic is freely
421 available under the <a
422 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
423 General Public License (<abbr
424 title="General Public License">GPL</abbr>)</a>.</li>
425 <li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz
426 database conversion utility</a> is like Vzic, except for the <a
427 href="https://dotnet.microsoft.com">.NET framework</a>
428 and with a <abbr>BSD</abbr>-style license.</li>
429 <li><a
430 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
431 contains a script <code>parse_olson</code> that compiles
432 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
433 modules. It is part of the Perl <a
434 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
435 which is freely
436 available under both the <abbr>GPL</abbr> and the Perl Artistic
437 License. DateTime::TimeZone also contains a script
438 <code>tests_from_zdump</code> that generates test cases for each clock
439 transition in the <code><abbr>tz</abbr></code> database.</li>
440 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
441 Database Parser</a> is a
442 <a href="https://en.wikipedia.org/wiki/C++">C++</a> parser and
443 runtime library with a <a
444 href="https://en.cppreference.com/w/cpp/chrono"><code>std::chrono</code> API</a>
445 that is a standard part of C++.
446 It is freely available under the
447 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
448 <li><a id="ICU" href="https://icu.unicode.org">International Components for
449 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
450 href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a>
451 libraries for internationalization that
452 has a compiler from <code><abbr>tz</abbr></code> source
453 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
454 (mentioned <a href="#CLDR">below</a>)
455 into an <abbr>ICU</abbr>-specific format.
456 <abbr>ICU</abbr> is freely available under a
457 <abbr>BSD</abbr>-style license.</li>
458 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
459 the <a href="https://elixir-lang.org">Elixir</a> language downloads
460 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
461 title="Application Program Interface">API</abbr>s for use. It is
462 freely available under the <abbr>MIT</abbr> license.</li>
463 <li>Java-based compilers and libraries include:
464 <ul>
465 <li>The <a
466 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
467 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
468 <a href="https://openjdk.org">OpenJDK</a> and
469 <a href="https://jdk.java.net">Oracle JDK</a>.
470 Although its source code is proprietary, its executable is available under the
471 <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
472 Timezone Updater License Agreement</a>.</li>
473 <li>The <a
474 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
475 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
476 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
477 which is freely available under a <abbr>BSD</abbr>-style license.</li>
478 <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
479 and time <abbr>API</abbr></a> contains a class
480 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
481 <code><abbr>tz</abbr></code> source into a binary format. It inspired
482 Java 8 <code>java.time</code>, which its users should migrate to once
483 they can assume Java 8 or later. It is available under the <a
484 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
485 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
486 href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
487 are alternatives to TZUpdater. IANA Updater's license is unclear;
488 ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
489 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
490 time library for Android</a> and
491 <a href="https://github.com/MenoData/Time4J">Time4J: Advanced date,
492 time and interval library for Java</a> compile
493 <code><abbr>tz</abbr></code> source into a binary format.
494 Time4A is available under the Apache License and Time4J is
495 available under the <a
496 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
497 General Public License (<abbr title="Lesser General Public
498 License">LGPL</abbr>)</a>.</li>
499 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
500 Java-based libraries.</li>
501 </ul>
502 <li><a href="https://nodatime.org">Noda Time – Date and
503 time <abbr>API</abbr> for .NET</a>
504 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
505 It is freely available under the Apache License.</li>
506 <li>Many modern
507 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
508 runtimes support <code><abbr>tz</abbr></code> natively via the
509 <code>timeZone</code> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><code>Intl.DateTimeFormat</code></a>.
510 This can be used as-is or with most of the following libraries,
511 many of which also support runtimes lacking the <code>timeZone</code> option.
512 <ul>
513 <li>The <a
514 href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code>
515 timezone polyfill</a>
516 is freely available under a <abbr>BSD</abbr>-style license.</li>
517 <li>The <a href="https://date-fns.org">date-fns</a>
518 library manipulates timezone-aware timestamps in browsers and
519 in <a href="https://nodejs.org/en/">Node.js</a>.
520 It is freely available under the <abbr>MIT</abbr> license.</li>
521 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
522 minimalist replacement for the date and time API of
523 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
524 manipulation library.
525 It is freely available under the <abbr>MIT</abbr> license.</li>
526 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
527 timezone support for the <code>Intl</code> API.
