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14 <h1>Time zone and daylight saving time data</h1>
15 <p>
16 <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
17 <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
18 rules are controlled by individual
19 governments. They are sometimes changed with little notice, and their
20 histories and planned futures are often recorded only fitfully. Here
21 is a summary of attempts to organize and record relevant data in this
22 area.
23 </p>
24 <nav>
25 <ul>
26 <li>The <code><abbr>tz</abbr></code> database product and process
27 <ul>
28 <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
29 <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
30 <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
31 <li><a href="#coordinating">Coordinating with governments and distributors</a></li>
32 <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
33 </ul>
34 </li>
35 <li>Uses of the <code><abbr>tz</abbr></code> database
36 <ul>
37 <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
38 <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
39 <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
40 <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
41 <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
42 </ul>
43 </li>
44 <li>Related data
45 <ul>
46 <li><a href="#other-dbs">Other time zone databases</a></li>
47 <li><a href="#maps">Maps</a></li>
48 <li><a href="#boundaries">Time zone boundaries</a></li>
49 </ul>
50 </li>
51 <li>Timekeeping concepts
52 <ul>
53 <li><a href="#civil">Civil time concepts and history</a></li>
54 <li><a href="#national">National histories of legal time</a></li>
55 <li><a href="#costs">Costs and benefits of time shifts</a></li>
56 <li><a href="#precision">Precision timekeeping</a></li>
57 <li><a href="#notation">Time notation</a></li>
58 </ul>
59 </li>
60 <li><a href="#see-also">See also</a></li>
61 </ul>
62 </nav>
63
64 <section>
65 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
66 <p>
67 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
68 time zone database contains code and data
69 that represent the history of local time
70 for many representative locations around the globe.
71 It is updated periodically to reflect changes made by political bodies
72 to time zone boundaries and daylight saving rules.
73 This database (known as <code><abbr>tz</abbr></code>,
74 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
75 is used by several implementations,
76 including
77 <a href="https://www.gnu.org/software/libc/">the
78 <abbr title="GNUs Not Unix">GNU</abbr>
79 C Library</a> (used in
80 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
81 <a href="https://www.android.com">Android</a>,
82 <a href="https://www.freebsd.org">Free<abbr
83 title="Berkeley Software Distribution">BSD</abbr></a>,
84 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
85 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
86 <a href="https://www.chromium.org/chromium-os/">ChromiumOS</a>,
87 <a href="https://cygwin.com">Cygwin</a>,
88 <a href="https://mariadb.org">MariaDB</a>,
89 <a href="https://musl.libc.org">musl libc</a>,
90 <a href="https://www.mysql.com">MySQL</a>,
91 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
92 title="Web Operating System">webOS</abbr></a>,
93 <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
94 title="Advanced Interactive eXecutive">AIX</abbr></a>,
95 <a href="https://www.apple.com/os/ios/"><abbr
96 title="iPhone OS">iOS</abbr></a>,
97 <a href="https://www.apple.com/os/macos/">macOS</a>,
98 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
99 <a href="https://vmssoftware.com">Open<abbr
100 title="Virtual Memory System">VMS</abbr></a>,
101 <a href="https://www.oracle.com/database/">Oracle Database</a>,
102 <a href="https://www.oracle.com/solaris/solaris11/">Oracle Solaris</a>,
103 and <a href="https://qnx.software/en">QNX</a>.</p>
104 <p>
105 Each main entry in the database represents a <dfn>timezone</dfn>
106 for a set of civil-time clocks that have all agreed since 1970.
107 Timezones are typically identified by continent or ocean and then by the
108 name of the largest city within the region containing the clocks.
109 For example, <code>America/New_York</code>
110 represents most of the <abbr title="United States">US</abbr> eastern time zone;
111 <code>America/Phoenix</code> represents most of Arizona, which
112 uses mountain time without daylight saving time (<abbr>DST</abbr>);
113 <code>America/Detroit</code> represents most of Michigan, which uses
114 eastern time but with different <abbr>DST</abbr> rules in 1975;
115 and other entries represent smaller regions like Starke County,
116 Indiana, which switched from central to eastern time in 1991
117 and switched back in 2006.
118 To use the database on a <a
119 href="https://en.wikipedia.org/wiki/POSIX"><abbr
120 title="Portable Operating System Interface">POSIX</abbr>.1-2024</a>
121 implementation set the <code><abbr>TZ</abbr></code>
122 environment variable to the locations full name,
123 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
124 <p>
125 Associated with each timezone is a history of offsets from
126 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
127 Time</a> (<abbr>UT</abbr>), which is <a
128 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
129 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
130 for timestamps after 1960 this is more precisely <a
131 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
132 Universal Time</a> (<abbr>UTC</abbr>).
133 The database also records when daylight saving time was in use,
134 along with some time zone abbreviations such as <abbr>EST</abbr>
135 for Eastern Standard Time in the <abbr>US</abbr>.</p>
136 </section>
137
138 <section>
139 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
140 <p>
141 The following <a
142 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
143 the latest releases two
144 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
145 to a <abbr>GNU</abbr>/Linux or similar host.</p>
146 <pre><code>mkdir tzdb
147 cd tzdb
148 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
149 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
150 <a href="https://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="https://www.gnu.org/software/tar/">tar</a> -xf -
151 gzip -dc tzdata-latest.tar.gz | tar -xf -
152 </code></pre>
153 <p>Alternatively, the following shell commands download the same
154 release in a single-tarball format containing extra data
155 useful for regression testing:</p>
156 <pre><code>wget <a href="https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz">https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz</a>
157 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
158 </code></pre>
159 <p>These commands use convenience links to the latest release
160 of the <code><abbr>tz</abbr></code> database hosted by the
161 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
162 of the <a href="https://www.iana.org">Internet Assigned Numbers
163 Authority (IANA)</a>.
164 Older releases are in files named
165 <code>tzcode<var>V</var>.tar.gz</code>,
166 <code>tzdata<var>V</var>.tar.gz</code>, and
167 <code>tzdb-<var>V</var>.tar.lz</code>,
168 where <code><var>V</var></code> is the version.
169 Since 1996, each version has been a four-digit year followed by
170 lower-case letter (<samp>a</samp> through <samp>z</samp>,
171 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
172 through <samp>zzz</samp>, and so on).
173 Since version 2022a, each release has been distributed in
174 <a href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/pax.html#tag_20_94_13_06">POSIX
175 ustar interchange format</a>, compressed as described above;
176 older releases use a nearly compatible format.
