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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="GNUs 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://musl.libc.org">musl libc</a>,
88 <a href="https://www.mysql.com">MySQL</a>,
89 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
90 title="Web Operating System">webOS</abbr></a>,
91 <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
92 title="Advanced Interactive eXecutive">AIX</abbr></a>,
93 <a href="https://www.apple.com/ios"><abbr
94 title="iPhone OS">iOS</abbr></a>,
95 <a href="https://www.apple.com/macos">macOS</a>,
96 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
97 <a href="https://www.vmssoftware.com">Open<abbr
98 title="Virtual Memory System">VMS</abbr></a>,
99 <a href="https://www.oracle.com/database/">Oracle Database</a>,
100 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>,
101 and <a href="https://blackberry.qnx.com/en">QNX</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 locations 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 releases 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/9799919799/utilities/pax.html#tag_20_94_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 <a href="https://ftp.iana.org/tz/releases/">Older archived releases are
178 available</a> via
179 <a href="https://en.wikipedia.org/wiki/HTTPS"><abbr
180 title="Hypertext Transfer Protocol Secure">HTTPS</abbr></a>,
181 <a href="https://en.wikipedia.org/wiki/Rsync"><abbr
182 title="remote sync">rsync</abbr></a>, and
183 <a href="https://en.wikipedia.org/wiki/FTP"><abbr
184 title="File Transfer Protocol">FTP</abbr></a>.
185 <p>Alternatively, a development repository of code and data can bem
186 retrieved from <a href="https://github.com">GitHub</a> via the shell
187 command:</p>
188 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
189 </code></pre>
190 <p>
191 Since version 2012e, each release has been tagged in development repositories.
192 Untagged commits are less well tested and probably contain
193 more errors.</p>
194 <p>
195 After obtaining the code and data files, see the
196 <code>README</code> file for what to do next.
197 The code lets you compile the <code><abbr>tz</abbr></code> source files into
198 machine-readable binary files, one for each location. The binary files
199 are in a special format specified by
200 <a href="https://www.rfc-editor.org/rfc/rfc9636">The
201 Time Zone Information Format (<abbr>TZif</abbr>)</a>
202 (Internet <abbr title="Request For Comments">RFC</abbr> 9636).
203 The code also lets
204 you read a <abbr>TZif</abbr> file and interpret timestamps for that
205 location.</p>
206 </section>
207
208 <section>
209 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
210 <p>
211 The <code><abbr>tz</abbr></code> code and data
212 are by no means authoritative. If you find errors, please
213 email changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
214 the time zone mailing list.
215 The mailing list and its archives are public,
216 so please do not send confidential information.
217 See
218 <a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing
219 lists main page</a> to subscribe or to browse its archive of old messages.
220 <a href="https://tzdata-meta.timtimeonline.com">Metadata for mailing list
221 discussions</a> and corresponding data changes can be
222 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
223 </p>
224 <p>
225 Changes to the <code><abbr>tz</abbr></code> code and data are often
226 propagated to clients via operating system updates, so
227 client <code><abbr>tz</abbr></code> data can often be corrected by
228 applying these updates. With GNU/Linux and similar systems, if your
229 maintenance provider has not yet adopted the
230 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
231 the process by tailoring the generic instructions in
232 the <code><abbr>tz</abbr> README</code> file and installing the latest
233 data yourself. System-specific instructions for installing the
234 latest <code><abbr>tz</abbr></code> data have also been published
235 for <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates"><abbr>AIX</abbr></a>,
236 <a
237 href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
238 <a
239 href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
240 title="International Components for Unicode">ICU</abbr></a>,
241 <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
242 JDK</a>,
243 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
244 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
245 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
246 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
247 </p>
248 <p>Since version 2013a,
249 sources for the <code><abbr>tz</abbr></code> database have been
250 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
251 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
252 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
253 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
254 title="linefeed">LF</abbr></a>,
255 which can be modified by common text editors such
256 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
257 <a href="https://gedit-text-editor.org">gedit</a>, and
258 <a href="https://www.vim.org">vim</a>.
259 Specialized source-file editing can be done via the
260 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
261 zoneinfo</a> package for <a
262 href="https://www.sublimetext.com">Sublime Text</a> and the <a
263 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
264 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
265 Studio Code</a>.
266 </p>
267 <p>
268 For further information about updates, please see
269 <a href="https://www.rfc-editor.org/rfc/rfc6557">Procedures for
270 Maintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557).
271 More detail can be
272 found in <a href="theory.html">Theory and pragmatics of the
273 <code><abbr>tz</abbr></code> code and data</a>.
274 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
275 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
276 </p>
277 </section>
278
279 <section>
280 <h2 id="coordinating">Coordinating with governments and distributors</h2>
281 <p>
282 As discussed in
283 <a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How
284 Time Zones Are Coordinated</a>, the time zone database relies on
285 collaboration among governments, the time zone database volunteer
286 community, and data distributors downstream.
