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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 2016h, each release has contained a text file named
171 "<samp>version</samp>" whose first (and currently only) line is the version.
172 Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
173 and are also available in an
174 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
175 title="File Transfer Protocol">FTP</abbr> directory</a> via a
176 less-secure protocol.</p>
177 <p>Alternatively, a development repository of code and data can be
178 retrieved from <a href="https://github.com">GitHub</a> via the shell
179 command:</p>
180 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a>
181 </code></pre>
182 <p>
183 Since version 2012e, each release has been tagged in development repositories.
184 Untagged commits are less well tested and probably contain
185 more errors.</p>
186 <p>
187 After obtaining the code and data files, see the
188 <code>README</code> file for what to do next.
189 The code lets you compile the <code><abbr>tz</abbr></code> source files into
190 machine-readable binary files, one for each location. The binary files
191 are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
192 specified by <a href="https://tools.ietf.org/html/8536">Internet
193 <abbr>RFC</abbr> 8536</a>.
194 The code also lets
195 you read a <abbr>TZif</abbr> file and interpret timestamps for that
196 location.</p>
197 </section>
198
199 <section>
200 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
201 <p>
202 The <code><abbr>tz</abbr></code> code and data
203 are by no means authoritative. If you find errors, please
204 send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
205 the time zone mailing list. You can also <a
206 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
207 and browse the <a
208 href="https://mm.icann.org/pipermail/tz/">archive of old
209 messages</a>.
210 <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
211 discussions</a> and corresponding data changes can be
212 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
213 </p>
214 <p>
215 If your government plans to change its time zone boundaries or
216 daylight saving rules, inform <code>tz (a] iana.org</code> well in
217 advance, as this will coordinate updates to many cell phones,
218 computers, and other devices around the world.
219 The change should be officially announced at least a year before it affects
220 how clocks operate; otherwise, there is a good chance that some
221 clocks will operate incorrectly after the change, due
222 to delays in propagating updates to software and data. The shorter
223 the notice, the more likely clock problems will arise; see "<a
224 href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
225 the Timing of Time Zone Changes</a>" for examples.
226 The <code><abbr>tz</abbr></code> data can represent planned changes
227 far into the future, and a long-planned change can easily be reverted
228 or otherwise altered with a year's notice before the change would have
229 affected clocks.
230 </p>
231 <p>
232 Changes to the <code><abbr>tz</abbr></code> code and data are often
233 propagated to clients via operating system updates, so
234 client <code><abbr>tz</abbr></code> data can often be corrected by
235 applying these updates. With GNU/Linux and similar systems, if your
236 maintenance provider has not yet adopted the
237 latest <code><abbr>tz</abbr></code> data, you can often short-circuit
238 the process by tailoring the generic instructions in
239 the <code><abbr>tz</abbr> README</code> file and installing the latest
240 data yourself. System-specific instructions for installing the
241 latest <code><abbr>tz</abbr></code> data have also been published
242 for <a href="https://www.ibm.com/developerworks/aix/library/au-aix-olson-time-zone/index.html"><abbr>AIX</abbr></a>,
243 <a
244 href="https://play.google.com/store/apps/details?id=com.google.android.timezone.data">Android</a>,
245 <a
246 href="http://userguide.icu-project.org/datetime/timezone"><abbr
247 title="International Components for Unicode">ICU</abbr></a>,
248 <a href="https://developer.ibm.com/javasdk/support/dst/jtzu/"><abbr>IBM</abbr></a>
249 and <a
250 href="https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html">Oracle</a>
251 Java, <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
252 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
253 and <a
254 href="https://nodatime.org/userguide/tzdb">Noda Time</a> (see below).
255 </p>
256 <p>Sources for the <code><abbr>tz</abbr></code> database are
257 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
258 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
259 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
260 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
261 title="linefeed">LF</abbr></a>,
262 which can be modified by common text editors such
263 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
264 <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
265 <a href="https://www.vim.org">vim</a>.
266 Specialized source-file editing can be done via the
267 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
268 zoneinfo</a> package for <a
269 href="https://www.sublimetext.com">Sublime Text</a> and the <a
270 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
271 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
272 Studio Code</a>.
273 </p>
274 <p>
275 For further information about updates, please see
276 <a href="https://tools.ietf.org/html/rfc6557">Procedures for
277 Maintaining the Time Zone Database</a> (Internet <abbr
278 title="Request For Comments">RFC</abbr> 6557). More detail can be
279 found in <a href="theory.html">Theory and pragmatics of the
280 <code><abbr>tz</abbr></code> code and data</a>.
