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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://en.wikipedia.org/wiki/BlackBerry_10">BlackBerry 10</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 1999g, 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://tools.ietf.org/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 If your government plans to change its time zone boundaries or
220 daylight saving rules, inform <code>tz (a] iana.org</code> well in
221 advance, as this will coordinate updates to many cell phones,
222 computers, and other devices around the world.
223 The change should be officially announced at least a year before it affects
224 how clocks operate; otherwise, there is a good chance that some
225 clocks will operate incorrectly after the change, due
226 to delays in propagating updates to software and data. The shorter
227 the notice, the more likely clock problems will arise; see "<a
228 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
229 the Timing of Time Zone Changes</a>" for examples.
230 The <code><abbr>tz</abbr></code> data can represent planned changes
231 far into the future, and a long-planned change can easily be reverted
232 or otherwise altered with a year's notice before the change would have
233 affected clocks.
234 </p>
235 <p>
236 Changes to the <code><abbr>tz</abbr></code> code and data are often
237 propagated to clients via operating system updates, so
238 client <code><abbr>tz</abbr></code> data can often be corrected by
239 applying these updates. With GNU/Linux and similar systems, if your
240 maintenance provider has not yet adopted the
241 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
242 the process by tailoring the generic instructions in
243 the <code><abbr>tz</abbr> README</code> file and installing the latest
244 data yourself. System-specific instructions for installing the
245 latest <code><abbr>tz</abbr></code> data have also been published
246 for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
247 <a
248 href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
249 <a
250 href="http://userguide.icu-project.org/datetime/timezone"><abbr
251 title="International Components for Unicode">ICU</abbr></a>,
252 <a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr>
253 JDK</a>,
254 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
255 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
256 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
257 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html#usage">OpenJDK/Oracle JDK</a>.
258 </p>
259 <p>Sources for the <code><abbr>tz</abbr></code> database are
260 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
261 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
262 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
263 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
264 title="linefeed">LF</abbr></a>,
265 which can be modified by common text editors such
266 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
267 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
268 <a href="https://www.vim.org">vim</a>.
269 Specialized source-file editing can be done via the
270 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
271 zoneinfo</a> package for <a
272 href="https://www.sublimetext.com">Sublime Text</a> and the <a
273 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
274 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
275 Studio Code</a>.
276 </p>
277 <p>
278 For further information about updates, please see
279 <a href="https://tools.ietf.org/html/rfc6557">Procedures for
280 Maintaining the Time Zone Database</a> (Internet <abbr
281 title="Request For Comments">RFC</abbr> 6557). More detail can be
282 found in <a href="theory.html">Theory and pragmatics of the
283 <code><abbr>tz</abbr></code> code and data</a>.
284 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
285 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
286 </p>
287 </section>
288
289 <section>
290 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
291 <ul>
292 <li>The article
293 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
294 an encyclopedic summary.</li>
295 <li><a href="tz-how-to.html">How to Read the
296 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
297 database format.</li>
298 <li><a
299 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
300 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
301 database's style.</li>
302 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
303 managing time in the internet</a> analyzes the database longitudinally.</li>
304 </ul>
305 </section>
306
307 <section>
308 <h2 id="web">Web sites using recent versions of the
309 <code><abbr>tz</abbr></code> database</h2>
310 <p>
311 These are listed roughly in ascending order of complexity and fanciness.
312 </p>
313 <ul>
314 <li><a href="https://time.is">Time.is</a> shows locations'
315 time and zones.</li>
316 <li><a href="https://www.timejones.com">TimeJones.com</a>,
317 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
318 <a href="https://www.worldclock.com">The World Clock</a>
319 are time zone converters.</li>
320 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
321 publishes <code><abbr>tzdb</abbr></code>-derived data in
322 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
323 title="comma-separated values">CSV</abbr></a> and
324 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
325 title="Structured Query Language">SQL</abbr></a> form.</li>
326 <li><a
327 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
328 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
329 <li><a
330 href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
331 Time in 1000 Places</a> uses descriptions of the values.</li>
332 <li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
333 uses a pulldown menu.</li>
334 <li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
335 timezone information for all countries</a>
336 displays tables of <abbr>DST</abbr> rules.
