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4 <title>Sources for time zone and daylight saving time data</title>
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10 <body>
11 <h1>Sources for time zone and daylight saving time data</h1>
12 <p>
13 <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
14 <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
15 rules are controlled by individual
16 governments. They are sometimes changed with little notice, and their
17 histories and planned futures are often recorded only fitfully. Here
18 is a summary of attempts to organize and record relevant data in this
19 area.
20 </p>
21 <h3>Outline</h3>
22 <nav>
23 <ul>
24 <li>The <code><abbr>tz</abbr></code> database product and process
25 <ul>
26 <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
27 <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
28 <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
29 <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
30 </ul>
31 </li>
32 <li>Uses of the <code><abbr>tz</abbr></code> database
33 <ul>
34 <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
35 <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
36 <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
37 <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
38 <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
39 </ul>
40 </li>
41 <li>Related data
42 <ul>
43 <li><a href="#other-dbs">Other time zone databases</a></li>
44 <li><a href="#maps">Maps</a></li>
45 <li><a href="#boundaries">Time zone boundaries</a></li>
46 </ul>
47 </li>
48 <li>Timekeeping concepts
49 <ul>
50 <li><a href="#civil">Civil time concepts and history</a></li>
51 <li><a href="#national">National histories of legal time</a></li>
52 <li><a href="#costs">Costs and benefits of time shifts</a></li>
53 <li><a href="#precision">Precision timekeeping</a></li>
54 <li><a href="#notation">Time notation</a></li>
55 <li><a href="#see-also">See also</a></li>
56 </ul>
57 </li>
58 </ul>
59 </nav>
60
61 <section>
62 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
63 <p>
64 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
65 time zone database contains code and data
66 that represent the history of local time
67 for many representative locations around the globe.
68 It is updated periodically to reflect changes made by political bodies
69 to time zone boundaries and daylight saving rules.
70 This database (known as <code><abbr>tz</abbr></code>,
71 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
72 is used by several implementations,
73 including
74 <a href="https://www.gnu.org/software/libc/">the
75 <abbr title="GNU's Not Unix">GNU</abbr>
76 C Library</a> (used in
77 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
78 <a href="https://www.android.com">Android</a>,
79 <a href="https://www.freebsd.org">Free<abbr
80 title="Berkeley Software Distribution">BSD</abbr></a>,
81 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
82 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
83 <a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
84 <a href="https://cygwin.com">Cygwin</a>,
85 <a href="https://mariadb.org">MariaDB</a>,
86 <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
87 <a href="https://www.mysql.com">MySQL</a>,
88 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
89 title="Web Operating System">webOS</abbr></a>,
90 <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
91 title="Advanced Interactive eXecutive">AIX</abbr></a>,
92 <a href="https://www.apple.com/ios"><abbr
93 title="iPhone OS">iOS</abbr></a>,
94 <a href="https://www.apple.com/macos">macOS</a>,
95 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
96 <a href="https://www.vmssoftware.com">Open<abbr
97 title="Virtual Memory System">VMS</abbr></a>,
98 <a href="https://www.oracle.com/database/">Oracle Database</a>, and
99 <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
100 <p>
101 Each main entry in the database represents a <dfn>timezone</dfn>
102 for a set of civil-time clocks that have all agreed since 1970.
103 Timezones are typically identified by continent or ocean and then by the
104 name of the largest city within the region containing the clocks.
105 For example, <code>America/New_York</code>
106 represents most of the <abbr title="United States">US</abbr> eastern time zone;
107 <code>America/Phoenix</code> represents most of Arizona, which
108 uses mountain time without daylight saving time (<abbr>DST</abbr>);
109 <code>America/Detroit</code> represents most of Michigan, which uses
110 eastern time but with different <abbr>DST</abbr> rules in 1975;
111 and other entries represent smaller regions like Starke County,
112 Indiana, which switched from central to eastern time in 1991
113 and switched back in 2006.
114 To use the database on an extended <a
115 href="https://en.wikipedia.org/wiki/POSIX"><abbr
116 title="Portable Operating System Interface">POSIX</abbr></a>
117 implementation set the <code><abbr>TZ</abbr></code>
118 environment variable to the location's full name,
119 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
120 <p>
121 Associated with each timezone is a history of offsets from
122 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
123 Time</a> (<abbr>UT</abbr>), which is <a
124 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
125 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
126 for timestamps after 1960 this is more precisely <a
127 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
128 Universal Time</a> (<abbr>UTC</abbr>).
129 The database also records when daylight saving time was in use,
130 along with some time zone abbreviations such as <abbr>EST</abbr>
131 for Eastern Standard Time in the <abbr>US</abbr>.</p>
132 </section>
133
134 <section>
135 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
136 <p>
137 The following <a
138 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
139 the latest release's two
140 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
141 to a <abbr>GNU</abbr>/Linux or similar host.</p>
142 <pre><code>mkdir tzdb
143 cd tzdb
144 <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
145 wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
146 <a href="https://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="https://www.gnu.org/software/tar/">tar</a> -xf -
147 gzip -dc tzdata-latest.tar.gz | tar -xf -
148 </code></pre>
149 <p>Alternatively, the following shell commands download the same
150 release in a single-tarball format containing extra data
151 useful for regression testing:</p>
152 <pre><code>wget <a href="https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz">https://www.iana.org/time-zones/repository/tzdb-latest.tar.lz</a>
153 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
154 </code></pre>
155 <p>These commands use convenience links to the latest release
156 of the <code><abbr>tz</abbr></code> database hosted by the
157 <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
158 of the <a href="https://www.iana.org">Internet Assigned Numbers
159 Authority (IANA)</a>.
160 Older releases are in files named
161 <code>tzcode<var>V</var>.tar.gz</code>,
162 <code>tzdata<var>V</var>.tar.gz</code>, and
163 <code>tzdb-<var>V</var>.tar.lz</code>,
164 where <code><var>V</var></code> is the version.
