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4 1.13 christos <title>Time zone and daylight saving time data</title>
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14 1.13 christos <h1>Time zone and daylight saving time data</h1>
15 1.1 christos <p>
16 1.1 christos <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and
17 1.1 christos <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a>
18 1.1 christos rules are controlled by individual
19 1.1 christos governments. They are sometimes changed with little notice, and their
20 1.1 christos histories and planned futures are often recorded only fitfully. Here
21 1.1 christos is a summary of attempts to organize and record relevant data in this
22 1.1 christos area.
23 1.1 christos </p>
24 1.6 christos <nav>
25 1.6 christos <ul>
26 1.6 christos <li>The <code><abbr>tz</abbr></code> database product and process
27 1.6 christos <ul>
28 1.6 christos <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li>
29 1.6 christos <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li>
30 1.6 christos <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li>
31 1.14 christos <li><a href="#coordinating">Coordinating with governments and distributors</a></li>
32 1.6 christos <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
33 1.6 christos </ul>
34 1.6 christos </li>
35 1.6 christos <li>Uses of the <code><abbr>tz</abbr></code> database
36 1.6 christos <ul>
37 1.6 christos <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
38 1.6 christos <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
39 1.6 christos <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
40 1.6 christos <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
41 1.6 christos <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
42 1.6 christos </ul>
43 1.6 christos </li>
44 1.6 christos <li>Related data
45 1.6 christos <ul>
46 1.6 christos <li><a href="#other-dbs">Other time zone databases</a></li>
47 1.6 christos <li><a href="#maps">Maps</a></li>
48 1.6 christos <li><a href="#boundaries">Time zone boundaries</a></li>
49 1.6 christos </ul>
50 1.6 christos </li>
51 1.6 christos <li>Timekeeping concepts
52 1.6 christos <ul>
53 1.6 christos <li><a href="#civil">Civil time concepts and history</a></li>
54 1.6 christos <li><a href="#national">National histories of legal time</a></li>
55 1.7 christos <li><a href="#costs">Costs and benefits of time shifts</a></li>
56 1.6 christos <li><a href="#precision">Precision timekeeping</a></li>
57 1.6 christos <li><a href="#notation">Time notation</a></li>
58 1.6 christos </ul>
59 1.6 christos </li>
60 1.13 christos <li><a href="#see-also">See also</a></li>
61 1.6 christos </ul>
62 1.6 christos </nav>
63 1.6 christos
64 1.6 christos <section>
65 1.1 christos <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
66 1.1 christos <p>
67 1.1 christos The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
68 1.1 christos time zone database contains code and data
69 1.1 christos that represent the history of local time
70 1.1 christos for many representative locations around the globe.
71 1.1 christos It is updated periodically to reflect changes made by political bodies
72 1.1 christos to time zone boundaries and daylight saving rules.
73 1.1 christos This database (known as <code><abbr>tz</abbr></code>,
74 1.1 christos <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
75 1.1 christos is used by several implementations,
76 1.1 christos including
77 1.1 christos <a href="https://www.gnu.org/software/libc/">the
78 1.20 christos <abbr title="GNUs Not Unix">GNU</abbr>
79 1.1 christos C Library</a> (used in
80 1.1 christos <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
81 1.1 christos <a href="https://www.android.com">Android</a>,
82 1.1 christos <a href="https://www.freebsd.org">Free<abbr
83 1.1 christos title="Berkeley Software Distribution">BSD</abbr></a>,
84 1.1 christos <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
85 1.1 christos <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
86 1.17 christos <a href="https://www.chromium.org/chromium-os/">ChromiumOS</a>,
87 1.1 christos <a href="https://cygwin.com">Cygwin</a>,
88 1.7 christos <a href="https://mariadb.org">MariaDB</a>,
89 1.17 christos <a href="https://musl.libc.org">musl libc</a>,
90 1.1 christos <a href="https://www.mysql.com">MySQL</a>,
91 1.1 christos <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
92 1.1 christos title="Web Operating System">webOS</abbr></a>,
93 1.7 christos <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
94 1.1 christos title="Advanced Interactive eXecutive">AIX</abbr></a>,
95 1.21 christos <a href="https://www.apple.com/os/ios/"><abbr
96 1.1 christos title="iPhone OS">iOS</abbr></a>,
97 1.21 christos <a href="https://www.apple.com/os/macos/">macOS</a>,
98 1.1 christos <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
99 1.21 christos <a href="https://vmssoftware.com">Open<abbr
100 1.1 christos title="Virtual Memory System">VMS</abbr></a>,
101 1.20 christos <a href="https://www.oracle.com/database/">Oracle Database</a>,
102 1.21 christos <a href="https://www.oracle.com/solaris/solaris11/">Oracle Solaris</a>,
103 1.21 christos and <a href="https://qnx.software/en">QNX</a>.</p>
104 1.1 christos <p>
105 1.2 christos Each main entry in the database represents a <dfn>timezone</dfn>
106 1.2 christos for a set of civil-time clocks that have all agreed since 1970.