281 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
282 displays changes between recent <code><abbr>tzdb</abbr></code> versions.
283 </p>
284 </section>
285
286 <section>
287 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
288 <ul>
289 <li>The article
290 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
291 an encyclopedic summary.</li>
292 <li><a href="tz-how-to.html">How to Read the
293 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
294 database format.</li>
295 <li><a
296 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
297 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
298 database's style.</li>
299 </ul>
300 </section>
301
302 <section>
303 <h2 id="web">Web sites using recent versions of the
304 <code><abbr>tz</abbr></code> database</h2>
305 <p>
306 These are listed roughly in ascending order of complexity and fanciness.
307 </p>
308 <ul>
309 <li><a href="https://time.is">Time.is</a> shows locations'
310 time and zones.</li>
311 <li><a href="https://www.timejones.com">TimeJones.com</a>,
312 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
313 <a href="https://www.worldclock.com">The World Clock</a>
314 are time zone converters.</li>
315 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
316 publishes <code><abbr>tzdb</abbr></code>-derived data in
317 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
318 title="comma-separated values">CSV</abbr></a> and
319 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
320 title="Structured Query Language">SQL</abbr></a> form.</li>
321 <li><a
322 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
323 lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
324 <li><a
325 href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
326 Time in 1000 Places</a> uses descriptions of the values.</li>
327 <li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
328 uses a pulldown menu.</li>
329 <li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
330 timezone information for all countries</a> displays tables of DST rules.
331 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
332 Worldwide</a> lets you sort zone names and convert times.</li>
333 <li><a href="https://24timezones.com">24TimeZones</a> has a world
334 time map and a time converter.</li>
335 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
336 calculates the current time difference between locations.</li>
337 <li><a href="http://www.wx-now.com">Weather Now</a> and
338 <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
339 </ul>
340 </section>
341
342 <section>
343 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
344 <ul>
345 <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
346 <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
347 Distribution Service (tzdist) working group</a> defined <a
348 href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
349 (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
350 along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
351 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
352 transferring time zone data by reference.
353 <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
354 implementations</a> are available.
355 The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
356 mailing list</a> discussed the Internet draft <a
357 id="TZDIST-Geolocate"
358 href="https://tools.ietf.org/html/draft-murchison-tzdist-geolocate">TZDIST
359 Geolocate Extension</a> (now expired), which let a client determine its timezone
360 from its geographic location using a <a
361 href="https://tools.ietf.org/html/rfc5870">'geo' URI</a>.</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 API <a
415 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">adopted
416 into the draft standard</a> for
417 <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
418 the next 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/technetwork/java/javase/tzupdater-readme-136440.html">TZUpdater
440 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
441 Oracle Java.</li>
442 <li>The <a
443 href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
444 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
445 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
446 which is freely available under a <abbr>BSD</abbr>-style license.</li>
447 <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
448 and time <abbr>API</abbr></a> contains a class
449 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
450 <code><abbr>tz</abbr></code> source into a binary format. It inspired
451 Java 8 <code>java.time</code>, which its users should migrate to once
452 they can assume Java 8 or later. It is available under the <a
453 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
454 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a>,
455 <a href="https://github.com/akashche/tzdbgen">tzdbgen</a>, and <a
456 href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
457 are other alternatives to TZUpdater. IANA Updater's license is unclear;
458 the others are licensed under the <abbr>GPL</abbr>.</li>
459 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
460 time library for Android</a> and
461 <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
462 time and interval library for Java</a> compile
463 <code><abbr>tz</abbr></code> source into a binary format.
464 Time4A is available under the Apache License and Time4J is
465 available under the <a
466 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
467 General Public License (<abbr title="Lesser General Public
468 License">LGPL</abbr>)</a>.</li>
469 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
470 Java-based libraries.</li>
471 </ul>
472 <li><a href="https://nodatime.org">Noda Time – Date and
473 time <abbr>API</abbr> for .NET</a>
474 is like Joda-Time and Time4J, but for the .NET framework instead of Java.
475 It is freely available under the Apache License.</li>
476 <li>Many modern
477 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
478 runtimes support <code><abbr>tz</abbr></code> natively via the
479 <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>.