165 Since 1996, each version has been a four-digit year followed by
166 lower-case letter (<samp>a</samp> through <samp>z</samp>,
167 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
168 through <samp>zzz</samp>, and so on).
169 Since version 1999g, each release has been distributed in
170 <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
171 ustar interchange format</a>, compressed as described above;
172 older releases use a nearly-compatible format.
173 Since version 2016h, each release has contained a text file named
174 "<samp>version</samp>" whose first (and currently only) line is the version.
175 Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
176 and are also available in an
177 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
178 title="File Transfer Protocol">FTP</abbr> directory</a> via a
179 less-secure protocol.</p>
180 <p>Alternatively, a development repository of code and data can be
181 retrieved from <a href="https://github.com">GitHub</a> via the shell
182 command:</p>
183 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
184 </code></pre>
185 <p>
186 Since version 2012e, each release has been tagged in development repositories.
187 Untagged commits are less well tested and probably contain
188 more errors.</p>
189 <p>
190 After obtaining the code and data files, see the
191 <code>README</code> file for what to do next.
192 The code lets you compile the <code><abbr>tz</abbr></code> source files into
193 machine-readable binary files, one for each location. The binary files
194 are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
195 specified by <a href="https://tools.ietf.org/html/8536">Internet
196 <abbr>RFC</abbr> 8536</a>.
197 The code also lets
198 you read a <abbr>TZif</abbr> file and interpret timestamps for that
199 location.</p>
200 </section>
201
202 <section>
203 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
204 <p>
205 The <code><abbr>tz</abbr></code> code and data
206 are by no means authoritative. If you find errors, please
207 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
208 the time zone mailing list. You can also <a
209 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
210 and browse the <a
211 href="https://mm.icann.org/pipermail/tz/">archive of old
212 messages</a>.
213 <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
214 discussions</a> and corresponding data changes can be
215 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
216 </p>
217 <p>
218 If your government plans to change its time zone boundaries or
219 daylight saving rules, inform <code>tz (a] iana.org</code> well in
220 advance, as this will coordinate updates to many cell phones,
221 computers, and other devices around the world.
222 The change should be officially announced at least a year before it affects
223 how clocks operate; otherwise, there is a good chance that some
224 clocks will operate incorrectly after the change, due
225 to delays in propagating updates to software and data. The shorter
226 the notice, the more likely clock problems will arise; see "<a
227 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
228 the Timing of Time Zone Changes</a>" for examples.
229 The <code><abbr>tz</abbr></code> data can represent planned changes
230 far into the future, and a long-planned change can easily be reverted
231 or otherwise altered with a year's notice before the change would have
232 affected clocks.
233 </p>
234 <p>
235 Changes to the <code><abbr>tz</abbr></code> code and data are often
236 propagated to clients via operating system updates, so
237 client <code><abbr>tz</abbr></code> data can often be corrected by
238 applying these updates. With GNU/Linux and similar systems, if your
239 maintenance provider has not yet adopted the
240 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
241 the process by tailoring the generic instructions in
242 the <code><abbr>tz</abbr> README</code> file and installing the latest
243 data yourself. System-specific instructions for installing the
244 latest <code><abbr>tz</abbr></code> data have also been published
245 for <a href="https://developer.ibm.com/articles/au-aix-olson-time-zone/"><abbr>AIX</abbr></a>,
246 <a
247 href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
248 <a
249 href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
250 title="International Components for Unicode">ICU</abbr></a>,
251 <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
252 JDK</a>,
253 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
254 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
255 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
256 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
257 </p>
258 <p>Sources for the <code><abbr>tz</abbr></code> database are
259 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
260 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
261 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
262 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
263 title="linefeed">LF</abbr></a>,
264 which can be modified by common text editors such
265 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
266 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
267 <a href="https://www.vim.org">vim</a>.
268 Specialized source-file editing can be done via the
269 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
270 zoneinfo</a> package for <a
271 href="https://www.sublimetext.com">Sublime Text</a> and the <a
272 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
273 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
274 Studio Code</a>.
275 </p>
276 <p>
277 For further information about updates, please see
278 <a href="https://tools.ietf.org/html/rfc6557">Procedures for
279 Maintaining the Time Zone Database</a> (Internet <abbr
280 title="Request For Comments">RFC</abbr> 6557). More detail can be
281 found in <a href="theory.html">Theory and pragmatics of the
282 <code><abbr>tz</abbr></code> code and data</a>.