107 1.2 christos Timezones are typically identified by continent or ocean and then by the
108 1.2 christos name of the largest city within the region containing the clocks.
109 1.1 christos For example, <code>America/New_York</code>
110 1.1 christos represents most of the <abbr title="United States">US</abbr> eastern time zone;
111 1.1 christos <code>America/Phoenix</code> represents most of Arizona, which
112 1.2 christos uses mountain time without daylight saving time (<abbr>DST</abbr>);
113 1.1 christos <code>America/Detroit</code> represents most of Michigan, which uses
114 1.1 christos eastern time but with different <abbr>DST</abbr> rules in 1975;
115 1.1 christos and other entries represent smaller regions like Starke County,
116 1.1 christos Indiana, which switched from central to eastern time in 1991
117 1.1 christos and switched back in 2006.
118 1.17 christos To use the database on a <a
119 1.1 christos href="https://en.wikipedia.org/wiki/POSIX"><abbr
120 1.17 christos title="Portable Operating System Interface">POSIX</abbr>.1-2024</a>
121 1.1 christos implementation set the <code><abbr>TZ</abbr></code>
122 1.20 christos environment variable to the locations full name,
123 1.1 christos e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
124 1.1 christos <p>
125 1.2 christos Associated with each timezone is a history of offsets from
126 1.1 christos <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
127 1.1 christos Time</a> (<abbr>UT</abbr>), which is <a
128 1.1 christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
129 1.1 christos Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
130 1.2 christos for timestamps after 1960 this is more precisely <a
131 1.1 christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
132 1.1 christos Universal Time</a> (<abbr>UTC</abbr>).
133 1.1 christos The database also records when daylight saving time was in use,
134 1.1 christos along with some time zone abbreviations such as <abbr>EST</abbr>
135 1.1 christos for Eastern Standard Time in the <abbr>US</abbr>.</p>
136 1.6 christos </section>
137 1.6 christos
138 1.6 christos <section>
139 1.1 christos <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
140 1.1 christos <p>
141 1.1 christos The following <a
142 1.1 christos href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
143 1.20 christos the latest releases two
144 1.1 christos <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
145 1.1 christos to a <abbr>GNU</abbr>/Linux or similar host.</p>
146 1.2 christos <pre><code>mkdir tzdb
147 1.1 christos cd tzdb
148 1.1 christos <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
149 1.1 christos wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
150 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 -
151 1.1 christos gzip -dc tzdata-latest.tar.gz | tar -xf -
152 1.1 christos </code></pre>
153 1.1 christos <p>Alternatively, the following shell commands download the same
154 1.1 christos release in a single-tarball format containing extra data
155 1.1 christos useful for regression testing:</p>
156 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>
157 1.1 christos <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
158 1.1 christos </code></pre>
159 1.1 christos <p>These commands use convenience links to the latest release
160 1.1 christos of the <code><abbr>tz</abbr></code> database hosted by the
161 1.1 christos <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
162 1.1 christos of the <a href="https://www.iana.org">Internet Assigned Numbers
163 1.1 christos Authority (IANA)</a>.
164 1.1 christos Older releases are in files named
165 1.1 christos <code>tzcode<var>V</var>.tar.gz</code>,
166 1.1 christos <code>tzdata<var>V</var>.tar.gz</code>, and
167 1.1 christos <code>tzdb-<var>V</var>.tar.lz</code>,
168 1.1 christos where <code><var>V</var></code> is the version.
169 1.1 christos Since 1996, each version has been a four-digit year followed by
170 1.1 christos lower-case letter (<samp>a</samp> through <samp>z</samp>,
171 1.1 christos then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
172 1.1 christos through <samp>zzz</samp>, and so on).
173 1.13 christos Since version 2022a, each release has been distributed in
174 1.20 christos <a href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/pax.html#tag_20_94_13_06">POSIX
175 1.10 christos ustar interchange format</a>, compressed as described above;
176 1.14 christos older releases use a nearly compatible format.