480 On other runtimes, you can use one of the following.
481 <ul>
482 <li>The <a
483 href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
484 timezone polyfill</a>
485 is freely available under a <abbr>BSD</abbr>-style license.</li>
486 <li><a
487 href="https://github.com/kshetline/compact-time-zone-generator">CompactTimeZoneGenerator</a>
488 compiles time zone data into a compact form designed for
489 JavaScript. It is freely available under a combination of
490 the <abbr>MIT</abbr> license and the Apache License.</li>
491 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
492 plugin for the <a href="https://momentjs.com">Moment.js</a> date
493 manipulation library. It is freely available under the <abbr>MIT</abbr>
494 license.</li>
495 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
496 JavaScript library that supports date arithmetic that is time zone
497 aware. It is freely available under the <abbr>MIT</abbr> license.</li>
498 <li><a href="https://github.com/mde/timezone-js">TimezoneJS.Date</a>'s
499 <abbr>API</abbr> is upward compatible with standard JavaScript
500 Dates. It is freely available under the Apache License.</li>
501 </ul>
502 <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
503 compiler from <code><abbr>tz</abbr></code> source into
504 <a href="https://julialang.org/">Julia</a>. It is freely available
505 under the <abbr>MIT</abbr> license.</li>
506 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
507 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
508 title="Free Pascal Compiler">FPC</abbr></a>
509 compiles from <code><abbr>tz</abbr></code> source into
510 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
511 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
512 and <a
513 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
514 It is freely available under a <abbr>BSD</abbr>-style license.</li>
515 <li><a href="http://pytz.sourceforge.net">pytz – World Timezone
516 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
517 <a href="https://www.python.org">Python</a>.
518 It is freely available under a <abbr>BSD</abbr>-style license.</li>
519 <li><a href="https://tzinfo.github.io">TZInfo –
520 Ruby Timezone Library</a>
521 compiles <code><abbr>tz</abbr></code> source into
522 <a href="https://www.ruby-lang.org/en/">Ruby</a>.
523 It is freely available under the <abbr>MIT</abbr> license.</li>
524 <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
525 Library</a> is
526 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
527 library that compiles <code><abbr>tz</abbr></code> source into a time
528 zone repository whose format
529 is either proprietary or an <abbr>XML</abbr>-encoded
530 representation.</li>
531 <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
532 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
533 source into text files, along with a runtime that can read those
534 files. Tcl is freely available under a <abbr>BSD</abbr>-style
535 license.</li>
536 </ul>
537 </section>
538
539 <section>
540 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
541 <ul>
542 <li>The <a
543 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
544 Library</a>
545 has an independent, thread-safe implementation of
546 a <abbr>TZif</abbr> file reader.
547 This library is freely available under the LGPL
548 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
549 <li><a href="https://www.gnome.org">GNOME</a>'s
550 <a href="https://developer.gnome.org/glib/">GLib</a> has
551 a <abbr>TZif</abbr> file reader written in C that
552 creates a <code>GTimeZone</code> object representing sets
553 of <abbr>UT</abbr> offsets.
554 It is freely available under the <abbr>LGPL</abbr>.</li>
555 <li>The
556 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
557 <code>baltzo::TimeZoneUtil</code> component contains a C++
558 implementation of a <abbr>TZif</abbr> file reader. It is freely available under
559 the Apache License.</li>
560 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
561 library that translates between <abbr>UT</abbr> and civil time and
562 can read <abbr>TZif</abbr> files. It is freely available under the Apache
563 License.</li>
564 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
565 is a <abbr>TZif</abbr> file reader written in Java.
566 It is freely available under the <abbr>LGPL</abbr>.</li>
567 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
568 library that reads <abbr>TZif</abbr> files and converts
569 timestamps from one time zone or format to another.
570 It is used by <a href="https://secure.php.net"><abbr
571 title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
572 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
573 and <a href="https://www.mongodb.com">MongoDB</a>.
574 It is freely available under the <abbr>MIT</abbr> license.</li>
575 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
576 <abbr>TZif</abbr> file reader.</li>
577 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
578 DateTime::TimeZone::Tzfile</a>
579 is a <abbr>TZif</abbr> file reader written in Perl.