528 It is freely available under the <abbr>MIT</abbr> license.</li>
529 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
530 Moment.js plugin.
531 It is freely available under the <abbr>MIT</abbr> license.</li>
532 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
533 JavaScript library that supports date arithmetic that is time zone
534 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
535 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
536 supports live <code><abbr>tzdb</abbr></code> updates,
537 astronomical and atomic time, a command-line interface,
538 and full <a
539 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
540 Its companion <a
541 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
542 can generate <abbr>TZif</abbr> and other files, and a companion website
543 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
544 convert timestamps, view transition histories, and download code and data.
545 It is freely available under the <abbr>MIT</abbr> license.</li>
546 </ul>
547 The proposed <a
548 href="https://github.com/tc39/proposal-temporal"><code>Temporal</code>
549 objects</a> let programs access an abstract view of
550 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
551 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
552 problematic <code>Date</code> objects</a> when working with dates and times.
553 <li><a href="https://github.com/JuliaTime">JuliaTime</a> contains a
554 compiler from <code><abbr>tz</abbr></code> source into
555 <a href="https://julialang.org">Julia</a>. It is freely available
556 under the <abbr>MIT</abbr> license.</li>
557 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
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://tcl.tk">Tcl</a>
586 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
587 source into text files, along with a runtime that can read those
588 files. Tcl is freely available under a <abbr>BSD</abbr>-style
589 license.</li>
590 </ul>
591 </section>
592
593 <section>
594 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
595 <ul>
596 <li>The <a
597 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
598 Library</a>
599 has an independent, thread-safe implementation of
600 a <abbr>TZif</abbr> file reader.
601 This library is freely available under the LGPL
602 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
603 <li><a href="https://www.gnome.org">GNOME</a>'s
604 <a href="https://developer.gnome.org/glib/">GLib</a> has
605 a <abbr>TZif</abbr> file reader written in C that
606 creates a <code>GTimeZone</code> object representing sets
607 of <abbr>UT</abbr> offsets.
608 It is freely available under the <abbr>LGPL</abbr>.</li>
609 <li>The
610 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
611 <code>baltzo::TimeZoneUtil</code> component contains a C++
612 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
613 the Apache License.</li>
614 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
615 library that translates between <abbr>UT</abbr> and civil time and
616 can read <abbr>TZif</abbr> files. It is freely available under the Apache
617 License.</li>
618 <li>The
619 <a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code>
620 package</a> contains Python code
621 to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map
622 <code><abbr>tz</abbr></code> settings to proleptic TZ approximations.
623 For example, it maps <code>"Africa/Cairo"</code>
624 to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>,
625 an approximation valid for Cairo timestamps from 2023 on.
626 This can help porting to platforms that support only proleptic TZ.
627 The package is freely available under the MIT license.</li>
628 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
629 library that reads <abbr>TZif</abbr> files and converts
630 timestamps from one time zone or format to another.
631 It is used by <a href="https://secure.php.net"><abbr
632 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
633 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
634 and <a href="https://www.mongodb.com">MongoDB</a>.
635 It is freely available under the <abbr>MIT</abbr> license.</li>
636 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
637 <abbr>TZif</abbr> file reader.</li>
638 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
639 DateTime::TimeZone::Tzfile</a>
640 is a <abbr>TZif</abbr> file reader written in Perl.
641 It is freely available under the same terms as Perl
642 (dual <abbr>GPL</abbr> and Artistic license).</li>
643 <li>Python has a <a id="python-zoneinfo"
644 href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
645 class</a> that reads <abbr>TZif</abbr> data and creates objects
646 that represent <code><abbr>tzdb</abbr></code> timezones.
