tz-link.html revision 1.15 1 1.1 christos <!DOCTYPE html>
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3 1.1 christos <head>
4 1.13 christos <title>Time zone and daylight saving time data</title>
5 1.1 christos <meta charset="UTF-8">
6 1.2 christos <style>
7 1.2 christos pre {margin-left: 2em; white-space: pre-wrap;}
8 1.2 christos </style>
9 1.1 christos </head>
10 1.1 christos <body>
11 1.13 christos <h1>Time zone and daylight saving time data</h1>
12 1.1 christos <p>
13 1.1 christos <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
14 1.1 christos <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
15 1.1 christos rules are controlled by individual
16 1.1 christos governments. They are sometimes changed with little notice, and their
17 1.1 christos histories and planned futures are often recorded only fitfully. Here
18 1.1 christos is a summary of attempts to organize and record relevant data in this
19 1.1 christos area.
20 1.1 christos </p>
21 1.6 christos <h3>Outline</h3>
22 1.6 christos <nav>
23 1.6 christos <ul>
24 1.6 christos <li>The <code><abbr>tz</abbr></code> database product and process
25 1.6 christos <ul>
26 1.6 christos <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
27 1.6 christos <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
28 1.6 christos <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
29 1.14 christos <li><a href="#coordinating">Coordinating with governments and distributors</a></li>
30 1.6 christos <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
31 1.6 christos </ul>
32 1.6 christos </li>
33 1.6 christos <li>Uses of the <code><abbr>tz</abbr></code> database
34 1.6 christos <ul>
35 1.6 christos <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
36 1.6 christos <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
37 1.6 christos <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
38 1.6 christos <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
39 1.6 christos <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
40 1.6 christos </ul>
41 1.6 christos </li>
42 1.6 christos <li>Related data
43 1.6 christos <ul>
44 1.6 christos <li><a href="#other-dbs">Other time zone databases</a></li>
45 1.6 christos <li><a href="#maps">Maps</a></li>
46 1.6 christos <li><a href="#boundaries">Time zone boundaries</a></li>
47 1.6 christos </ul>
48 1.6 christos </li>
49 1.6 christos <li>Timekeeping concepts
50 1.6 christos <ul>
51 1.6 christos <li><a href="#civil">Civil time concepts and history</a></li>
52 1.6 christos <li><a href="#national">National histories of legal time</a></li>
53 1.7 christos <li><a href="#costs">Costs and benefits of time shifts</a></li>
54 1.6 christos <li><a href="#precision">Precision timekeeping</a></li>
55 1.6 christos <li><a href="#notation">Time notation</a></li>
56 1.6 christos </ul>
57 1.6 christos </li>
58 1.13 christos <li><a href="#see-also">See also</a></li>
59 1.6 christos </ul>
60 1.6 christos </nav>
61 1.6 christos
62 1.6 christos <section>
63 1.1 christos <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
64 1.1 christos <p>
65 1.1 christos The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
66 1.1 christos time zone database contains code and data
67 1.1 christos that represent the history of local time
68 1.1 christos for many representative locations around the globe.
69 1.1 christos It is updated periodically to reflect changes made by political bodies
70 1.1 christos to time zone boundaries and daylight saving rules.
71 1.1 christos This database (known as <code><abbr>tz</abbr></code>,
72 1.1 christos <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
73 1.1 christos is used by several implementations,
74 1.1 christos including
75 1.1 christos <a href="https://www.gnu.org/software/libc/">the
76 1.1 christos <abbr title="GNU's Not Unix">GNU</abbr>
77 1.1 christos C Library</a> (used in
78 1.1 christos <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
79 1.1 christos <a href="https://www.android.com">Android</a>,
80 1.1 christos <a href="https://www.freebsd.org">Free<abbr
81 1.1 christos title="Berkeley Software Distribution">BSD</abbr></a>,
82 1.1 christos <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
83 1.1 christos <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
84 1.13 christos <a href="https://www.chromium.org/chromium-os/">Chromium OS</a>,
85 1.1 christos <a href="https://cygwin.com">Cygwin</a>,
86 1.7 christos <a href="https://mariadb.org">MariaDB</a>,
87 1.1 christos <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
88 1.1 christos <a href="https://www.mysql.com">MySQL</a>,
89 1.1 christos <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
90 1.1 christos title="Web Operating System">webOS</abbr></a>,
91 1.7 christos <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
92 1.1 christos title="Advanced Interactive eXecutive">AIX</abbr></a>,
93 1.6 christos <a href="https://www.apple.com/ios"><abbr
94 1.1 christos title="iPhone OS">iOS</abbr></a>,
95 1.6 christos <a href="https://www.apple.com/macos">macOS</a>,
96 1.1 christos <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
97 1.6 christos <a href="https://www.vmssoftware.com">Open<abbr
98 1.1 christos title="Virtual Memory System">VMS</abbr></a>,
99 1.6 christos <a href="https://www.oracle.com/database/">Oracle Database</a>, and
100 1.1 christos <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
101 1.1 christos <p>
102 1.2 christos Each main entry in the database represents a <dfn>timezone</dfn>
103 1.2 christos for a set of civil-time clocks that have all agreed since 1970.
104 1.2 christos Timezones are typically identified by continent or ocean and then by the
105 1.2 christos name of the largest city within the region containing the clocks.
106 1.1 christos For example, <code>America/New_York</code>
107 1.1 christos represents most of the <abbr title="United States">US</abbr> eastern time zone;
108 1.1 christos <code>America/Phoenix</code> represents most of Arizona, which
109 1.2 christos uses mountain time without daylight saving time (<abbr>DST</abbr>);
110 1.1 christos <code>America/Detroit</code> represents most of Michigan, which uses
111 1.1 christos eastern time but with different <abbr>DST</abbr> rules in 1975;
112 1.1 christos and other entries represent smaller regions like Starke County,
113 1.1 christos Indiana, which switched from central to eastern time in 1991
114 1.1 christos and switched back in 2006.
115 1.1 christos To use the database on an extended <a
116 1.1 christos href="https://en.wikipedia.org/wiki/POSIX"><abbr
117 1.1 christos title="Portable Operating System Interface">POSIX</abbr></a>
118 1.1 christos implementation set the <code><abbr>TZ</abbr></code>
119 1.1 christos environment variable to the location's full name,
120 1.1 christos e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
121 1.1 christos <p>
122 1.2 christos Associated with each timezone is a history of offsets from
123 1.1 christos <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
124 1.1 christos Time</a> (<abbr>UT</abbr>), which is <a
125 1.1 christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
126 1.1 christos Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
127 1.2 christos for timestamps after 1960 this is more precisely <a
128 1.1 christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
129 1.1 christos Universal Time</a> (<abbr>UTC</abbr>).
130 1.1 christos The database also records when daylight saving time was in use,
131 1.1 christos along with some time zone abbreviations such as <abbr>EST</abbr>
132 1.1 christos for Eastern Standard Time in the <abbr>US</abbr>.</p>
133 1.6 christos </section>
134 1.6 christos
135 1.6 christos <section>
136 1.1 christos <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
137 1.1 christos <p>
138 1.1 christos The following <a
139 1.1 christos href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
140 1.1 christos the latest release's two
141 1.1 christos <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
142 1.1 christos to a <abbr>GNU</abbr>/Linux or similar host.</p>
143 1.2 christos <pre><code>mkdir tzdb
144 1.1 christos cd tzdb
145 1.1 christos <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
146 1.1 christos wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
147 1.1 christos <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 1.1 christos gzip -dc tzdata-latest.tar.gz | tar -xf -
149 1.1 christos </code></pre>
150 1.1 christos <p>Alternatively, the following shell commands download the same
151 1.1 christos release in a single-tarball format containing extra data
152 1.1 christos useful for regression testing:</p>
153 1.2 christos <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 1.1 christos <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
155 1.1 christos </code></pre>
156 1.1 christos <p>These commands use convenience links to the latest release
157 1.1 christos of the <code><abbr>tz</abbr></code> database hosted by the
158 1.1 christos <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
159 1.1 christos of the <a href="https://www.iana.org">Internet Assigned Numbers
160 1.1 christos Authority (IANA)</a>.