177 Since version 2016h, each release has contained a text file named
178 <code>version</code> whose first (and currently only) line is the version.
179 <a href="https://ftp.iana.org/tz/releases/">Older archived releases are
180 available</a> via
181 <a href="https://en.wikipedia.org/wiki/HTTPS"><abbr
182 title="Hypertext Transfer Protocol Secure">HTTPS</abbr></a>,
183 <a href="https://en.wikipedia.org/wiki/Rsync"><abbr
184 title="remote sync">rsync</abbr></a>, and
185 <a href="https://en.wikipedia.org/wiki/FTP"><abbr
186 title="File Transfer Protocol">FTP</abbr></a>.
187 <p>Alternatively, a development repository of code and data can be
188 retrieved from <a href="https://github.com">GitHub</a> via the shell
189 command:</p>
190 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
191 </code></pre>
192 <p>
193 Since version 2012e, each release has been tagged in development repositories.
194 Untagged commits are less well tested and probably contain
195 more errors.</p>
196 <p>
197 After obtaining the code and data files, see the
198 <code>README</code> file for what to do next.
199 The code lets you compile the <code><abbr>tz</abbr></code> source files into
200 machine-readable binary files, one for each location.
201 The binary file format is specified by
202 <a href="https://www.rfc-editor.org/info/rfc9636/">The
203 Time Zone Information Format (<abbr>TZif</abbr>)</a>
204 (Internet <abbr title="Request For Comments">RFC</abbr> 9636).
205 The code also lets
206 you read a <abbr>TZif</abbr> file and interpret timestamps for that
207 location.</p>
208 </section>
209
210 <section>
211 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
212 <p>
213 The <code><abbr>tz</abbr></code> code and data
214 are by no means authoritative. If you find errors, please
215 email changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
216 the time zone mailing list.
217 The mailing list and its archives are public,
218 so please do not send confidential information.
219 See
220 <a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing
221 lists main page</a> to subscribe or to browse its archive of old messages.
222 <a href="https://tzdata-meta.timtimeonline.com">Metadata for mailing list
223 discussions</a> and corresponding data changes can be
224 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
225 </p>
226 <p>
227 Changes to the <code><abbr>tz</abbr></code> code and data are often
228 propagated to clients via operating system updates, so
229 client <code><abbr>tz</abbr></code> data can often be corrected by
230 applying these updates. With GNU/Linux and similar systems, if your
231 maintenance provider has not yet adopted the
232 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
233 the process by tailoring the generic instructions in
234 the <code><abbr>tz</abbr> README</code> file and installing the latest
235 data yourself. System-specific instructions for installing the
236 latest <code><abbr>tz</abbr></code> data have also been published
237 for <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates"><abbr>AIX</abbr></a>,
238 <a
239 href="https://source.android.com/docs/core/permissions/timezone-rules">Android</a>,
240 <a
241 href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
242 title="International Components for Unicode">ICU</abbr></a>,
243 <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
244 JDK</a>,
245 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
246 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
247 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
248 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
249 </p>
250 <p>Since version 2013a,
251 sources for the <code><abbr>tz</abbr></code> database have been
252 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
253 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
254 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
255 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
256 title="linefeed">LF</abbr></a>,
257 which can be modified by common text editors such
258 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
259 <a href="https://gedit-text-editor.org">gedit</a>, and
260 <a href="https://www.vim.org">vim</a>.
261 Specialized source-file editing can be done via the
262 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
263 zoneinfo</a> package for <a
264 href="https://www.sublimetext.com">Sublime Text</a> and the <a
265 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
266 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
267 Studio Code</a>.
268 </p>
269 <p>
270 For further information about updates, please see
271 <a href="https://www.rfc-editor.org/info/rfc6557/">Procedures for
272 Maintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557).
273 More detail can be
274 found in <a href="theory.html">Theory and pragmatics of the
275 <code><abbr>tz</abbr></code> code and data</a>.
276 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
277 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
278 </p>
279 </section>
280
281 <section>
282 <h2 id="coordinating">Coordinating with governments and distributors</h2>
283 <p>
284 As discussed in
285 <a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How
286 Time Zones Are Coordinated</a>, the time zone database relies on
287 collaboration among governments, the time zone database volunteer
288 community, and data distributors downstream.
289 <p>
290 If your government plans to change its time zone boundaries or
291 daylight saving rules, please send email as described in
292 <a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a>.
293 Do this well in advance,
294 as this will lessen confusion and will coordinate updates to many cell phones,
295 computers, and other devices around the world.
296 In your email, please cite the legislation or regulation that specifies
297 the change, so that it can be checked for details such as the exact times
298 when clock transitions occur.
299 It is OK if a rule change is planned to affect clocks
300 far into the future, as a long-planned change can easily be reverted
301 or otherwise altered with a years notice before the change would have
302 affected clocks.</p>
303 <p>
304 There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.
305 However, typically a release occurs every few months.
306 Many downstream timezone data distributors wait for
307 a <code><abbr>tzdb</abbr></code> release before they produce an update
308 to time zone behavior in consumer devices and software products.
309 After a release, various parties must integrate, test,
310 and roll out an update before <a
311 href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.
312 These updates can be expensive, for both the <a
313 href="https://en.wikipedia.org/wiki/Quality_assurance">quality
314 assurance</a> process and the overall cost of shipping and installing
315 updates to each devices copy of <code><abbr>tzdb</abbr></code>.
316 Updates may be batched with other updates and may take substantial
317 time to reach end users after a release.
318 Older devices may no longer be supported and thus may never be updated,
319 which means they will continue to use out-of-date rules.</p>
320 <p>
321 For these reasons any rule change should be promulgated at least a
322 year before it affects how clocks operate; otherwise, there is a good
323 chance that many clocks will be wrong due to delays in propagating updates,
324 and that residents will be confused or even actively resist the change.
325 The shorter the notice, the more likely clock problems will arise; see <a
326 href="https://codeofmatt.com/on-the-timing-of-time-zone-changes/">On
327 the Timing of Time Zone Changes</a> for examples.
