tz-link.html revision 1.15 1 <!DOCTYPE html>
2 <html lang="en">
3 <head>
4 <title>Time zone and daylight saving time data</title>
5 <meta charset="UTF-8">
6 <style>
7 pre {margin-left: 2em; white-space: pre-wrap;}
8 </style>
9 </head>
10 <body>
11 <h1>Time zone and daylight saving time data</h1>
12 <p>
13 <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
14 <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
15 rules are controlled by individual
16 governments. They are sometimes changed with little notice, and their
17 histories and planned futures are often recorded only fitfully. Here
18 is a summary of attempts to organize and record relevant data in this
19 area.
20 </p>
21 <h3>Outline</h3>
22 <nav>
23 <ul>
24 <li>The <code><abbr>tz</abbr></code> database product and process
25 <ul>
26 <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
27 <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
28 <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
29 <li><a href="#coordinating">Coordinating with governments and distributors</a></li>
30 <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
31 </ul>
32 </li>
33 <li>Uses of the <code><abbr>tz</abbr></code> database
34 <ul>
35 <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
36 <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
37 <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
38 <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
39 <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
40 </ul>
41 </li>
42 <li>Related data
43 <ul>
44 <li><a href="#other-dbs">Other time zone databases</a></li>
45 <li><a href="#maps">Maps</a></li>
46 <li><a href="#boundaries">Time zone boundaries</a></li>
47 </ul>
48 </li>
49 <li>Timekeeping concepts
50 <ul>
51 <li><a href="#civil">Civil time concepts and history</a></li>
52 <li><a href="#national">National histories of legal time</a></li>
53 <li><a href="#costs">Costs and benefits of time shifts</a></li>
54 <li><a href="#precision">Precision timekeeping</a></li>
55 <li><a href="#notation">Time notation</a></li>
56 </ul>
57 </li>
58 <li><a href="#see-also">See also</a></li>
59 </ul>
60 </nav>
61
62 <section>
63 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
64 <p>
65 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
66 time zone database contains code and data
67 that represent the history of local time
68 for many representative locations around the globe.
69 It is updated periodically to reflect changes made by political bodies
70 to time zone boundaries and daylight saving rules.
71 This database (known as <code><abbr>tz</abbr></code>,
72 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
73 is used by several implementations,
74 including
75 <a href="https://www.gnu.org/software/libc/">the
76 <abbr title="GNU's Not Unix">GNU</abbr>
77 C Library</a> (used in
78 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
79 <a href="https://www.android.com">Android</a>,
80 <a href="https://www.freebsd.org">Free<abbr
81 title="Berkeley Software Distribution">BSD</abbr></a>,
82 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
83 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
84 <a href="https://www.chromium.org/chromium-os/">Chromium OS</a>,
85 <a href="https://cygwin.com">Cygwin</a>,
86 <a href="https://mariadb.org">MariaDB</a>,
87 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
88 <a href="https://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>, and
100 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
101 <p>
102 Each main entry in the database represents a <dfn>timezone</dfn>
103 for a set of civil-time clocks that have all agreed since 1970.
104 Timezones are typically identified by continent or ocean and then by the
105 name of the largest city within the region containing the clocks.
106 For example, <code>America/New_York</code>
107 represents most of the <abbr title="United States">US</abbr> eastern time zone;
108 <code>America/Phoenix</code> represents most of Arizona, which
109 uses mountain time without daylight saving time (<abbr>DST</abbr>);
110 <code>America/Detroit</code> represents most of Michigan, which uses
111 eastern time but with different <abbr>DST</abbr> rules in 1975;
112 and other entries represent smaller regions like Starke County,
113 Indiana, which switched from central to eastern time in 1991
114 and switched back in 2006.
115 To use the database on an extended <a
116 href="https://en.wikipedia.org/wiki/POSIX"><abbr
117 title="Portable Operating System Interface">POSIX</abbr></a>
118 implementation set the <code><abbr>TZ</abbr></code>
119 environment variable to the location's full name,
120 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
121 <p>
122 Associated with each timezone is a history of offsets from
123 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
124 Time</a> (<abbr>UT</abbr>), which is <a
125 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
126 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
127 for timestamps after 1960 this is more precisely <a
128 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
129 Universal Time</a> (<abbr>UTC</abbr>).
130 The database also records when daylight saving time was in use,
131 along with some time zone abbreviations such as <abbr>EST</abbr>
132 for Eastern Standard Time in the <abbr>US</abbr>.</p>
133 </section>
134
135 <section>
136 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
137 <p>
138 The following <a
139 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
140 the latest release's two
141 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
142 to a <abbr>GNU</abbr>/Linux or similar host.</p>
143 <pre><code>mkdir tzdb
144 cd tzdb
145 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
146 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
147 <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 -
148 gzip -dc tzdata-latest.tar.gz | tar -xf -
149 </code></pre>
150 <p>Alternatively, the following shell commands download the same
151 release in a single-tarball format containing extra data
152 useful for regression testing:</p>
153 <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>
154 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
155 </code></pre>
156 <p>These commands use convenience links to the latest release
157 of the <code><abbr>tz</abbr></code> database hosted by the
158 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
159 of the <a href="https://www.iana.org">Internet Assigned Numbers
160 Authority (IANA)</a>.
161 Older releases are in files named
162 <code>tzcode<var>V</var>.tar.gz</code>,
163 <code>tzdata<var>V</var>.tar.gz</code>, and
164 <code>tzdb-<var>V</var>.tar.lz</code>,
165 where <code><var>V</var></code> is the version.