337 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
338 Worldwide</a> lets you sort zone names and convert times.</li>
339 <li><a href="https://24timezones.com">24TimeZones</a> has a world
340 time map and a time converter.</li>
341 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
342 calculates the current time difference between locations.</li>
343 <li><a href="http://www.wx-now.com">Weather Now</a> and
344 <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
345 </ul>
346 </section>
347
348 <section>
349 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
350 <ul>
351 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
352 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
353 Distribution Service (tzdist) working group</a> defined <a
354 href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
355 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
356 along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
357 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
358 transferring time zone data by reference.
359 <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
360 implementations</a> are available.
361 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
362 mailing list</a> discusses possible extensions.</li>
363 <li>The <a href="https://tools.ietf.org/html/rfc5545">
364 Internet Calendaring and Scheduling Core Object Specification
365 (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
366 covers time zone
367 data; see its VTIMEZONE calendar component.
368 The iCalendar format requires specialized parsers and generators; a
369 variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
370 (Internet <abbr>RFC</abbr> 6321) uses
371 <a href="https://www.w3.org/XML/"><abbr
372 title="Extensible Markup Language">XML</abbr></a> format, and a variant
373 <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
374 (Internet <abbr>RFC</abbr> 7265)
375 uses <a href="https://www.json.org"><abbr
376 title="JavaScript Object Notation">JSON</abbr></a> format.</li>
377 </ul>
378 </section>
379
380 <section>
381 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
382 <p>Although some of these do not fully support
383 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
384 distributions you can generally work around compatibility problems by
385 running the command <code>make rearguard_tarballs</code> and compiling
386 from the resulting tarballs instead.</p>
387 <ul>
388 <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
389 href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
390 program that compiles
391 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
392 Vzic is freely
393 available under the <a
394 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
395 General Public License (<abbr
396 title="General Public License">GPL</abbr>)</a>.</li>
397 <li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz
398 database conversion utility</a> is like Vzic, except for the <a
399 href="https://dotnet.microsoft.com">.NET framework</a>
400 and with a <abbr>BSD</abbr>-style license.</li>
401 <li><a
402 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
403 contains a script <code>parse_olson</code> that compiles
404 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
405 modules. It is part of the Perl <a
406 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
407 which is freely
408 available under both the <abbr>GPL</abbr> and the Perl Artistic
409 License. DateTime::TimeZone also contains a script
410 <code>tests_from_zdump</code> that generates test cases for each clock
411 transition in the <code><abbr>tz</abbr></code> database.</li>
412 <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
413 Database Parser</a> is a
414 <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
415 runtime library with <a
416 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
417 adopted by
418 <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
419 the current iteration of the C++ standard.
420 It is freely available under the
421 <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
422 <li><a id="ICU" href="http://site.icu-project.org">International Components for
423 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
424 href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
425 libraries for internationalization that
426 has a compiler from <code><abbr>tz</abbr></code> source
427 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
428 (mentioned <a href="#CLDR">below</a>)
429 into an <abbr>ICU</abbr>-specific format.
430 <abbr>ICU</abbr> is freely available under a
431 <abbr>BSD</abbr>-style license.</li>
432 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
433 the <a href="https://elixir-lang.org">Elixir</a> language downloads
434 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
435 title="Application Program Interface">API</abbr>s for use. It is
436 freely available under the <abbr>MIT</abbr> license.</li>
437 <li>Java-based compilers and libraries include:
438 <ul>
439 <li>The <a
440 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
441 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
442 <a href="https://openjdk.java.net/">OpenJDK</a> and
443 <a href="https://jdk.java.net/">Oracle JDK</a>.
444 Although its source code is proprietary, its executable is available under the
445 <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
446 Timezone Updater License Agreement</a>.</li>
447 <li>The <a
448 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
449 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
450 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
451 which is freely available under a <abbr>BSD</abbr>-style license.</li>
452 <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
453 and time <abbr>API</abbr></a> contains a class
454 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
455 <code><abbr>tz</abbr></code> source into a binary format. It inspired
456 Java 8 <code>java.time</code>, which its users should migrate to once
457 they can assume Java 8 or later. It is available under the <a
458 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
459 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
460 href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
461 are alternatives to TZUpdater. IANA Updater's license is unclear;
462 ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
463 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
464 time library for Android</a> and
465 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
466 time and interval library for Java</a> compile
467 <code><abbr>tz</abbr></code> source into a binary format.