580 It is freely available under the same terms as Perl
581 (dual <abbr>GPL</abbr> and Artistic license).</li>
582 <li>Python has a <a
583 href="https://docs.python.org/3.9/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
584 class</a> that reads <abbr>TZif</abbr> data and creates objects
585 that represent <code><abbr>tzdb</abbr></code> timezones.
586 Python is freely available under the
587 <a href="https://docs.python.org/3.9/license.html">Python Software Foundation
588 License</a>.
589 A companion <a href="https://pypi.org/">PyPI</a> module
590 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
591 supplies TZif data if the underlying system data cannot be found;
592 it is freely available under the Apache License.</li>
593 <li>The
594 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
595 library contains a Python tool that
596 converts <abbr>TZif</abbr> data into
597 <abbr>JSON</abbr>-format data suitable for use
598 in its JavaScript library for time zone conversion. Dates before 1970
599 are not supported.</li>
600 <li>The <a
601 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
602 package contains <a href="https://www.haskell.org">Haskell</a> code that
603 parses and uses <abbr>TZif</abbr> data. It is freely
604 available under a <abbr>BSD</abbr>-style license.</li>
605 </ul>
606 </section>
607
608 <section>
609 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
610 <ul>
611 <li><a href="https://foxclocks.org">FoxClocks</a>
612 is an extension for <a href="https://www.google.com/chrome/">Google
613 Chrome</a> and for <a
614 href="https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Toolkit_API">Mozilla
615 Toolkit</a> applications like <a
616 href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
617 href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
618 It displays multiple clocks in the application window, and has a mapping
619 interface to <a href="https://www.google.com/earth/">Google Earth</a>.
620 It is freely available under the <abbr>GPL</abbr>.</li>
621 <li><a href="https://golang.org">Go programming language</a>
622 implementations contain a copy of a 32-bit subset of a recent
623 <code><abbr>tz</abbr></code> database in a
624 Go-specific format.</li>
625 <li><a
626 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
627 clock (intclock)</a> is a clock that displays multiple time zones on
628 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
629 under the <abbr>GPL</abbr>.</li>
630 <li>Microsoft Windows 8.1
631 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
632 data (mentioned <a href="#CLDR">below</a>) used by the
633 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
634 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
635 <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
636 <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
637 <a id="System.TimeZoneInfo"
638 href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
639 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
640 the older, proprietary method of Microsoft Windows 2000 and later,
641 which stores time zone data in the
642 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
643 <a
644 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
645 Tzid table</a> or <a
646 href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
647 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
648 to <code><abbr>tz</abbr></code> names.
649 These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
650 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
651 <li><a
652 href="https://www.oracle.com/java/index.html">Oracle
653 Java</a> contains a copy of a subset of a recent
654 <code><abbr>tz</abbr></code> database in a
655 Java-specific format.</li>
656 <li><a href="https://relativedata.com/page/Time-Zone-Master">Time Zone
657 Master</a> is a Microsoft Windows clock program that can automatically
658 download, compile and use <code><abbr>tz</abbr></code> releases.
659 The Basic version is free.</li>
660 <li><a
661 href="http://veladg.com/velaterra.html">VelaTerra</a> is
662 a macOS program. Its developers
663 <a href="http://veladg.com/tzoffer.html">offer free
664 licenses</a> to <code><abbr>tz</abbr></code> contributors.</li>
665 </ul>
666 </section>
667
668 <section>
669 <h2 id="other-dbs">Other time zone databases</h2>
670 <ul>
671 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
672 is Astrodienst's Web version of Shanks and Pottenger's out-of-print
673 time zone history atlases
674 <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
675 <a href="https://www.worldcat.org/oclc/76950459">for the world</a>,
676 now published in <a
677 href="https://astrocom.com/astrology-products/software/acs-atlas-software">software</a>
678 form by <a href="https://astrocom.com">ACS-Starcrafts</a>.
679 Although these extensive atlases
680 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
681 sources for much of the older <code><abbr>tz</abbr></code> data</a>,
682 they are unreliable as Shanks appears to have
683 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
684 sources and do not indicate which entries are guesswork.</li>
685 <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
686 its own <code>tztab</code>(4) format.</li>
687 <li>Microsoft Windows has proprietary data mentioned
688 <a href="#System.TimeZoneInfo">above</a>.</li>
689 <li><a href="https://www.worldtimeserver.com">World Time Server</a>
690 is another time zone database.</li>
691 <li>The <a
692 href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
693 Schedules Information Manual</a> of the
694 International Air Transport Association
695 gives current time zone rules for airports served by commercial aviation.</li>
696 </ul>
697 </section>
698
699 <section>
700 <h2 id="maps">Maps</h2>
701 <ul>
702 <li>The <a href="https://www.cia.gov/index.html">United States Central
703 Intelligence Agency (<abbr
704 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
705 href="https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf">time
706 zone map</a>; the
707 <a
708 href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda
709 Library Map Collection</a>
710 of the University of Texas at Austin has copies of
711 recent editions.