287 <p>
288 If your government plans to change its time zone boundaries or
289 daylight saving rules, please send email as described in
290 "<a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a>".
291 Do this well in advance,
292 as this will lessen confusion and will coordinate updates to many cell phones,
293 computers, and other devices around the world.
294 In your email, please cite the legislation or regulation that specifies
295 the change, so that it can be checked for details such as the exact times
296 when clock transitions occur.
297 It is OK if a rule change is planned to affect clocks
298 far into the future, as a long-planned change can easily be reverted
299 or otherwise altered with a years notice before the change would have
300 affected clocks.</p>
301 <p>
302 There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.
303 However, typically a release occurs every few months.
304 Many downstream timezone data distributors wait for
305 a <code><abbr>tzdb</abbr></code> release before they produce an update
306 to time zone behavior in consumer devices and software products.
307 After a release, various parties must integrate, test,
308 and roll out an update before <a
309 href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.
310 These updates can be expensive, for both the <a
311 href="https://en.wikipedia.org/wiki/Quality_assurance">quality
312 assurance</a> process and the overall cost of shipping and installing
313 updates to each devices copy of <code><abbr>tzdb</abbr></code>.
314 Updates may be batched with other updates and may take substantial
315 time to reach end users after a release.
316 Older devices may no longer be supported and thus may never be updated,
317 which means they will continue to use out-of-date rules.</p>
318 <p>
319 For these reasons any rule change should be promulgated at least a
320 year before it affects how clocks operate; otherwise, there is a good
321 chance that many clocks will be wrong due to delays in propagating updates,
322 and that residents will be confused or even actively resist the change.
323 The shorter the notice, the more likely clock problems will arise; see <a
324 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
325 the Timing of Time Zone Changes</a> for examples.
326 </p>
327 </section>
328
329 <section>
330 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
331 <ul>
332 <li>The article
333 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
334 an encyclopedic summary.</li>
335 <li><a href="tz-how-to.html">How to Read the
336 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
337 database format.</li>
338 <li><a
339 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
340 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
341 databases style.</li>
342 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
343 managing time in the internet</a> analyzes the database longitudinally.</li>
344 </ul>
345 </section>
346
347 <section>
348 <h2 id="web">Web sites using recent versions of the
349 <code><abbr>tz</abbr></code> database</h2>
350 <p>
351 These are listed roughly in ascending order of complexity and fanciness.
352 </p>
353 <ul>
354 <li><a href="https://time.is">Time.is</a> shows locations
355 time and zones.</li>
356 <li><a href="https://www.timejones.com">TimeJones.com</a>,
357 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
358 <a href="https://www.worldclock.com">The World Clock</a>
359 are time zone converters.</li>
360 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
361 publishes <code><abbr>tzdb</abbr></code>-derived data in
362 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
363 title="comma-separated values">CSV</abbr></a> and
364 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
365 title="Structured Query Language">SQL</abbr></a> form.</li>
366 <li><a
367 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
368 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
369 <li><a
370 href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
371 Time in 1000 Places</a> uses descriptions of the values.</li>
372 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock
373 Worldwide</a> lets you sort zone names and convert times.</li>
374 <li><a href="https://24timezones.com">24TimeZones</a> has a world
375 time map and a time converter.</li>
376 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
377 calculates the current time difference between locations.</li>
378 <li><a href="https://www.wx-now.com">Weather Now</a> and
379 <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
380 </ul>
381 </section>
382
383 <section>
384 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
385 <ul>
386 <li><a href="https://www.rfc-editor.org/rfc/rfc7808">Time Zone
387 Data Distribution Service</a> (TZDIST, Internet <abbr>RFC</abbr> 7808)
388 is associated with
389 <a href="https://www.rfc-editor.org/rfc/rfc7809">CalDAV</a>
390 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
391 transferring time zone data by reference.
392 <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
393 implementations</a> are available.</li>
394 <li>The <a href="https://www.rfc-editor.org/rfc/rfc5545">iCalendar format</a>
395 (Internet <abbr>RFC</abbr> 5445)
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/rfc/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/rfc/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/vzic">Vzic</a> is a <a
419 href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>
420 program that compiles
421 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
422 Vzic is freely
423 available under the <a
424 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
425 General Public License (<abbr
426 title="General Public License">GPL</abbr>)</a>.</li>
427 <li><a
428 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
429 contains a script <code>parse_olson</code> that compiles
430 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
431 modules. It is part of the Perl <a
432 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
433 which is freely
434 available under both the <abbr>GPL</abbr> and the Perl Artistic
435 License. DateTime::TimeZone also contains a script
436 <code>tests_from_zdump</code> that generates test cases for each clock
437 transition in the <code><abbr>tz</abbr></code> database.</li>
438 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
439 Database Parser</a> is a
440 <a href="https://en.wikipedia.org/wiki/C++">C++</a> parser and
441 runtime library with a <a
442 href="https://en.cppreference.com/w/cpp/chrono"><code>std::chrono</code> API</a>
443 that is a standard part of C++.