283 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
284 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
285 </p>
286 </section>
287
288 <section>
289 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
290 <ul>
291 <li>The article
292 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
293 an encyclopedic summary.</li>
294 <li><a href="tz-how-to.html">How to Read the
295 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
296 database format.</li>
297 <li><a
298 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
299 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
300 database's style.</li>
301 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
302 managing time in the internet</a> analyzes the database longitudinally.</li>
303 </ul>
304 </section>
305
306 <section>
307 <h2 id="web">Web sites using recent versions of the
308 <code><abbr>tz</abbr></code> database</h2>
309 <p>
310 These are listed roughly in ascending order of complexity and fanciness.
311 </p>
312 <ul>
313 <li><a href="https://time.is">Time.is</a> shows locations'
314 time and zones.</li>
315 <li><a href="https://www.timejones.com">TimeJones.com</a>,
316 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
317 <a href="https://www.worldclock.com">The World Clock</a>
318 are time zone converters.</li>
319 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
320 publishes <code><abbr>tzdb</abbr></code>-derived data in
321 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
322 title="comma-separated values">CSV</abbr></a> and
323 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
324 title="Structured Query Language">SQL</abbr></a> form.</li>
325 <li><a
326 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
327 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
328 <li><a
329 href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
330 Time in 1000 Places</a> uses descriptions of the values.</li>
331 <li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
332 uses a pulldown menu.</li>
333 <li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
334 timezone information for all countries</a>
335 displays tables of <abbr>DST</abbr> rules.
336 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
337 Worldwide</a> lets you sort zone names and convert times.</li>
338 <li><a href="https://24timezones.com">24TimeZones</a> has a world
339 time map and a time converter.</li>
340 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
341 calculates the current time difference between locations.</li>
342 <li><a href="http://www.wx-now.com">Weather Now</a> and
343 <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
344 </ul>
345 </section>
346
347 <section>
348 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
349 <ul>
350 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
351 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
352 Distribution Service (tzdist) working group</a> defined <a
353 href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
354 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
355 along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
356 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
357 transferring time zone data by reference.
358 <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
359 implementations</a> are available.
360 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
361 mailing list</a> discusses possible extensions.</li>
362 <li>The <a href="https://tools.ietf.org/html/rfc5545">
363 Internet Calendaring and Scheduling Core Object Specification
364 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
365 covers time zone
366 data; see its VTIMEZONE calendar component.
367 The iCalendar format requires specialized parsers and generators; a
368 variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
369 (Internet <abbr>RFC</abbr> 6321) uses
370 <a href="https://www.w3.org/XML/"><abbr
371 title="Extensible Markup Language">XML</abbr></a> format, and a variant
372 <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
373 (Internet <abbr>RFC</abbr> 7265)
374 uses <a href="https://www.json.org"><abbr
375 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
376 </ul>
377 </section>
378
379 <section>
380 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
381 <p>Although some of these do not fully support
382 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
383 distributions you can generally work around compatibility problems by
384 running the command <code>make rearguard_tarballs</code> and compiling
385 from the resulting tarballs instead.</p>
386 <ul>
387 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
388 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
389 program that compiles
390 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
391 Vzic is freely
392 available under the <a
393 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
394 General Public License (<abbr
395 title="General Public License">GPL</abbr>)</a>.</li>
396 <li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz
397 database conversion utility</a> is like Vzic, except for the <a
398 href="https://dotnet.microsoft.com">.NET framework</a>
399 and with a <abbr>BSD</abbr>-style license.</li>
400 <li><a
401 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
402 contains a script <code>parse_olson</code> that compiles
403 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
404 modules. It is part of the Perl <a
405 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
406 which is freely
407 available under both the <abbr>GPL</abbr> and the Perl Artistic
408 License. DateTime::TimeZone also contains a script
409 <code>tests_from_zdump</code> that generates test cases for each clock
410 transition in the <code><abbr>tz</abbr></code> database.</li>
411 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
412 Database Parser</a> is a
413 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
414 runtime library with <a
415 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
416 adopted by
417 <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
418 the current iteration of the C++ standard.
419 It is freely available under the
420 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
421 <li><a id="ICU" href="http://site.icu-project.org">International Components for
422 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
423 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
424 libraries for internationalization that
425 has a compiler from <code><abbr>tz</abbr></code> source
426 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
427 (mentioned <a href="#CLDR">below</a>)
428 into an <abbr>ICU</abbr>-specific format.
429 <abbr>ICU</abbr> is freely available under a
430 <abbr>BSD</abbr>-style license.</li>
431 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
432 the <a href="https://elixir-lang.org">Elixir</a> language downloads
433 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
434 title="Application Program Interface">API</abbr>s for use. It is
435 freely available under the <abbr>MIT</abbr> license.</li>
436 <li>Java-based compilers and libraries include:
437 <ul>
438 <li>The <a
439 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
440 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
441 <a href="https://openjdk.java.net/">OpenJDK</a> and
442 <a href="https://jdk.java.net/">Oracle JDK</a>.