468 Time4A is available under the Apache License and Time4J is
469 available under the <a
470 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
471 General Public License (<abbr title="Lesser General Public
472 License">LGPL</abbr>)</a>.</li>
473 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
474 Java-based libraries.</li>
475 </ul>
476 <li><a href="https://nodatime.org">Noda Time – Date and
477 time <abbr>API</abbr> for .NET</a>
478 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
479 It is freely available under the Apache License.</li>
480 <li>Many modern
481 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
482 runtimes support <code><abbr>tz</abbr></code> natively via the
483 <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>.
484 This can be used as-is or with most of the following libraries,
485 many of which also support runtimes lacking the <samp>timeZone</samp> option.
486 <ul>
487 <li>The <a
488 href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
489 timezone polyfill</a>
490 is freely available under a <abbr>BSD</abbr>-style license.</li>
491 <li><a
492 href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
493 compiles time zone data into a compact form designed for
494 JavaScript. It is freely available under a combination of
495 the <abbr>MIT</abbr> license and the Apache License.</li>
496 <li>The <a href="https://date-fns.org/">date-fns</a>
497 library manipulates timezone-aware timestamps in browsers and
498 in <a href="https://nodejs.org/en/">Node.js</a>.
499 It is freely available under the <abbr>MIT</abbr> license.</li>
500 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
501 minimalist replacement for the date and time API of
502 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
503 manipulation library.
504 It is freely available under the <abbr>MIT</abbr> license.</li>
505 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
506 timezone support for the <samp>Intl</samp> API.
507 It is freely available under the <abbr>MIT</abbr> license.</li>
508 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
509 Moment.js plugin.
510 It is freely available under the <abbr>MIT</abbr> license.</li>
511 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
512 JavaScript library that supports date arithmetic that is time zone
513 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
514 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
515 supports live <code><abbr>tzdb</abbr></code> updates,
516 astronomical and atomic time, a command-line interface,
517 and full <a
518 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
519 Its companion <a
520 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
521 can generate <abbr>TZif</abbr> and other files, and a companion website
522 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
523 convert timestamps, view transition histories, and download code and data.
524 It is freely available under the <abbr>MIT</abbr> license.</li>
525 </ul>
526 The proposed <a
527 href="https://github.com/tc39/proposal-temporal"><samp>Temporal</samp>
528 objects</a> let programs access an abstract view of
529 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
530 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
531 problematic <samp>Date</samp> objects</a> when working with dates and times.
532 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
533 compiler from <code><abbr>tz</abbr></code> source into
534 <a href="https://julialang.org/">Julia</a>. It is freely available
535 under the <abbr>MIT</abbr> license.</li>
536 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
537 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
538 title="Free Pascal Compiler">FPC</abbr></a>
539 compiles from <code><abbr>tz</abbr></code> source into
540 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
541 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
542 and <a
543 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
544 It is freely available under a <abbr>BSD</abbr>-style license.</li>
545 <li><a href="http://pytz.sourceforge.net">pytz – World Timezone
546 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
547 <a href="https://www.python.org">Python</a>.
548 It is freely available under a <abbr>BSD</abbr>-style license.
549 In code that can assume Python 3.9 or later it is superseded by
550 <a href="#python-zoneinfo"><code>zoneinfo.ZoneInfo</code></a>
551 and the <a href="#pypi-tzdata"><code>tzdata</code> Python package</a>.</li>
552 <li><a href="https://tzinfo.github.io">TZInfo –
553 Ruby Timezone Library</a>
554 compiles <code><abbr>tz</abbr></code> source into
555 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
556 It is freely available under the <abbr>MIT</abbr> license.</li>
557 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
558 Library</a> is
559 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
560 library that compiles <code><abbr>tz</abbr></code> source into a time
561 zone repository whose format
562 is either proprietary or an <abbr>XML</abbr>-encoded
563 representation.</li>
564 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
565 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
566 source into text files, along with a runtime that can read those
567 files. Tcl is freely available under a <abbr>BSD</abbr>-style
568 license.</li>
569 </ul>
570 </section>
571
572 <section>
573 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
574 <ul>
575 <li>The <a
576 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
577 Library</a>
578 has an independent, thread-safe implementation of
579 a <abbr>TZif</abbr> file reader.