444 It is freely available under the
445 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
446 <li><a id="ICU" href="https://icu.unicode.org">International Components for
447 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
448 href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a>
449 libraries for internationalization that
450 has a compiler from <code><abbr>tz</abbr></code> source
451 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
452 (mentioned <a href="#CLDR">below</a>)
453 into an <abbr>ICU</abbr>-specific format.
454 <abbr>ICU</abbr> is freely available under a
455 <abbr>BSD</abbr>-style license.</li>
456 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
457 the <a href="https://elixir-lang.org">Elixir</a> language downloads
458 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
459 title="Application Program Interface">API</abbr>s for use. It is
460 freely available under the <abbr>MIT</abbr> license.</li>
461 <li>Java-based compilers and libraries include:
462 <ul>
463 <li>The <a
464 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
465 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
466 <a href="https://openjdk.org">OpenJDK</a> and
467 <a href="https://jdk.java.net">Oracle JDK</a>.
468 Although its source code is proprietary, its executable is available under the
469 <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
470 Timezone Updater License Agreement</a>.</li>
471 <li>The <a
472 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
473 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
474 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
475 which is freely available under a <abbr>BSD</abbr>-style license.</li>
476 <li><a href="https://www.joda.org/joda-time/">Joda-Time Java date
477 and time <abbr>API</abbr></a> contains a class
478 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
479 <code><abbr>tz</abbr></code> source into a binary format. It inspired
480 Java 8 <code>java.time</code>, which its users should migrate to once
481 they can assume Java 8 or later. It is available under the <a
482 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
483 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
484 href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
485 are alternatives to TZUpdater. IANA Updaters license is unclear;
486 ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
487 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
488 time library for Android</a> and
489 <a href="https://github.com/MenoData/Time4J">Time4J: Advanced date,
490 time and interval library for Java</a> compile
491 <code><abbr>tz</abbr></code> source into a binary format.
492 Time4A is available under the Apache License and Time4J is
493 available under the <a
494 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
495 General Public License (<abbr title="Lesser General Public
496 License">LGPL</abbr>)</a>.</li>
497 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
498 Java-based libraries.</li>
499 </ul>
500 <li><a href="https://nodatime.org">Noda Time Date and
501 time <abbr>API</abbr> for .NET</a>
502 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
503 It is freely available under the Apache License.</li>
504 <li>Many modern
505 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
506 runtimes support <code><abbr>tz</abbr></code> natively via the
507 <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>.
508 This can be used as-is or with most of the following libraries,
509 many of which also support runtimes lacking the <code>timeZone</code> option.
510 <ul>
511 <li>The <a
512 href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code>
513 timezone polyfill</a>
514 is freely available under a <abbr>BSD</abbr>-style license.</li>
515 <li>The <a href="https://date-fns.org">date-fns</a>
516 library manipulates timezone-aware timestamps in browsers and
517 in <a href="https://nodejs.org/en/">Node.js</a>.
518 It is freely available under the <abbr>MIT</abbr> license.</li>
519 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
520 minimalist replacement for the date and time API of
521 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
522 manipulation library.
523 It is freely available under the <abbr>MIT</abbr> license.</li>
524 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
525 timezone support for the <code>Intl</code> API.
526 It is freely available under the <abbr>MIT</abbr> license.</li>
527 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
528 Moment.js plugin.
529 It is freely available under the <abbr>MIT</abbr> license.</li>
530 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
531 JavaScript library that supports date arithmetic that is time zone
532 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
533 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
534 supports live <code><abbr>tzdb</abbr></code> updates,
535 astronomical and atomic time, a command-line interface,
536 and full <a
537 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
538 Its companion <a
539 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
540 can generate <abbr>TZif</abbr> and other files, and a companion website
541 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
542 convert timestamps, view transition histories, and download code and data.
543 It is freely available under the <abbr>MIT</abbr> license.</li>
544 </ul>
545 The proposed <a
546 href="https://github.com/tc39/proposal-temporal"><code>Temporal</code>
547 objects</a> let programs access an abstract view of
548 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
549 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScripts
550 problematic <code>Date</code> objects</a> when working with dates and times.