712 The pictorial quality is good,
713 but the maps do not indicate daylight saving time,
714 and parts of the data are a few years out of date.</li>
715 <li><a href="https://www.worldtimezone.com">World Time Zone Map
716 with current time</a>
717 has several fancy time zone maps; it covers Russia particularly well.
718 The maps' pictorial quality is not quite as good as the
719 <abbr>CIA</abbr>'s
720 but the maps are more up to date.</li>
721 <li><a
722 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
723 much is time wrong around the world?</a> maps the difference between
724 mean solar and standard time, highlighting areas such as western China
725 where the two differ greatly. It's a bit out of date, unfortunately.</li>
726 </ul>
727 </section>
728
729 <section>
730 <h2 id="boundaries">Time zone boundaries</h2>
731 <p>Geographical boundaries between timezones are available
732 from several <a href="https://en.wikipedia.org/wiki/Geolocation">geolocation</a>
733 services and other sources.</p>
734 <ul>
735 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
736 Boundary Builder</a> extracts
737 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
738 boundaries of <code><abbr>tzdb</abbr></code> timezones.
739 Its code is freely available under the <abbr>MIT</abbr> license, and
740 its data entries are freely available under the
741 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
742 Open Database License</a>. The maps' borders appear to be quite accurate.</li>
743 <li>Programmatic interfaces that map geographical coordinates via tz_world to
744 <code><abbr>tzdb</abbr></code> timezones include:
745 <ul>
746 <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
747 written in <a
748 href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
749 and is freely available under the <abbr>MIT</abbr> license.</li>
750 <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
751 is written in Go and is freely available under the Apache License.</li>
752 <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
753 in both Java and
754 <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
755 form, is freely available under the MIT license.</li>
756 <li>For <a href="https://nodejs.org/en/">Node.js</a>,
757 the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
758 is freely available under the MIT license, and
759 the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
760 is in the public domain.</li>
761 <li>The <a
762 href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
763 library for Python is freely available under the MIT license.
764 <li>The <a
765 href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
766 library for Ruby is freely available under the MIT license.</li>
767 </ul></li>
768 <li>Free access via a network API, if you register a key, is provided by
769 the <a
770 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
771 Timezone web service</a>, the <a
772 href="https://developers.google.com/maps/documentation/timezone/intro">Google
773 Maps Time Zone API</a>, and
774 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
775 Commercial network API access is provided
776 by <a href="https://askgeo.com">AskGeo</a>
777 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
778 </li>
779 <li>"<a
780 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
781 to get a time zone from a location using latitude and longitude
782 coordinates?</a>" discusses other geolocation possibilities.</li>
783 <li><a href="http://statoids.com/statoids.html">Administrative
784 Divisions of Countries ("Statoids")</a> lists
785 political subdivision data related to time zones.</li>
786 <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
787 zone boundaries for multizone countries</a> summarizes legal
788 boundaries between time zones within countries.</li>
789 <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
790 – GIS and Database Tools</a> includes a Manifold-format map of
791 world time zone boundaries distributed under the
792 <abbr>GPL</abbr>.</li>
793 <li>A ship within the <a
794 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
795 waters</a> of any nation uses that nation's time. In international
796 waters, time zone boundaries are meridians 15° apart, except that
797 <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
798 wide and are separated by
799 the 180° meridian (not by the International Date Line, which is
800 for land and territorial waters only). A captain can change ship's
801 clocks any time after entering a new time zone; midnight changes are
802 common.</li>
803 </ul>
804 </section>
805
806 <section>
807 <h2 id="civil">Civil time concepts and history</h2>
808 <ul>
809 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
810 Walk through Time</a>
811 surveys the evolution of timekeeping.</li>
812 <li>The history of daylight saving time is surveyed in <a
813 href="http://www.webexhibits.org/daylightsaving/">About Daylight
814 Saving Time – History, rationale, laws & dates</a> and summarized in
815 <a href="http://seizethedaylight.com/dst/">A Brief
816 History of Daylight Saving Time</a>.</li>
817 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
818 Lords</a> discusses how authoritarians manipulate civil time.</li>
819 <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
820 contains guidelines and best practices for software applications that
821 deal with civil time.</li>
822 <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
823 the International Date Line</a> tells the story of the most important
824 time zone boundary.</li>
825 <li><a href="http://statoids.com/tconcept.html">Basic Time
826 Zone Concepts</a> discusses terminological issues behind time zones.</li>
827 </ul>
828 </section>
829
830 <section>
831 <h2 id="national">National histories of legal time</h2>
832 <dl>
833 <dt>Australia</dt>
834 <dd>The Parliamentary Library commissioned a <a
835 href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
836 paper on daylight saving time in Australia</a>.