166 Since 1996, each version has been a four-digit year followed by
167 lower-case letter (<samp>a</samp> through <samp>z</samp>,
168 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
169 through <samp>zzz</samp>, and so on).
170 Since version 2022a, each release has been distributed in
171 <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
172 ustar interchange format</a>, compressed as described above;
173 older releases use a nearly compatible format.
174 Since version 2016h, each release has contained a text file named
175 "<samp>version</samp>" whose first (and currently only) line is the version.
176 Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
177 and are also available in an
178 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
179 title="File Transfer Protocol">FTP</abbr> directory</a> via a
180 less secure protocol.</p>
181 <p>Alternatively, a development repository of code and data can be
182 retrieved from <a href="https://github.com">GitHub</a> via the shell
183 command:</p>
184 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
185 </code></pre>
186 <p>
187 Since version 2012e, each release has been tagged in development repositories.
188 Untagged commits are less well tested and probably contain
189 more errors.</p>
190 <p>
191 After obtaining the code and data files, see the
192 <code>README</code> file for what to do next.
193 The code lets you compile the <code><abbr>tz</abbr></code> source files into
194 machine-readable binary files, one for each location. The binary files
195 are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
196 specified by <a href="https://datatracker.ietf.org/doc/html/8536">Internet
197 <abbr>RFC</abbr> 8536</a>.
198 The code also lets
199 you read a <abbr>TZif</abbr> file and interpret timestamps for that
200 location.</p>
201 </section>
202
203 <section>
204 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
205 <p>
206 The <code><abbr>tz</abbr></code> code and data
207 are by no means authoritative. If you find errors, please
208 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
209 the time zone mailing list. You can also <a
210 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
211 and browse the <a
212 href="https://mm.icann.org/pipermail/tz/">archive of old
213 messages</a>.
214 <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
215 discussions</a> and corresponding data changes can be
216 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
217 </p>
218 <p>
219 Changes to the <code><abbr>tz</abbr></code> code and data are often
220 propagated to clients via operating system updates, so
221 client <code><abbr>tz</abbr></code> data can often be corrected by
222 applying these updates. With GNU/Linux and similar systems, if your
223 maintenance provider has not yet adopted the
224 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
225 the process by tailoring the generic instructions in
226 the <code><abbr>tz</abbr> README</code> file and installing the latest
227 data yourself. System-specific instructions for installing the
228 latest <code><abbr>tz</abbr></code> data have also been published
229 for <a href="https://developer.ibm.com/articles/au-aix-olson-time-zone/"><abbr>AIX</abbr></a>,
230 <a
231 href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
232 <a
233 href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
234 title="International Components for Unicode">ICU</abbr></a>,
235 <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
236 JDK</a>,
237 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
238 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
239 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
240 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
241 </p>
242 <p>Since version 2013a,
243 sources for the <code><abbr>tz</abbr></code> database have been
244 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
245 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
246 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
247 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
248 title="linefeed">LF</abbr></a>,
249 which can be modified by common text editors such
250 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
251 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
252 <a href="https://www.vim.org">vim</a>.
253 Specialized source-file editing can be done via the
254 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
255 zoneinfo</a> package for <a
256 href="https://www.sublimetext.com">Sublime Text</a> and the <a
257 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
258 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
259 Studio Code</a>.
260 </p>
261 <p>
262 For further information about updates, please see
263 <a href="https://datatracker.ietf.org/doc/html/rfc6557">Procedures for
264 Maintaining the Time Zone Database</a> (Internet <abbr
265 title="Request For Comments">RFC</abbr> 6557). More detail can be
266 found in <a href="theory.html">Theory and pragmatics of the
267 <code><abbr>tz</abbr></code> code and data</a>.
268 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
269 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
270 </p>
271 </section>
272
273 <section>
274 <h2 id="coordinating">Coordinating with governments and distributors</h2>
275 <p>
276 As discussed in
277 "<a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How
278 Time Zones Are Coordinated</a>", the time zone database relies on
279 collaboration among governments, the time zone database volunteer
280 community, and data distributors downstream.
281 <p>
282 If your government plans to change its time zone boundaries or
283 daylight saving rules, please send email to <a
284 href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a> well in advance,
285 as this will lessen confusion and will coordinate updates to many cell phones,
286 computers, and other devices around the world.