580 This library is freely available under the LGPL
581 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
582 <li><a href="https://www.gnome.org">GNOME</a>'s
583 <a href="https://developer.gnome.org/glib/">GLib</a> has
584 a <abbr>TZif</abbr> file reader written in C that
585 creates a <code>GTimeZone</code> object representing sets
586 of <abbr>UT</abbr> offsets.
587 It is freely available under the <abbr>LGPL</abbr>.</li>
588 <li>The
589 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
590 <code>baltzo::TimeZoneUtil</code> component contains a C++
591 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
592 the Apache License.</li>
593 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
594 library that translates between <abbr>UT</abbr> and civil time and
595 can read <abbr>TZif</abbr> files. It is freely available under the Apache
596 License.</li>
597 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
598 library that reads <abbr>TZif</abbr> files and converts
599 timestamps from one time zone or format to another.
600 It is used by <a href="https://secure.php.net"><abbr
601 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
602 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
603 and <a href="https://www.mongodb.com">MongoDB</a>.
604 It is freely available under the <abbr>MIT</abbr> license.</li>
605 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
606 <abbr>TZif</abbr> file reader.</li>
607 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
608 DateTime::TimeZone::Tzfile</a>
609 is a <abbr>TZif</abbr> file reader written in Perl.
610 It is freely available under the same terms as Perl
611 (dual <abbr>GPL</abbr> and Artistic license).</li>
612 <li>Python has a <a id="python-zoneinfo"
613 href="https://docs.python.org/3.9/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
614 class</a> that reads <abbr>TZif</abbr> data and creates objects
615 that represent <code><abbr>tzdb</abbr></code> timezones.
616 Python is freely available under the
617 <a href="https://docs.python.org/3.9/license.html">Python Software Foundation
618 License</a>.
619 A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
620 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
621 supplies TZif data if the underlying system data cannot be found;
622 it is freely available under the Apache License.</li>
623 <li>The
624 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
625 library contains a Python tool that
626 converts <abbr>TZif</abbr> data into
627 <abbr>JSON</abbr>-format data suitable for use
628 in its JavaScript library for time zone conversion. Dates before 1970
629 are not supported.</li>
630 <li>The <a
631 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
632 package contains <a href="https://www.haskell.org">Haskell</a> code that
633 parses and uses <abbr>TZif</abbr> data. It is freely
634 available under a <abbr>BSD</abbr>-style license.</li>
635 </ul>
636 </section>
637
638 <section>
639 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
640 <ul>
641 <li><a href="https://foxclocks.org">FoxClocks</a>
642 is an extension for <a href="https://www.google.com/chrome/">Google
643 Chrome</a> and for <a
644 href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
645 Toolkit</a> applications like <a
646 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
647 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
648 It displays multiple clocks in the application window, and has a mapping
649 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
650 It is freely available under the <abbr>GPL</abbr>.</li>
651 <li><a href="https://golang.org">Go programming language</a>
652 implementations contain a copy of a 32-bit subset of a recent
653 <code><abbr>tz</abbr></code> database in a
654 Go-specific format.</li>
655 <li>Microsoft Windows 8.1
656 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
657 data (mentioned <a href="#CLDR">below</a>) used by the
658 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
659 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
660 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
661 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
662 <a id="System.TimeZoneInfo"
663 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
664 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
665 the older, proprietary method of Microsoft Windows 2000 and later,
666 which stores time zone data in the
667 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
668 <a
669 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
670 Tzid table</a> or <a
671 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
672 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
673 to <code><abbr>tz</abbr></code> names.
674 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
675 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
676 <li><a
677 href="https://www.oracle.com/java/index.html">Oracle
678 Java</a> contains a copy of a subset of a recent
679 <code><abbr>tz</abbr></code> database in a
680 Java-specific format.</li>
681 </ul>
682 </section>
683
684 <section>
685 <h2 id="other-dbs">Other time zone databases</h2>
686 <ul>
687 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
688 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
689 time zone history atlases
690 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
691 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
692 Although these extensive atlases
693 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
694 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
695 they are unreliable as Shanks appears to have
696 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
697 sources and do not indicate which entries are guesswork.</li>
698 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
699 its own <code>tztab</code>(4) format.</li>
700 <li>Microsoft Windows has proprietary data mentioned
701 <a href="#System.TimeZoneInfo">above</a>.</li>
702 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
703 is another time zone database.</li>
704 <li>The <a
705 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
706 Schedules Information Manual</a> of the
707 International Air Transport Association
708 gives current time zone rules for airports served by commercial aviation.</li>
709 </ul>
710 </section>
711
712 <section>
713 <h2 id="maps">Maps</h2>
714 <ul>
715 <li>The <a
716 href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
717 and Regional Maps section</a> of <em>The World Factbook</em>, published by the
718 <a href="https://www.cia.gov">US Central Intelligence
719 Agency (<abbr
720 title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
721 zone map; the
722 <a
723 href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda
724 Library Map Collection</a>
725 of the University of Texas at Austin has copies of
726 recent editions.