443 Although its source code is proprietary, its executable is available under the
444 <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
445 Timezone Updater License Agreement</a>.</li>
446 <li>The <a
447 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
448 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
449 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
450 which is freely available under a <abbr>BSD</abbr>-style license.</li>
451 <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
452 and time <abbr>API</abbr></a> contains a class
453 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
454 <code><abbr>tz</abbr></code> source into a binary format. It inspired
455 Java 8 <code>java.time</code>, which its users should migrate to once
456 they can assume Java 8 or later. It is available under the <a
457 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
458 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
459 href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
460 are alternatives to TZUpdater. IANA Updater's license is unclear;
461 ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
462 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
463 time library for Android</a> and
464 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
465 time and interval library for Java</a> compile
466 <code><abbr>tz</abbr></code> source into a binary format.
467 Time4A is available under the Apache License and Time4J is
468 available under the <a
469 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
470 General Public License (<abbr title="Lesser General Public
471 License">LGPL</abbr>)</a>.</li>
472 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
473 Java-based libraries.</li>
474 </ul>
475 <li><a href="https://nodatime.org">Noda Time – Date and
476 time <abbr>API</abbr> for .NET</a>
477 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
478 It is freely available under the Apache License.</li>
479 <li>Many modern
480 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
481 runtimes support <code><abbr>tz</abbr></code> natively via the
482 <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>.
483 This can be used as-is or with most of the following libraries,
484 many of which also support runtimes lacking the <samp>timeZone</samp> option.
485 <ul>
486 <li>The <a
487 href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
488 timezone polyfill</a>
489 is freely available under a <abbr>BSD</abbr>-style license.</li>
490 <li>The <a href="https://date-fns.org/">date-fns</a>
491 library manipulates timezone-aware timestamps in browsers and
492 in <a href="https://nodejs.org/en/">Node.js</a>.
493 It is freely available under the <abbr>MIT</abbr> license.</li>
494 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
495 minimalist replacement for the date and time API of
496 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
497 manipulation library.
498 It is freely available under the <abbr>MIT</abbr> license.</li>
499 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
500 timezone support for the <samp>Intl</samp> API.
501 It is freely available under the <abbr>MIT</abbr> license.</li>
502 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
503 Moment.js plugin.
504 It is freely available under the <abbr>MIT</abbr> license.</li>
505 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
506 JavaScript library that supports date arithmetic that is time zone
507 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
508 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
509 supports live <code><abbr>tzdb</abbr></code> updates,
510 astronomical and atomic time, a command-line interface,
511 and full <a
512 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
513 Its companion <a
514 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
515 can generate <abbr>TZif</abbr> and other files, and a companion website
516 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
517 convert timestamps, view transition histories, and download code and data.
518 It is freely available under the <abbr>MIT</abbr> license.</li>
519 </ul>
520 The proposed <a
521 href="https://github.com/tc39/proposal-temporal"><samp>Temporal</samp>
522 objects</a> let programs access an abstract view of
523 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
524 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
525 problematic <samp>Date</samp> objects</a> when working with dates and times.
526 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
527 compiler from <code><abbr>tz</abbr></code> source into
528 <a href="https://julialang.org/">Julia</a>. It is freely available
529 under the <abbr>MIT</abbr> license.</li>
530 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
531 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
532 title="Free Pascal Compiler">FPC</abbr></a>
533 compiles from <code><abbr>tz</abbr></code> source into
534 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
535 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
536 and <a
537 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
538 It is freely available under a <abbr>BSD</abbr>-style license.</li>
539 <li><a href="http://pytz.sourceforge.net">pytz – World Timezone
540 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
541 <a href="https://www.python.org">Python</a>.
542 It is freely available under a <abbr>BSD</abbr>-style license.
543 In code that can assume Python 3.6 or later it is largely superseded; see <a
544 href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
545 The Fastest Footgun in the West</a>.</li>
546 <li><a href="https://tzinfo.github.io">TZInfo –
547 Ruby Timezone Library</a>
548 compiles <code><abbr>tz</abbr></code> source into
549 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
550 It is freely available under the <abbr>MIT</abbr> license.</li>
551 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
552 Library</a> is
553 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
554 library that compiles <code><abbr>tz</abbr></code> source into a time
555 zone repository whose format
556 is either proprietary or an <abbr>XML</abbr>-encoded
557 representation.</li>
558 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
559 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
560 source into text files, along with a runtime that can read those
561 files. Tcl is freely available under a <abbr>BSD</abbr>-style
562 license.</li>
563 </ul>
564 </section>
565
566 <section>
567 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
568 <ul>
569 <li>The <a
570 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
571 Library</a>
572 has an independent, thread-safe implementation of
573 a <abbr>TZif</abbr> file reader.
574 This library is freely available under the LGPL
575 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
576 <li><a href="https://www.gnome.org">GNOME</a>'s
577 <a href="https://developer.gnome.org/glib/">GLib</a> has
578 a <abbr>TZif</abbr> file reader written in C that
579 creates a <code>GTimeZone</code> object representing sets
580 of <abbr>UT</abbr> offsets.
581 It is freely available under the <abbr>LGPL</abbr>.</li>
582 <li>The
583 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
584 <code>baltzo::TimeZoneUtil</code> component contains a C++
585 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
586 the Apache License.</li>
587 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
588 library that translates between <abbr>UT</abbr> and civil time and
589 can read <abbr>TZif</abbr> files. It is freely available under the Apache
590 License.</li>
591 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
592 library that reads <abbr>TZif</abbr> files and converts
593 timestamps from one time zone or format to another.