551 <li><a href="https://github.com/JuliaTime">JuliaTime</a> contains a
552 compiler from <code><abbr>tz</abbr></code> source into
553 <a href="https://julialang.org">Julia</a>. It is freely available
554 under the <abbr>MIT</abbr> license.</li>
555 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr>
556 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
557 title="Free Pascal Compiler">FPC</abbr></a>
558 compiles from <code><abbr>tz</abbr></code> source into
559 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
560 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
561 and <a
562 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
563 It is freely available under a <abbr>BSD</abbr>-style license.</li>
564 <li><a href="https://pythonhosted.org/pytz/">pytz World Timezone
565 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
566 <a href="https://www.python.org">Python</a>.
567 It is freely available under a <abbr>BSD</abbr>-style license.
568 In code that can assume Python 3.6 or later it is largely superseded; see <a
569 href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
570 The Fastest Footgun in the West</a>.</li>
571 <li><a href="https://tzinfo.github.io">TZInfo
572 Ruby Timezone Library</a>
573 compiles <code><abbr>tz</abbr></code> source into
574 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
575 It is freely available under the <abbr>MIT</abbr> license.</li>
576 <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
577 Library</a> is
578 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
579 library that compiles <code><abbr>tz</abbr></code> source into a time
580 zone repository whose format
581 is either proprietary or an <abbr>XML</abbr>-encoded
582 representation.</li>
583 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
584 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
585 source into text files, along with a runtime that can read those
586 files. Tcl is freely available under a <abbr>BSD</abbr>-style
587 license.</li>
588 </ul>
589 </section>
590
591 <section>
592 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
593 <ul>
594 <li>The <a
595 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
596 Library</a>
597 has an independent, thread-safe implementation of
598 a <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://developer.gnome.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://golang.org">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://secure.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.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
677 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
678 It displays multiple clocks in the application window, and has a mapping
679 interface to <a href="https://www.google.com/earth/">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://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
687 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
688 <a id="System.TimeZoneInfo"
689 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/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/master/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 <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
701 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
702 <li><a
703 href="https://www.oracle.com/java/index.html">Oracle
704 Java</a> contains a copy of a subset of a recent
705 <code><abbr>tz</abbr></code> database in a
706 Java-specific format.</li>
707 </ul>
708 </section>
709
710 <section>
711 <h2 id="other-dbs">Other time zone databases</h2>
712 <ul>
713 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
714 is Astrodiensts Web version of Shanks and Pottengers out-of-print
715 time zone history atlases
716 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
717 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
718 Although these extensive atlases
719 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
720 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
721 they are unreliable as Shanks appears to have
722 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
723 sources and do not indicate which entries are guesswork.</li>
724 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
725 its own <code>tztab</code>(4) format.</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/publications/store/Pages/standard-schedules-information.aspx">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>The <a
742 href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
743 and Regional Maps section</a> of <em>The World Factbook</em>, published by the
744 <a href="https://www.cia.gov">US Central Intelligence
745 Agency (<abbr
746 title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
747 zone map; the
748 <a
749 href="https://legacy.lib.utexas.edu/maps/world.html">PerryCastaeda
750 Library Map Collection</a>
751 of the University of Texas at Austin has copies of
752 recent editions.
753 The pictorial quality is good,
754 but the maps do not indicate daylight saving time,
755 and parts of the data are a few years out of date.</li>
756 <li><a href="https://www.worldtimezone.com">World Time Zone Map
757 with current time</a>
758 has several fancy time zone maps; it covers Russia particularly well.
759 The maps pictorial quality is not quite as good as the <abbr>CIA</abbr>s
760 but the maps are more up to date.</li>
761 <li><a
762 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
763 much is time wrong around the world?</a> maps the difference between
764 mean solar and standard time, highlighting areas such as western China
765 where the two differ greatly. Its a bit out of date, unfortunately.</li>
766 </ul>
767 </section>
768
769 <section>
770 <h2 id="boundaries">Time zone boundaries</h2>
771 <p>Geographical boundaries between timezones are available
772 from several <a
773 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
774 geolocation</a>
775 services and other sources.</p>
776 <ul>
777 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
778 Boundary Builder</a> extracts
779 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
780 boundaries of <code><abbr>tzdb</abbr></code> timezones.