161 1.1 christos Older releases are in files named
162 1.1 christos <code>tzcode<var>V</var>.tar.gz</code>,
163 1.1 christos <code>tzdata<var>V</var>.tar.gz</code>, and
164 1.1 christos <code>tzdb-<var>V</var>.tar.lz</code>,
165 1.1 christos where <code><var>V</var></code> is the version.
166 1.1 christos Since 1996, each version has been a four-digit year followed by
167 1.1 christos lower-case letter (<samp>a</samp> through <samp>z</samp>,
168 1.1 christos then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
169 1.1 christos through <samp>zzz</samp>, and so on).
170 1.13 christos Since version 2022a, each release has been distributed in
171 1.10 christos <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
172 1.10 christos ustar interchange format</a>, compressed as described above;
173 1.14 christos older releases use a nearly compatible format.
174 1.1 christos Since version 2016h, each release has contained a text file named
175 1.1 christos "<samp>version</samp>" whose first (and currently only) line is the version.
176 1.7 christos Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
177 1.7 christos and are also available in an
178 1.1 christos <a href="ftp://ftp.iana.org/tz/releases/"><abbr
179 1.1 christos title="File Transfer Protocol">FTP</abbr> directory</a> via a
180 1.14 christos less secure protocol.</p>
181 1.1 christos <p>Alternatively, a development repository of code and data can be
182 1.1 christos retrieved from <a href="https://github.com">GitHub</a> via the shell
183 1.1 christos command:</p>
184 1.2 christos <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 1.1 christos </code></pre>
186 1.1 christos <p>
187 1.1 christos Since version 2012e, each release has been tagged in development repositories.
188 1.1 christos Untagged commits are less well tested and probably contain
189 1.1 christos more errors.</p>
190 1.1 christos <p>
191 1.1 christos After obtaining the code and data files, see the
192 1.1 christos <code>README</code> file for what to do next.
193 1.1 christos The code lets you compile the <code><abbr>tz</abbr></code> source files into
194 1.2 christos machine-readable binary files, one for each location. The binary files
195 1.5 christos are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
196 1.13 christos specified by <a href="https://datatracker.ietf.org/doc/html/8536">Internet
197 1.5 christos <abbr>RFC</abbr> 8536</a>.
198 1.2 christos The code also lets
199 1.2 christos you read a <abbr>TZif</abbr> file and interpret timestamps for that
200 1.1 christos location.</p>
201 1.6 christos </section>
202 1.6 christos
203 1.6 christos <section>
204 1.1 christos <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
205 1.1 christos <p>
206 1.1 christos The <code><abbr>tz</abbr></code> code and data
207 1.1 christos are by no means authoritative. If you find errors, please
208 1.1 christos send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
209 1.1 christos the time zone mailing list. You can also <a
210 1.1 christos href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
211 1.1 christos and browse the <a
212 1.1 christos href="https://mm.icann.org/pipermail/tz/">archive of old
213 1.5 christos messages</a>.
214 1.5 christos <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
215 1.5 christos discussions</a> and corresponding data changes can be
216 1.5 christos generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
217 1.5 christos </p>
218 1.1 christos <p>
219 1.1 christos Changes to the <code><abbr>tz</abbr></code> code and data are often
220 1.1 christos propagated to clients via operating system updates, so
221 1.1 christos client <code><abbr>tz</abbr></code> data can often be corrected by
222 1.1 christos applying these updates. With GNU/Linux and similar systems, if your
223 1.1 christos maintenance provider has not yet adopted the
224 1.1 christos latest <code><abbr>tz</abbr></code> data, you can often short-circuit
225 1.1 christos the process by tailoring the generic instructions in
226 1.1 christos the <code><abbr>tz</abbr> README</code> file and installing the latest
227 1.1 christos data yourself. System-specific instructions for installing the
228 1.1 christos latest <code><abbr>tz</abbr></code> data have also been published
229 1.11 christos for <a href="https://developer.ibm.com/articles/au-aix-olson-time-zone/"><abbr>AIX</abbr></a>,
230 1.1 christos <a
231 1.10 christos href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
232 1.1 christos <a
233 1.11 christos href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
234 1.1 christos title="International Components for Unicode">ICU</abbr></a>,
235 1.11 christos <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
236 1.9 christos JDK</a>,
237 1.9 christos <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
238 1.1 christos href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
239 1.9 christos <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
240 1.11 christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
241 1.1 christos </p>
242 1.12 christos <p>Since version 2013a,
243 1.12 christos sources for the <code><abbr>tz</abbr></code> database have been
244 1.1 christos <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
245 1.1 christos title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
246 1.1 christos <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
247 1.1 christos with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
248 1.1 christos title="linefeed">LF</abbr></a>,
249 1.1 christos which can be modified by common text editors such
250 1.1 christos as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
251 1.1 christos <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
252 1.1 christos <a href="https://www.vim.org">vim</a>.
253 1.1 christos Specialized source-file editing can be done via the
254 1.1 christos <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
255 1.1 christos zoneinfo</a> package for <a
256 1.1 christos href="https://www.sublimetext.com">Sublime Text</a> and the <a
257 1.1 christos href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
258 1.1 christos zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
259 1.1 christos Studio Code</a>.
260 1.1 christos </p>
261 1.1 christos <p>
262 1.1 christos For further information about updates, please see
263 1.13 christos <a href="https://datatracker.ietf.org/doc/html/rfc6557">Procedures for
264 1.1 christos Maintaining the Time Zone Database</a> (Internet <abbr
265 1.1 christos title="Request For Comments">RFC</abbr> 6557). More detail can be
266 1.7 christos found in <a href="theory.html">Theory and pragmatics of the
267 1.7 christos <code><abbr>tz</abbr></code> code and data</a>.
268 1.2 christos <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
269 1.2 christos displays changes between recent <code><abbr>tzdb</abbr></code> versions.
270 1.1 christos </p>
271 1.6 christos </section>
272 1.6 christos
273 1.6 christos <section>
274 1.14 christos <h2 id="coordinating">Coordinating with governments and distributors</h2>
275 1.14 christos <p>
276 1.14 christos As discussed in
277 1.14 christos "<a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How
278 1.14 christos Time Zones Are Coordinated</a>", the time zone database relies on
279 1.14 christos collaboration among governments, the time zone database volunteer
280 1.14 christos community, and data distributors downstream.
281 1.14 christos <p>
282 1.14 christos If your government plans to change its time zone boundaries or
283 1.14 christos daylight saving rules, please send email to <a
284 1.14 christos href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a> well in advance,
285 1.14 christos as this will lessen confusion and will coordinate updates to many cell phones,
286 1.14 christos computers, and other devices around the world.
287 1.14 christos In your email, please cite the legislation or regulation that specifies
288 1.14 christos the change, so that it can be checked for details such as the exact times
289 1.14 christos when clock transitions occur.
290 1.14 christos It is OK if a rule change is planned to affect clocks
291 1.14 christos far into the future, as a long-planned change can easily be reverted
292 1.14 christos or otherwise altered with a year's notice before the change would have
293 1.14 christos affected clocks.</p>
294 1.14 christos <p>
295 1.14 christos There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.
296 1.14 christos However, typically a release occurs every few months.
297 1.14 christos Many downstream timezone data distributors wait for
298 1.14 christos a <code><abbr>tzdb</abbr></code> release before they produce an update
299 1.14 christos to time zone behavior in consumer devices and software products.
300 1.14 christos After a release, various parties must integrate, test,
301 1.14 christos and roll out an update before <a
302 1.14 christos href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.
303 1.14 christos These updates can be expensive, for both the <a
304 1.14 christos href="https://en.wikipedia.org/wiki/Quality_assurance">quality
305 1.14 christos assurance</a> process and the overall cost of shipping and installing
306 1.14 christos updates to each device's copy of <code><abbr>tzdb</abbr></code>.
307 1.14 christos Updates may be batched with other updates and may take substantial
308 1.14 christos time to reach end users after a release.