727 The pictorial quality is good,
728 but the maps do not indicate daylight saving time,
729 and parts of the data are a few years out of date.</li>
730 <li><a href="https://www.worldtimezone.com">World Time Zone Map
731 with current time</a>
732 has several fancy time zone maps; it covers Russia particularly well.
733 The maps' pictorial quality is not quite as good as the
734 <abbr>CIA</abbr>'s
735 but the maps are more up to date.</li>
736 <li><a
737 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
738 much is time wrong around the world?</a> maps the difference between
739 mean solar and standard time, highlighting areas such as western China
740 where the two differ greatly. It's a bit out of date, unfortunately.</li>
741 </ul>
742 </section>
743
744 <section>
745 <h2 id="boundaries">Time zone boundaries</h2>
746 <p>Geographical boundaries between timezones are available
747 from several <a
748 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
749 geolocation</a>
750 services and other sources.</p>
751 <ul>
752 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
753 Boundary Builder</a> extracts
754 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
755 boundaries of <code><abbr>tzdb</abbr></code> timezones.
756 Its code is freely available under the <abbr>MIT</abbr> license, and
757 its data entries are freely available under the
758 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
759 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
760 <li>Programmatic interfaces that map geographical coordinates via tz_world to
761 <code><abbr>tzdb</abbr></code> timezones include:
762 <ul>
763 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
764 written in <a
765 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
766 and is freely available under the <abbr>MIT</abbr> license.</li>
767 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
768 is written in Go and is freely available under the Apache License.</li>
769 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
770 in both Java and
771 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
772 form, is freely available under the MIT license.</li>
773 <li>For Node.js,
774 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
775 is freely available under the MIT license, and
776 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
777 is in the public domain.</li>
778 <li>The <a
779 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
780 library for Python is freely available under the MIT license.
781 <li>The <a
782 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
783 library for Ruby is freely available under the MIT license.</li>
784 </ul></li>
785 <li>Free access via a network API, if you register a key, is provided by
786 the <a
787 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
788 Timezone web service</a>, the <a
789 href="https://developers.google.com/maps/documentation/timezone/intro">Google
790 Maps Time Zone API</a>, and
791 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
792 Commercial network API access is provided
793 by <a href="https://askgeo.com">AskGeo</a>
794 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
795 </li>
796 <li>"<a
797 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
798 to get a time zone from a location using latitude and longitude
799 coordinates?</a>" discusses other geolocation possibilities.</li>
800 <li><a href="http://statoids.com/statoids.html">Administrative
801 Divisions of Countries ("Statoids")</a> lists
802 political subdivision data related to time zones.</li>
803 <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
804 zone boundaries for multizone countries</a> summarizes legal
805 boundaries between time zones within countries.</li>
806 <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
807 – GIS and Database Tools</a> includes a Manifold-format map of
808 world time zone boundaries distributed under the
809 <abbr>GPL</abbr>.</li>
810 <li>A ship within the <a
811 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
812 waters</a> of any nation uses that nation's time. In international
813 waters, time zone boundaries are meridians 15° apart, except that
814 <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
815 wide and are separated by
816 the 180° meridian (not by the International Date Line, which is
817 for land and territorial waters only). A captain can change ship's
818 clocks any time after entering a new time zone; midnight changes are
819 common.</li>
820 </ul>
821 </section>
822
823 <section>
824 <h2 id="civil">Civil time concepts and history</h2>
825 <ul>
826 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
827 Walk through Time</a>
828 surveys the evolution of timekeeping.</li>
829 <li>The history of daylight saving time is surveyed in <a
830 href="http://www.webexhibits.org/daylightsaving/">About Daylight
831 Saving Time – History, rationale, laws & dates</a> and summarized in
832 <a href="http://seizethedaylight.com/dst/">A Brief
833 History of Daylight Saving Time</a>.</li>
834 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
835 Lords</a> discusses how authoritarians manipulate civil time.</li>
836 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
837 contains guidelines and best practices for software applications that
838 deal with civil time.</li>
839 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
840 the International Date Line</a> tells the story of the most important
841 time zone boundary.</li>
842 <li><a href="http://statoids.com/tconcept.html">Basic Time
843 Zone Concepts</a> discusses terminological issues behind time zones.</li>
844 </ul>
845 </section>
846
847 <section>
848 <h2 id="national">National histories of legal time</h2>
849 <dl>
850 <dt>Australia</dt>
851 <dd>The Parliamentary Library commissioned a <a
852 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
853 paper on daylight saving time in Australia</a>.