647 Python is freely available under the
648 <a href="https://docs.python.org/3/license.html">Python Software Foundation
649 License</a>.
650 A companion <a id="pypi-tzdata" href="https://pypi.org">PyPI</a> module
651 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
652 supplies TZif data if the underlying system data cannot be found;
653 it is freely available under the Apache License.</li>
654 <li>The
655 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
656 library contains a Python tool that
657 converts <abbr>TZif</abbr> data into
658 <abbr>JSON</abbr>-format data suitable for use
659 in its JavaScript library for time zone conversion. Dates before 1970
660 are not supported.</li>
661 <li>The <a
662 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
663 package contains <a href="https://www.haskell.org">Haskell</a> code that
664 parses and uses <abbr>TZif</abbr> data. It is freely
665 available under a <abbr>BSD</abbr>-style license.</li>
666 </ul>
667 </section>
668
669 <section>
670 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
671 <ul>
672 <li><a href="https://foxclocks.org">FoxClocks</a>
673 is an extension for <a href="https://www.google.com/chrome/">Google
674 Chrome</a>, <a
675 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
676 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
677 It displays multiple clocks in the application window, and has a mapping
678 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
679 It is freely available under the <abbr>GPL</abbr>.</li>
680 <li><a href="https://golang.org">Go programming language</a>
681 implementations contain a copy of a 32-bit subset of a recent
682 <code><abbr>tz</abbr></code> database in a
683 Go-specific format.</li>
684 <li>Microsoft Windows 8.1
685 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
686 data (mentioned <a href="#CLDR">below</a>) used by the
687 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
688 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
689 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
690 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
691 <a id="System.TimeZoneInfo"
692 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
693 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
694 the older, proprietary method of Microsoft Windows 2000 and later,
695 which stores time zone data in the
696 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
697 <a
698 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
699 Tzid table</a> or <a
700 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
701 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
702 to <code><abbr>tz</abbr></code> names.
703 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
704 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
705 <li><a
706 href="https://www.oracle.com/java/index.html">Oracle
707 Java</a> contains a copy of a subset of a recent
708 <code><abbr>tz</abbr></code> database in a
709 Java-specific format.</li>
710 </ul>
711 </section>
712
713 <section>
714 <h2 id="other-dbs">Other time zone databases</h2>
715 <ul>
716 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
717 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
718 time zone history atlases
719 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
720 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
721 Although these extensive atlases
722 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
723 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
724 they are unreliable as Shanks appears to have
725 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
726 sources and do not indicate which entries are guesswork.</li>
727 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
728 its own <code>tztab</code>(4) format.</li>
729 <li>Microsoft Windows has proprietary data mentioned
730 <a href="#System.TimeZoneInfo">above</a>.</li>
731 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
732 is another time zone database.</li>
733 <li>The <a
734 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
735 Schedules Information Manual</a> of the
736 International Air Transport Association
737 gives current time zone rules for airports served by commercial aviation.</li>
738 </ul>
739 </section>
740
741 <section>
742 <h2 id="maps">Maps</h2>
743 <ul>
744 <li>The <a
745 href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
746 and Regional Maps section</a> of <em>The World Factbook</em>, published by the
747 <a href="https://www.cia.gov">US Central Intelligence
748 Agency (<abbr
749 title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
750 zone map; the
751 <a
752 href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda
753 Library Map Collection</a>
754 of the University of Texas at Austin has copies of
755 recent editions.
756 The pictorial quality is good,
757 but the maps do not indicate daylight saving time,
758 and parts of the data are a few years out of date.</li>
759 <li><a href="https://www.worldtimezone.com">World Time Zone Map
760 with current time</a>
761 has several fancy time zone maps; it covers Russia particularly well.
762 The maps' pictorial quality is not quite as good as the
763 <abbr>CIA</abbr>'s
764 but the maps are more up to date.</li>
765 <li><a
766 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
767 much is time wrong around the world?</a> maps the difference between
768 mean solar and standard time, highlighting areas such as western China
769 where the two differ greatly. It's a bit out of date, unfortunately.</li>
770 </ul>
771 </section>
772
773 <section>
774 <h2 id="boundaries">Time zone boundaries</h2>
775 <p>Geographical boundaries between timezones are available
776 from several <a
777 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
778 geolocation</a>
779 services and other sources.</p>
780 <ul>
781 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
782 Boundary Builder</a> extracts
783 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
784 boundaries of <code><abbr>tzdb</abbr></code> timezones.