177 1.1 christos Since version 2016h, each release has contained a text file named
178 1.20 christos <code>version</code> whose first (and currently only) line is the version.
179 1.20 christos <a href="https://ftp.iana.org/tz/releases/">Older archived releases are
180 1.20 christos available</a> via
181 1.20 christos <a href="https://en.wikipedia.org/wiki/HTTPS"><abbr
182 1.20 christos title="Hypertext Transfer Protocol Secure">HTTPS</abbr></a>,
183 1.20 christos <a href="https://en.wikipedia.org/wiki/Rsync"><abbr
184 1.20 christos title="remote sync">rsync</abbr></a>, and
185 1.20 christos <a href="https://en.wikipedia.org/wiki/FTP"><abbr
186 1.20 christos title="File Transfer Protocol">FTP</abbr></a>.
187 1.21 christos <p>Alternatively, a development repository of code and data can be
188 1.1 christos retrieved from <a href="https://github.com">GitHub</a> via the shell
189 1.1 christos command:</p>
190 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>
191 1.1 christos </code></pre>
192 1.1 christos <p>
193 1.1 christos Since version 2012e, each release has been tagged in development repositories.
194 1.1 christos Untagged commits are less well tested and probably contain
195 1.1 christos more errors.</p>
196 1.1 christos <p>
197 1.1 christos After obtaining the code and data files, see the
198 1.1 christos <code>README</code> file for what to do next.
199 1.1 christos The code lets you compile the <code><abbr>tz</abbr></code> source files into
200 1.2 christos machine-readable binary files, one for each location. The binary files
201 1.17 christos are in a special format specified by
202 1.20 christos <a href="https://www.rfc-editor.org/rfc/rfc9636">The
203 1.17 christos Time Zone Information Format (<abbr>TZif</abbr>)</a>
204 1.18 christos (Internet <abbr title="Request For Comments">RFC</abbr> 9636).
205 1.2 christos The code also lets
206 1.2 christos you read a <abbr>TZif</abbr> file and interpret timestamps for that
207 1.1 christos location.</p>
208 1.6 christos </section>
209 1.6 christos
210 1.6 christos <section>
211 1.1 christos <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
212 1.1 christos <p>
213 1.1 christos The <code><abbr>tz</abbr></code> code and data
214 1.1 christos are by no means authoritative. If you find errors, please
215 1.17 christos email changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
216 1.20 christos the time zone mailing list.
217 1.20 christos The mailing list and its archives are public,
218 1.20 christos so please do not send confidential information.
219 1.20 christos See
220 1.17 christos <a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing
221 1.20 christos lists main page</a> to subscribe or to browse its archive of old messages.
222 1.17 christos <a href="https://tzdata-meta.timtimeonline.com">Metadata for mailing list
223 1.5 christos discussions</a> and corresponding data changes can be
224 1.5 christos generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
225 1.5 christos </p>
226 1.1 christos <p>
227 1.1 christos Changes to the <code><abbr>tz</abbr></code> code and data are often
228 1.1 christos propagated to clients via operating system updates, so
229 1.1 christos client <code><abbr>tz</abbr></code> data can often be corrected by
230 1.1 christos applying these updates. With GNU/Linux and similar systems, if your
231 1.1 christos maintenance provider has not yet adopted the
232 1.1 christos latest <code><abbr>tz</abbr></code> data, you can often short-circuit
233 1.1 christos the process by tailoring the generic instructions in
234 1.1 christos the <code><abbr>tz</abbr> README</code> file and installing the latest
235 1.1 christos data yourself. System-specific instructions for installing the
236 1.1 christos latest <code><abbr>tz</abbr></code> data have also been published
237 1.17 christos for <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates"><abbr>AIX</abbr></a>,
238 1.1 christos <a
239 1.21 christos href="https://source.android.com/docs/core/permissions/timezone-rules">Android</a>,
240 1.1 christos <a
241 1.11 christos href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
242 1.1 christos title="International Components for Unicode">ICU</abbr></a>,
243 1.11 christos <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
244 1.9 christos JDK</a>,
245 1.9 christos <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
246 1.1 christos href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
247 1.9 christos <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
248 1.11 christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
249 1.1 christos </p>
250 1.12 christos <p>Since version 2013a,
251 1.12 christos sources for the <code><abbr>tz</abbr></code> database have been
252 1.1 christos <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
253 1.1 christos title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
254 1.1 christos <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
255 1.1 christos with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
256 1.1 christos title="linefeed">LF</abbr></a>,
257 1.1 christos which can be modified by common text editors such
258 1.1 christos as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
259 1.20 christos <a href="https://gedit-text-editor.org">gedit</a>, and
260 1.1 christos <a href="https://www.vim.org">vim</a>.