309 1.14 christos Older devices may no longer be supported and thus may never be updated,
310 1.14 christos which means they will continue to use out-of-date rules.</p>
311 1.14 christos <p>
312 1.14 christos For these reasons any rule change should be promulgated at least a
313 1.14 christos year before it affects how clocks operate; otherwise, there is a good
314 1.14 christos chance that many clocks will be wrong due to delays in propagating updates,
315 1.14 christos and that residents will be confused or even actively resist the change.
316 1.14 christos The shorter the notice, the more likely clock problems will arise; see "<a
317 1.14 christos href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
318 1.14 christos the Timing of Time Zone Changes</a>" for examples.
319 1.14 christos </p>
320 1.14 christos </section>
321 1.14 christos
322 1.14 christos <section>
323 1.1 christos <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
324 1.1 christos <ul>
325 1.1 christos <li>The article
326 1.1 christos <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
327 1.1 christos an encyclopedic summary.</li>
328 1.1 christos <li><a href="tz-how-to.html">How to Read the
329 1.1 christos tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
330 1.1 christos database format.</li>
331 1.1 christos <li><a
332 1.1 christos href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
333 1.1 christos literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
334 1.1 christos database's style.</li>
335 1.10 christos <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
336 1.10 christos managing time in the internet</a> analyzes the database longitudinally.</li>
337 1.1 christos </ul>
338 1.6 christos </section>
339 1.6 christos
340 1.6 christos <section>
341 1.1 christos <h2 id="web">Web sites using recent versions of the
342 1.1 christos <code><abbr>tz</abbr></code> database</h2>
343 1.1 christos <p>
344 1.1 christos These are listed roughly in ascending order of complexity and fanciness.
345 1.1 christos </p>
346 1.1 christos <ul>
347 1.1 christos <li><a href="https://time.is">Time.is</a> shows locations'
348 1.1 christos time and zones.</li>
349 1.1 christos <li><a href="https://www.timejones.com">TimeJones.com</a>,
350 1.1 christos <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
351 1.7 christos <a href="https://www.worldclock.com">The World Clock</a>
352 1.1 christos are time zone converters.</li>
353 1.8 christos <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
354 1.8 christos publishes <code><abbr>tzdb</abbr></code>-derived data in
355 1.8 christos <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
356 1.8 christos title="comma-separated values">CSV</abbr></a> and
357 1.8 christos in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
358 1.8 christos title="Structured Query Language">SQL</abbr></a> form.</li>
359 1.1 christos <li><a
360 1.6 christos href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
361 1.1 christos lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
362 1.1 christos <li><a
363 1.13 christos href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
364 1.1 christos Time in 1000 Places</a> uses descriptions of the values.</li>
365 1.7 christos <li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
366 1.10 christos timezone information for all countries</a>
367 1.10 christos displays tables of <abbr>DST</abbr> rules.
368 1.1 christos <li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
369 1.1 christos Worldwide</a> lets you sort zone names and convert times.</li>
370 1.1 christos <li><a href="https://24timezones.com">24TimeZones</a> has a world
371 1.1 christos time map and a time converter.</li>
372 1.1 christos <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
373 1.1 christos calculates the current time difference between locations.</li>
374 1.13 christos <li><a href="https://www.wx-now.com">Weather Now</a> and
375 1.13 christos <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
376 1.1 christos </ul>
377 1.6 christos </section>
378 1.6 christos
379 1.6 christos <section>
380 1.1 christos <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
381 1.1 christos <ul>
382 1.1 christos <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
383 1.1 christos <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
384 1.1 christos Distribution Service (tzdist) working group</a> defined <a
385 1.13 christos href="https://datatracker.ietf.org/doc/html/rfc7808">TZDIST</a>
386 1.1 christos (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
387 1.13 christos along with <a href="https://datatracker.ietf.org/doc/html/rfc7809">CalDAV</a>
388 1.1 christos (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
389 1.2 christos transferring time zone data by reference.
390 1.8 christos <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
391 1.8 christos implementations</a> are available.
392 1.2 christos The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
393 1.9 christos mailing list</a> discusses possible extensions.</li>
394 1.13 christos <li>The <a href="https://datatracker.ietf.org/doc/html/rfc5545">
395 1.1 christos Internet Calendaring and Scheduling Core Object Specification
396 1.1 christos (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
397 1.1 christos covers time zone
398 1.1 christos data; see its VTIMEZONE calendar component.
399 1.1 christos The iCalendar format requires specialized parsers and generators; a
400 1.13 christos variant <a href="https://datatracker.ietf.org/doc/html/rfc6321">xCal</a>
401 1.1 christos (Internet <abbr>RFC</abbr> 6321) uses
402 1.1 christos <a href="https://www.w3.org/XML/"><abbr
403 1.1 christos title="Extensible Markup Language">XML</abbr></a> format, and a variant
404 1.13 christos <a href="https://datatracker.ietf.org/doc/html/rfc7265">jCal</a>
405 1.1 christos (Internet <abbr>RFC</abbr> 7265)
406 1.1 christos uses <a href="https://www.json.org"><abbr
407 1.1 christos title="JavaScript Object Notation">JSON</abbr></a> format.</li>
408 1.1 christos </ul>
409 1.6 christos </section>
410 1.6 christos
411 1.6 christos <section>
412 1.1 christos <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
413 1.7 christos <p>Although some of these do not fully support
414 1.7 christos <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
415 1.7 christos distributions you can generally work around compatibility problems by
416 1.7 christos running the command <code>make rearguard_tarballs</code> and compiling
417 1.7 christos from the resulting tarballs instead.</p>
418 1.1 christos <ul>
419 1.1 christos <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
420 1.14 christos href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>
421 1.1 christos program that compiles
422 1.1 christos <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
423 1.1 christos Vzic is freely
424 1.1 christos available under the <a
425 1.1 christos href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
426 1.1 christos General Public License (<abbr
427 1.1 christos title="General Public License">GPL</abbr>)</a>.</li>
428 1.1 christos <li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz
429 1.1 christos database conversion utility</a> is like Vzic, except for the <a
430 1.6 christos href="https://dotnet.microsoft.com">.NET framework</a>
431 1.1 christos and with a <abbr>BSD</abbr>-style license.</li>
432 1.1 christos <li><a
433 1.4 christos href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
434 1.1 christos contains a script <code>parse_olson</code> that compiles
435 1.1 christos <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
436 1.1 christos modules. It is part of the Perl <a
437 1.4 christos href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
438 1.4 christos which is freely
439 1.1 christos available under both the <abbr>GPL</abbr> and the Perl Artistic
440 1.1 christos License. DateTime::TimeZone also contains a script
441 1.1 christos <code>tests_from_zdump</code> that generates test cases for each clock
442 1.1 christos transition in the <code><abbr>tz</abbr></code> database.</li>
443 1.1 christos <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
444 1.1 christos Database Parser</a> is a
445 1.14 christos <a href="https://en.wikipedia.org/wiki/C++">C++</a> parser and
446 1.9 christos runtime library with <a
447 1.13 christos href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
448 1.9 christos adopted by
449 1.6 christos <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
450 1.9 christos the current iteration of the C++ standard.
451 1.1 christos It is freely available under the
452 1.1 christos <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
453 1.13 christos <li><a id="ICU" href="https://icu.unicode.org">International Components for
454 1.1 christos Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
455 1.14 christos href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a>
456 1.1 christos libraries for internationalization that
457 1.1 christos has a compiler from <code><abbr>tz</abbr></code> source
458 1.1 christos and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
459 1.1 christos (mentioned <a href="#CLDR">below</a>)
460 1.1 christos into an <abbr>ICU</abbr>-specific format.
461 1.1 christos <abbr>ICU</abbr> is freely available under a
462 1.1 christos <abbr>BSD</abbr>-style license.</li>
463 1.1 christos <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
464 1.1 christos the <a href="https://elixir-lang.org">Elixir</a> language downloads
465 1.7 christos and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
466 1.1 christos title="Application Program Interface">API</abbr>s for use. It is
467 1.1 christos freely available under the <abbr>MIT</abbr> license.</li>
468 1.1 christos <li>Java-based compilers and libraries include:
469 1.1 christos <ul>
470 1.1 christos <li>The <a
471 1.9 christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
472 1.1 christos tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
473 1.9 christos <a href="https://openjdk.java.net/">OpenJDK</a> and
474 1.9 christos <a href="https://jdk.java.net/">Oracle JDK</a>.