837 The Bureau of Meteorology publishes a list of <a
838 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
839 Dates of Daylight Savings Time within Australia</a>.</dd>
840 <dt>Belgium</dt>
841 <dd>The Royal Observatory of Belgium maintains a table of time in
842 Belgium (in
843 <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
844 hreflang="nl">Dutch</a> and <a
845 href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
846 hreflang="fr">French</a>).</dd>
847 <dt>Brazil</dt>
848 <dd>The Time Service Department of the National Observatory
849 records <a href="http://pcdsh01.on.br/DecHV.html"
850 hreflang="pt-BR">Brazil's daylight saving time decrees (in
851 Portuguese)</a>.</dd>
852 <dt>Canada</dt>
853 <dd>National Research Council Canada publishes current
854 and some older information about <a
855 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
856 zones and daylight saving time</a>.</dd>
857 <dt>Chile</dt>
858 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
859 <a href="https://www.horaoficial.cl/historia_hora.html" hreflang="es">history of
860 Chile's official time (in Spanish)</a>.</dd>
861 <dt>China</dt>
862 <dd>The Hong Kong Observatory maintains a
863 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
864 summer time in Hong Kong</a>,
865 and Macau's Meteorological and Geophysical Bureau maintains a <a
866 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
867 history for Macau</a>.
868 Unfortunately the latter is incomplete and has errors.</dd>
869 <dt>Czech Republic</dt>
870 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
871 hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
872 summarizes and cites historical DST regulations.</dd>
873 <dt>Germany</dt>
874 <dd>The National Institute for Science and Technology maintains the <a
875 href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
876 of Legal Time in Germany</a>.</dd>
877 <dt>Israel</dt>
878 <dd>The Interior Ministry periodically issues <a
879 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
880 hreflang="he">announcements (in Hebrew)</a>.</dd>
881 <dt>Italy</dt>
882 <dd>The National Institute of Metrological Research publishes a
883 <a href="http://oldsite.inrim.it/res/tf/ora_legale_i.shtml" hreflang="it">table
884 of civil time (in Italian)</a>.</dd>
885 <dt>Malaysia</dt>
886 <dd>See Singapore <a href="#Singapore">below</a>.</dd>
887 <dt>Mexico</dt>
888 <dd>The Investigation and Analysis Service of the Mexican Library of
889 Congress has published a <a
890 href="http://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
891 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
892 <dt>Netherlands</dt>
893 <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
894 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
895 covers the history of local time in the Netherlands from ancient times.</dd>
896 <dt>New Zealand</dt>
897 <dd>The Department of Internal Affairs maintains a brief <a
898 href="https://www.dia.govt.nz/Daylight-Saving-History">History of
899 Daylight Saving</a>.</dd>
900 <dt>Singapore</dt>
901 <dd><a id="Singapore"
902 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
903 is Singapore in the "Wrong" Time Zone?</a> details the
904 history of legal time in Singapore and Malaysia.</dd>
905 <dt>United Kingdom</dt>
906 <dd><a
907 href="https://www.polyomino.org.uk/british-time/">History of
908 legal time in Britain</a> discusses in detail the country
909 with perhaps the best-documented history of clock adjustments.</dd>
910 <dt>United States</dt>
911 <dd>The Department of Transportation's <a
912 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
913 Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
914 <dt>Uruguay</dt>
915 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
916 Navy (SOHMA) publishes an annual <a
917 href="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
918 (in Spanish)</a>.</dd>
919 </dl>
920 </section>
921
922 <section>
923 <h2 id="costs">Costs and benefits of time shifts</h2>
924 <p>Various sources argue for and against daylight saving time and time
925 zone shifts, and many scientific studies have been conducted. This
926 section summarizes reviews and position statements based on
927 scientific literature in the area.</p>
928 <ul>
929 <li>Carey RN, Sarma KM.