785 Its code is freely available under the <abbr>MIT</abbr> license, and
786 its data entries are freely available under the
787 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
788 Open Database License</a>. The borders appear to be quite accurate.
789 Its main web page lists more than twenty libraries
790 for looking up a timezone name from a GPS coordinate.</li>
791 <li>Free access via a network API, if you register a key, is provided by
792 the <a
793 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
794 Timezone web service</a>, the <a
795 href="https://developers.google.com/maps/documentation/timezone/intro">Google
796 Maps Time Zone API</a>, and
797 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
798 Commercial network API access is provided
799 by <a href="https://askgeo.com">AskGeo</a>
800 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
801 </li>
802 <li>"<a
803 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
804 to get a time zone from a location using latitude and longitude
805 coordinates?</a>" discusses other geolocation possibilities.</li>
806 <li><a href="http://statoids.com/statoids.html">Administrative
807 Divisions of Countries ("Statoids")</a> lists
808 political subdivision data related to time zones.</li>
809 <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
810 – GIS and Database Tools</a> includes a Manifold-format map of
811 world time zone boundaries circa 2007, distributed under the
812 <abbr>GPL</abbr>.</li>
813 <li>A ship within the <a
814 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
815 waters</a> of any nation uses that nation's time. In international
816 waters, time zone boundaries are meridians 15° apart, except that
817 <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
818 wide and are separated by
819 the 180° meridian (not by the International Date Line, which is
820 for land and territorial waters only). A captain can change ship's
821 clocks any time after entering a new time zone; midnight changes are
822 common.</li>
823 </ul>
824 </section>
825
826 <section>
827 <h2 id="civil">Civil time concepts and history</h2>
828 <ul>
829 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
830 Walk through Time</a>
831 surveys the evolution of timekeeping.</li>
832 <li>The history of daylight saving time is surveyed in <a
833 href="http://www.webexhibits.org/daylightsaving/">About Daylight
834 Saving Time – History, rationale, laws & dates</a> and summarized in
835 <a href="http://seizethedaylight.com/dst/">A Brief
836 History of Daylight Saving Time</a>.</li>
837 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
838 Lords</a> discusses how authoritarians manipulate civil time.</li>
839 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
840 contains guidelines and best practices for software applications that
841 deal with civil time.</li>
842 <li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of
843 the International Date Line</a> tells the story of the most important
844 time zone boundary.</li>
845 <li><a href="http://statoids.com/tconcept.html">Basic Time
846 Zone Concepts</a> discusses terminological issues behind time zones.</li>
847 </ul>
848 </section>
849
850 <section>
851 <h2 id="national">National histories of legal time</h2>
852 <dl>
853 <dt>Australia</dt>
854 <dd>The Parliamentary Library commissioned a <a
855 href="https://parlinfo.aph.gov.au/parlInfo/download/library/prspub/359V6/upload_binary/359v60.pdf">research
856 paper on daylight saving time in Australia</a>.
857 The Bureau of Meteorology publishes a list of <a
858 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
859 Dates of Daylight Savings Time within Australia</a>.</dd>
860 <dt>Belgium</dt>
861 <dd>The Royal Observatory of Belgium maintains a table of time in
862 Belgium (in
863 <a href="https://robinfo.oma.be/nl/astro-info/tijd/"
864 hreflang="nl">Dutch</a> and <a
865 href="https://robinfo.oma.be/fr/astro-info/heure/"
866 hreflang="fr">French</a>).</dd>
867 <dt>Brazil</dt>
868 <dd>The Time Service Department of the National Observatory
869 records <a href="http://pcdsh01.on.br/DecHV.html"
870 hreflang="pt-BR">Brazil's daylight saving time decrees (in
871 Portuguese)</a>.</dd>
872 <dt>Canada</dt>
873 <dd>National Research Council Canada publishes current
874 and some older information about <a
875 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
876 zones and daylight saving time</a>.</dd>
877 <dt>Chile</dt>
878 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
879 <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
880 Chile's official time (in Spanish)</a>.</dd>
881 <dt>China</dt>
882 <dd>The Hong Kong Observatory maintains a
883 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
884 summer time in Hong Kong</a>,
885 and Macau's Meteorological and Geophysical Bureau maintains a <a
886 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
887 history for Macau</a>.