594 It is used by <a href="https://secure.php.net"><abbr
595 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
596 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
597 and <a href="https://www.mongodb.com">MongoDB</a>.
598 It is freely available under the <abbr>MIT</abbr> license.</li>
599 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
600 <abbr>TZif</abbr> file reader.</li>
601 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
602 DateTime::TimeZone::Tzfile</a>
603 is a <abbr>TZif</abbr> file reader written in Perl.
604 It is freely available under the same terms as Perl
605 (dual <abbr>GPL</abbr> and Artistic license).</li>
606 <li>Python has a <a id="python-zoneinfo"
607 href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
608 class</a> that reads <abbr>TZif</abbr> data and creates objects
609 that represent <code><abbr>tzdb</abbr></code> timezones.
610 Python is freely available under the
611 <a href="https://docs.python.org/3/license.html">Python Software Foundation
612 License</a>.
613 A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
614 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
615 supplies TZif data if the underlying system data cannot be found;
616 it is freely available under the Apache License.</li>
617 <li>The
618 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
619 library contains a Python tool that
620 converts <abbr>TZif</abbr> data into
621 <abbr>JSON</abbr>-format data suitable for use
622 in its JavaScript library for time zone conversion. Dates before 1970
623 are not supported.</li>
624 <li>The <a
625 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
626 package contains <a href="https://www.haskell.org">Haskell</a> code that
627 parses and uses <abbr>TZif</abbr> data. It is freely
628 available under a <abbr>BSD</abbr>-style license.</li>
629 </ul>
630 </section>
631
632 <section>
633 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
634 <ul>
635 <li><a href="https://foxclocks.org">FoxClocks</a>
636 is an extension for <a href="https://www.google.com/chrome/">Google
637 Chrome</a> and for <a
638 href="https://wiki.mozilla.org/Modules/Toolkit">Mozilla
639 Toolkit</a> applications like <a
640 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
641 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
642 It displays multiple clocks in the application window, and has a mapping
643 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
644 It is freely available under the <abbr>GPL</abbr>.</li>
645 <li><a href="https://golang.org">Go programming language</a>
646 implementations contain a copy of a 32-bit subset of a recent
647 <code><abbr>tz</abbr></code> database in a
648 Go-specific format.</li>
649 <li>Microsoft Windows 8.1
650 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
651 data (mentioned <a href="#CLDR">below</a>) used by the
652 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
653 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
654 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
655 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
656 <a id="System.TimeZoneInfo"
657 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
658 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
659 the older, proprietary method of Microsoft Windows 2000 and later,
660 which stores time zone data in the
661 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
662 <a
663 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
664 Tzid table</a> or <a
665 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
666 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
667 to <code><abbr>tz</abbr></code> names.
668 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
669 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
670 <li><a
671 href="https://www.oracle.com/java/index.html">Oracle
672 Java</a> contains a copy of a subset of a recent
673 <code><abbr>tz</abbr></code> database in a
674 Java-specific format.</li>
675 </ul>
676 </section>
677
678 <section>
679 <h2 id="other-dbs">Other time zone databases</h2>
680 <ul>
681 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
682 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
683 time zone history atlases
684 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
685 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
686 Although these extensive atlases
687 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
688 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
689 they are unreliable as Shanks appears to have
690 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
691 sources and do not indicate which entries are guesswork.</li>
692 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
693 its own <code>tztab</code>(4) format.</li>
694 <li>Microsoft Windows has proprietary data mentioned
695 <a href="#System.TimeZoneInfo">above</a>.</li>
696 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
697 is another time zone database.</li>
698 <li>The <a
699 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
700 Schedules Information Manual</a> of the
701 International Air Transport Association
702 gives current time zone rules for airports served by commercial aviation.</li>
703 </ul>
704 </section>
705
706 <section>
707 <h2 id="maps">Maps</h2>
708 <ul>
709 <li>The <a
710 href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
711 and Regional Maps section</a> of <em>The World Factbook</em>, published by the
712 <a href="https://www.cia.gov">US Central Intelligence
713 Agency (<abbr
714 title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
715 zone map; the
716 <a
717 href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda
718 Library Map Collection</a>
719 of the University of Texas at Austin has copies of
720 recent editions.
721 The pictorial quality is good,
722 but the maps do not indicate daylight saving time,
723 and parts of the data are a few years out of date.</li>
724 <li><a href="https://www.worldtimezone.com">World Time Zone Map
725 with current time</a>
726 has several fancy time zone maps; it covers Russia particularly well.
727 The maps' pictorial quality is not quite as good as the
728 <abbr>CIA</abbr>'s
729 but the maps are more up to date.</li>
730 <li><a
731 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
732 much is time wrong around the world?</a> maps the difference between
733 mean solar and standard time, highlighting areas such as western China
734 where the two differ greatly. It's a bit out of date, unfortunately.</li>
735 </ul>
736 </section>
737
738 <section>
739 <h2 id="boundaries">Time zone boundaries</h2>
740 <p>Geographical boundaries between timezones are available
741 from several <a
742 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
743 geolocation</a>
744 services and other sources.</p>
745 <ul>
746 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
747 Boundary Builder</a> extracts
748 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
749 boundaries of <code><abbr>tzdb</abbr></code> timezones.