930 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
931 daylight saving time on road traffic collision risk: a systematic
932 review</a>.
933 <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>.
934 This reviews research literature and concludes that the evidence
935 neither supports nor refutes road safety benefits from
936 shifts in time zones.</li>
937 <li>Havranek T, Herman D, Irsova D.
938 <a href="https://www.econstor.eu/handle/10419/174191">Does daylight
939 saving save electricity? A meta-analysis</a>. <em>Energy J.</em>
940 2018;39(2).
941 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
942 This analyzes research literature and concludes, "Electricity savings
943 are larger for countries farther away from the equator, while
944 subtropical regions consume more electricity because of DST."</li>
945 <li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
946 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
947 an American Academy of Sleep Medicine position statement</a>.
948 <em>J Clin Sleep Med.</em>
949 2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
950 This argues for permanent standard time due to health risks of both
951 DST transitions and permanent DST.</li>
952 <li>Roenneberg T, Winnebeck EC, Klerman EB.
953 <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
954 saving time and artificial time zones – a battle between
955 biological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
956 doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
957 This reviews evidence about the health effects of DST and concludes,
958 "In summary, the scientific literature strongly argues against the
959 switching between DST and Standard Time and even more so against
960 adopting DST permanently."</li>
961 </ul>
962 </section>
963
964 <section>
965 <h2 id="precision">Precision timekeeping</h2>
966 <ul>
967 <li><a
968 href="http://leapsecond.com/hpan/an1289.pdf">The
969 Science of Timekeeping</a> is a thorough introduction
970 to the theory and practice of precision timekeeping.</li>
971 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
972 Time 2016</a> contains several freely-readable papers.</li>
973 <li><a href="http://www.ntp.org"><abbr
974 title="Network Time Protocol">NTP</abbr>: The Network
975 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
976 discusses how to synchronize clocks of
977 Internet hosts.</li>
978 <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
979 family of software algorithms can achieve accuracy to a few tens of
980 nanoseconds in scalable server farms without special hardware.</li>
981 <li>The <a
982 href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
983 Time Protocol</a> (<abbr
984 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
985 can achieve submicrosecond clock accuracy on a local area network
986 with special-purpose hardware.</li>
987 <li><a
988 href="https://tools.ietf.org/html/rfc4833">Timezone
989 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
990 (Internet <abbr>RFC</abbr> 4833)
991 specifies a <a
992 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
993 option for a server to configure
994 a client's time zone and daylight saving settings automatically.</li>
995 <li><a
996 href="https://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
997 Times</a> explains more abstruse astronomical time scales like
998 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
999 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
1000 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
1001 <a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
1002 Scales</a> goes into more detail, particularly for historical variants.</li>
1003 <li>The <a href="https://www.iau.org"><abbr
1004 title="International Astronomical Union">IAU</abbr></a>'s <a
1005 href="http://www.iausofa.org"><abbr
1006 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
1007 collection contains C and <a
1008 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
1009 code for converting among time scales like
1010 <abbr title="International Atomic Time">TAI</abbr>,
1011 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
1012 <abbr>UTC</abbr>.</li>
1013 <li><a
1014 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
1015 – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
1016 diverse local time
1017 scales used by each landed mission on Mars.</li>
1018 <li><a href="http://leapsecond.com">LeapSecond.com</a> is
1019 dedicated not only to leap seconds but to precise time and frequency
1020 in general. It covers the state of the art in amateur timekeeping, and
1021 how the art has progressed over the past few decades.</li>
1022 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
1023 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
1024 and time-signal emissions</a>, International Telecommunication Union –
1025 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
1026 <li><a
1027 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
1028 title="International Earth Rotation and Reference Systems Service">IERS</abbr>
1029 Bulletins</a> contains official publications of the International
1030 Earth Rotation and Reference Systems Service, which decides when leap
1031 seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
1032 via an optional "<code>right</code>" configuration, as opposed to the
1033 default "<code>posix</code>" configuration.</li>
1034 <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1035 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1036 leap second so that they disagree with <abbr>UTC</abbr> by at most a
1037 half second, even though every <abbr>POSIX</abbr> minute has exactly
1038 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1039 "<code>posix</code>" configuration, is <a
1040 href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
1041 the <abbr>NTP</abbr> reference implementation, and is used by major
1042 cloud service providers. However, according to
1043 <a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">§3.7.1 of
1044 Network Time Protocol Best Current Practices</a>
1045 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1046 applications requiring accurate UTC or civil time,
1047 and is intended for use only in single, well-controlled environments.</li>
1048 <li>The <a
1049 href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
1050 Second Discussion List</a> covers <a
1051 href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
1052 and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
1053 discussed further in
1054 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1055 leap second: its history and possible future</a>.