888 Unfortunately the latter is incomplete and has errors.</dd>
889 <dt>Czech Republic</dt>
890 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
891 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
892 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
893 <dt>Germany</dt>
894 <dd>The National Institute for Science and Technology maintains the <a
895 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
896 of Legal Time in Germany</a>.</dd>
897 <dt>Israel</dt>
898 <dd><a
899 href="https://tz.cs.huji.ac.il">Israel Timezone Files</a>
900 lists official time-change announcements and laws since 1940,
901 almost all in Hebrew.</dd>
902 <dt>Malaysia</dt>
903 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
904 <dt>Mexico</dt>
905 <dd>The Investigation and Analysis Service of the Mexican Library of
906 Congress has published a <a
907 href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
908 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
909 <dt>Netherlands</dt>
910 <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"
911 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
912 covers the history of local time in the Netherlands from ancient times.</dd>
913 <dt>New Zealand</dt>
914 <dd>The Department of Internal Affairs maintains a brief <a
915 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
916 Daylight Saving</a>.</dd>
917 <dt>Palestine</dt>
918 <dd>The Ministry of Telecom and IT publishes a <a
919 href="https://mtit.pna.ps/home/TimeZone"
920 hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
921 <dt>Portugal</dt>
922 <dd>The Lisbon Astronomical Observatory publishes a
923 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
924 legal time (in Portuguese)</a>.</dd>
925 <dt>Singapore</dt>
926 <dd><a id="Singapore"
927 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
928 is Singapore in the "Wrong" Time Zone?</a> details the
929 history of legal time in Singapore and Malaysia.</dd>
930 <dt>United Kingdom</dt>
931 <dd><a
932 href="https://www.polyomino.org.uk/british-time/">History of
933 legal time in Britain</a> discusses in detail the country
934 with perhaps the best-documented history of clock adjustments.</dd>
935 <dt>United States</dt>
936 <dd>The Department of Transportation's <a
937 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
938 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
939 <dt>Uruguay</dt>
940 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
941 Navy (SOHMA) publishes an annual <a
942 href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
943 (in Spanish)</a>.</dd>
944 </dl>
945 </section>
946
947 <section>
948 <h2 id="costs">Costs and benefits of time shifts</h2>
949 <p>Various sources argue for and against daylight saving time and time
950 zone shifts, and many scientific studies have been conducted. This
951 section summarizes reviews and position statements based on
952 scientific literature in the area.</p>
953 <ul>
954 <li>In 2022 the American Medical Association issued a
955 <a href="https://www.ama-assn.org/press-center/press-releases/ama-calls-permanent-standard-time">statement
956 supporting permanent standard time</a> on health grounds.</li>
957 <li>Carey RN, Sarma KM.
958 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
959 daylight saving time on road traffic collision risk: a systematic
960 review</a>.
961 <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>.
962 This reviews research literature and concludes that the evidence
963 neither supports nor refutes road safety benefits from
964 shifts in time zones.</li>
965 <li>Havranek T, Herman D, Irsova D.
966 <a href="https://www.iaee.org/en/publications/ejarticle.aspx?id=3051">Does
967 daylight saving save electricity? A meta-analysis</a>.
968 <em>Energy J.</em> 2018;39(2):35–61.
969 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
970 This analyzes research literature and concludes, "Electricity savings
971 are larger for countries farther away from the equator, while
972 subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
973 <li>Malow BA. <a
974 href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time
975 to abolish the clock change and adopt permanent
976 standard time in the United States:
977 a Sleep Research Society position statement</a>.
978 <em>Sleep.</em> 2022;45(12):zsac236.
979 doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
980 After reviewing the scientific literature, the Sleep Research Society
981 advocates permanent standard time due to its health benefits.
982 <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
983 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
984 is the optimal choice for health and safety:
985 an American Academy of Sleep Medicine position statement</a>.
986 <em>J Clin Sleep Med.</em> 2024;20(1):121–125.
987 doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
988 The AASM argues for permanent standard time due to health and safety risks
989 and economic costs of both <abbr>DST</abbr> transitions and
990 permanent <abbr>DST</abbr>.</li>
991 <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
992 <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why
993 should we abolish Daylight Saving Time?</a>
994 <em>J Biol Rhythms.</em> 2019;34(3):227–230.