854 The Bureau of Meteorology publishes a list of <a
855 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
856 Dates of Daylight Savings Time within Australia</a>.</dd>
857 <dt>Belgium</dt>
858 <dd>The Royal Observatory of Belgium maintains a table of time in
859 Belgium (in
860 <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
861 hreflang="nl">Dutch</a> and <a
862 href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
863 hreflang="fr">French</a>).</dd>
864 <dt>Brazil</dt>
865 <dd>The Time Service Department of the National Observatory
866 records <a href="http://pcdsh01.on.br/DecHV.html"
867 hreflang="pt-BR">Brazil's daylight saving time decrees (in
868 Portuguese)</a>.</dd>
869 <dt>Canada</dt>
870 <dd>National Research Council Canada publishes current
871 and some older information about <a
872 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
873 zones and daylight saving time</a>.</dd>
874 <dt>Chile</dt>
875 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
876 <a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
877 Chile's official time (in Spanish)</a>.</dd>
878 <dt>China</dt>
879 <dd>The Hong Kong Observatory maintains a
880 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
881 summer time in Hong Kong</a>,
882 and Macau's Meteorological and Geophysical Bureau maintains a <a
883 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
884 history for Macau</a>.
885 Unfortunately the latter is incomplete and has errors.</dd>
886 <dt>Czech Republic</dt>
887 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
888 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
889 summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
890 <dt>Germany</dt>
891 <dd>The National Institute for Science and Technology maintains the <a
892 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
893 of Legal Time in Germany</a>.</dd>
894 <dt>Israel</dt>
895 <dd>The Interior Ministry periodically issues <a
896 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
897 hreflang="he">announcements (in Hebrew)</a>.</dd>
898 <dt>Malaysia</dt>
899 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
900 <dt>Mexico</dt>
901 <dd>The Investigation and Analysis Service of the Mexican Library of
902 Congress has published a <a
903 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
904 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
905 <dt>Netherlands</dt>
906 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
907 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
908 covers the history of local time in the Netherlands from ancient times.</dd>
909 <dt>New Zealand</dt>
910 <dd>The Department of Internal Affairs maintains a brief <a
911 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
912 Daylight Saving</a>.</dd>
913 <dt>Portugal</dt>
914 <dd>The Lisbon Astronomical Observatory publishes a
915 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
916 legal time (in Portuguese)</a>.</dd>
917 <dt>Singapore</dt>
918 <dd><a id="Singapore"
919 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
920 is Singapore in the "Wrong" Time Zone?</a> details the
921 history of legal time in Singapore and Malaysia.</dd>
922 <dt>United Kingdom</dt>
923 <dd><a
924 href="https://www.polyomino.org.uk/british-time/">History of
925 legal time in Britain</a> discusses in detail the country
926 with perhaps the best-documented history of clock adjustments.</dd>
927 <dt>United States</dt>
928 <dd>The Department of Transportation's <a
929 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
930 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
931 <dt>Uruguay</dt>
932 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
933 Navy (SOHMA) publishes an annual <a
934 href="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
935 (in Spanish)</a>.</dd>
936 </dl>
937 </section>
938
939 <section>
940 <h2 id="costs">Costs and benefits of time shifts</h2>
941 <p>Various sources argue for and against daylight saving time and time
942 zone shifts, and many scientific studies have been conducted. This
943 section summarizes reviews and position statements based on
944 scientific literature in the area.</p>
945 <ul>
946 <li>Carey RN, Sarma KM.