328 </p>
329 </section>
330
331 <section>
332 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
333 <ul>
334 <li>The article
335 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
336 an encyclopedic summary.</li>
337 <li><a href="tz-how-to.html">How to Read the
338 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
339 database source format.</li>
340 <li><a
341 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
342 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
343 databases style.</li>
344 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
345 managing time in the internet</a> analyzes the database longitudinally.</li>
346 </ul>
347 </section>
348
349 <section>
350 <h2 id="web">Web sites using recent versions of the
351 <code><abbr>tz</abbr></code> database</h2>
352 <p>
353 These are listed roughly in ascending order of complexity and fanciness.
354 </p>
355 <ul>
356 <li><a href="https://time.is">Time.is</a> shows locations
357 time and zones.</li>
358 <li><a href="https://www.timejones.com">TimeJones.com</a>,
359 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
360 <a href="https://www.worldclock.com">The World Clock</a>
361 are time zone converters.</li>
362 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
363 publishes <code><abbr>tzdb</abbr></code>-derived data in
364 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
365 title="comma-separated values">CSV</abbr></a> and
366 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
367 title="Structured Query Language">SQL</abbr></a> form.</li>
368 <li><a
369 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
370 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
371 <li><a
372 href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
373 Time in 1000 Places</a> uses descriptions of the values.</li>
374 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock
375 Worldwide</a> lets you sort zone names and convert times.</li>
376 <li><a href="https://24timezones.com">24TimeZones</a> has a world
377 time map and a time converter.</li>
378 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
379 calculates the current time difference between locations.</li>
380 <li><a href="https://www.wx-now.com">Weather Now</a> and
381 <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
382 </ul>
383 </section>
384
385 <section>
386 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
387 <ul>
388 <li><a href="https://www.rfc-editor.org/info/rfc7808/">Time Zone
389 Data Distribution Service</a> (Internet <abbr>RFC</abbr> 7808)
390 is associated with
391 <a href="https://www.rfc-editor.org/info/rfc7809/">CalDAV</a>
392 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
393 transferring time zone data by reference.</li>
394 <li>The <a href="https://www.rfc-editor.org/info/rfc5545/">iCalendar format</a>
395 (Internet <abbr>RFC</abbr> 5545)
396 covers time zone
397 data; see its VTIMEZONE calendar component.
398 The iCalendar format requires specialized parsers and generators; a
399 variant <a href="https://www.rfc-editor.org/info/rfc6321/">xCal</a>
400 (Internet <abbr>RFC</abbr> 6321) uses
401 <a href="https://www.w3.org/XML/"><abbr
402 title="Extensible Markup Language">XML</abbr></a> format, and a variant
403 <a href="https://www.rfc-editor.org/info/rfc7265/">jCal</a>
404 (Internet <abbr>RFC</abbr> 7265)
405 uses <a href="https://www.json.org/json-en.html"><abbr
406 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
407 </ul>
408 </section>
409
410 <section>
411 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
412 <p>Although some of these do not fully support
413 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
414 distributions you can generally work around compatibility problems by
415 running the command <code>make rearguard_tarballs</code> and compiling
416 from the resulting tarballs instead.</p>
417 <ul>
418 <li><a href="https://github.com/libical">Libical</a>
419 contains <code>vzic</code>, <a
420 href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>
421 program that compiles
422 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
423 Vzic is freely
424 available under the <a
425 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
426 General Public License (<abbr
427 title="General Public License">GPL</abbr>)</a>.</li>
428 <li><a
429 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
430 contains a script <code>parse_olson</code> that compiles
431 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
432 modules. It is part of the Perl <a
433 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
434 which is freely
435 available under both the <abbr>GPL</abbr> and the Perl Artistic
436 License. DateTime::TimeZone also contains a script
437 <code>tests_from_zdump</code> that generates test cases for each clock
438 transition in the <code><abbr>tz</abbr></code> database.</li>
439 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
440 Database Parser</a> is a
441 <a href="https://en.wikipedia.org/wiki/C++">C++</a> parser and
442 runtime library with a <a
443 href="https://en.cppreference.com/cpp/chrono"><code>std::chrono</code> API</a>
444 that is a standard part of C++.
445 It is freely available under the
446 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
447 <li><a id="ICU" href="https://icu.unicode.org">International Components for
448 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
449 href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a>
450 libraries for internationalization that
451 has a compiler from <code><abbr>tz</abbr></code> source
452 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
453 (mentioned <a href="#CLDR">below</a>)
454 into an <abbr>ICU</abbr>-specific format.
455 <abbr>ICU</abbr> is freely available under a
456 <abbr>BSD</abbr>-style license.</li>
457 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
458 the <a href="https://elixir-lang.org">Elixir</a> language downloads
459 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
460 title="Application Program Interface">API</abbr>s for use. It is
461 freely available under the <abbr>MIT</abbr> license.</li>
462 <li>Java-based compilers and libraries include:
463 <ul>
464 <li>The <a
465 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
466 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
467 <a href="https://openjdk.org">OpenJDK</a> and
468 <a href="https://jdk.java.net">Oracle JDK</a>.
469 Although its source code is proprietary, its executable is available under the
470 <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
471 Timezone Updater License Agreement</a>.</li>
472 <li>The <a
473 href="https://www.oracle.com/technical-resources/articles/java/jf14-date-time.html">Java
474 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
475 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
476 which is freely available under a <abbr>BSD</abbr>-style license.</li>
477 <li><a href="https://www.joda.org/joda-time/">Joda-Time Java date
478 and time <abbr>API</abbr></a> contains a class
479 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
480 <code><abbr>tz</abbr></code> source into a binary format. It inspired
481 Java 8 <code>java.time</code>, which its users should migrate to once
482 they can assume Java 8 or later. It is available under the <a
483 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
484 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
485 href="https://www.azul.com/products/components/ziupdater-time-zone-tool/">ZIUpdater</a>
486 are alternatives to TZUpdater. IANA Updaters license is unclear;
487 ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
488 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
489 time library for Android</a> and
490 <a href="https://github.com/MenoData/Time4J">Time4J: Advanced date,
491 time and interval library for Java</a> compile
492 <code><abbr>tz</abbr></code> source into a binary format.
493 Time4A is available under the Apache License and Time4J is
494 available under the <a
495 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
496 General Public License (<abbr title="Lesser General Public
497 License">LGPL</abbr>)</a>.</li>
498 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
499 Java-based libraries.</li>
500 </ul>
501 <li><a href="https://nodatime.org">Noda Time Date and
502 time <abbr>API</abbr> for .NET</a>
503 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
504 It is freely available under the Apache License.</li>
505 <li>Many modern
506 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
507 runtimes support <code><abbr>tz</abbr></code> natively via the
508 <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>.
509 This can be used as-is or with most of the following libraries,
510 many of which also support runtimes lacking the <code>timeZone</code> option.