475 1.9 christos Although its source code is proprietary, its executable is available under the
476 1.9 christos <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
477 1.9 christos Timezone Updater License Agreement</a>.</li>
478 1.1 christos <li>The <a
479 1.4 christos href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
480 1.6 christos SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
481 1.4 christos href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
482 1.1 christos which is freely available under a <abbr>BSD</abbr>-style license.</li>
483 1.4 christos <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
484 1.1 christos and time <abbr>API</abbr></a> contains a class
485 1.1 christos <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
486 1.1 christos <code><abbr>tz</abbr></code> source into a binary format. It inspired
487 1.1 christos Java 8 <code>java.time</code>, which its users should migrate to once
488 1.1 christos they can assume Java 8 or later. It is available under the <a
489 1.1 christos href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
490 1.9 christos <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
491 1.7 christos href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
492 1.9 christos are alternatives to TZUpdater. IANA Updater's license is unclear;
493 1.9 christos ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
494 1.4 christos <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
495 1.4 christos time library for Android</a> and
496 1.4 christos <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
497 1.4 christos time and interval library for Java</a> compile
498 1.4 christos <code><abbr>tz</abbr></code> source into a binary format.
499 1.4 christos Time4A is available under the Apache License and Time4J is
500 1.1 christos available under the <a
501 1.1 christos href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
502 1.1 christos General Public License (<abbr title="Lesser General Public
503 1.1 christos License">LGPL</abbr>)</a>.</li>
504 1.1 christos <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
505 1.1 christos Java-based libraries.</li>
506 1.1 christos </ul>
507 1.1 christos <li><a href="https://nodatime.org">Noda Time – Date and
508 1.1 christos time <abbr>API</abbr> for .NET</a>
509 1.4 christos is like Joda-Time and Time4J, but for the .NET framework instead of Java.
510 1.4 christos It is freely available under the Apache License.</li>
511 1.8 christos <li>Many modern
512 1.8 christos <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
513 1.8 christos runtimes support <code><abbr>tz</abbr></code> natively via the
514 1.8 christos <samp>timeZone</samp> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><samp>Intl.DateTimeFormat</samp></a>.
515 1.10 christos This can be used as-is or with most of the following libraries,
516 1.10 christos many of which also support runtimes lacking the <samp>timeZone</samp> option.
517 1.1 christos <ul>
518 1.8 christos <li>The <a
519 1.8 christos href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
520 1.8 christos timezone polyfill</a>
521 1.8 christos is freely available under a <abbr>BSD</abbr>-style license.</li>
522 1.10 christos <li>The <a href="https://date-fns.org/">date-fns</a>
523 1.10 christos library manipulates timezone-aware timestamps in browsers and
524 1.10 christos in <a href="https://nodejs.org/en/">Node.js</a>.
525 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
526 1.10 christos <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
527 1.10 christos minimalist replacement for the date and time API of
528 1.10 christos the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
529 1.10 christos manipulation library.
530 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
531 1.10 christos <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
532 1.10 christos timezone support for the <samp>Intl</samp> API.
533 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
534 1.1 christos <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
535 1.10 christos Moment.js plugin.
536 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
537 1.8 christos <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
538 1.8 christos JavaScript library that supports date arithmetic that is time zone
539 1.8 christos aware. It is freely available under the <abbr>MIT</abbr> license.</li>
540 1.10 christos <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
541 1.10 christos supports live <code><abbr>tzdb</abbr></code> updates,
542 1.10 christos astronomical and atomic time, a command-line interface,
543 1.10 christos and full <a
544 1.10 christos href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
545 1.10 christos Its companion <a
546 1.10 christos href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
547 1.10 christos can generate <abbr>TZif</abbr> and other files, and a companion website
548 1.10 christos <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
549 1.10 christos convert timestamps, view transition histories, and download code and data.
550 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
551 1.1 christos </ul>
552 1.10 christos The proposed <a
553 1.10 christos href="https://github.com/tc39/proposal-temporal"><samp>Temporal</samp>
554 1.10 christos objects</a> let programs access an abstract view of
555 1.10 christos <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
556 1.10 christos href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
557 1.10 christos problematic <samp>Date</samp> objects</a> when working with dates and times.
558 1.1 christos <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
559 1.1 christos compiler from <code><abbr>tz</abbr></code> source into
560 1.1 christos <a href="https://julialang.org/">Julia</a>. It is freely available
561 1.1 christos under the <abbr>MIT</abbr> license.</li>
562 1.6 christos <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
563 1.6 christos <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
564 1.6 christos title="Free Pascal Compiler">FPC</abbr></a>
565 1.2 christos compiles from <code><abbr>tz</abbr></code> source into
566 1.2 christos <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
567 1.2 christos as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
568 1.2 christos and <a
569 1.2 christos href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
570 1.2 christos It is freely available under a <abbr>BSD</abbr>-style license.</li>
571 1.1 christos <li><a href="http://pytz.sourceforge.net">pytz – World Timezone
572 1.1 christos Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
573 1.1 christos <a href="https://www.python.org">Python</a>.
574 1.9 christos It is freely available under a <abbr>BSD</abbr>-style license.
575 1.11 christos In code that can assume Python 3.6 or later it is largely superseded; see <a
576 1.11 christos href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
577 1.11 christos The Fastest Footgun in the West</a>.</li>
578 1.1 christos <li><a href="https://tzinfo.github.io">TZInfo –
579 1.1 christos Ruby Timezone Library</a>
580 1.1 christos compiles <code><abbr>tz</abbr></code> source into
581 1.1 christos <a href="https://www.ruby-lang.org/en/">Ruby</a>.
582 1.1 christos It is freely available under the <abbr>MIT</abbr> license.</li>
583 1.13 christos <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
584 1.1 christos Library</a> is
585 1.1 christos a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
586 1.1 christos library that compiles <code><abbr>tz</abbr></code> source into a time
587 1.1 christos zone repository whose format
588 1.1 christos is either proprietary or an <abbr>XML</abbr>-encoded
589 1.1 christos representation.</li>
590 1.1 christos <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
591 1.1 christos contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
592 1.1 christos source into text files, along with a runtime that can read those
593 1.1 christos files. Tcl is freely available under a <abbr>BSD</abbr>-style
594 1.1 christos license.</li>
595 1.1 christos </ul>
596 1.6 christos </section>
597 1.6 christos
598 1.6 christos <section>
599 1.2 christos <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
600 1.1 christos <ul>
601 1.1 christos <li>The <a
602 1.1 christos href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
603 1.1 christos Library</a>
604 1.1 christos has an independent, thread-safe implementation of
605 1.2 christos a <abbr>TZif</abbr> file reader.
606 1.1 christos This library is freely available under the LGPL
607 1.1 christos and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
608 1.1 christos <li><a href="https://www.gnome.org">GNOME</a>'s
609 1.1 christos <a href="https://developer.gnome.org/glib/">GLib</a> has
610 1.2 christos a <abbr>TZif</abbr> file reader written in C that
611 1.1 christos creates a <code>GTimeZone</code> object representing sets
612 1.1 christos of <abbr>UT</abbr> offsets.
613 1.1 christos It is freely available under the <abbr>LGPL</abbr>.</li>
614 1.1 christos <li>The
615 1.1 christos <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
616 1.1 christos <code>baltzo::TimeZoneUtil</code> component contains a C++
617 1.2 christos implementation of a <abbr>TZif</abbr> file reader. It is freely available under
618 1.1 christos the Apache License.</li>
619 1.1 christos <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
620 1.1 christos library that translates between <abbr>UT</abbr> and civil time and
621 1.2 christos can read <abbr>TZif</abbr> files. It is freely available under the Apache
622 1.1 christos License.</li>
623 1.1 christos <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
624 1.2 christos library that reads <abbr>TZif</abbr> files and converts
625 1.2 christos timestamps from one time zone or format to another.