287 In your email, please cite the legislation or regulation that specifies
288 the change, so that it can be checked for details such as the exact times
289 when clock transitions occur.
290 It is OK if a rule change is planned to affect clocks
291 far into the future, as a long-planned change can easily be reverted
292 or otherwise altered with a year's notice before the change would have
293 affected clocks.</p>
294 <p>
295 There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.
296 However, typically a release occurs every few months.
297 Many downstream timezone data distributors wait for
298 a <code><abbr>tzdb</abbr></code> release before they produce an update
299 to time zone behavior in consumer devices and software products.
300 After a release, various parties must integrate, test,
301 and roll out an update before <a
302 href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.
303 These updates can be expensive, for both the <a
304 href="https://en.wikipedia.org/wiki/Quality_assurance">quality
305 assurance</a> process and the overall cost of shipping and installing
306 updates to each device's copy of <code><abbr>tzdb</abbr></code>.
307 Updates may be batched with other updates and may take substantial
308 time to reach end users after a release.
309 Older devices may no longer be supported and thus may never be updated,
310 which means they will continue to use out-of-date rules.</p>
311 <p>
312 For these reasons any rule change should be promulgated at least a
313 year before it affects how clocks operate; otherwise, there is a good
314 chance that many clocks will be wrong due to delays in propagating updates,
315 and that residents will be confused or even actively resist the change.
316 The shorter the notice, the more likely clock problems will arise; see "<a
317 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
318 the Timing of Time Zone Changes</a>" for examples.
319 </p>
320 </section>
321
322 <section>
323 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
324 <ul>
325 <li>The article
326 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
327 an encyclopedic summary.</li>
328 <li><a href="tz-how-to.html">How to Read the
329 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
330 database format.</li>
331 <li><a
332 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
333 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
334 database's style.</li>
335 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
336 managing time in the internet</a> analyzes the database longitudinally.</li>
337 </ul>
338 </section>
339
340 <section>
341 <h2 id="web">Web sites using recent versions of the
342 <code><abbr>tz</abbr></code> database</h2>
343 <p>
344 These are listed roughly in ascending order of complexity and fanciness.
345 </p>
346 <ul>
347 <li><a href="https://time.is">Time.is</a> shows locations'
348 time and zones.</li>
349 <li><a href="https://www.timejones.com">TimeJones.com</a>,
350 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
351 <a href="https://www.worldclock.com">The World Clock</a>
352 are time zone converters.</li>
353 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
354 publishes <code><abbr>tzdb</abbr></code>-derived data in
355 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
356 title="comma-separated values">CSV</abbr></a> and
357 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
358 title="Structured Query Language">SQL</abbr></a> form.</li>
359 <li><a
360 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
361 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
362 <li><a
363 href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
364 Time in 1000 Places</a> uses descriptions of the values.</li>
365 <li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
366 timezone information for all countries</a>
367 displays tables of <abbr>DST</abbr> rules.
368 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
369 Worldwide</a> lets you sort zone names and convert times.</li>
370 <li><a href="https://24timezones.com">24TimeZones</a> has a world
371 time map and a time converter.</li>
372 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
373 calculates the current time difference between locations.</li>
374 <li><a href="https://www.wx-now.com">Weather Now</a> and
375 <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
376 </ul>
377 </section>
378
379 <section>
380 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
381 <ul>
382 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
383 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
384 Distribution Service (tzdist) working group</a> defined <a
385 href="https://datatracker.ietf.org/doc/html/rfc7808">TZDIST</a>
386 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
387 along with <a href="https://datatracker.ietf.org/doc/html/rfc7809">CalDAV</a>
388 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
389 transferring time zone data by reference.
390 <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
391 implementations</a> are available.
392 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
393 mailing list</a> discusses possible extensions.</li>
394 <li>The <a href="https://datatracker.ietf.org/doc/html/rfc5545">
395 Internet Calendaring and Scheduling Core Object Specification
396 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
397 covers time zone
398 data; see its VTIMEZONE calendar component.