750 Its code is freely available under the <abbr>MIT</abbr> license, and
751 its data entries are freely available under the
752 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
753 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
754 <li>Programmatic interfaces that map geographical coordinates via tz_world to
755 <code><abbr>tzdb</abbr></code> timezones include:
756 <ul>
757 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
758 written in <a
759 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
760 and is freely available under the <abbr>MIT</abbr> license.</li>
761 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
762 is written in Go and is freely available under the Apache License.</li>
763 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
764 in both Java and
765 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
766 form, is freely available under the MIT license.</li>
767 <li>For Node.js,
768 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
769 is freely available under the MIT license, and
770 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
771 is in the public domain.</li>
772 <li>The <a
773 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
774 library for Python is freely available under the MIT license.
775 <li>The <a
776 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
777 library for Ruby is freely available under the MIT license.</li>
778 </ul></li>
779 <li>Free access via a network API, if you register a key, is provided by
780 the <a
781 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
782 Timezone web service</a>, the <a
783 href="https://developers.google.com/maps/documentation/timezone/intro">Google
784 Maps Time Zone API</a>, and
785 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
786 Commercial network API access is provided
787 by <a href="https://askgeo.com">AskGeo</a>
788 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
789 </li>
790 <li>"<a
791 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
792 to get a time zone from a location using latitude and longitude
793 coordinates?</a>" discusses other geolocation possibilities.</li>
794 <li><a href="http://statoids.com/statoids.html">Administrative
795 Divisions of Countries ("Statoids")</a> lists
796 political subdivision data related to time zones.</li>
797 <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
798 zone boundaries for multizone countries</a> summarizes legal
799 boundaries between time zones within countries.</li>
800 <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
801 – GIS and Database Tools</a> includes a Manifold-format map of
802 world time zone boundaries distributed under the
803 <abbr>GPL</abbr>.</li>
804 <li>A ship within the <a
805 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
806 waters</a> of any nation uses that nation's time. In international
807 waters, time zone boundaries are meridians 15° apart, except that
808 <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
809 wide and are separated by
810 the 180° meridian (not by the International Date Line, which is
811 for land and territorial waters only). A captain can change ship's
812 clocks any time after entering a new time zone; midnight changes are
813 common.</li>
814 </ul>
815 </section>
816
817 <section>
818 <h2 id="civil">Civil time concepts and history</h2>
819 <ul>
820 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
821 Walk through Time</a>
822 surveys the evolution of timekeeping.</li>
823 <li>The history of daylight saving time is surveyed in <a
824 href="http://www.webexhibits.org/daylightsaving/">About Daylight
825 Saving Time – History, rationale, laws & dates</a> and summarized in
826 <a href="http://seizethedaylight.com/dst/">A Brief
827 History of Daylight Saving Time</a>.</li>
828 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
829 Lords</a> discusses how authoritarians manipulate civil time.</li>
830 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
831 contains guidelines and best practices for software applications that
832 deal with civil time.</li>
833 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
834 the International Date Line</a> tells the story of the most important
835 time zone boundary.</li>
836 <li><a href="http://statoids.com/tconcept.html">Basic Time
837 Zone Concepts</a> discusses terminological issues behind time zones.</li>
838 </ul>
839 </section>
840
841 <section>
842 <h2 id="national">National histories of legal time</h2>
843 <dl>
844 <dt>Australia</dt>
845 <dd>The Parliamentary Library commissioned a <a
846 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
847 paper on daylight saving time in Australia</a>.
848 The Bureau of Meteorology publishes a list of <a
849 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
850 Dates of Daylight Savings Time within Australia</a>.</dd>
851 <dt>Belgium</dt>
852 <dd>The Royal Observatory of Belgium maintains a table of time in
853 Belgium (in
854 <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
855 hreflang="nl">Dutch</a> and <a
856 href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
857 hreflang="fr">French</a>).</dd>
858 <dt>Brazil</dt>
859 <dd>The Time Service Department of the National Observatory
860 records <a href="http://pcdsh01.on.br/DecHV.html"
861 hreflang="pt-BR">Brazil's daylight saving time decrees (in
862 Portuguese)</a>.</dd>
863 <dt>Canada</dt>
864 <dd>National Research Council Canada publishes current
865 and some older information about <a
866 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
867 zones and daylight saving time</a>.</dd>
868 <dt>Chile</dt>
869 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
870 <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
871 Chile's official time (in Spanish)</a>.</dd>
872 <dt>China</dt>
873 <dd>The Hong Kong Observatory maintains a
874 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
875 summer time in Hong Kong</a>,
876 and Macau's Meteorological and Geophysical Bureau maintains a <a
877 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
878 history for Macau</a>.