626 1.1 christos It is used by <a href="https://secure.php.net"><abbr
627 1.1 christos title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
628 1.1 christos <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
629 1.1 christos and <a href="https://www.mongodb.com">MongoDB</a>.
630 1.1 christos It is freely available under the <abbr>MIT</abbr> license.</li>
631 1.1 christos <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
632 1.2 christos <abbr>TZif</abbr> file reader.</li>
633 1.4 christos <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
634 1.1 christos DateTime::TimeZone::Tzfile</a>
635 1.2 christos is a <abbr>TZif</abbr> file reader written in Perl.
636 1.1 christos It is freely available under the same terms as Perl
637 1.1 christos (dual <abbr>GPL</abbr> and Artistic license).</li>
638 1.9 christos <li>Python has a <a id="python-zoneinfo"
639 1.11 christos href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
640 1.8 christos class</a> that reads <abbr>TZif</abbr> data and creates objects
641 1.8 christos that represent <code><abbr>tzdb</abbr></code> timezones.
642 1.8 christos Python is freely available under the
643 1.11 christos <a href="https://docs.python.org/3/license.html">Python Software Foundation
644 1.8 christos License</a>.
645 1.9 christos A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
646 1.8 christos <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
647 1.8 christos supplies TZif data if the underlying system data cannot be found;
648 1.8 christos it is freely available under the Apache License.</li>
649 1.1 christos <li>The
650 1.1 christos public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
651 1.1 christos library contains a Python tool that
652 1.2 christos converts <abbr>TZif</abbr> data into
653 1.1 christos <abbr>JSON</abbr>-format data suitable for use
654 1.1 christos in its JavaScript library for time zone conversion. Dates before 1970
655 1.1 christos are not supported.</li>
656 1.1 christos <li>The <a
657 1.1 christos href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
658 1.1 christos package contains <a href="https://www.haskell.org">Haskell</a> code that
659 1.2 christos parses and uses <abbr>TZif</abbr> data. It is freely
660 1.1 christos available under a <abbr>BSD</abbr>-style license.</li>
661 1.1 christos </ul>
662 1.6 christos </section>
663 1.6 christos
664 1.6 christos <section>
665 1.1 christos <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
666 1.1 christos <ul>
667 1.1 christos <li><a href="https://foxclocks.org">FoxClocks</a>
668 1.1 christos is an extension for <a href="https://www.google.com/chrome/">Google
669 1.1 christos Chrome</a> and for <a
670 1.11 christos href="https://wiki.mozilla.org/Modules/Toolkit">Mozilla
671 1.1 christos Toolkit</a> applications like <a
672 1.1 christos href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
673 1.1 christos href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
674 1.1 christos It displays multiple clocks in the application window, and has a mapping
675 1.1 christos interface to <a href="https://www.google.com/earth/">Google Earth</a>.
676 1.1 christos It is freely available under the <abbr>GPL</abbr>.</li>
677 1.1 christos <li><a href="https://golang.org">Go programming language</a>
678 1.1 christos implementations contain a copy of a 32-bit subset of a recent
679 1.1 christos <code><abbr>tz</abbr></code> database in a
680 1.1 christos Go-specific format.</li>
681 1.1 christos <li>Microsoft Windows 8.1
682 1.1 christos and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
683 1.3 christos data (mentioned <a href="#CLDR">below</a>) used by the
684 1.3 christos <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
685 1.3 christos <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
686 1.3 christos <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
687 1.3 christos <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
688 1.1 christos <a id="System.TimeZoneInfo"
689 1.1 christos href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
690 1.1 christos Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
691 1.1 christos the older, proprietary method of Microsoft Windows 2000 and later,
692 1.1 christos which stores time zone data in the
693 1.1 christos <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
694 1.1 christos <a
695 1.1 christos href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
696 1.1 christos Tzid table</a> or <a
697 1.6 christos href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
698 1.1 christos file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
699 1.1 christos to <code><abbr>tz</abbr></code> names.
700 1.3 christos These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
701 1.3 christos or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
702 1.1 christos <li><a
703 1.1 christos href="https://www.oracle.com/java/index.html">Oracle
704 1.1 christos Java</a> contains a copy of a subset of a recent
705 1.1 christos <code><abbr>tz</abbr></code> database in a
706 1.1 christos Java-specific format.</li>
707 1.1 christos </ul>
708 1.6 christos </section>
709 1.6 christos
710 1.6 christos <section>
711 1.1 christos <h2 id="other-dbs">Other time zone databases</h2>
712 1.1 christos <ul>
713 1.1 christos <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
714 1.4 christos is Astrodienst's Web version of Shanks and Pottenger's out-of-print
715 1.4 christos time zone history atlases
716 1.4 christos <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
717 1.9 christos <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
718 1.4 christos Although these extensive atlases
719 1.4 christos <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
720 1.4 christos sources for much of the older <code><abbr>tz</abbr></code> data</a>,
721 1.4 christos they are unreliable as Shanks appears to have
722 1.1 christos guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
723 1.1 christos sources and do not indicate which entries are guesswork.</li>
724 1.1 christos <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
725 1.1 christos its own <code>tztab</code>(4) format.</li>
726 1.1 christos <li>Microsoft Windows has proprietary data mentioned
727 1.1 christos <a href="#System.TimeZoneInfo">above</a>.</li>
728 1.1 christos <li><a href="https://www.worldtimeserver.com">World Time Server</a>
729 1.1 christos is another time zone database.</li>
730 1.1 christos <li>The <a
731 1.1 christos href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
732 1.1 christos Schedules Information Manual</a> of the
733 1.1 christos International Air Transport Association
734 1.1 christos gives current time zone rules for airports served by commercial aviation.</li>
735 1.1 christos </ul>
736 1.6 christos </section>
737 1.6 christos
738 1.6 christos <section>
739 1.1 christos <h2 id="maps">Maps</h2>
740 1.1 christos <ul>
741 1.9 christos <li>The <a
742 1.9 christos href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
743 1.9 christos and Regional Maps section</a> of <em>The World Factbook</em>, published by the
744 1.9 christos <a href="https://www.cia.gov">US Central Intelligence
745 1.9 christos Agency (<abbr
746 1.9 christos title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
747 1.9 christos zone map; the
748 1.1 christos <a
749 1.6 christos href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda
750 1.1 christos Library Map Collection</a>
751 1.1 christos of the University of Texas at Austin has copies of
752 1.1 christos recent editions.
753 1.1 christos The pictorial quality is good,
754 1.1 christos but the maps do not indicate daylight saving time,
755 1.1 christos and parts of the data are a few years out of date.</li>
756 1.6 christos <li><a href="https://www.worldtimezone.com">World Time Zone Map
757 1.6 christos with current time</a>
758 1.1 christos has several fancy time zone maps; it covers Russia particularly well.
759 1.1 christos The maps' pictorial quality is not quite as good as the
760 1.1 christos <abbr>CIA</abbr>'s
761 1.1 christos but the maps are more up to date.</li>
762 1.1 christos <li><a
763 1.1 christos href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
764 1.1 christos much is time wrong around the world?</a> maps the difference between
765 1.1 christos mean solar and standard time, highlighting areas such as western China
766 1.1 christos where the two differ greatly. It's a bit out of date, unfortunately.</li>
767 1.1 christos </ul>
768 1.6 christos </section>
769 1.6 christos
770 1.6 christos <section>
771 1.1 christos <h2 id="boundaries">Time zone boundaries</h2>
772 1.2 christos <p>Geographical boundaries between timezones are available
773 1.9 christos from several <a
774 1.9 christos href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
775 1.9 christos geolocation</a>
776 1.1 christos services and other sources.</p>
777 1.1 christos <ul>
778 1.1 christos <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
779 1.1 christos Boundary Builder</a> extracts
780 1.1 christos <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
781 1.2 christos boundaries of <code><abbr>tzdb</abbr></code> timezones.