781 Its code is freely available under the <abbr>MIT</abbr> license, and
782 its data entries are freely available under the
783 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
784 Open Database License</a>. The borders appear to be quite accurate.
785 Its main web page lists more than twenty libraries
786 for looking up a timezone name from a GPS coordinate.</li>
787 <li>Free access via a network API, if you register a key, is provided by
788 the <a
789 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
790 Timezone web service</a>, the <a
791 href="https://developers.google.com/maps/documentation/timezone/intro">Google
792 Maps Time Zone API</a>, and
793 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
794 Commercial network API access is provided
795 by <a href="https://askgeo.com">AskGeo</a>
796 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
797 </li>
798 <li><a
799 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
800 to get a time zone from a location using latitude and longitude
801 coordinates?</a> discusses other geolocation possibilities.</li>
802 <li><a href="http://statoids.com/statoids.html">Administrative
803 Divisions of Countries (Statoids)</a> lists
804 political subdivision data related to time zones.</li>
805 <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
806 GIS and Database Tools</a> includes a Manifold-format map of
807 world time zone boundaries circa 2007, distributed under the
808 <abbr>GPL</abbr>.</li>
809 <li>A ship within the <a
810 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
811 waters</a> of any nation uses that nations time. In international
812 waters, time zone boundaries are meridians 15 apart, except that
813 <abbr>UT</abbr>12 and <abbr>UT</abbr>+12 are each 7.5
814 wide and are separated by
815 the 180 meridian (not by the International Date Line, which is
816 for land and territorial waters only). A captain can change ships
817 clocks any time after entering a new time zone; midnight changes are
818 common.</li>
819 </ul>
820 </section>
821
822 <section>
823 <h2 id="civil">Civil time concepts and history</h2>
824 <ul>
825 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
826 Walk through Time</a>
827 surveys the evolution of timekeeping.</li>
828 <li>The history of daylight saving time is surveyed in <a
829 href="http://www.webexhibits.org/daylightsaving/">About Daylight
830 Saving Time History, rationale, laws & dates</a> and summarized in
831 <a href="http://seizethedaylight.com/dst/">A Brief
832 History of Daylight Saving Time</a>.</li>
833 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
834 Lords</a> discusses how authoritarians manipulate civil time.</li>
835 <li><a href="https://www.w3.org/TR/timezone/">Working with Time
836 and Time Zones</a>
837 contains guidelines and best practices for software applications that
838 deal with civil time.</li>
839 <li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of
840 the International Date Line</a> tells the story of the most important
841 time zone boundary.</li>
842 <li><a href="http://statoids.com/tconcept.html">Basic Time
843 Zone Concepts</a> discusses terminological issues behind time zones.</li>
844 </ul>
845 </section>
846
847 <section>
848 <h2 id="national">National histories of legal time</h2>
849 <dl>
850 <dt>Australia</dt>
851 <dd>The Bureau of Meteorology publishes a list of <a
852 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
853 Dates of Daylight Savings Time within Australia</a>.</dd>
854 <dt>Belgium</dt>
855 <dd>The Royal Observatory of Belgium maintains a table of time in
856 Belgium (in
857 <a href="https://robinfo.oma.be/nl/astro-info/tijd/"
858 hreflang="nl">Dutch</a> and <a
859 href="https://robinfo.oma.be/fr/astro-info/heure/"
860 hreflang="fr">French</a>).</dd>
861 <dt>Brazil</dt>
862 <dd>The Time Service Department of the National Observatory
863 records <a href="http://pcdsh01.on.br/DecHV.html"
864 hreflang="pt-BR">Brazils daylight saving time decrees (in
865 Portuguese)</a>.</dd>
866 <dt>Canada</dt>
867 <dd>National Research Council Canada publishes current
868 and some older information about <a
869 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
870 zones and daylight saving time</a>.</dd>
871 <dt>Chile</dt>
872 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
873 <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
874 Chiles official time (in Spanish)</a>.</dd>
875 <dt>China</dt>
876 <dd>The Hong Kong Observatory maintains a
877 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
878 summer time in Hong Kong</a>,
879 and Macaus Meteorological and Geophysical Bureau maintains a <a
880 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
881 history for Macau</a>.
882 Unfortunately the latter is incomplete and has errors.</dd>
883 <dt>Czech Republic</dt>
884 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
885 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
886 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
887 <dt>Germany</dt>
888 <dd>The National Institute for Science and Technology maintains the <a
889 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
890 of Legal Time in Germany</a>.</dd>
891 <dt>Israel</dt>
892 <dd><a
893 href="https://tz.cs.huji.ac.il">Israel Timezone Files</a>
894 lists official time-change announcements and laws since 1940,
895 almost all in Hebrew.</dd>
896 <dt>Malaysia</dt>
897 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
898 <dt>Mexico</dt>
899 <dd>The Investigation and Analysis Service of the Mexican Library of
900 Congress has published a <a
901 href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
902 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
903 <dt>Netherlands</dt>
904 <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"
905 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
906 covers the history of local time in the Netherlands from ancient times.</dd>
907 <dt>New Zealand</dt>
908 <dd>The Department of Internal Affairs maintains a brief <a
909 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
910 Daylight Saving</a>.</dd>
911 <dt>Palestine</dt>
912 <dd>The Ministry of Telecom and Digital Economy publishes a <a
913 href="https://mtde.gov.ps/home/TimeZone"
914 hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
915 <dt>Portugal</dt>
916 <dd>The Lisbon Astronomical Observatory publishes a
917 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
918 legal time (in Portuguese)</a>.</dd>
919 <dt>Singapore</dt>
920 <dd><a id="Singapore"