995 doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
996 The Society for Research on Biological Rhythms
997 opposes DST changes and permanent DST, and advocates that governments adopt
998 "permanent Standard Time for the health and safety of their citizens".</li>
999 </ul>
1000 </section>
1001
1002 <section>
1003 <h2 id="precision">Precision timekeeping</h2>
1004 <ul>
1005 <li><a
1006 href="http://leapsecond.com/hpan/an1289.pdf">The
1007 Science of Timekeeping</a> is a thorough introduction
1008 to the theory and practice of precision timekeeping.</li>
1009 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
1010 Time 2016</a> contains several freely readable papers.</li>
1011 <li><a href="https://www.ntp.org"><abbr
1012 title="Network Time Protocol">NTP</abbr>: The Network
1013 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
1014 discusses how to synchronize clocks of
1015 Internet hosts.</li>
1016 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
1017 family of software algorithms can achieve accuracy to a few tens of
1018 nanoseconds in scalable server farms without special hardware.</li>
1019 <li>The <a
1020 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
1021 Time Protocol</a> (<abbr
1022 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
1023 can achieve submicrosecond clock accuracy on a local area network
1024 with special-purpose hardware.</li>
1025 <li><a
1026 href="https://datatracker.ietf.org/doc/html/rfc4833">Timezone
1027 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
1028 (Internet <abbr>RFC</abbr> 4833)
1029 specifies a <a
1030 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
1031 option for a server to configure
1032 a client's time zone and daylight saving settings automatically.</li>
1033 <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
1034 Scales</a> describes astronomical time scales like
1035 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
1036 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
1037 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
1038 <li>The <a href="https://www.iau.org"><abbr
1039 title="International Astronomical Union">IAU</abbr></a>'s <a
1040 href="https://www.iausofa.org"><abbr
1041 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
1042 collection contains C and <a
1043 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
1044 code for converting among time scales like
1045 <abbr title="International Atomic Time">TAI</abbr>,
1046 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
1047 <abbr>UTC</abbr>. It is freely available under the
1048 <a href="https://www.iausofa.org/tandc.html">SOFA license</a>.</li>
1049 <li><a
1050 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
1051 – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
1052 diverse local time
1053 scales used by each landed mission on Mars.</li>
1054 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
1055 dedicated not only to leap seconds but to precise time and frequency
1056 in general. It covers the state of the art in amateur timekeeping, and
1057 how the art has progressed over the past few decades.</li>
1058 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
1059 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
1060 and time-signal emissions</a>, International Telecommunication Union –
1061 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
1062 <li><a
1063 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
1064 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
1065 Bulletins</a> contains official publications of the International
1066 Earth Rotation and Reference Systems Service, which decides when leap
1067 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
1068 via an optional "<code>right</code>" configuration where a computer's internal
1069 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
1070 as opposed to the default "<code>posix</code>" configuration
1071 where the internal clock ignores leap seconds.
1072 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
1073 <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for
1074 timestamps starting with <code>time_t</code> 78 796 800,
1075 which corresponds to the first leap second
1076 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
1077 and to
1078 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
1079 In practice the two configurations also agree for timestamps before
1080 1972 even though the historical situation is messy, partly because
1081 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
1082 is well-defined for sufficiently old timestamps.</li>
1083 <li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The
1084 <abbr>NTP</abbr> Leap Second File</a> covers the text file
1085 <code>leap-seconds.list</code>, which lists the currently known leap seconds.
1086 The <abbr>IERS</abbr> maintains this file, and a copy is distributed by
1087 <code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like
1088 <a href="https://www.ntp.org">classic
1089 <code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a>
1090 and <a href="https://ntpsec.org">NTPsec</a>.
1091 The <code><abbr>tz</abbr></code> database also distributes leap second
1092 information in a differently-formatted <code>leapseconds</code> text file,
1093 as well as in the "<code>right</code>" configuration in binary form; for
1094 example, <code>right/UTC</code> can be used
1095 by <a href="https://chrony-project.org"><code>chrony</code></a>,
1096 another <abbr>NTP</abbr> implementation.</li>
1097 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1098 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1099 leap second so that they disagree with <abbr>UTC</abbr> by at most a
1100 half second, even though every <abbr>POSIX</abbr> minute has exactly
1101 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1102 "<code>posix</code>" configuration, is <a
1103 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
1104 the abovementioned <abbr>NTP</abbr> implementations, <a
1105 href="https://github.com/google/unsmear">supports</a> conversion between
1106 <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
1107 cloud service providers. However, according to
1108 <a href="https://datatracker.ietf.org/doc/html/rfc8633#section-3.7.1">§3.7.1 of
1109 Network Time Protocol Best Current Practices</a>
1110 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1111 applications requiring accurate <abbr>UTC</abbr> or civil time,
1112 and is intended for use only in single, well-controlled environments.</li>
1113 <li>The <a
1114 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
1115 Second Discussion List</a> covers <a
1116 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
1117 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
1118 discussed further in
1119 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1120 leap second: its history and possible future</a>.
1121 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1122 might be redefined
1123 without Leap Seconds</a> gives pointers on this
1124 contentious issue.