879 Unfortunately the latter is incomplete and has errors.</dd>
880 <dt>Czech Republic</dt>
881 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
882 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
883 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
884 <dt>Germany</dt>
885 <dd>The National Institute for Science and Technology maintains the <a
886 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
887 of Legal Time in Germany</a>.</dd>
888 <dt>Israel</dt>
889 <dd>The Interior Ministry periodically issues <a
890 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
891 hreflang="he">announcements (in Hebrew)</a>.</dd>
892 <dt>Malaysia</dt>
893 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
894 <dt>Mexico</dt>
895 <dd>The Investigation and Analysis Service of the Mexican Library of
896 Congress has published a <a
897 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
898 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
899 <dt>Netherlands</dt>
900 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
901 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
902 covers the history of local time in the Netherlands from ancient times.</dd>
903 <dt>New Zealand</dt>
904 <dd>The Department of Internal Affairs maintains a brief <a
905 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
906 Daylight Saving</a>.</dd>
907 <dt>Portugal</dt>
908 <dd>The Lisbon Astronomical Observatory publishes a
909 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
910 legal time (in Portuguese)</a>.</dd>
911 <dt>Singapore</dt>
912 <dd><a id="Singapore"
913 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
914 is Singapore in the "Wrong" Time Zone?</a> details the
915 history of legal time in Singapore and Malaysia.</dd>
916 <dt>United Kingdom</dt>
917 <dd><a
918 href="https://www.polyomino.org.uk/british-time/">History of
919 legal time in Britain</a> discusses in detail the country
920 with perhaps the best-documented history of clock adjustments.</dd>
921 <dt>United States</dt>
922 <dd>The Department of Transportation's <a
923 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
924 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
925 <dt>Uruguay</dt>
926 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
927 Navy (SOHMA) publishes an annual <a
928 href="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
929 (in Spanish)</a>.</dd>
930 </dl>
931 </section>
932
933 <section>
934 <h2 id="costs">Costs and benefits of time shifts</h2>
935 <p>Various sources argue for and against daylight saving time and time
936 zone shifts, and many scientific studies have been conducted. This
937 section summarizes reviews and position statements based on
938 scientific literature in the area.</p>
939 <ul>
940 <li>Carey RN, Sarma KM.
941 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
942 daylight saving time on road traffic collision risk: a systematic
943 review</a>.
944 <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>.
945 This reviews research literature and concludes that the evidence
946 neither supports nor refutes road safety benefits from
947 shifts in time zones.</li>
948 <li>Havranek T, Herman D, Irsova D.
949 <a href="https://www.econstor.eu/handle/10419/174191">Does daylight
950 saving save electricity? A meta-analysis</a>. <em>Energy J.</em>
951 2018;39(2).
952 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
953 This analyzes research literature and concludes, "Electricity savings
954 are larger for countries farther away from the equator, while
955 subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
956 <li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
957 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
958 an American Academy of Sleep Medicine position statement</a>.
959 <em>J Clin Sleep Med.</em>
960 2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
961 This argues for permanent standard time due to health risks of both
962 <abbr>DST</abbr> transitions and permanent <abbr>DST</abbr>.</li>
963 <li>Roenneberg T, Winnebeck EC, Klerman EB.
964 <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
965 saving time and artificial time zones – a battle between
966 biological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
967 doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
968 This reviews evidence about the health effects of <abbr>DST</abbr>
969 and concludes,
970 "In summary, the scientific literature strongly argues against the
971 switching between <abbr>DST</abbr> and Standard Time and even more so against
972 adopting <abbr>DST</abbr> permanently."</li>
973 </ul>
974 </section>
975
976 <section>
977 <h2 id="precision">Precision timekeeping</h2>
978 <ul>
979 <li><a
980 href="http://leapsecond.com/hpan/an1289.pdf">The
981 Science of Timekeeping</a> is a thorough introduction
982 to the theory and practice of precision timekeeping.</li>
983 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
984 Time 2016</a> contains several freely-readable papers.</li>
985 <li><a href="http://www.ntp.org"><abbr
986 title="Network Time Protocol">NTP</abbr>: The Network
987 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
988 discusses how to synchronize clocks of
989 Internet hosts.</li>
990 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
991 family of software algorithms can achieve accuracy to a few tens of
992 nanoseconds in scalable server farms without special hardware.</li>
993 <li>The <a
994 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
995 Time Protocol</a> (<abbr
996 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
997 can achieve submicrosecond clock accuracy on a local area network
998 with special-purpose hardware.</li>
999 <li><a
1000 href="https://tools.ietf.org/html/rfc4833">Timezone
1001 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
1002 (Internet <abbr>RFC</abbr> 4833)
1003 specifies a <a
1004 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
1005 option for a server to configure
1006 a client's time zone and daylight saving settings automatically.</li>
1007 <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
1008 Scales</a> describes astronomical time scales like
1009 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
1010 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
1011 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
1012 <li>The <a href="https://www.iau.org"><abbr
1013 title="International Astronomical Union">IAU</abbr></a>'s <a
1014 href="http://www.iausofa.org"><abbr
1015 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
1016 collection contains C and <a
1017 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
1018 code for converting among time scales like
1019 <abbr title="International Atomic Time">TAI</abbr>,
1020 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
1021 <abbr>UTC</abbr>.</li>
1022 <li><a
1023 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
1024 – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
1025 diverse local time
1026 scales used by each landed mission on Mars.</li>
1027 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
1028 dedicated not only to leap seconds but to precise time and frequency
1029 in general. It covers the state of the art in amateur timekeeping, and
1030 how the art has progressed over the past few decades.</li>
1031 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
1032 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
1033 and time-signal emissions</a>, International Telecommunication Union –
1034 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
1035 <li><a
1036 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
1037 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
1038 Bulletins</a> contains official publications of the International
1039 Earth Rotation and Reference Systems Service, which decides when leap
1040 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
1041 via an optional "<code>right</code>" configuration where a computer's internal
1042 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
1043 as opposed to the default "<code>posix</code>" configuration
1044 where the internal clock ignores leap seconds.