399 The iCalendar format requires specialized parsers and generators; a
400 variant <a href="https://datatracker.ietf.org/doc/html/rfc6321">xCal</a>
401 (Internet <abbr>RFC</abbr> 6321) uses
402 <a href="https://www.w3.org/XML/"><abbr
403 title="Extensible Markup Language">XML</abbr></a> format, and a variant
404 <a href="https://datatracker.ietf.org/doc/html/rfc7265">jCal</a>
405 (Internet <abbr>RFC</abbr> 7265)
406 uses <a href="https://www.json.org"><abbr
407 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
408 </ul>
409 </section>
410
411 <section>
412 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
413 <p>Although some of these do not fully support
414 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
415 distributions you can generally work around compatibility problems by
416 running the command <code>make rearguard_tarballs</code> and compiling
417 from the resulting tarballs instead.</p>
418 <ul>
419 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <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 href="https://sourceforge.net/projects/tzical/">tziCal – tz
429 database conversion utility</a> is like Vzic, except for the <a
430 href="https://dotnet.microsoft.com">.NET framework</a>
431 and with a <abbr>BSD</abbr>-style license.</li>
432 <li><a
433 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
434 contains a script <code>parse_olson</code> that compiles
435 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
436 modules. It is part of the Perl <a
437 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
438 which is freely
439 available under both the <abbr>GPL</abbr> and the Perl Artistic
440 License. DateTime::TimeZone also contains a script
441 <code>tests_from_zdump</code> that generates test cases for each clock
442 transition in the <code><abbr>tz</abbr></code> database.</li>
443 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
444 Database Parser</a> is a
445 <a href="https://en.wikipedia.org/wiki/C++">C++</a> parser and
446 runtime library with <a
447 href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
448 adopted by
449 <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
450 the current iteration of the C++ standard.
451 It is freely available under the
452 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
453 <li><a id="ICU" href="https://icu.unicode.org">International Components for
454 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
455 href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a>
456 libraries for internationalization that
457 has a compiler from <code><abbr>tz</abbr></code> source
458 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
459 (mentioned <a href="#CLDR">below</a>)
460 into an <abbr>ICU</abbr>-specific format.
461 <abbr>ICU</abbr> is freely available under a
462 <abbr>BSD</abbr>-style license.</li>
463 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
464 the <a href="https://elixir-lang.org">Elixir</a> language downloads
465 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
466 title="Application Program Interface">API</abbr>s for use. It is
467 freely available under the <abbr>MIT</abbr> license.</li>
468 <li>Java-based compilers and libraries include:
469 <ul>
470 <li>The <a
471 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
472 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
473 <a href="https://openjdk.java.net/">OpenJDK</a> and
474 <a href="https://jdk.java.net/">Oracle JDK</a>.
475 Although its source code is proprietary, its executable is available under the
476 <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
477 Timezone Updater License Agreement</a>.</li>
478 <li>The <a
479 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
480 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
481 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
482 which is freely available under a <abbr>BSD</abbr>-style license.</li>
483 <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
484 and time <abbr>API</abbr></a> contains a class
485 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
486 <code><abbr>tz</abbr></code> source into a binary format. It inspired
487 Java 8 <code>java.time</code>, which its users should migrate to once
488 they can assume Java 8 or later. It is available under the <a
489 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
490 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
491 href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
492 are alternatives to TZUpdater. IANA Updater's license is unclear;
493 ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
494 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
495 time library for Android</a> and
496 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
497 time and interval library for Java</a> compile
498 <code><abbr>tz</abbr></code> source into a binary format.
499 Time4A is available under the Apache License and Time4J is
500 available under the <a
501 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
502 General Public License (<abbr title="Lesser General Public
503 License">LGPL</abbr>)</a>.</li>
504 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
505 Java-based libraries.</li>
506 </ul>
507 <li><a href="https://nodatime.org">Noda Time – Date and
508 time <abbr>API</abbr> for .NET</a>
509 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
510 It is freely available under the Apache License.</li>
511 <li>Many modern
512 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
513 runtimes support <code><abbr>tz</abbr></code> natively via the
514 <samp>timeZone</samp> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><samp>Intl.DateTimeFormat</samp></a>.
515 This can be used as-is or with most of the following libraries,
516 many of which also support runtimes lacking the <samp>timeZone</samp> option.
517 <ul>
518 <li>The <a
519 href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
520 timezone polyfill</a>
521 is freely available under a <abbr>BSD</abbr>-style license.</li>
522 <li>The <a href="https://date-fns.org/">date-fns</a>
523 library manipulates timezone-aware timestamps in browsers and
524 in <a href="https://nodejs.org/en/">Node.js</a>.
525 It is freely available under the <abbr>MIT</abbr> license.</li>
526 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
527 minimalist replacement for the date and time API of
528 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
529 manipulation library.
530 It is freely available under the <abbr>MIT</abbr> license.</li>
531 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
532 timezone support for the <samp>Intl</samp> API.
533 It is freely available under the <abbr>MIT</abbr> license.</li>
534 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
535 Moment.js plugin.
536 It is freely available under the <abbr>MIT</abbr> license.</li>
537 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
538 JavaScript library that supports date arithmetic that is time zone
539 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
540 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
541 supports live <code><abbr>tzdb</abbr></code> updates,
542 astronomical and atomic time, a command-line interface,
543 and full <a
544 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
545 Its companion <a
546 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
547 can generate <abbr>TZif</abbr> and other files, and a companion website
548 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
549 convert timestamps, view transition histories, and download code and data.
550 It is freely available under the <abbr>MIT</abbr> license.</li>
551 </ul>
552 The proposed <a
553 href="https://github.com/tc39/proposal-temporal"><samp>Temporal</samp>
554 objects</a> let programs access an abstract view of
555 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
556 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
557 problematic <samp>Date</samp> objects</a> when working with dates and times.