261 1.1 christos Specialized source-file editing can be done via the
262 1.1 christos <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
263 1.1 christos zoneinfo</a> package for <a
264 1.1 christos href="https://www.sublimetext.com">Sublime Text</a> and the <a
265 1.1 christos href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
266 1.1 christos zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
267 1.1 christos Studio Code</a>.
268 1.1 christos </p>
269 1.1 christos <p>
270 1.1 christos For further information about updates, please see
271 1.18 christos <a href="https://www.rfc-editor.org/rfc/rfc6557">Procedures for
272 1.17 christos Maintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557).
273 1.17 christos More detail can be
274 1.7 christos found in <a href="theory.html">Theory and pragmatics of the
275 1.7 christos <code><abbr>tz</abbr></code> code and data</a>.
276 1.2 christos <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
277 1.2 christos displays changes between recent <code><abbr>tzdb</abbr></code> versions.
278 1.1 christos </p>
279 1.6 christos </section>
280 1.6 christos
281 1.6 christos <section>
282 1.14 christos <h2 id="coordinating">Coordinating with governments and distributors</h2>
283 1.14 christos <p>
284 1.14 christos As discussed in
285 1.20 christos <a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How
286 1.20 christos Time Zones Are Coordinated</a>, the time zone database relies on
287 1.14 christos collaboration among governments, the time zone database volunteer
288 1.14 christos community, and data distributors downstream.
289 1.14 christos <p>
290 1.14 christos If your government plans to change its time zone boundaries or
291 1.20 christos daylight saving rules, please send email as described in
292 1.21 christos <a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a>.
293 1.20 christos Do this well in advance,
294 1.14 christos as this will lessen confusion and will coordinate updates to many cell phones,
295 1.14 christos computers, and other devices around the world.
296 1.14 christos In your email, please cite the legislation or regulation that specifies
297 1.14 christos the change, so that it can be checked for details such as the exact times
298 1.14 christos when clock transitions occur.
299 1.14 christos It is OK if a rule change is planned to affect clocks
300 1.14 christos far into the future, as a long-planned change can easily be reverted
301 1.20 christos or otherwise altered with a years notice before the change would have
302 1.14 christos affected clocks.</p>
303 1.14 christos <p>
304 1.14 christos There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases.
305 1.14 christos However, typically a release occurs every few months.
306 1.14 christos Many downstream timezone data distributors wait for
307 1.14 christos a <code><abbr>tzdb</abbr></code> release before they produce an update
308 1.14 christos to time zone behavior in consumer devices and software products.
309 1.14 christos After a release, various parties must integrate, test,
310 1.14 christos and roll out an update before <a
311 1.14 christos href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes.
312 1.14 christos These updates can be expensive, for both the <a
313 1.14 christos href="https://en.wikipedia.org/wiki/Quality_assurance">quality
314 1.14 christos assurance</a> process and the overall cost of shipping and installing
315 1.20 christos updates to each devices copy of <code><abbr>tzdb</abbr></code>.
316 1.14 christos Updates may be batched with other updates and may take substantial
317 1.14 christos time to reach end users after a release.
318 1.14 christos Older devices may no longer be supported and thus may never be updated,
319 1.14 christos which means they will continue to use out-of-date rules.</p>
320 1.14 christos <p>
321 1.14 christos For these reasons any rule change should be promulgated at least a
322 1.14 christos year before it affects how clocks operate; otherwise, there is a good
323 1.14 christos chance that many clocks will be wrong due to delays in propagating updates,
324 1.14 christos and that residents will be confused or even actively resist the change.
325 1.20 christos The shorter the notice, the more likely clock problems will arise; see <a
326 1.21 christos href="https://codeofmatt.com/on-the-timing-of-time-zone-changes/">On
327 1.20 christos the Timing of Time Zone Changes</a> for examples.