782 1.1 christos Its code is freely available under the <abbr>MIT</abbr> license, and
783 1.1 christos its data entries are freely available under the
784 1.1 christos <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
785 1.1 christos Open Database License</a>. The maps' borders appear to be quite accurate.</li>
786 1.1 christos <li>Programmatic interfaces that map geographical coordinates via tz_world to
787 1.2 christos <code><abbr>tzdb</abbr></code> timezones include:
788 1.1 christos <ul>
789 1.1 christos <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
790 1.1 christos written in <a
791 1.1 christos href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
792 1.1 christos and is freely available under the <abbr>MIT</abbr> license.</li>
793 1.1 christos <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
794 1.1 christos is written in Go and is freely available under the Apache License.</li>
795 1.1 christos <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
796 1.1 christos in both Java and
797 1.1 christos <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
798 1.1 christos form, is freely available under the MIT license.</li>
799 1.10 christos <li>For Node.js,
800 1.1 christos the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
801 1.1 christos is freely available under the MIT license, and
802 1.1 christos the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
803 1.1 christos is in the public domain.</li>
804 1.1 christos <li>The <a
805 1.1 christos href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
806 1.1 christos library for Python is freely available under the MIT license.
807 1.1 christos <li>The <a
808 1.1 christos href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
809 1.1 christos library for Ruby is freely available under the MIT license.</li>
810 1.1 christos </ul></li>
811 1.1 christos <li>Free access via a network API, if you register a key, is provided by
812 1.4 christos the <a
813 1.4 christos href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
814 1.4 christos Timezone web service</a>, the <a
815 1.4 christos href="https://developers.google.com/maps/documentation/timezone/intro">Google
816 1.4 christos Maps Time Zone API</a>, and
817 1.8 christos the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
818 1.1 christos Commercial network API access is provided
819 1.1 christos by <a href="https://askgeo.com">AskGeo</a>
820 1.1 christos and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
821 1.1 christos </li>
822 1.1 christos <li>"<a
823 1.1 christos href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How
824 1.1 christos to get a time zone from a location using latitude and longitude
825 1.1 christos coordinates?</a>" discusses other geolocation possibilities.</li>
826 1.1 christos <li><a href="http://statoids.com/statoids.html">Administrative
827 1.1 christos Divisions of Countries ("Statoids")</a> lists
828 1.1 christos political subdivision data related to time zones.</li>
829 1.7 christos <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
830 1.1 christos zone boundaries for multizone countries</a> summarizes legal
831 1.1 christos boundaries between time zones within countries.</li>
832 1.13 christos <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
833 1.1 christos – GIS and Database Tools</a> includes a Manifold-format map of
834 1.1 christos world time zone boundaries distributed under the
835 1.1 christos <abbr>GPL</abbr>.</li>
836 1.1 christos <li>A ship within the <a
837 1.1 christos href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
838 1.1 christos waters</a> of any nation uses that nation's time. In international
839 1.1 christos waters, time zone boundaries are meridians 15° apart, except that
840 1.1 christos <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
841 1.1 christos wide and are separated by
842 1.1 christos the 180° meridian (not by the International Date Line, which is
843 1.1 christos for land and territorial waters only). A captain can change ship's
844 1.1 christos clocks any time after entering a new time zone; midnight changes are
845 1.1 christos common.</li>
846 1.1 christos </ul>
847 1.6 christos </section>
848 1.6 christos
849 1.6 christos <section>
850 1.1 christos <h2 id="civil">Civil time concepts and history</h2>
851 1.1 christos <ul>
852 1.1 christos <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
853 1.1 christos Walk through Time</a>
854 1.1 christos surveys the evolution of timekeeping.</li>
855 1.4 christos <li>The history of daylight saving time is surveyed in <a
856 1.4 christos href="http://www.webexhibits.org/daylightsaving/">About Daylight
857 1.4 christos Saving Time – History, rationale, laws & dates</a> and summarized in
858 1.4 christos <a href="http://seizethedaylight.com/dst/">A Brief
859 1.4 christos History of Daylight Saving Time</a>.</li>
860 1.4 christos <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
861 1.4 christos Lords</a> discusses how authoritarians manipulate civil time.</li>
862 1.1 christos <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
863 1.1 christos contains guidelines and best practices for software applications that
864 1.1 christos deal with civil time.</li>
865 1.1 christos <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
866 1.1 christos the International Date Line</a> tells the story of the most important
867 1.1 christos time zone boundary.</li>
868 1.1 christos <li><a href="http://statoids.com/tconcept.html">Basic Time
869 1.1 christos Zone Concepts</a> discusses terminological issues behind time zones.</li>
870 1.1 christos </ul>
871 1.6 christos </section>
872 1.6 christos
873 1.6 christos <section>
874 1.1 christos <h2 id="national">National histories of legal time</h2>
875 1.1 christos <dl>
876 1.1 christos <dt>Australia</dt>
877 1.6 christos <dd>The Parliamentary Library commissioned a <a
878 1.1 christos href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
879 1.1 christos paper on daylight saving time in Australia</a>.
880 1.1 christos The Bureau of Meteorology publishes a list of <a
881 1.1 christos href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
882 1.1 christos Dates of Daylight Savings Time within Australia</a>.</dd>
883 1.1 christos <dt>Belgium</dt>
884 1.7 christos <dd>The Royal Observatory of Belgium maintains a table of time in
885 1.7 christos Belgium (in
886 1.7 christos <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
887 1.7 christos hreflang="nl">Dutch</a> and <a
888 1.7 christos href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
889 1.7 christos hreflang="fr">French</a>).</dd>
890 1.1 christos <dt>Brazil</dt>
891 1.1 christos <dd>The Time Service Department of the National Observatory
892 1.1 christos records <a href="http://pcdsh01.on.br/DecHV.html"
893 1.1 christos hreflang="pt-BR">Brazil's daylight saving time decrees (in
894 1.1 christos Portuguese)</a>.</dd>
895 1.1 christos <dt>Canada</dt>
896 1.1 christos <dd>National Research Council Canada publishes current
897 1.1 christos and some older information about <a
898 1.6 christos href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
899 1.6 christos zones and daylight saving time</a>.</dd>
900 1.1 christos <dt>Chile</dt>
901 1.1 christos <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
902 1.11 christos <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
903 1.1 christos Chile's official time (in Spanish)</a>.</dd>
904 1.3 christos <dt>China</dt>
905 1.3 christos <dd>The Hong Kong Observatory maintains a
906 1.7 christos <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
907 1.3 christos summer time in Hong Kong</a>,
908 1.3 christos and Macau's Meteorological and Geophysical Bureau maintains a <a
909 1.7 christos href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
910 1.3 christos history for Macau</a>.