511 <ul>
512 <li>The <a
513 href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code>
514 timezone polyfill</a>
515 is freely available under a <abbr>BSD</abbr>-style license.</li>
516 <li>The <a href="https://date-fns.org">date-fns</a>
517 library manipulates timezone-aware timestamps in browsers and
518 in <a href="https://nodejs.org/en/">Node.js</a>.
519 It is freely available under the <abbr>MIT</abbr> license.</li>
520 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
521 minimalist replacement for the date and time API of
522 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
523 manipulation library.
524 It is freely available under the <abbr>MIT</abbr> license.</li>
525 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
526 timezone support for the <code>Intl</code> API.
527 It is freely available under the <abbr>MIT</abbr> license.</li>
528 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
529 Moment.js plugin.
530 It is freely available under the <abbr>MIT</abbr> license.</li>
531 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
532 JavaScript library that supports date arithmetic that is time zone
533 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
534 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
535 supports live <code><abbr>tzdb</abbr></code> updates,
536 astronomical and atomic time, a command-line interface,
537 and full <a
538 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
539 Its companion <a
540 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
541 can generate <abbr>TZif</abbr> and other files, and a companion website
542 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
543 convert timestamps, view transition histories, and download code and data.
544 It is freely available under the <abbr>MIT</abbr> license.</li>
545 </ul>
546 The proposed <a
547 href="https://github.com/tc39/proposal-temporal"><code>Temporal</code>
548 objects</a> let programs access an abstract view of
549 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
550 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScripts
551 problematic <code>Date</code> objects</a> when working with dates and times.
552 <li><a href="https://github.com/JuliaTime">JuliaTime</a> contains a
553 compiler from <code><abbr>tz</abbr></code> source into
554 <a href="https://julialang.org">Julia</a>. It is freely available
555 under the <abbr>MIT</abbr> license.</li>
556 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr>
557 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
558 title="Free Pascal Compiler">FPC</abbr></a>
559 compiles from <code><abbr>tz</abbr></code> source into
560 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
561 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
562 and <a
563 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
564 It is freely available under a <abbr>BSD</abbr>-style license.</li>
565 <li><a href="https://pythonhosted.org/pytz/">pytz World Timezone
566 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
567 <a href="https://www.python.org">Python</a>.
568 It is freely available under a <abbr>BSD</abbr>-style license.
569 In code that can assume Python 3.6 or later it is largely superseded; see <a
570 href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
571 The Fastest Footgun in the West</a>.</li>
572 <li><a href="https://tzinfo.github.io">TZInfo
573 Ruby Timezone Library</a>
574 compiles <code><abbr>tz</abbr></code> source into
575 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
576 It is freely available under the <abbr>MIT</abbr> license.</li>
577 <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
578 Library</a> is
579 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
580 library that compiles <code><abbr>tz</abbr></code> source into a time
581 zone repository whose format
582 is either proprietary or an <abbr>XML</abbr>-encoded
583 representation.</li>
584 <li><a id="Tcl" href="https://www.tcl-lang.org">Tcl</a>
585 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
586 source into text files, along with a runtime that can read those
587 files. Tcl is freely available under a <abbr>BSD</abbr>-style
588 license.</li>
589 </ul>
590 </section>
591
592 <section>
593 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
594 <ul>
595 <li>The <a
596 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
597 Library</a>
598 has a thread-safe <abbr>TZif</abbr> file reader.
599 This library is freely available under the LGPL
600 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
601 <li><a href="https://www.gnome.org">GNOME</a>s
602 <a href="https://docs.gtk.org/glib/">GLib</a> has
603 a <abbr>TZif</abbr> file reader written in C that
604 creates a <code>GTimeZone</code> object representing sets
605 of <abbr>UT</abbr> offsets.
606 It is freely available under the <abbr>LGPL</abbr>.</li>
607 <li>The
608 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>s
609 <code>baltzo::TimeZoneUtil</code> component contains a C++
610 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
611 the Apache License.</li>
612 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
613 library that translates between <abbr>UT</abbr> and civil time and
614 can read <abbr>TZif</abbr> files. It is freely available under the Apache
615 License.</li>
616 <li>The <a href="https://go.dev">Go programming language</a>
617 has a <abbr>TZif</abbr> file reader <a
618 href="https://pkg.go.dev/time#LoadLocationFromTZData"><code>LoadLocationFromTZData</code></a>.</li>
619 <li>The
620 <a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code>
621 package</a> contains Python code
622 to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map
623 <code><abbr>tz</abbr></code> settings to proleptic TZ approximations.
624 For example, it maps <code>"Africa/Cairo"</code>
625 to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>,
626 an approximation valid for Cairo timestamps from 2023 on.
627 This can help porting to platforms that support only proleptic TZ.
628 The package is freely available under the MIT license.</li>
629 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
630 library that reads <abbr>TZif</abbr> files and converts
631 timestamps from one time zone or format to another.
632 It is used by <a href="https://www.php.net"><abbr
633 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
634 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
635 and <a href="https://www.mongodb.com">MongoDB</a>.
636 It is freely available under the <abbr>MIT</abbr> license.</li>
637 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
638 <abbr>TZif</abbr> file reader.</li>
639 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
640 DateTime::TimeZone::Tzfile</a>
641 is a <abbr>TZif</abbr> file reader written in Perl.
642 It is freely available under the same terms as Perl
643 (dual <abbr>GPL</abbr> and Artistic license).</li>
644 <li>Python has a <a id="python-zoneinfo"
645 href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
646 class</a> that reads <abbr>TZif</abbr> data and creates objects
647 that represent <code><abbr>tzdb</abbr></code> timezones.
648 Python is freely available under the
649 <a href="https://docs.python.org/3/license.html">Python Software Foundation
650 License</a>.
651 A companion <a id="pypi-tzdata" href="https://pypi.org">PyPI</a> module
652 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
653 supplies TZif data if the underlying system data cannot be found;
654 it is freely available under the Apache License.</li>
655 <li>The
656 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
657 library contains a Python tool that
658 converts <abbr>TZif</abbr> data into
659 <abbr>JSON</abbr>-format data suitable for use
660 in its JavaScript library for time zone conversion. Dates before 1970
661 are not supported.</li>
662 <li>The <a
663 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
664 package contains <a href="https://www.haskell.org">Haskell</a> code that
665 parses and uses <abbr>TZif</abbr> data. It is freely
666 available under a <abbr>BSD</abbr>-style license.</li>
667 </ul>
668 </section>
669
670 <section>
671 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
672 <ul>
673 <li><a href="https://foxclocks.org">FoxClocks</a>
674 is an extension for <a href="https://www.google.com/chrome/">Google
675 Chrome</a>, <a
676 href="https://www.firefox.com/en-US/">Firefox</a> and <a
677 href="https://www.thunderbird.net/en-US/">Thunderbird</a>.