1045 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
1046 <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for
1047 timestamps starting with <code>time_t</code> 78 796 800,
1048 which corresponds to the first leap second
1049 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
1050 and to
1051 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
1052 In practice the two configurations also agree for timestamps before
1053 1972 even though the historical situation is messy, partly because
1054 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
1055 is well-defined for sufficiently-old timestamps.</li>
1056 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1057 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1058 leap second so that they disagree with <abbr>UTC</abbr> by at most a
1059 half second, even though every <abbr>POSIX</abbr> minute has exactly
1060 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1061 "<code>posix</code>" configuration, is <a
1062 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
1063 the <abbr>NTP</abbr> reference implementation, and is used by major
1064 cloud service providers. However, according to
1065 <a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">§3.7.1 of
1066 Network Time Protocol Best Current Practices</a>
1067 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1068 applications requiring accurate <abbr>UTC</abbr> or civil time,
1069 and is intended for use only in single, well-controlled environments.</li>
1070 <li>The <a
1071 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
1072 Second Discussion List</a> covers <a
1073 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
1074 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
1075 discussed further in
1076 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1077 leap second: its history and possible future</a>.
1078 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1079 might be redefined
1080 without Leap Seconds</a> gives pointers on this
1081 contentious issue, which was active until 2015 and could become active
1082 again.</li>
1083 </ul>
1084 </section>
1085
1086 <section>
1087 <h2 id="notation">Time notation</h2>
1088 <ul>
1089 <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
1090 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1091 zone names, abbreviations, identifiers, and formats. For example, it
1092 contains French translations for "Eastern European Summer Time",
1093 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
1094 "Bucharest". Its
1095 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1096 charts</a> show these values for many locales. Data values are available in
1097 both <abbr title="Locale Data Markup Language">LDML</abbr>
1098 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1099 <li>
1100 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1101 the international standard date and time notation</a> covers
1102 <a
1103 href="https://www.iso.org/standard/70907.html"><em><abbr
1104 title="International Organization for Standardization">ISO</abbr>
1105 8601-1:2019 – Date and time – Representations for information
1106 interchange – Part 1: Basic rules</em></a>.</li>
1107 <li>
1108 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1109 Schema: Datatypes – dateTime</a> specifies a format inspired by
1110 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1111 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">§3.3 of
1112 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1113 specifies the time notation used in email and <a
1114 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1115 headers.</li>
1116 <li>
1117 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
1118 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1119 specifies an <abbr>ISO</abbr> 8601
1120 profile for use in new Internet
1121 protocols.</li>
1122 <li>
1123 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1124 Formats on the Web</a> surveys web- and Internet-oriented date and time
1125 formats.</li>
1126 <li>Alphabetic time zone abbreviations should not be used as unique
1127 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1128 practice. For example, in English-speaking North America
1129 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
1130 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1131 and French-speaking North Americans prefer
1132 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
1133 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
1134 database contains English abbreviations for many timestamps;
1135 unfortunately some of these abbreviations were merely the database maintainers'
1136 inventions, and these have been removed when possible.</li>
1137 <li>Numeric time zone abbreviations typically count hours east of
1138 <abbr>UT</abbr>, e.g., +09 for Japan and
1139 −10 for Hawaii. However, the <abbr>POSIX</abbr>
1140 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
1141 For example, one might use <code><abbr>TZ</abbr>="<abbr
1142 title="Japan Standard Time">JST</abbr>-9"</code> and
1143 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1144 for Japan and Hawaii, respectively. If the
1145 <code><abbr>tz</abbr></code> database is available, it is usually better to use
1146 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1147 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1148 confusion, handle old timestamps better, and insulate you better from
1149 any future changes to the rules. One should never set
1150 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1151 <code>"GMT-9"</code>, though, since this would incorrectly imply that
1152 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1153 is called "<abbr>GMT</abbr>".</li>
1154 </ul>
1155 </section>
1156
1157 <section>
1158 <h2 id="see-also">See also</h2>
1159 <ul>
1160 <li><a href="theory.html">Theory and pragmatics of the
1161 <code><abbr>tz</abbr></code> code and data</a></li>
1162 <li><a href="tz-art.html">Time and the Arts</a></li>
1163 </ul>
1164 </section>
1165
1166 <footer>
1167 <hr>
1168 This web page is in the public domain, so clarified as of
1169 2009-05-17 by Arthur David Olson.
1170 <br>
1171 Please send corrections to this web page to the
1172 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1173 </footer>
1174 </body>
1175 </html>
1176