558 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
559 compiler from <code><abbr>tz</abbr></code> source into
560 <a href="https://julialang.org/">Julia</a>. It is freely available
561 under the <abbr>MIT</abbr> license.</li>
562 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
563 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
564 title="Free Pascal Compiler">FPC</abbr></a>
565 compiles from <code><abbr>tz</abbr></code> source into
566 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
567 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
568 and <a
569 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
570 It is freely available under a <abbr>BSD</abbr>-style license.</li>
571 <li><a href="http://pytz.sourceforge.net">pytz – World Timezone
572 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
573 <a href="https://www.python.org">Python</a>.
574 It is freely available under a <abbr>BSD</abbr>-style license.
575 In code that can assume Python 3.6 or later it is largely superseded; see <a
576 href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
577 The Fastest Footgun in the West</a>.</li>
578 <li><a href="https://tzinfo.github.io">TZInfo –
579 Ruby Timezone Library</a>
580 compiles <code><abbr>tz</abbr></code> source into
581 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
582 It is freely available under the <abbr>MIT</abbr> license.</li>
583 <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
584 Library</a> is
585 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
586 library that compiles <code><abbr>tz</abbr></code> source into a time
587 zone repository whose format
588 is either proprietary or an <abbr>XML</abbr>-encoded
589 representation.</li>
590 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
591 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
592 source into text files, along with a runtime that can read those
593 files. Tcl is freely available under a <abbr>BSD</abbr>-style
594 license.</li>
595 </ul>
596 </section>
597
598 <section>
599 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
600 <ul>
601 <li>The <a
602 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
603 Library</a>
604 has an independent, thread-safe implementation of
605 a <abbr>TZif</abbr> file reader.
606 This library is freely available under the LGPL
607 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
608 <li><a href="https://www.gnome.org">GNOME</a>'s
609 <a href="https://developer.gnome.org/glib/">GLib</a> has
610 a <abbr>TZif</abbr> file reader written in C that
611 creates a <code>GTimeZone</code> object representing sets
612 of <abbr>UT</abbr> offsets.
613 It is freely available under the <abbr>LGPL</abbr>.</li>
614 <li>The
615 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
616 <code>baltzo::TimeZoneUtil</code> component contains a C++
617 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
618 the Apache License.</li>
619 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
620 library that translates between <abbr>UT</abbr> and civil time and
621 can read <abbr>TZif</abbr> files. It is freely available under the Apache
622 License.</li>
623 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
624 library that reads <abbr>TZif</abbr> files and converts
625 timestamps from one time zone or format to another.
626 It is used by <a href="https://secure.php.net"><abbr
627 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
628 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
629 and <a href="https://www.mongodb.com">MongoDB</a>.
630 It is freely available under the <abbr>MIT</abbr> license.</li>
631 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
632 <abbr>TZif</abbr> file reader.</li>
633 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
634 DateTime::TimeZone::Tzfile</a>
635 is a <abbr>TZif</abbr> file reader written in Perl.
636 It is freely available under the same terms as Perl
637 (dual <abbr>GPL</abbr> and Artistic license).</li>
638 <li>Python has a <a id="python-zoneinfo"
639 href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
640 class</a> that reads <abbr>TZif</abbr> data and creates objects
641 that represent <code><abbr>tzdb</abbr></code> timezones.
642 Python is freely available under the
643 <a href="https://docs.python.org/3/license.html">Python Software Foundation
644 License</a>.
645 A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
646 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
647 supplies TZif data if the underlying system data cannot be found;
648 it is freely available under the Apache License.</li>
649 <li>The
650 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
651 library contains a Python tool that
652 converts <abbr>TZif</abbr> data into
653 <abbr>JSON</abbr>-format data suitable for use
654 in its JavaScript library for time zone conversion. Dates before 1970
655 are not supported.</li>
656 <li>The <a
657 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
658 package contains <a href="https://www.haskell.org">Haskell</a> code that
659 parses and uses <abbr>TZif</abbr> data. It is freely
660 available under a <abbr>BSD</abbr>-style license.</li>
661 </ul>
662 </section>
663
664 <section>
665 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
666 <ul>
667 <li><a href="https://foxclocks.org">FoxClocks</a>
668 is an extension for <a href="https://www.google.com/chrome/">Google
669 Chrome</a> and for <a
670 href="https://wiki.mozilla.org/Modules/Toolkit">Mozilla
671 Toolkit</a> applications like <a
672 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
673 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
674 It displays multiple clocks in the application window, and has a mapping
675 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
676 It is freely available under the <abbr>GPL</abbr>.</li>
677 <li><a href="https://golang.org">Go programming language</a>
678 implementations contain a copy of a 32-bit subset of a recent
679 <code><abbr>tz</abbr></code> database in a
680 Go-specific format.</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 Astrodienst's Web version of Shanks and Pottenger's 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">Perry–Castañeda
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
760 <abbr>CIA</abbr>'s
761 but the maps are more up to date.</li>
762 <li><a
763 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
764 much is time wrong around the world?</a> maps the difference between
765 mean solar and standard time, highlighting areas such as western China
766 where the two differ greatly. It's a bit out of date, unfortunately.</li>
767 </ul>
768 </section>
769
770 <section>
771 <h2 id="boundaries">Time zone boundaries</h2>
772 <p>Geographical boundaries between timezones are available
773 from several <a
774 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
775 geolocation</a>
776 services and other sources.</p>
777 <ul>
778 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
779 Boundary Builder</a> extracts
780 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
781 boundaries of <code><abbr>tzdb</abbr></code> timezones.