678 It displays multiple clocks in the application window, and has a mapping
679 interface to <a href="https://earth.google.com/web/">Google Earth</a>.
680 It is freely available under the <abbr>GPL</abbr>.</li>
681 <li>Microsoft Windows 8.1
682 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
683 data (mentioned <a href="#CLDR">below</a>) used by the
684 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
685 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
686 <a href="https://learn.microsoft.com/en-us/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
687 <a href="https://learn.microsoft.com/en-us/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
688 <a id="System.TimeZoneInfo"
689 href="https://learn.microsoft.com/en-us/archive/blogs/bclteam/exploring-windows-time-zones-with-system-timezoneinfo-josh-free">Exploring
690 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
691 the older, proprietary method of Microsoft Windows 2000 and later,
692 which stores time zone data in the
693 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
694 <a
695 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone
696 Tzid table</a> or <a
697 href="https://github.com/unicode-org/cldr/blob/main/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
698 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
699 to <code><abbr>tz</abbr></code> names.
700 These mappings can be performed programmatically via the
701 <a href="https://github.com/mattjohnsonpint/TimeZoneConverter">TimeZoneConverter</a>
702 .NET library,
703 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
704 <li><a
705 href="https://www.oracle.com/java/">Oracle
706 Java</a> contains a copy of a subset of a recent
707 <code><abbr>tz</abbr></code> database in a
708 Java-specific format.</li>
709 </ul>
710 </section>
711
712 <section>
713 <h2 id="other-dbs">Other time zone databases</h2>
714 <ul>
715 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
716 is Astrodiensts Web version of Shanks and Pottengers out-of-print
717 time zone history atlases
718 <a href="https://search.worldcat.org/title/468828649">for the US</a> and
719 <a href="https://search.worldcat.org/title/76950459">for the world</a>.
720 Although these extensive atlases
721 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
722 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
723 they are unreliable as Shanks appears to have
724 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
725 sources and do not indicate which entries are guesswork.</li>
726 <li>Microsoft Windows has proprietary data mentioned
727 <a href="#System.TimeZoneInfo">above</a>.</li>
728 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
729 is another time zone database.</li>
730 <li>The <a
731 href="https://www.iata.org/en/publications/manuals/standard-schedules-information">Standard
732 Schedules Information Manual</a> of the
733 International Air Transport Association
734 gives current time zone rules for airports served by commercial aviation.</li>
735 </ul>
736 </section>
737
738 <section>
739 <h2 id="maps">Maps</h2>
740 <ul>
741 <li><a href="https://www.worldtimezone.com">World Time Zone Map
742 with current time</a>
743 has several fancy time zone maps; it covers Russia particularly well.</li>
744 <li><a
745 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
746 much is time wrong around the world?</a> maps the difference between
747 mean solar and standard time, highlighting areas such as western China
748 where the two differ greatly. Its a bit out of date, unfortunately.</li>
749 <li>The
750 <a
751 href="https://maps.lib.utexas.edu/maps/world.html">PerryCastaeda
752 Library Map Collection</a> of the University of Texas at Austin has
753 copies of old maps taken from <a
754 href="https://en.wikipedia.org/wiki/The_World_Factbook"><em>The
755 World Factbook</em></a>, formerly published by the
756 <a href="https://www.cia.gov">US Central Intelligence Agency</a>.
757 Although the maps pictorial quality is good,
758 the maps do not indicate daylight saving time.</li>
759 </ul>
760 </section>
761
762 <section>
763 <h2 id="boundaries">Time zone boundaries</h2>
764 <p>Geographical boundaries between timezones are available
765 from several <a
766 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
767 geolocation</a>
768 services and other sources.</p>
769 <ul>
770 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
771 Boundary Builder</a> extracts
772 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
773 boundaries of <code><abbr>tzdb</abbr></code> timezones.
774 Its code is freely available under the <abbr>MIT</abbr> license, and
775 its data entries are freely available under the
776 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
777 Open Database License</a>. The borders appear to be quite accurate.
778 Its main web page lists more than twenty libraries
779 for looking up a timezone name from a GPS coordinate.</li>
780 <li>Free access via a network API, if you register a key, is provided by
781 the <a
782 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
783 Timezone web service</a>, the <a
784 href="https://developers.google.com/maps/documentation/timezone/overview">Google
785 Maps Time Zone API</a>, and
786 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
787 Commercial network API access is provided
788 by <a href="https://askgeo.com">AskGeo</a>
789 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
790 </li>
791 <li><a
792 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
793 to get a time zone from a location using latitude and longitude
794 coordinates?</a> discusses other geolocation possibilities.</li>
795 <li><a href="https://statoids.com/statoids.html">Administrative
796 Divisions of Countries (Statoids)</a> lists
797 political subdivision data related to time zones.</li>
798 <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
799 GIS and Database Tools</a> includes a Manifold-format map of
800 world time zone boundaries circa 2007, distributed under the
801 <abbr>GPL</abbr>.</li>
802 <li>A ship within the <a
803 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
804 waters</a> of any nation uses that nations time. In international
805 waters, time zone boundaries are meridians 15 apart, except that
806 <abbr>UT</abbr>12 and <abbr>UT</abbr>+12 are each 7.5
807 wide and are separated by
808 the 180 meridian (not by the International Date Line, which is
809 for land and territorial waters only). A captain can change ships
810 clocks any time after entering a new time zone; midnight changes are
811 common.</li>
812 </ul>
813 </section>
814
815 <section>
816 <h2 id="civil">Civil time concepts and history</h2>
817 <ul>
818 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
819 Walk through Time</a>
820 surveys the evolution of timekeeping.</li>
821 <li>The history of daylight saving time is surveyed in <a
822 href="https://www.webexhibits.org/daylightsaving/">About Daylight
823 Saving Time History, rationale, laws & dates</a> and summarized in
824 <a href="http://seizethedaylight.com/dst/">A Brief
825 History of Daylight Saving Time</a>.</li>
826 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
827 Lords</a> discusses how authoritarians manipulate civil time.</li>
828 <li><a href="https://www.w3.org/TR/timezone/">Working with Time
829 and Time Zones</a>
830 contains guidelines and best practices for software applications that
831 deal with civil time.</li>
832 <li><a href="https://webspace.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="https://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 Bureau of Meteorology publishes a list of <a
845 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
846 Dates of Daylight Savings Time within Australia</a>.</dd>
847 <dt>Belgium</dt>
848 <dd>The Royal Observatory of Belgium maintains a table of time in
849 Belgium (in
850 <a href="https://robinfo.oma.be/nl/astro-info/tijd/"
851 hreflang="nl">Dutch</a> and <a
852 href="https://robinfo.oma.be/fr/astro-info/heure/"
853 hreflang="fr">French</a>).</dd>
854 <dt>Brazil</dt>
855 <dd>The Time Service Department of the National Observatory
856 records <a href="http://pcdsh01.on.br/DecHV.html"
857 hreflang="pt-BR">Brazils daylight saving time decrees (in
858 Portuguese)</a>.</dd>
859 <dt>Canada</dt>
860 <dd>National Research Council Canada publishes current
861 and some older information about <a
862 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
863 zones and daylight saving time</a>.</dd>
864 <dt>Chile</dt>
865 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
866 <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
867 Chiles official time (in Spanish)</a>.</dd>
868 <dt>China</dt>
869 <dd>The Hong Kong Observatory maintains a
870 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
871 summer time in Hong Kong</a>,
872 and Macaus Meteorological and Geophysical Bureau maintains a <a
873 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
874 history for Macau</a>.