911 1.3 christos Unfortunately the latter is incomplete and has errors.</dd>
912 1.1 christos <dt>Czech Republic</dt>
913 1.1 christos <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
914 1.1 christos hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
915 1.10 christos summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
916 1.1 christos <dt>Germany</dt>
917 1.1 christos <dd>The National Institute for Science and Technology maintains the <a
918 1.1 christos href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
919 1.1 christos of Legal Time in Germany</a>.</dd>
920 1.1 christos <dt>Israel</dt>
921 1.1 christos <dd>The Interior Ministry periodically issues <a
922 1.1 christos href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
923 1.1 christos hreflang="he">announcements (in Hebrew)</a>.</dd>
924 1.2 christos <dt>Malaysia</dt>
925 1.2 christos <dd>See Singapore <a href="#Singapore">below</a>.</dd>
926 1.1 christos <dt>Mexico</dt>
927 1.1 christos <dd>The Investigation and Analysis Service of the Mexican Library of
928 1.1 christos Congress has published a <a
929 1.13 christos href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
930 1.1 christos hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
931 1.1 christos <dt>Netherlands</dt>
932 1.1 christos <dd><a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
933 1.1 christos hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
934 1.1 christos covers the history of local time in the Netherlands from ancient times.</dd>
935 1.1 christos <dt>New Zealand</dt>
936 1.1 christos <dd>The Department of Internal Affairs maintains a brief <a
937 1.1 christos href="https://www.dia.govt.nz/Daylight-Saving-History">History of
938 1.6 christos Daylight Saving</a>.</dd>
939 1.14 christos <dt>Palestine</dt>
940 1.14 christos <dd>The Ministry of Telecom and IT publishes a <a
941 1.14 christos href="https://mtit.pna.ps/Site/TimeZoon"
942 1.14 christos hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
943 1.10 christos <dt>Portugal</dt>
944 1.10 christos <dd>The Lisbon Astronomical Observatory publishes a
945 1.10 christos <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
946 1.10 christos legal time (in Portuguese)</a>.</dd>
947 1.1 christos <dt>Singapore</dt>
948 1.1 christos <dd><a id="Singapore"
949 1.7 christos href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
950 1.1 christos is Singapore in the "Wrong" Time Zone?</a> details the
951 1.1 christos history of legal time in Singapore and Malaysia.</dd>
952 1.1 christos <dt>United Kingdom</dt>
953 1.1 christos <dd><a
954 1.1 christos href="https://www.polyomino.org.uk/british-time/">History of
955 1.1 christos legal time in Britain</a> discusses in detail the country
956 1.6 christos with perhaps the best-documented history of clock adjustments.</dd>
957 1.1 christos <dt>United States</dt>
958 1.1 christos <dd>The Department of Transportation's <a
959 1.1 christos href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
960 1.1 christos Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
961 1.1 christos <dt>Uruguay</dt>
962 1.1 christos <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
963 1.1 christos Navy (SOHMA) publishes an annual <a
964 1.13 christos href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
965 1.1 christos (in Spanish)</a>.</dd>
966 1.1 christos </dl>
967 1.6 christos </section>
968 1.6 christos
969 1.6 christos <section>
970 1.7 christos <h2 id="costs">Costs and benefits of time shifts</h2>
971 1.7 christos <p>Various sources argue for and against daylight saving time and time
972 1.7 christos zone shifts, and many scientific studies have been conducted. This
973 1.8 christos section summarizes reviews and position statements based on
974 1.8 christos scientific literature in the area.</p>
975 1.7 christos <ul>
976 1.15 christos <li>In 2022 the American Medical Association issued a
977 1.15 christos <a href="https://www.ama-assn.org/press-center/press-releases/ama-calls-permanent-standard-time">statement
978 1.15 christos supporting permanent standard time</a> on health grounds.</li>
979 1.7 christos <li>Carey RN, Sarma KM.
980 1.7 christos <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
981 1.7 christos daylight saving time on road traffic collision risk: a systematic
982 1.7 christos review</a>.
983 1.7 christos <em>BMJ Open.</em> 2017;7(6):e014319. doi:<a href="https://doi.org/10.1136/bmjopen-2016-014319">10.1136/bmjopen-2016-014319</a>.
984 1.7 christos This reviews research literature and concludes that the evidence
985 1.7 christos neither supports nor refutes road safety benefits from
986 1.7 christos shifts in time zones.</li>
987 1.7 christos <li>Havranek T, Herman D, Irsova D.
988 1.13 christos <a href="https://www.iaee.org/en/publications/ejarticle.aspx?id=3051">Does
989 1.13 christos daylight saving save electricity? A meta-analysis.</a>
990 1.13 christos <em>Energy J.</em> 2018;39(2):35–61.
991 1.7 christos doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
992 1.7 christos This analyzes research literature and concludes, "Electricity savings
993 1.7 christos are larger for countries farther away from the equator, while
994 1.10 christos subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
995 1.15 christos <li>Malow BA. It is time to abolish the clock change and adopt permanent
996 1.15 christos standard time in the United States: a Sleep Research Society position statement.
997 1.15 christos <em>Sleep.</em> 2022;45(12):zsac236.
998 1.15 christos doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
999 1.15 christos After reviewing the scientific literature, the Sleep Research Society
1000 1.15 christos advocates permanent standard time due to its health benefits.
1001 1.15 christos <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
1002 1.15 christos <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
1003 1.15 christos is the optimal choice for health and safety:
1004 1.8 christos an American Academy of Sleep Medicine position statement</a>.
1005 1.15 christos <em>J Clin Sleep Med.</em> 2023-10-31.
1006 1.15 christos doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
1007 1.15 christos The AASM argues for permanent standard time due to health and safety risks
1008 1.15 christos and economic costs of both <abbr>DST</abbr> transitions and
1009 1.15 christos permanent <abbr>DST</abbr>.</li>
1010 1.13 christos <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
1011 1.13 christos <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7205184/">Why
1012 1.13 christos should we abolish Daylight Saving Time?</a>
1013 1.15 christos <em>J Biol Rhythms.</em> 2019;34(3):227–230.
1014 1.13 christos doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
1015 1.15 christos The Society for Research on Biological Rhythms
1016 1.13 christos opposes DST changes and permanent DST, and advocates that governments adopt
1017 1.13 christos "permanent Standard Time for the health and safety of their citizens".</li>
1018 1.7 christos </ul>
1019 1.7 christos </section>
1020 1.7 christos
1021 1.7 christos <section>
1022 1.1 christos <h2 id="precision">Precision timekeeping</h2>
1023 1.1 christos <ul>
1024 1.1 christos <li><a
1025 1.1 christos href="http://leapsecond.com/hpan/an1289.pdf">The
1026 1.1 christos Science of Timekeeping</a> is a thorough introduction
1027 1.1 christos to the theory and practice of precision timekeeping.</li>
1028 1.1 christos <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
1029 1.14 christos Time 2016</a> contains several freely readable papers.</li>
1030 1.13 christos <li><a href="https://www.ntp.org"><abbr
1031 1.1 christos title="Network Time Protocol">NTP</abbr>: The Network
1032 1.1 christos Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
1033 1.1 christos discusses how to synchronize clocks of
1034 1.1 christos Internet hosts.</li>
1035 1.2 christos <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a>
1036 1.2 christos family of software algorithms can achieve accuracy to a few tens of
1037 1.2 christos nanoseconds in scalable server farms without special hardware.</li>
1038 1.1 christos <li>The <a
1039 1.1 christos href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
1040 1.1 christos Time Protocol</a> (<abbr
1041 1.1 christos title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
1042 1.2 christos can achieve submicrosecond clock accuracy on a local area network
1043 1.2 christos with special-purpose hardware.</li>
1044 1.1 christos <li><a
1045 1.13 christos href="https://datatracker.ietf.org/doc/html/rfc4833">Timezone
1046 1.1 christos Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
1047 1.1 christos (Internet <abbr>RFC</abbr> 4833)
1048 1.1 christos specifies a <a
1049 1.1 christos href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
1050 1.1 christos option for a server to configure
1051 1.1 christos a client's time zone and daylight saving settings automatically.</li>
1052 1.11 christos <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
1053 1.11 christos Scales</a> describes astronomical time scales like
1054 1.1 christos <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
1055 1.1 christos <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
1056 1.1 christos <abbr title="Barycentric Dynamic Time">TDB</abbr>.
1057 1.1 christos <li>The <a href="https://www.iau.org"><abbr
1058 1.1 christos title="International Astronomical Union">IAU</abbr></a>'s <a
1059 1.13 christos href="https://www.iausofa.org"><abbr
1060 1.1 christos title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
1061 1.1 christos collection contains C and <a
1062 1.1 christos href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
1063 1.1 christos code for converting among time scales like
1064 1.1 christos <abbr title="International Atomic Time">TAI</abbr>,
1065 1.1 christos <abbr>TDB</abbr>, <abbr>TDT</abbr> and
1066 1.13 christos <abbr>UTC</abbr>. It is freely available under the
1067 1.13 christos <a href="https://www.iausofa.org/tandc.html">SOFA license</a>.</li>
1068 1.1 christos <li><a
1069 1.1 christos href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
1070 1.1 christos – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
1071 1.1 christos diverse local time
1072 1.1 christos scales used by each landed mission on Mars.</li>
1073 1.1 christos <li><a href="http://leapsecond.com">LeapSecond.com</a> is
1074 1.1 christos dedicated not only to leap seconds but to precise time and frequency
1075 1.1 christos in general. It covers the state of the art in amateur timekeeping, and
1076 1.1 christos how the art has progressed over the past few decades.</li>
1077 1.7 christos <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
1078 1.7 christos href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
1079 1.7 christos and time-signal emissions</a>, International Telecommunication Union –
1080 1.7 christos Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
1081 1.1 christos <li><a
1082 1.1 christos href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
1083 1.1 christos title="International Earth Rotation and Reference Systems Service">IERS</abbr>
1084 1.1 christos Bulletins</a> contains official publications of the International
1085 1.1 christos Earth Rotation and Reference Systems Service, which decides when leap
1086 1.4 christos seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
1087 1.9 christos via an optional "<code>right</code>" configuration where a computer's internal
1088 1.9 christos <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
1089 1.9 christos as opposed to the default "<code>posix</code>" configuration
1090 1.9 christos where the internal clock ignores leap seconds.