947 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
948 daylight saving time on road traffic collision risk: a systematic
949 review</a>.
950 <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>.
951 This reviews research literature and concludes that the evidence
952 neither supports nor refutes road safety benefits from
953 shifts in time zones.</li>
954 <li>Havranek T, Herman D, Irsova D.
955 <a href="https://www.econstor.eu/handle/10419/174191">Does daylight
956 saving save electricity? A meta-analysis</a>. <em>Energy J.</em>
957 2018;39(2).
958 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
959 This analyzes research literature and concludes, "Electricity savings
960 are larger for countries farther away from the equator, while
961 subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
962 <li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
963 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
964 an American Academy of Sleep Medicine position statement</a>.
965 <em>J Clin Sleep Med.</em>
966 2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
967 This argues for permanent standard time due to health risks of both
968 <abbr>DST</abbr> transitions and permanent <abbr>DST</abbr>.</li>
969 <li>Roenneberg T, Winnebeck EC, Klerman EB.
970 <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
971 saving time and artificial time zones – a battle between
972 biological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
973 doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
974 This reviews evidence about the health effects of <abbr>DST</abbr>
975 and concludes,
976 "In summary, the scientific literature strongly argues against the
977 switching between <abbr>DST</abbr> and Standard Time and even more so against
978 adopting <abbr>DST</abbr> permanently."</li>
979 </ul>
980 </section>
981
982 <section>
983 <h2 id="precision">Precision timekeeping</h2>
984 <ul>
985 <li><a
986 href="http://leapsecond.com/hpan/an1289.pdf">The
987 Science of Timekeeping</a> is a thorough introduction
988 to the theory and practice of precision timekeeping.</li>
989 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
990 Time 2016</a> contains several freely-readable papers.</li>
991 <li><a href="http://www.ntp.org"><abbr
992 title="Network Time Protocol">NTP</abbr>: The Network
993 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
994 discusses how to synchronize clocks of
995 Internet hosts.</li>
996 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
997 family of software algorithms can achieve accuracy to a few tens of
998 nanoseconds in scalable server farms without special hardware.</li>
999 <li>The <a
1000 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
1001 Time Protocol</a> (<abbr
1002 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
1003 can achieve submicrosecond clock accuracy on a local area network
1004 with special-purpose hardware.</li>
1005 <li><a
1006 href="https://tools.ietf.org/html/rfc4833">Timezone
1007 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
1008 (Internet <abbr>RFC</abbr> 4833)
1009 specifies a <a
1010 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
1011 option for a server to configure
1012 a client's time zone and daylight saving settings automatically.</li>
1013 <li><a
1014 href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
1015 Times</a> explains more abstruse astronomical time scales like
1016 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
1017 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
1018 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
1019 <a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
1020 Scales</a> goes into more detail, particularly for historical variants.</li>
1021 <li>The <a href="https://www.iau.org"><abbr
1022 title="International Astronomical Union">IAU</abbr></a>'s <a
1023 href="http://www.iausofa.org"><abbr
1024 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
1025 collection contains C and <a
1026 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
1027 code for converting among time scales like
1028 <abbr title="International Atomic Time">TAI</abbr>,
1029 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
1030 <abbr>UTC</abbr>.</li>
1031 <li><a
1032 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
1033 – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
1034 diverse local time
1035 scales used by each landed mission on Mars.</li>
1036 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
1037 dedicated not only to leap seconds but to precise time and frequency
1038 in general. It covers the state of the art in amateur timekeeping, and
1039 how the art has progressed over the past few decades.</li>
1040 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
1041 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
1042 and time-signal emissions</a>, International Telecommunication Union –
1043 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
1044 <li><a
1045 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
1046 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
1047 Bulletins</a> contains official publications of the International
1048 Earth Rotation and Reference Systems Service, which decides when leap
1049 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
1050 via an optional "<code>right</code>" configuration where a computer's internal
1051 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
1052 as opposed to the default "<code>posix</code>" configuration
1053 where the internal clock ignores leap seconds.