875 Unfortunately the latter is incomplete and has errors.</dd>
876 <dt>Czech Republic</dt>
877 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
878 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
879 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
880 <dt>Germany</dt>
881 <dd>The National Institute for Science and Technology maintains the <a
882 href="https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
883 of Legal Time in Germany</a>.</dd>
884 <dt>Israel</dt>
885 <dd><a
886 href="https://tz.cs.huji.ac.il">Israel Timezone Files</a>
887 lists official time-change announcements and laws since 1940,
888 almost all in Hebrew.</dd>
889 <dt>Malaysia</dt>
890 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
891 <dt>Mexico</dt>
892 <dd>The Investigation and Analysis Service of the Mexican Library of
893 Congress has published a <a
894 href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
895 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
896 <dt>Netherlands</dt>
897 <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"
898 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
899 covers the history of local time in the Netherlands from ancient times.</dd>
900 <dt>New Zealand</dt>
901 <dd>The Department of Internal Affairs maintains a brief <a
902 href="https://www.govt.nz/browse/recreation-and-the-environment/daylight-saving/history-of-daylight-saving-in-nz/">History
903 of Daylight Saving in NZ</a>.</dd>
904 <dt>Palestine</dt>
905 <dd>The Ministry of Telecom and Digital Economy publishes a <a
906 href="https://mtde.gov.ps/home/TimeZone"
907 hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
908 <dt>Portugal</dt>
909 <dd>The Lisbon Astronomical Observatory publishes a
910 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
911 legal time (in Portuguese)</a>.</dd>
912 <dt>Singapore</dt>
913 <dd><a id="Singapore"
914 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
915 is Singapore in the Wrong Time Zone?</a> details the
916 history of legal time in Singapore and Malaysia.</dd>
917 <dt>United Kingdom</dt>
918 <dd><a
919 href="https://www.polyomino.org.uk/british-time/">History of
920 legal time in Britain</a> discusses in detail the country
921 with perhaps the best-documented history of clock adjustments.</dd>
922 <dt>United States</dt>
923 <dd>The Department of Transportations <a
924 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
925 Time Zone Proceedings</a> lists changes to
926 official written time zone boundaries, and its <a
927 href="https://geodata.bts.gov/datasets/usdot::time-zones/about">Time
928 Zones dataset</a> maps current boundaries.
929 These boundaries are only for standard time, so the current map puts
930 all of Arizona in one time zone even though part of Arizona
931 observes <abbr>DST</abbr> and part does not.</dd>
932 <dt>Uruguay</dt>
933 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
934 Navy (SOHMA) publishes an annual <a
935 href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
936 (in Spanish)</a>.</dd>
937 </dl>
938 </section>
939
940 <section>
941 <h2 id="costs">Costs and benefits of time shifts</h2>
942 <p>Various sources argue for and against daylight saving time and time
943 zone shifts, and many scientific studies have been conducted. This
944 section summarizes reviews and position statements based on
945 scientific literature in the area.</p>
946 <ul>
947 <li>Carey RN, Sarma KM.
948 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
949 daylight saving time on road traffic collision risk: a systematic
950 review</a>.
951 <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>.
952 This reviews research literature and concludes that the evidence
953 neither supports nor refutes road safety benefits from
954 shifts in time zones.</li>
955 <li>Havranek T, Herman D, Irsova D.
956 Does daylight saving save electricity? A meta-analysis.
957 <em>Energy J.</em> 2018;39(2):3561.
958 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
959 This analyzes research literature and concludes, Electricity savings
960 are larger for countries farther away from the equator, while
961 subtropical regions consume more electricity because of <abbr>DST</abbr>.</li>
962 <li>Neumann P, von Blanckenburg K. <a
963 href="https://journals.sagepub.com/doi/full/10.1177/0961463X241310562">What
964 time will it be? A comprehensive literature review on daylight saving time</a>.
965 <em>Time Soc</em>. 2025;34(4):684745.
966 doi:<a href="https://doi.org/10.1177/0961463X241310562">10.1177/0961463X241310562</a>.
967 This reviews <abbr>DST</abbr>s effects on electricity, health, crime, road
968 safety, and the economy, focusing on research since 2010, and concludes that
969 year-round standard time is preferable overall.</li>
970 <li>Romigi A, Franco V, Scoditti E <em>et al</em>.
971 The effects of daylight saving time and clock time transitions on sleep and
972 sleepiness: a systematic review. <em>Sleep Med Rev.</em> 2025;84:102161. doi:<a
973 href="https://doi.org/10.1016/j.smrv.2025.102161">10.1016/j.smrv.2025.102161</a>.
974 This reviews <abbr>DST</abbr> and <abbr>DST</abbr> transitions,
975 and concludes that they both harm sleep, health and behavior.</li>
976 </ul>
977
978 <p>The following medical societies have taken positions on the
979 advisability of clock shifts:</p>
980
981 <ul>
982 <li>In 2022 the American Medical Association issued a
983 <a href="https://www.ama-assn.org/press-center/ama-press-releases/ama-calls-permanent-standard-time">statement
984 supporting permanent standard time</a> on health grounds.</li>
985 <li>Crawford MR, Winnebeck EC, von Schantz M <em>et al</em>.