921 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
922 is Singapore in the Wrong Time Zone?</a> details the
923 history of legal time in Singapore and Malaysia.</dd>
924 <dt>United Kingdom</dt>
925 <dd><a
926 href="https://www.polyomino.org.uk/british-time/">History of
927 legal time in Britain</a> discusses in detail the country
928 with perhaps the best-documented history of clock adjustments.</dd>
929 <dt>United States</dt>
930 <dd>The Department of Transportations <a
931 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
932 Time Zone Proceedings</a> lists changes to
933 official written time zone boundaries, and its <a
934 href="https://geodata.bts.gov/datasets/usdot::time-zones/about">Time
935 Zones dataset</a> maps current boundaries.
936 These boundaries are only for standard time, so the current map puts
937 all of Arizona in one time zone even though part of Arizona
938 observes <abbr>DST</abbr> and part does not.</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>Carey RN, Sarma KM.
955 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
956 daylight saving time on road traffic collision risk: a systematic
957 review</a>.
958 <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>.
959 This reviews research literature and concludes that the evidence
960 neither supports nor refutes road safety benefits from
961 shifts in time zones.</li>
962 <li>Havranek T, Herman D, Irsova D.
963 Does daylight saving save electricity? A meta-analysis.
964 <em>Energy J.</em> 2018;39(2):3561.
965 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
966 This analyzes research literature and concludes, Electricity savings
967 are larger for countries farther away from the equator, while
968 subtropical regions consume more electricity because of <abbr>DST</abbr>.</li>
969 <li>Neumann P, von Blanckenburg K. <a
970 href="https://journals.sagepub.com/doi/full/10.1177/0961463X241310562">What
971 time will it be? A comprehensive literature review on daylight saving time</a>.
972 <em>Time Soc</em>. 2025-01-21.
973 doi:<a href="https://doi.org/10.1177/0961463X241310562">10.1177/0961463X241310562</a>.
974 This reviews DSTs effects on electricity, health, crime, road safety,
975 and the economy, focusing on research since 2010, and concludes that
976 year-round standard time is preferable overall.
977 </ul>
978
979 <p>The following medical societies have taken positions on the
980 advisability of clock shifts:</p>
981
982 <ul>
983 <li>In 2022 the American Medical Association issued a
984 <a href="https://www.ama-assn.org/press-center/press-releases/ama-calls-permanent-standard-time">statement
985 supporting permanent standard time</a> on health grounds.</li>
986 <li>Crawford MR, Winnebeck EC, von Schantz M <em>et al</em>.
987 <a href="https://onlinelibrary.wiley.com/doi/10.1111/jsr.14352">The
988 British Sleep Society position statement on Daylight Saving Time in the UK</a>.
989 <em>J Sleep Res.</em> 2025;34(3):e14352.
990 doi:<a href="https://doi.org/10.1111/jsr.14352">10.1111/jsr.14352</a>.
991 This recommends that the UK abolish DST for health reasons.</li>
992 <li>Malow BA. <a
993 href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time
994 to abolish the clock change and adopt permanent
995 standard time in the United States:
996 a Sleep Research Society position statement</a>.
997 <em>Sleep.</em> 2022;45(12):zsac236.
998 doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
999 After reviewing the scientific literature, the Sleep Research Society
1000 advocates permanent standard time due to its health benefits.</li>
1001 <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
1002 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
1003 is the optimal choice for health and safety:
1004 an American Academy of Sleep Medicine position statement</a>.
1005 <em>J Clin Sleep Med.</em> 2024;20(1):121125.
1006 doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
1007 The AASM argues for permanent standard time due to health and safety risks
1008 and economic costs of both <abbr>DST</abbr> transitions and
1009 permanent <abbr>DST</abbr>.</li>
1010 <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
1011 <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why
1012 should we abolish Daylight Saving Time?</a>
1013 <em>J Biol Rhythms.</em> 2019;34(3):227230.