782 Its code is freely available under the <abbr>MIT</abbr> license, and
783 its data entries are freely available under the
784 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
785 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
786 <li>Programmatic interfaces that map geographical coordinates via tz_world to
787 <code><abbr>tzdb</abbr></code> timezones include:
788 <ul>
789 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
790 written in <a
791 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
792 and is freely available under the <abbr>MIT</abbr> license.</li>
793 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
794 is written in Go and is freely available under the Apache License.</li>
795 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
796 in both Java and
797 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
798 form, is freely available under the MIT license.</li>
799 <li>For Node.js,
800 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
801 is freely available under the MIT license, and
802 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
803 is in the public domain.</li>
804 <li>The <a
805 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
806 library for Python is freely available under the MIT license.
807 <li>The <a
808 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
809 library for Ruby is freely available under the MIT license.</li>
810 </ul></li>
811 <li>Free access via a network API, if you register a key, is provided by
812 the <a
813 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
814 Timezone web service</a>, the <a
815 href="https://developers.google.com/maps/documentation/timezone/intro">Google
816 Maps Time Zone API</a>, and
817 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
818 Commercial network API access is provided
819 by <a href="https://askgeo.com">AskGeo</a>
820 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
821 </li>
822 <li>"<a
823 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
824 to get a time zone from a location using latitude and longitude
825 coordinates?</a>" discusses other geolocation possibilities.</li>
826 <li><a href="http://statoids.com/statoids.html">Administrative
827 Divisions of Countries ("Statoids")</a> lists
828 political subdivision data related to time zones.</li>
829 <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
830 zone boundaries for multizone countries</a> summarizes legal
831 boundaries between time zones within countries.</li>
832 <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
833 – GIS and Database Tools</a> includes a Manifold-format map of
834 world time zone boundaries distributed under the
835 <abbr>GPL</abbr>.</li>
836 <li>A ship within the <a
837 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
838 waters</a> of any nation uses that nation's time. In international
839 waters, time zone boundaries are meridians 15° apart, except that
840 <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
841 wide and are separated by
842 the 180° meridian (not by the International Date Line, which is
843 for land and territorial waters only). A captain can change ship's
844 clocks any time after entering a new time zone; midnight changes are
845 common.</li>
846 </ul>
847 </section>
848
849 <section>
850 <h2 id="civil">Civil time concepts and history</h2>
851 <ul>
852 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
853 Walk through Time</a>
854 surveys the evolution of timekeeping.</li>
855 <li>The history of daylight saving time is surveyed in <a
856 href="http://www.webexhibits.org/daylightsaving/">About Daylight
857 Saving Time – History, rationale, laws & dates</a> and summarized in
858 <a href="http://seizethedaylight.com/dst/">A Brief
859 History of Daylight Saving Time</a>.</li>
860 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
861 Lords</a> discusses how authoritarians manipulate civil time.</li>
862 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
863 contains guidelines and best practices for software applications that
864 deal with civil time.</li>
865 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
866 the International Date Line</a> tells the story of the most important
867 time zone boundary.</li>
868 <li><a href="http://statoids.com/tconcept.html">Basic Time
869 Zone Concepts</a> discusses terminological issues behind time zones.</li>
870 </ul>
871 </section>
872
873 <section>
874 <h2 id="national">National histories of legal time</h2>
875 <dl>
876 <dt>Australia</dt>
877 <dd>The Parliamentary Library commissioned a <a
878 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
879 paper on daylight saving time in Australia</a>.
880 The Bureau of Meteorology publishes a list of <a
881 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
882 Dates of Daylight Savings Time within Australia</a>.</dd>
883 <dt>Belgium</dt>
884 <dd>The Royal Observatory of Belgium maintains a table of time in
885 Belgium (in
886 <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
887 hreflang="nl">Dutch</a> and <a
888 href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
889 hreflang="fr">French</a>).</dd>
890 <dt>Brazil</dt>
891 <dd>The Time Service Department of the National Observatory
892 records <a href="http://pcdsh01.on.br/DecHV.html"
893 hreflang="pt-BR">Brazil's daylight saving time decrees (in
894 Portuguese)</a>.</dd>
895 <dt>Canada</dt>
896 <dd>National Research Council Canada publishes current
897 and some older information about <a
898 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
899 zones and daylight saving time</a>.</dd>
900 <dt>Chile</dt>
901 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
902 <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
903 Chile's official time (in Spanish)</a>.</dd>
904 <dt>China</dt>
905 <dd>The Hong Kong Observatory maintains a
906 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
907 summer time in Hong Kong</a>,
908 and Macau's Meteorological and Geophysical Bureau maintains a <a
909 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
910 history for Macau</a>.