986 <a href="https://onlinelibrary.wiley.com/doi/10.1111/jsr.14352">The
987 British Sleep Society position statement on Daylight Saving Time in the UK</a>.
988 <em>J Sleep Res.</em> 2025;34(3):e14352.
989 doi:<a href="https://doi.org/10.1111/jsr.14352">10.1111/jsr.14352</a>.
990 This recommends that the UK abolish <abbr>DST</abbr> for health reasons.</li>
991 <li>Malow BA. <a
992 href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time
993 to abolish the clock change and adopt permanent
994 standard time in the United States:
995 a Sleep Research Society position statement</a>.
996 <em>Sleep.</em> 2022;45(12):zsac236.
997 doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
998 The Sleep Research Society
999 advocates permanent standard time due to its health benefits.</li>
1000 <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
1001 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
1002 is the optimal choice for health and safety:
1003 an American Academy of Sleep Medicine position statement</a>.
1004 <em>J Clin Sleep Med.</em> 2024;20(1):121125.
1005 doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
1006 The AASM argues for permanent standard time due to health and safety risks
1007 and economic costs of both <abbr>DST</abbr> transitions and
1008 permanent <abbr>DST</abbr>.</li>
1009 <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
1010 <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why
1011 should we abolish Daylight Saving Time?</a>
1012 <em>J Biol Rhythms.</em> 2019;34(3):227230.
1013 doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
1014 The Society for Research on Biological Rhythms
1015 opposes <abbr>DST</abbr> changes and permanent <abbr>DST</abbr>,
1016 and advocates that governments adopt
1017 permanent Standard Time for the health and safety of their citizens.</li>
1018 </ul>
1019 </section>
1020
1021 <section>
1022 <h2 id="precision">Precision timekeeping</h2>
1023 <ul>
1024 <li><a
1025 href="http://leapsecond.com/hpan/an1289.pdf">The
1026 Science of Timekeeping</a> is a thorough introduction
1027 to the theory and practice of precision timekeeping.</li>
1028 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
1029 Time 2016</a> contains several freely readable papers.</li>
1030 <li><a href="https://www.ntp.org"><abbr
1031 title="Network Time Protocol">NTP</abbr>: The Network
1032 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
1033 discusses how to synchronize clocks of
1034 Internet hosts.</li>
1035 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
1036 family of software algorithms can achieve accuracy to a few tens of
1037 nanoseconds in scalable server farms without special hardware.</li>
1038 <li>The <a
1039 href="https://www.nist.gov/el/intelligent-systems-division-73500/ieee-1588">Precision
1040 Time Protocol</a> (<abbr
1041 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
1042 can achieve submicrosecond clock accuracy on a local area network
1043 with special-purpose hardware.</li>
1044 <li><a
1045 href="https://www.rfc-editor.org/info/rfc4833/">Timezone
1046 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
1047 (Internet <abbr>RFC</abbr> 4833)
1048 specifies a <a
1049 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
1050 option for a server to configure
1051 a clients time zone and daylight saving settings automatically.</li>
1052 <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
1053 Scales</a> describes astronomical time scales like
1054 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
1055 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
1056 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
1057 <li>The <a href="https://www.iau.org"><abbr
1058 title="International Astronomical Union">IAU</abbr></a>s <a
1059 href="https://www.iausofa.org"><abbr
1060 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
1061 collection contains C and <a
1062 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
1063 code for converting among time scales like
1064 <abbr title="International Atomic Time">TAI</abbr>,
1065 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
1066 <abbr>UTC</abbr>. It is freely available under the
1067 <a href="https://www.iausofa.org/terms-and-conditions">SOFA license</a>.</li>
1068 <li><a
1069 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
1070 Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
1071 diverse local time
1072 scales used by each landed mission on Mars.</li>
1073 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
1074 dedicated not only to leap seconds but to precise time and frequency
1075 in general. It covers the state of the art in amateur timekeeping, and
1076 how the art has progressed over the past few decades.</li>
1077 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
1078 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
1079 and time-signal emissions</a>, International Telecommunication Union
1080 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
1081 <li><a
1082 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
1083 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
1084 Bulletins</a> contains official publications of the International
1085 Earth Rotation and Reference Systems Service, which decides when leap
1086 seconds occur.
1087 The <code><abbr>tz</abbr></code> code and data support leap seconds
1088 via an optional <code>"right"</code> configuration where a computers internal
1089 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
1090 as opposed to the default <code>"posix"</code> configuration
1091 where the internal clock ignores leap seconds.
1092 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
1093 <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for
1094 timestamps starting with <code>time_t</code> 78 796 800,
1095 which corresponds to the first leap second
1096 1972-06-30 23:59:60 <abbr>UTC</abbr> in the <code>"right"</code> configuration,
1097 and to
1098 1972-07-01 00:00:00 <abbr>UTC</abbr> in the <code>"posix"</code> configuration.
1099 In practice the two configurations also agree for timestamps before
1100 1972 even though the historical situation is messy, partly because
1101 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
1102 is well-defined for sufficiently old timestamps.</li>
1103 <li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The
1104 <abbr>NTP</abbr> Leap Second File</a> covers the text file
1105 <code>leap-seconds.list</code>, which lists the currently known leap seconds.
1106 The <abbr>IERS</abbr> maintains this file, and a copy is distributed by
1107 <code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like
1108 <a href="https://www.ntp.org">classic
1109 <code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a>
1110 and <a href="https://ntpsec.org">NTPsec</a>.