1056 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1057 might be redefined
1058 without Leap Seconds</a> gives pointers on this
1059 contentious issue, which was active until 2015 and could become active
1060 again.</li>
1061 </ul>
1062 </section>
1063
1064 <section>
1065 <h2 id="notation">Time notation</h2>
1066 <ul>
1067 <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
1068 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1069 zone names, abbreviations, identifiers, and formats. For example, it
1070 contains French translations for "Eastern European Summer Time",
1071 "<abbr title="Eastern European Summer Time">EEST</abbr>", and
1072 "Bucharest". Its
1073 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1074 charts</a> show these values for many locales. Data values are available in
1075 both <abbr title="Locale Data Markup Language">LDML</abbr>
1076 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1077 <li>
1078 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1079 the international standard date and time notation</a> covers
1080 <a
1081 href="https://www.iso.org/standard/70907.html"><em><abbr
1082 title="International Organization for Standardization">ISO</abbr>
1083 8601-1:2019 – Date and time – Representations for information
1084 interchange – Part 1: Basic rules</em></a>.</li>
1085 <li>
1086 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1087 Schema: Datatypes – dateTime</a> specifies a format inspired by
1088 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1089 <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">§3.3 of
1090 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1091 specifies the time notation used in email and <a
1092 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1093 headers.</li>
1094 <li>
1095 <a href="https://tools.ietf.org/html/rfc3339">Date and Time
1096 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1097 specifies an <abbr>ISO</abbr> 8601
1098 profile for use in new Internet
1099 protocols.</li>
1100 <li>
1101 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1102 Formats on the Web</a> surveys web- and Internet-oriented date and time
1103 formats.</li>
1104 <li>Alphabetic time zone abbreviations should not be used as unique
1105 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1106 practice. For example, in English-speaking North America
1107 "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
1108 but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1109 and French-speaking North Americans prefer
1110 "<abbr title="Heure Normale du Centre">HNC</abbr>" to
1111 "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
1112 database contains English abbreviations for many timestamps;
1113 unfortunately some of these abbreviations were merely the database maintainers'
1114 inventions, and these have been removed when possible.</li>
1115 <li>Numeric time zone abbreviations typically count hours east of
1116 <abbr>UT</abbr>, e.g., +09 for Japan and
1117 −10 for Hawaii. However, the <abbr>POSIX</abbr>
1118 <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
1119 For example, one might use <code><abbr>TZ</abbr>="<abbr
1120 title="Japan Standard Time">JST</abbr>-9"</code> and
1121 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1122 for Japan and Hawaii, respectively. If the
1123 <code><abbr>tz</abbr></code> database is available, it is usually better to use
1124 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1125 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1126 confusion, handle old timestamps better, and insulate you better from
1127 any future changes to the rules. One should never set
1128 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1129 <code>"GMT-9"</code>, though, since this would incorrectly imply that
1130 local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1131 is called "<abbr>GMT</abbr>".</li>
1132 </ul>
1133 </section>
1134
1135 <section>
1136 <h2 id="see-also">See also</h2>
1137 <ul>
1138 <li><a href="theory.html">Theory and pragmatics of the
1139 <code><abbr>tz</abbr></code> code and data</a></li>
1140 <li><a href="tz-art.html">Time and the Arts</a></li>
1141 </ul>
1142 </section>
1143
1144 <footer>
1145 <hr>
1146 This web page is in the public domain, so clarified as of
1147 2009-05-17 by Arthur David Olson.
1148 <br>
1149 Please send corrections to this web page to the
1150 <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1151 </footer>
1152 </body>
1153 </html>
1154