1014 doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
1015 The Society for Research on Biological Rhythms
1016 opposes <abbr>DST</abbr> changes and permanent <abbr>DST</abbr>,
1017 and advocates that governments adopt
1018 permanent Standard Time for the health and safety of their citizens.</li>
1019 </ul>
1020 </section>
1021
1022 <section>
1023 <h2 id="precision">Precision timekeeping</h2>
1024 <ul>
1025 <li><a
1026 href="http://leapsecond.com/hpan/an1289.pdf">The
1027 Science of Timekeeping</a> is a thorough introduction
1028 to the theory and practice of precision timekeeping.</li>
1029 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
1030 Time 2016</a> contains several freely readable papers.</li>
1031 <li><a href="https://www.ntp.org"><abbr
1032 title="Network Time Protocol">NTP</abbr>: The Network
1033 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
1034 discusses how to synchronize clocks of
1035 Internet hosts.</li>
1036 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
1037 family of software algorithms can achieve accuracy to a few tens of
1038 nanoseconds in scalable server farms without special hardware.</li>
1039 <li>The <a
1040 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
1041 Time Protocol</a> (<abbr
1042 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
1043 can achieve submicrosecond clock accuracy on a local area network
1044 with special-purpose hardware.</li>
1045 <li><a
1046 href="https://www.rfc-editor.org/rfc/rfc4833">Timezone
1047 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
1048 (Internet <abbr>RFC</abbr> 4833)
1049 specifies a <a
1050 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
1051 option for a server to configure
1052 a clients time zone and daylight saving settings automatically.</li>
1053 <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
1054 Scales</a> describes astronomical time scales like
1055 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
1056 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
1057 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
1058 <li>The <a href="https://www.iau.org"><abbr
1059 title="International Astronomical Union">IAU</abbr></a>s <a
1060 href="https://www.iausofa.org"><abbr
1061 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
1062 collection contains C and <a
1063 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
1064 code for converting among time scales like
1065 <abbr title="International Atomic Time">TAI</abbr>,
1066 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
1067 <abbr>UTC</abbr>. It is freely available under the
1068 <a href="https://www.iausofa.org/tandc.html">SOFA license</a>.</li>
1069 <li><a
1070 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
1071 Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
1072 diverse local time
1073 scales used by each landed mission on Mars.</li>
1074 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
1075 dedicated not only to leap seconds but to precise time and frequency
1076 in general. It covers the state of the art in amateur timekeeping, and
1077 how the art has progressed over the past few decades.</li>
1078 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
1079 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
1080 and time-signal emissions</a>, International Telecommunication Union
1081 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
1082 <li><a
1083 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
1084 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
1085 Bulletins</a> contains official publications of the International
1086 Earth Rotation and Reference Systems Service, which decides when leap
1087 seconds occur.
1088 The <code><abbr>tz</abbr></code> code and data support leap seconds
1089 via an optional <code>"right"</code> configuration where a computers internal
1090 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
1091 as opposed to the default <code>"posix"</code> configuration
1092 where the internal clock ignores leap seconds.
1093 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
1094 <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for
1095 timestamps starting with <code>time_t</code> 78 796 800,
1096 which corresponds to the first leap second
1097 1972-06-30 23:59:60 <abbr>UTC</abbr> in the <code>"right"</code> configuration,
1098 and to
1099 1972-07-01 00:00:00 <abbr>UTC</abbr> in the <code>"posix"</code> configuration.
1100 In practice the two configurations also agree for timestamps before
1101 1972 even though the historical situation is messy, partly because
1102 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
1103 is well-defined for sufficiently old timestamps.</li>
1104 <li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The
1105 <abbr>NTP</abbr> Leap Second File</a> covers the text file
1106 <code>leap-seconds.list</code>, which lists the currently known leap seconds.
1107 The <abbr>IERS</abbr> maintains this file, and a copy is distributed by
1108 <code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like
1109 <a href="https://www.ntp.org">classic
1110 <code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a>
1111 and <a href="https://ntpsec.org">NTPsec</a>.
1112 The <code><abbr>tz</abbr></code> database also distributes leap second
1113 information in a differently-formatted <code>leapseconds</code> text file,
1114 as well as in the <code>"right"</code> configuration in binary form; for
1115 example, <code>right/UTC</code> can be used
1116 by <a href="https://chrony-project.org"><code>chrony</code></a>,
1117 another <abbr>NTP</abbr> implementation.</li>
1118 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1119 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1120 leap second so that they disagree with <abbr>UTC</abbr> by at most a
1121 half second, even though every <abbr>POSIX</abbr> minute has exactly
1122 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1123 <code>"posix"</code> configuration, is <a
1124 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
1125 the abovementioned <abbr>NTP</abbr> implementations, <a
1126 href="https://github.com/google/unsmear">supports</a> conversion between
1127 <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
1128 cloud service providers. However, according to
1129 <a href="https://www.rfc-editor.org/rfc/rfc8633#section-3.7.1">3.7.1 of
1130 Network Time Protocol Best Current Practices</a>
1131 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1132 applications requiring accurate <abbr>UTC</abbr> or civil time,
1133 and is intended for use only in single, well-controlled environments.</li>
1134 <li>The <a
1135 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
1136 Second Discussion List</a> covers <a
1137 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
1138 and Klepczynskis 1999 proposal to discontinue leap seconds</a>,
1139 discussed further in
1140 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1141 leap second: its history and possible future</a>.
1142 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1143 might be redefined
1144 without Leap Seconds</a> gives pointers on this
1145 contentious issue.
1146 The General Conference on Weights and Measures
1147 <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>
1148 to discontinue the use of leap seconds by 2035, and requested that no
1149 discontinuous adjustments be made to UTC for at least a century.