1091 1.9 christos The two configurations agree for timestamps starting with 1972-01-01 00:00:00
1092 1.9 christos <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for
1093 1.9 christos timestamps starting with <code>time_t</code> 78 796 800,
1094 1.9 christos which corresponds to the first leap second
1095 1.9 christos 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
1096 1.9 christos and to
1097 1.9 christos 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
1098 1.9 christos In practice the two configurations also agree for timestamps before
1099 1.9 christos 1972 even though the historical situation is messy, partly because
1100 1.9 christos neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
1101 1.14 christos is well-defined for sufficiently old timestamps.</li>
1102 1.1 christos <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1103 1.1 christos discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1104 1.1 christos leap second so that they disagree with <abbr>UTC</abbr> by at most a
1105 1.1 christos half second, even though every <abbr>POSIX</abbr> minute has exactly
1106 1.4 christos sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1107 1.1 christos "<code>posix</code>" configuration, is <a
1108 1.1 christos href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
1109 1.12 christos the <abbr>NTP</abbr> reference implementation, <a
1110 1.12 christos href="https://github.com/google/unsmear">supports</a> conversion between
1111 1.12 christos <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
1112 1.7 christos cloud service providers. However, according to
1113 1.13 christos <a href="https://datatracker.ietf.org/doc/html/rfc8633#section-3.7.1">§3.7.1 of
1114 1.7 christos Network Time Protocol Best Current Practices</a>
1115 1.7 christos (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1116 1.9 christos applications requiring accurate <abbr>UTC</abbr> or civil time,
1117 1.7 christos and is intended for use only in single, well-controlled environments.</li>
1118 1.1 christos <li>The <a
1119 1.1 christos href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
1120 1.1 christos Second Discussion List</a> covers <a
1121 1.1 christos href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
1122 1.1 christos and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
1123 1.1 christos discussed further in
1124 1.1 christos <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1125 1.1 christos leap second: its history and possible future</a>.
1126 1.1 christos <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1127 1.1 christos might be redefined
1128 1.1 christos without Leap Seconds</a> gives pointers on this
1129 1.14 christos contentious issue.
1130 1.14 christos The General Conference on Weights and Measures
1131 1.14 christos <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">voted in 2022</a>
1132 1.14 christos to discontinue the use of leap seconds by 2035, replacing them with an
1133 1.14 christos as-yet-undetermined scheme some time after the year 2135.
1134 1.14 christos </li>
1135 1.1 christos </ul>
1136 1.6 christos </section>
1137 1.6 christos
1138 1.6 christos <section>
1139 1.1 christos <h2 id="notation">Time notation</h2>
1140 1.1 christos <ul>
1141 1.13 christos <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
1142 1.1 christos Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1143 1.1 christos zone names, abbreviations, identifiers, and formats. For example, it
1144 1.1 christos contains French translations for "Eastern European Summer Time",
1145 1.1 christos "<abbr title="Eastern European Summer Time">EEST</abbr>", and
1146 1.1 christos "Bucharest". Its
1147 1.1 christos <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1148 1.1 christos charts</a> show these values for many locales. Data values are available in
1149 1.1 christos both <abbr title="Locale Data Markup Language">LDML</abbr>
1150 1.1 christos (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1151 1.1 christos <li>
1152 1.1 christos <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1153 1.6 christos the international standard date and time notation</a> covers
1154 1.1 christos <a
1155 1.6 christos href="https://www.iso.org/standard/70907.html"><em><abbr
1156 1.6 christos title="International Organization for Standardization">ISO</abbr>
1157 1.6 christos 8601-1:2019 – Date and time – Representations for information
1158 1.6 christos interchange – Part 1: Basic rules</em></a>.</li>
1159 1.1 christos <li>
1160 1.6 christos <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1161 1.1 christos Schema: Datatypes – dateTime</a> specifies a format inspired by
1162 1.1 christos <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1163 1.13 christos <li><a href="https://datatracker.ietf.org/doc/html/rfc5322#section-3.3">§3.3 of
1164 1.1 christos Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1165 1.1 christos specifies the time notation used in email and <a
1166 1.1 christos href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1167 1.1 christos headers.</li>
1168 1.1 christos <li>
1169 1.13 christos <a href="https://datatracker.ietf.org/doc/html/rfc3339">Date and Time
1170 1.1 christos on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1171 1.1 christos specifies an <abbr>ISO</abbr> 8601
1172 1.1 christos profile for use in new Internet
1173 1.1 christos protocols.</li>
1174 1.1 christos <li>
1175 1.7 christos <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1176 1.1 christos Formats on the Web</a> surveys web- and Internet-oriented date and time
1177 1.1 christos formats.</li>
1178 1.1 christos <li>Alphabetic time zone abbreviations should not be used as unique
1179 1.1 christos identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1180 1.1 christos practice. For example, in English-speaking North America
1181 1.1 christos "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
1182 1.1 christos but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1183 1.1 christos and French-speaking North Americans prefer
1184 1.1 christos "<abbr title="Heure Normale du Centre">HNC</abbr>" to
1185 1.1 christos "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
1186 1.2 christos database contains English abbreviations for many timestamps;
1187 1.1 christos unfortunately some of these abbreviations were merely the database maintainers'
1188 1.1 christos inventions, and these have been removed when possible.</li>
1189 1.1 christos <li>Numeric time zone abbreviations typically count hours east of
1190 1.1 christos <abbr>UT</abbr>, e.g., +09 for Japan and
1191 1.1 christos −10 for Hawaii. However, the <abbr>POSIX</abbr>
1192 1.1 christos <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
1193 1.1 christos For example, one might use <code><abbr>TZ</abbr>="<abbr
1194 1.1 christos title="Japan Standard Time">JST</abbr>-9"</code> and
1195 1.1 christos <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1196 1.1 christos for Japan and Hawaii, respectively. If the
1197 1.1 christos <code><abbr>tz</abbr></code> database is available, it is usually better to use
1198 1.1 christos settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1199 1.1 christos <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1200 1.2 christos confusion, handle old timestamps better, and insulate you better from
1201 1.1 christos any future changes to the rules. One should never set
1202 1.1 christos <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1203 1.1 christos <code>"GMT-9"</code>, though, since this would incorrectly imply that
1204 1.1 christos local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1205 1.1 christos is called "<abbr>GMT</abbr>".</li>
1206 1.1 christos </ul>
1207 1.6 christos </section>
1208 1.6 christos
1209 1.6 christos <section>
1210 1.1 christos <h2 id="see-also">See also</h2>
1211 1.1 christos <ul>
1212 1.7 christos <li><a href="theory.html">Theory and pragmatics of the
1213 1.7 christos <code><abbr>tz</abbr></code> code and data</a></li>
1214 1.1 christos <li><a href="tz-art.html">Time and the Arts</a></li>
1215 1.1 christos </ul>
1216 1.6 christos </section>
1217 1.6 christos
1218 1.6 christos <footer>
1219 1.1 christos <hr>
1220 1.1 christos This web page is in the public domain, so clarified as of
1221 1.1 christos 2009-05-17 by Arthur David Olson.
1222 1.1 christos <br>
1223 1.1 christos Please send corrections to this web page to the
1224 1.1 christos <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1225 1.6 christos </footer>
1226 1.1 christos </body>
1227 1.1 christos </html>
1228