911 Unfortunately the latter is incomplete and has errors.</dd>
912 <dt>Czech Republic</dt>
913 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
914 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
915 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
916 <dt>Germany</dt>
917 <dd>The National Institute for Science and Technology maintains the <a
918 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
919 of Legal Time in Germany</a>.</dd>
920 <dt>Israel</dt>
921 <dd>The Interior Ministry periodically issues <a
922 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
923 hreflang="he">announcements (in Hebrew)</a>.</dd>
924 <dt>Malaysia</dt>
925 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
926 <dt>Mexico</dt>
927 <dd>The Investigation and Analysis Service of the Mexican Library of
928 Congress has published a <a
929 href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
930 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
931 <dt>Netherlands</dt>
932 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
933 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
934 covers the history of local time in the Netherlands from ancient times.</dd>
935 <dt>New Zealand</dt>
936 <dd>The Department of Internal Affairs maintains a brief <a
937 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
938 Daylight Saving</a>.</dd>
939 <dt>Palestine</dt>
940 <dd>The Ministry of Telecom and IT publishes a <a
941 href="https://mtit.pna.ps/Site/TimeZoon"
942 hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
943 <dt>Portugal</dt>
944 <dd>The Lisbon Astronomical Observatory publishes a
945 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
946 legal time (in Portuguese)</a>.</dd>
947 <dt>Singapore</dt>
948 <dd><a id="Singapore"
949 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
950 is Singapore in the "Wrong" Time Zone?</a> details the
951 history of legal time in Singapore and Malaysia.</dd>
952 <dt>United Kingdom</dt>
953 <dd><a
954 href="https://www.polyomino.org.uk/british-time/">History of
955 legal time in Britain</a> discusses in detail the country
956 with perhaps the best-documented history of clock adjustments.</dd>
957 <dt>United States</dt>
958 <dd>The Department of Transportation's <a
959 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
960 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
961 <dt>Uruguay</dt>
962 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
963 Navy (SOHMA) publishes an annual <a
964 href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
965 (in Spanish)</a>.</dd>
966 </dl>
967 </section>
968
969 <section>
970 <h2 id="costs">Costs and benefits of time shifts</h2>
971 <p>Various sources argue for and against daylight saving time and time
972 zone shifts, and many scientific studies have been conducted. This
973 section summarizes reviews and position statements based on
974 scientific literature in the area.</p>
975 <ul>
976 <li>In 2022 the American Medical Association issued a
977 <a href="https://www.ama-assn.org/press-center/press-releases/ama-calls-permanent-standard-time">statement
978 supporting permanent standard time</a> on health grounds.</li>
979 <li>Carey RN, Sarma KM.
980 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
981 daylight saving time on road traffic collision risk: a systematic
982 review</a>.
983 <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>.
984 This reviews research literature and concludes that the evidence
985 neither supports nor refutes road safety benefits from
986 shifts in time zones.</li>
987 <li>Havranek T, Herman D, Irsova D.
988 <a href="https://www.iaee.org/en/publications/ejarticle.aspx?id=3051">Does
989 daylight saving save electricity? A meta-analysis.</a>
990 <em>Energy J.</em> 2018;39(2):35–61.
991 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
992 This analyzes research literature and concludes, "Electricity savings
993 are larger for countries farther away from the equator, while
994 subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
995 <li>Malow BA. It is time to abolish the clock change and adopt permanent
996 standard time in the United States: a Sleep Research Society position statement.
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.
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> 2023-10-31.
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://www.ncbi.nlm.nih.gov/pmc/articles/PMC7205184/">Why
1012 should we abolish Daylight Saving Time?</a>
1013 <em>J Biol Rhythms.</em> 2019;34(3):227–230.
1014 doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
1015 The Society for Research on Biological Rhythms
1016 opposes DST changes and permanent DST, 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/intelligent-systems-division/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://datatracker.ietf.org/doc/html/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 client's 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/tandc.html">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. The <code><abbr>tz</abbr></code> code and data support leap seconds
1087 via an optional "<code>right</code>" configuration where a computer's internal
1088 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
1089 as opposed to the default "<code>posix</code>" configuration
1090 where the internal clock ignores leap seconds.