1150 The World Radiocommunication Conference <a
1151 href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved
1152 in 2023</a> to cooperate with this process. One proposal to implement this
1153 would replace leap seconds with seven 13-second leap smears occurring once per
1154 decade until 2100, with leap smears after that gradually increasing in size.
1155 See:
1156 <ul>
1157 <li>Levine J. <a href="https://www.preprints.org/manuscript/202406.0043/v1">A
1158 proposal to change the leap-second adjustments to
1159 coordinated universal time</a>. <em>Metrologia.</em> 2024;61(5):055002. doi:<a
1160 href="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a>.</li>
1161 </ul>
1162 However, there is still no consensus on whether this is the best way
1163 to replace leap seconds.
1164 </li>
1165 </ul>
1166 </section>
1167
1168 <section>
1169 <h2 id="notation">Time notation</h2>
1170 <ul>
1171 <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
1172 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1173 zone names, abbreviations, identifiers, and formats. For example, it
1174 contains French translations for Eastern European Summer Time,
1175 <abbr title="Eastern European Summer Time">EEST</abbr>, and Bucharest. Its
1176 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1177 charts</a> show these values for many locales. Data values are available in
1178 both <abbr title="Locale Data Markup Language">LDML</abbr>
1179 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1180 <li>
1181 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1182 the international standard date and time notation</a> covers
1183 <a
1184 href="https://www.iso.org/standard/70907.html"><em><abbr
1185 title="International Organization for Standardization">ISO</abbr>
1186 8601-1:2019 Date and time Representations for information
1187 interchange Part 1: Basic rules</em></a>.</li>
1188 <li>
1189 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1190 Schema: Datatypes dateTime</a> specifies a format inspired by
1191 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1192 <li><a href="https://www.rfc-editor.org/rfc/rfc5322#section-3.3">3.3 of
1193 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1194 specifies the time notation used in email and <a
1195 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1196 headers.</li>
1197 <li>
1198 <a href="https://www.rfc-editor.org/rfc/rfc3339">Date and Time
1199 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1200 specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.
1201 An extension, <a href="https://www.rfc-editor.org/rfc/rfc9557">Date
1202 and Time on the Internet: Timestamps with Additional Information</a>
1203 (Internet <abbr>RFC</abbr> 9557) extends this profile
1204 to let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp
1205 via suffixes like <code>[Asia/Tokyo]</code>.
1206 <li>
1207 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1208 Formats on the Web</a> surveys web- and Internet-oriented date and time
1209 formats.</li>
1210 <li>Alphabetic time zone abbreviations should not be used as unique
1211 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1212 practice. For example, in English-speaking North America
1213 <abbr>CST</abbr> denotes 6 hours behind <abbr>UT</abbr>,
1214 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1215 and French-speaking North Americans prefer
1216 <abbr title="Heure Normale du Centre">HNC</abbr> to
1217 <abbr>CST</abbr>. The <code><abbr>tz</abbr></code>
1218 database contains English abbreviations for many timestamps;
1219 unfortunately some of these abbreviations were merely the database maintainers
1220 inventions, and these have been removed when possible.</li>
1221 <li>Numeric time zone abbreviations typically count hours east of
1222 <abbr>UT</abbr>, e.g., +09 for Japan and
1223 10 for Hawaii. However, <abbr>POSIX</abbr> proleptic
1224 <code><abbr>TZ</abbr></code> settings use the opposite convention.
1225 For example, one might use <code><abbr>TZ</abbr>="<abbr
1226 title="Japan Standard Time">JST</abbr>-9"</code> and
1227 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1228 for Japan and Hawaii, respectively. If the
1229 <code><abbr>tz</abbr></code> database is available, it is usually better to use
1230 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1231 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1232 confusion, handle old timestamps better, and insulate you better from
1233 any future changes to the rules. One should never set
1234 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1235 <code>"GMT-9"</code>, though, since this would incorrectly imply that
1236 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1237 is called <abbr>GMT</abbr>.</li>
1238 </ul>
1239 </section>
1240
1241 <section>
1242 <h2 id="see-also">See also</h2>
1243 <ul>
1244 <li><a href="theory.html">Theory and pragmatics of the
1245 <code><abbr>tz</abbr></code> code and data</a></li>
1246 <li><a href="tz-art.html">Time and the Arts</a></li>
1247 </ul>
1248 </section>
1249
1250 <footer>
1251 <hr>
1252 This web page is in the public domain, so clarified as of
1253 2009-05-17 by Arthur David Olson.
1254 <br>
1255 Please send corrections to this web page to the
1256 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1257 The mailing list and its archives are public,
1258 so please do not send confidential information.
1259 </footer>
1260 </body>
1261 </html>
1262