1125 The General Conference on Weights and Measures
1126 <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>
1127 to discontinue the use of leap seconds by 2035, and requested that no
1128 discontinuous adjustments be made to UTC for at least a century.
1129 The World Radiocommunication Conference <a
1130 href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved
1131 in 2023</a> to cooperate with this process.
1132 <a href="https://www.preprints.org/manuscript/202406.0043/v1">A proposal
1133 to change the leap-second adjustments to Coordinated Universal Time</a>
1134 (doi:<a href="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a>)
1135 would replace leap seconds with 13-second leap smears occurring once per
1136 decade until 2100, with leap smears after that gradually increasing in size.
1137 However, there is still no consensus on whether this is the best way
1138 to replace leap seconds.
1139 </li>
1140 </ul>
1141 </section>
1142
1143 <section>
1144 <h2 id="notation">Time notation</h2>
1145 <ul>
1146 <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
1147 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1148 zone names, abbreviations, identifiers, and formats. For example, it
1149 contains French translations for "Eastern European Summer Time",
1150 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
1151 "Bucharest". Its
1152 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1153 charts</a> show these values for many locales. Data values are available in
1154 both <abbr title="Locale Data Markup Language">LDML</abbr>
1155 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1156 <li>
1157 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1158 the international standard date and time notation</a> covers
1159 <a
1160 href="https://www.iso.org/standard/70907.html"><em><abbr
1161 title="International Organization for Standardization">ISO</abbr>
1162 8601-1:2019 – Date and time – Representations for information
1163 interchange – Part 1: Basic rules</em></a>.</li>
1164 <li>
1165 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1166 Schema: Datatypes – dateTime</a> specifies a format inspired by
1167 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1168 <li><a href="https://datatracker.ietf.org/doc/html/rfc5322#section-3.3">§3.3 of
1169 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1170 specifies the time notation used in email and <a
1171 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1172 headers.</li>
1173 <li>
1174 <a href="https://datatracker.ietf.org/doc/html/rfc3339">Date and Time
1175 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1176 specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.
1177 An extension, <a href="https://datatracker.ietf.org/doc/html/rfc9557">Date
1178 and Time on the Internet: Timestamps with Additional Information</a>
1179 (Internet <abbr>RFC</abbr> 9557) extends this profile
1180 to let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp
1181 via suffixes like "<code>[Asia/Tokyo]</code>".
1182 <li>
1183 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1184 Formats on the Web</a> surveys web- and Internet-oriented date and time
1185 formats.</li>
1186 <li>Alphabetic time zone abbreviations should not be used as unique
1187 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1188 practice. For example, in English-speaking North America
1189 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
1190 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1191 and French-speaking North Americans prefer
1192 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
1193 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
1194 database contains English abbreviations for many timestamps;
1195 unfortunately some of these abbreviations were merely the database maintainers'
1196 inventions, and these have been removed when possible.</li>
1197 <li>Numeric time zone abbreviations typically count hours east of
1198 <abbr>UT</abbr>, e.g., +09 for Japan and
1199 −10 for Hawaii. However, <abbr>POSIX</abbr> proleptic
1200 <code><abbr>TZ</abbr></code> settings use the opposite convention.
1201 For example, one might use <code><abbr>TZ</abbr>="<abbr
1202 title="Japan Standard Time">JST</abbr>-9"</code> and
1203 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1204 for Japan and Hawaii, respectively. If the
1205 <code><abbr>tz</abbr></code> database is available, it is usually better to use
1206 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1207 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1208 confusion, handle old timestamps better, and insulate you better from
1209 any future changes to the rules. One should never set
1210 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1211 <code>"GMT-9"</code>, though, since this would incorrectly imply that
1212 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1213 is called "<abbr>GMT</abbr>".</li>
1214 </ul>
1215 </section>
1216
1217 <section>
1218 <h2 id="see-also">See also</h2>
1219 <ul>
1220 <li><a href="theory.html">Theory and pragmatics of the
1221 <code><abbr>tz</abbr></code> code and data</a></li>
1222 <li><a href="tz-art.html">Time and the Arts</a></li>
1223 </ul>
1224 </section>
1225
1226 <footer>
1227 <hr>
1228 This web page is in the public domain, so clarified as of
1229 2009-05-17 by Arthur David Olson.
1230 <br>
1231 Please send corrections to this web page to the
1232 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1233 </footer>
1234 </body>
1235 </html>
1236