1111 The <code><abbr>tz</abbr></code> database also distributes leap second
1112 information in a differently-formatted <code>leapseconds</code> text file,
1113 as well as in the <code>"right"</code> configuration in binary form; for
1114 example, <code>right/UTC</code> can be used
1115 by <a href="https://chrony-project.org"><code>chrony</code></a>,
1116 another <abbr>NTP</abbr> implementation.</li>
1117 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1118 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1119 leap second so that they disagree with <abbr>UTC</abbr> by at most a
1120 half second, even though every <abbr>POSIX</abbr> minute has exactly
1121 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1122 <code>"posix"</code> configuration, is <a
1123 href="https://gitlab.com/NTPsec/ntpsec/-/blob/master/docs/leapsmear.adoc">supported</a>
1124 by the <abbr>NTP</abbr> implementations mentioned above, <a
1125 href="https://github.com/google/unsmear">supports</a> conversion between
1126 <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
1127 cloud service providers. However, according to
1128 <a href="https://www.rfc-editor.org/info/rfc8633#section-3.7.1">3.7.1 of
1129 Network Time Protocol Best Current Practices</a>
1130 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1131 applications requiring accurate <abbr>UTC</abbr> or civil time,
1132 and is intended for use only in single, well-controlled environments.</li>
1133 <li>The <a
1134 href="https://groups.io/g/LEAPSECS">LEAPSECS List</a> covers <a
1135 href="https://gge.ext.unb.ca/Resources/gpsworld.november99.pdf">McCarthy
1136 and Klepczynskis 1999 proposal to discontinue leap seconds</a>,
1137 discussed further in
1138 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1139 leap second: its history and possible future</a>.
1140 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1141 might be redefined
1142 without Leap Seconds</a> gives pointers on this
1143 contentious issue.
1144 The General Conference on Weights and Measures (CGPM)
1145 <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>
1146 to discontinue the use of leap seconds by 2035, and requested that no
1147 discontinuous adjustments be made to UTC for at least a century.
1148 The World Radiocommunication Conference <a
1149 href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved
1150 in 2023</a> to cooperate with this process. A draft <a
1151 href="https://www.bipm.org/documents/d/guest/cgpm-2026-draft-resolutions">Resolution
1152 C to make continuous UTC effective on 2027-05-20</a>,
1153 and thereby discontinue leap seconds,
1154 has been scheduled for the 28th CGPM starting 2026-10-13 in Paris.
1155 One proposal to implement this
1156 would replace leap seconds with seven 13-second leap smears occurring once per
1157 decade until 2100, with leap smears after that gradually increasing in size.
1158 See:
1159 <ul>
1160 <li>Levine J. <a href="https://tf.nist.gov/general/pdf/3242.pdf">A
1161 proposal to change the leap-second adjustments to
1162 coordinated universal time</a>. <em>Metrologia.</em> 2024;61(5):055002. doi:<a
1163 href="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a>
1164 with followups in doi:<a
1165 href="https://doi.org/10.1088/1681-7575/ade314">10.1088/1681-7575/ade314</a>
1166 and doi:<a
1167 href="https://doi.org/10.1088/1681-7575/ade315">10.1088/1681-7575/ade315</a>.</li>
1168 </ul>
1169 However, there is still no consensus on whether this is the best way
1170 to replace leap seconds.
1171 </li>
1172 </ul>
1173 </section>
1174
1175 <section>
1176 <h2 id="notation">Time notation</h2>
1177 <ul>
1178 <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
1179 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1180 zone names, abbreviations, identifiers, and formats. For example, it
1181 contains French translations for Eastern European Summer Time,
1182 <abbr title="Eastern European Summer Time">EEST</abbr>, and Bucharest. Its
1183 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1184 charts</a> show these values for many locales. Data values are available in
1185 both <abbr title="Locale Data Markup Language">LDML</abbr>
1186 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1187 <li>
1188 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1189 the international standard date and time notation</a> covers
1190 <a
1191 href="https://www.iso.org/standard/70907.html"><em><abbr
1192 title="International Organization for Standardization">ISO</abbr>
1193 8601-1:2019 Date and time Representations for information
1194 interchange Part 1: Basic rules</em></a>.</li>
1195 <li>
1196 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1197 Schema: Datatypes dateTime</a> specifies a format inspired by
1198 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1199 <li><a href="https://www.rfc-editor.org/info/rfc5322#section-3.3">3.3 of
1200 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1201 specifies the time notation used in email and <a
1202 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1203 headers.</li>
1204 <li>
1205 <a href="https://www.rfc-editor.org/info/rfc3339/">Date and Time
1206 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1207 specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.
1208 An extension, <a href="https://www.rfc-editor.org/info/rfc9557/">Date
1209 and Time on the Internet: Timestamps with Additional Information</a>
1210 (Internet <abbr>RFC</abbr> 9557) extends this profile
1211 to let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp
1212 via suffixes like <code>[Asia/Tokyo]</code>.
1213 <li>
1214 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1215 Formats on the Web</a> surveys web- and Internet-oriented date and time
1216 formats.</li>
1217 <li>Alphabetic time zone abbreviations should not be used as unique
1218 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1219 practice. For example, in English-speaking North America
1220 <abbr>CST</abbr> denotes 6 hours behind <abbr>UT</abbr>,
1221 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1222 and French-speaking North Americans prefer
1223 <abbr title="Heure Normale du Centre">HNC</abbr> to
1224 <abbr>CST</abbr>. The <code><abbr>tz</abbr></code>
1225 database contains English abbreviations for many timestamps;
1226 unfortunately some of these abbreviations were merely the database maintainers
1227 inventions, and these have been removed when possible.</li>
1228 <li>Numeric time zone abbreviations typically count hours east of
1229 <abbr>UT</abbr>, e.g., +09 for Japan and
1230 10 for Hawaii. However, <abbr>POSIX</abbr> proleptic
1231 <code><abbr>TZ</abbr></code> settings use the opposite convention.
1232 For example, one might use <code><abbr>TZ</abbr>="<abbr
1233 title="Japan Standard Time">JST</abbr>-9"</code> and
1234 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1235 for Japan and Hawaii, respectively. If the
1236 <code><abbr>tz</abbr></code> database is available, it is usually better to use
1237 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1238 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1239 confusion, handle old timestamps better, and insulate you better from
1240 any future changes to the rules. One should never set
1241 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1242 <code>"GMT-9"</code>, though, since this would incorrectly imply that
1243 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1244 is called <abbr>GMT</abbr>.</li>
1245 </ul>
1246 </section>
1247
1248 <section>
1249 <h2 id="see-also">See also</h2>
1250 <ul>
1251 <li><a href="theory.html">Theory and pragmatics of the
1252 <code><abbr>tz</abbr></code> code and data</a></li>
1253 <li><a href="tz-art.html">Time and the arts</a></li>
1254 </ul>
1255 </section>
1256
1257 <footer>
1258 <hr>
1259 This web page is in the public domain, so clarified as of
1260 2009-05-17 by Arthur David Olson.
1261 <br>
1262 Please send corrections to this web page to the
1263 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1264 The mailing list and its archives are public,
1265 so please do not send confidential information.
1266 </footer>
1267 </body>
1268 </html>
1269