1091 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
1092 <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for
1093 timestamps starting with <code>time_t</code> 78 796 800,
1094 which corresponds to the first leap second
1095 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
1096 and to
1097 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
1098 In practice the two configurations also agree for timestamps before
1099 1972 even though the historical situation is messy, partly because
1100 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
1101 is well-defined for sufficiently old timestamps.</li>
1102 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1103 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1104 leap second so that they disagree with <abbr>UTC</abbr> by at most a
1105 half second, even though every <abbr>POSIX</abbr> minute has exactly
1106 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1107 "<code>posix</code>" configuration, is <a
1108 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
1109 the <abbr>NTP</abbr> reference implementation, <a
1110 href="https://github.com/google/unsmear">supports</a> conversion between
1111 <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
1112 cloud service providers. However, according to
1113 <a href="https://datatracker.ietf.org/doc/html/rfc8633#section-3.7.1">§3.7.1 of
1114 Network Time Protocol Best Current Practices</a>
1115 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1116 applications requiring accurate <abbr>UTC</abbr> or civil time,
1117 and is intended for use only in single, well-controlled environments.</li>
1118 <li>The <a
1119 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
1120 Second Discussion List</a> covers <a
1121 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
1122 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
1123 discussed further in
1124 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1125 leap second: its history and possible future</a>.
1126 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1127 might be redefined
1128 without Leap Seconds</a> gives pointers on this
1129 contentious issue.
1130 The General Conference on Weights and Measures
1131 <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">voted in 2022</a>
1132 to discontinue the use of leap seconds by 2035, replacing them with an
1133 as-yet-undetermined scheme some time after the year 2135.
1134 </li>
1135 </ul>
1136 </section>
1137
1138 <section>
1139 <h2 id="notation">Time notation</h2>
1140 <ul>
1141 <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
1142 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1143 zone names, abbreviations, identifiers, and formats. For example, it
1144 contains French translations for "Eastern European Summer Time",
1145 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
1146 "Bucharest". Its
1147 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1148 charts</a> show these values for many locales. Data values are available in
1149 both <abbr title="Locale Data Markup Language">LDML</abbr>
1150 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1151 <li>
1152 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1153 the international standard date and time notation</a> covers
1154 <a
1155 href="https://www.iso.org/standard/70907.html"><em><abbr
1156 title="International Organization for Standardization">ISO</abbr>
1157 8601-1:2019 – Date and time – Representations for information
1158 interchange – Part 1: Basic rules</em></a>.</li>
1159 <li>
1160 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1161 Schema: Datatypes – dateTime</a> specifies a format inspired by
1162 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1163 <li><a href="https://datatracker.ietf.org/doc/html/rfc5322#section-3.3">§3.3 of
1164 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1165 specifies the time notation used in email and <a
1166 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1167 headers.</li>
1168 <li>
1169 <a href="https://datatracker.ietf.org/doc/html/rfc3339">Date and Time
1170 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1171 specifies an <abbr>ISO</abbr> 8601
1172 profile for use in new Internet
1173 protocols.</li>
1174 <li>
1175 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1176 Formats on the Web</a> surveys web- and Internet-oriented date and time
1177 formats.</li>
1178 <li>Alphabetic time zone abbreviations should not be used as unique
1179 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1180 practice. For example, in English-speaking North America
1181 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
1182 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1183 and French-speaking North Americans prefer
1184 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
1185 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
1186 database contains English abbreviations for many timestamps;
1187 unfortunately some of these abbreviations were merely the database maintainers'
1188 inventions, and these have been removed when possible.</li>
1189 <li>Numeric time zone abbreviations typically count hours east of
1190 <abbr>UT</abbr>, e.g., +09 for Japan and
1191 −10 for Hawaii. However, the <abbr>POSIX</abbr>
1192 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
1193 For example, one might use <code><abbr>TZ</abbr>="<abbr
1194 title="Japan Standard Time">JST</abbr>-9"</code> and
1195 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1196 for Japan and Hawaii, respectively. If the
1197 <code><abbr>tz</abbr></code> database is available, it is usually better to use
1198 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1199 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1200 confusion, handle old timestamps better, and insulate you better from
1201 any future changes to the rules. One should never set
1202 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1203 <code>"GMT-9"</code>, though, since this would incorrectly imply that
1204 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1205 is called "<abbr>GMT</abbr>".</li>
1206 </ul>
1207 </section>
1208
1209 <section>
1210 <h2 id="see-also">See also</h2>
1211 <ul>
1212 <li><a href="theory.html">Theory and pragmatics of the
1213 <code><abbr>tz</abbr></code> code and data</a></li>
1214 <li><a href="tz-art.html">Time and the Arts</a></li>
1215 </ul>
1216 </section>
1217
1218 <footer>
1219 <hr>
1220 This web page is in the public domain, so clarified as of
1221 2009-05-17 by Arthur David Olson.
1222 <br>
1223 Please send corrections to this web page to the
1224 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1225 </footer>
1226 </body>
1227 </html>
1228