1054 The two configurations agree for timestamps starting with 1972-01-01 00:00:00
1055 <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for
1056 timestamps starting with <code>time_t</code> 78 796 800,
1057 which corresponds to the first leap second
1058 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
1059 and to
1060 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
1061 In practice the two configurations also agree for timestamps before
1062 1972 even though the historical situation is messy, partly because
1063 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
1064 is well-defined for sufficiently-old timestamps.</li>
1065 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1066 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1067 leap second so that they disagree with <abbr>UTC</abbr> by at most a
1068 half second, even though every <abbr>POSIX</abbr> minute has exactly
1069 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1070 "<code>posix</code>" configuration, is <a
1071 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
1072 the <abbr>NTP</abbr> reference implementation, and is used by major
1073 cloud service providers. However, according to
1074 <a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">§3.7.1 of
1075 Network Time Protocol Best Current Practices</a>
1076 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1077 applications requiring accurate <abbr>UTC</abbr> or civil time,
1078 and is intended for use only in single, well-controlled environments.</li>
1079 <li>The <a
1080 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
1081 Second Discussion List</a> covers <a
1082 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
1083 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
1084 discussed further in
1085 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1086 leap second: its history and possible future</a>.
1087 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1088 might be redefined
1089 without Leap Seconds</a> gives pointers on this
1090 contentious issue, which was active until 2015 and could become active
1091 again.</li>
1092 </ul>
1093 </section>
1094
1095 <section>
1096 <h2 id="notation">Time notation</h2>
1097 <ul>
1098 <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
1099 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1100 zone names, abbreviations, identifiers, and formats. For example, it
1101 contains French translations for "Eastern European Summer Time",
1102 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
1103 "Bucharest". Its
1104 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1105 charts</a> show these values for many locales. Data values are available in
1106 both <abbr title="Locale Data Markup Language">LDML</abbr>
1107 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1108 <li>
1109 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1110 the international standard date and time notation</a> covers
1111 <a
1112 href="https://www.iso.org/standard/70907.html"><em><abbr
1113 title="International Organization for Standardization">ISO</abbr>
1114 8601-1:2019 – Date and time – Representations for information
1115 interchange – Part 1: Basic rules</em></a>.</li>
1116 <li>
1117 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1118 Schema: Datatypes – dateTime</a> specifies a format inspired by
1119 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1120 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">§3.3 of
1121 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1122 specifies the time notation used in email and <a
1123 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1124 headers.</li>
1125 <li>
1126 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
1127 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1128 specifies an <abbr>ISO</abbr> 8601
1129 profile for use in new Internet
1130 protocols.</li>
1131 <li>
1132 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1133 Formats on the Web</a> surveys web- and Internet-oriented date and time
1134 formats.</li>
1135 <li>Alphabetic time zone abbreviations should not be used as unique
1136 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1137 practice. For example, in English-speaking North America
1138 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
1139 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1140 and French-speaking North Americans prefer
1141 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
1142 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
1143 database contains English abbreviations for many timestamps;
1144 unfortunately some of these abbreviations were merely the database maintainers'
1145 inventions, and these have been removed when possible.</li>
1146 <li>Numeric time zone abbreviations typically count hours east of
1147 <abbr>UT</abbr>, e.g., +09 for Japan and
1148 −10 for Hawaii. However, the <abbr>POSIX</abbr>
1149 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
1150 For example, one might use <code><abbr>TZ</abbr>="<abbr
1151 title="Japan Standard Time">JST</abbr>-9"</code> and
1152 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1153 for Japan and Hawaii, respectively. If the
1154 <code><abbr>tz</abbr></code> database is available, it is usually better to use
1155 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1156 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1157 confusion, handle old timestamps better, and insulate you better from
1158 any future changes to the rules. One should never set
1159 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1160 <code>"GMT-9"</code>, though, since this would incorrectly imply that
1161 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1162 is called "<abbr>GMT</abbr>".</li>
1163 </ul>
1164 </section>
1165
1166 <section>
1167 <h2 id="see-also">See also</h2>
1168 <ul>
1169 <li><a href="theory.html">Theory and pragmatics of the
1170 <code><abbr>tz</abbr></code> code and data</a></li>
1171 <li><a href="tz-art.html">Time and the Arts</a></li>
1172 </ul>
1173 </section>
1174
1175 <footer>
1176 <hr>
1177 This web page is in the public domain, so clarified as of
1178 2009-05-17 by Arthur David Olson.
1179 <br>
1180 Please send corrections to this web page to the
1181 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1182 </footer>
1183 </body>
1184 </html>
1185