328 1.14 christos </p>
329 1.14 christos </section>
330 1.14 christos
331 1.14 christos <section>
332 1.1 christos <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
333 1.1 christos <ul>
334 1.1 christos <li>The article
335 1.1 christos <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
336 1.1 christos an encyclopedic summary.</li>
337 1.1 christos <li><a href="tz-how-to.html">How to Read the
338 1.1 christos tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
339 1.1 christos database format.</li>
340 1.1 christos <li><a
341 1.1 christos href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
342 1.1 christos literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
343 1.20 christos databases style.</li>
344 1.10 christos <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
345 1.10 christos managing time in the internet</a> analyzes the database longitudinally.</li>
346 1.1 christos </ul>
347 1.6 christos </section>
348 1.6 christos
349 1.6 christos <section>
350 1.1 christos <h2 id="web">Web sites using recent versions of the
351 1.1 christos <code><abbr>tz</abbr></code> database</h2>
352 1.1 christos <p>
353 1.1 christos These are listed roughly in ascending order of complexity and fanciness.
354 1.1 christos </p>
355 1.1 christos <ul>
356 1.20 christos <li><a href="https://time.is">Time.is</a> shows locations
357 1.1 christos time and zones.</li>
358 1.1 christos <li><a href="https://www.timejones.com">TimeJones.com</a>,
359 1.1 christos <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
360 1.7 christos <a href="https://www.worldclock.com">The World Clock</a>
361 1.1 christos are time zone converters.</li>
362 1.8 christos <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
363 1.8 christos publishes <code><abbr>tzdb</abbr></code>-derived data in
364 1.8 christos <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
365 1.8 christos title="comma-separated values">CSV</abbr></a> and
366 1.8 christos in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
367 1.8 christos title="Structured Query Language">SQL</abbr></a> form.</li>
368 1.1 christos <li><a
369 1.6 christos href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
370 1.1 christos lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
371 1.1 christos <li><a
372 1.13 christos href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
373 1.1 christos Time in 1000 Places</a> uses descriptions of the values.</li>
374 1.20 christos <li><a href="https://www.timeanddate.com/worldclock/">The World Clock
375 1.1 christos Worldwide</a> lets you sort zone names and convert times.</li>
376 1.1 christos <li><a href="https://24timezones.com">24TimeZones</a> has a world
377 1.1 christos time map and a time converter.</li>
378 1.1 christos <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
379 1.1 christos calculates the current time difference between locations.</li>
380 1.13 christos <li><a href="https://www.wx-now.com">Weather Now</a> and
381 1.13 christos <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li>
382 1.1 christos </ul>
383 1.6 christos </section>
384 1.6 christos
385 1.6 christos <section>
386 1.1 christos <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
387 1.1 christos <ul>
388 1.20 christos <li><a href="https://www.rfc-editor.org/rfc/rfc7808">Time Zone
389 1.20 christos Data Distribution Service</a> (TZDIST, Internet <abbr>RFC</abbr> 7808)
390 1.20 christos is associated with
391 1.20 christos <a href="https://www.rfc-editor.org/rfc/rfc7809">CalDAV</a>
392 1.1 christos (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
393 1.2 christos transferring time zone data by reference.
394 1.8 christos <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
395 1.20 christos implementations</a> are available.</li>
396 1.20 christos <li>The <a href="https://www.rfc-editor.org/rfc/rfc5545">iCalendar format</a>
397 1.20 christos (Internet <abbr>RFC</abbr> 5445)
398 1.1 christos covers time zone
399 1.1 christos data; see its VTIMEZONE calendar component.
400 1.1 christos The iCalendar format requires specialized parsers and generators; a
401 1.18 christos variant <a href="https://www.rfc-editor.org/rfc/rfc6321">xCal</a>
402 1.1 christos (Internet <abbr>RFC</abbr> 6321) uses
403 1.1 christos <a href="https://www.w3.org/XML/"><abbr
404 1.1 christos title="Extensible Markup Language">XML</abbr></a> format, and a variant
405 1.18 christos <a href="https://www.rfc-editor.org/rfc/rfc7265">jCal</a>
406 1.1 christos (Internet <abbr>RFC</abbr> 7265)
407 1.17 christos uses <a href="https://www.json.org/json-en.html"><abbr
408 1.1 christos title="JavaScript Object Notation">JSON</abbr></a> format.</li>
409 1.1 christos </ul>
410 1.6 christos </section>
411 1.6 christos
412 1.6 christos <section>
413 1.1 christos <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
414 1.7 christos <p>Although some of these do not fully support
415 1.7 christos <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
416 1.7 christos distributions you can generally work around compatibility problems by
417 1.7 christos running the command <code>make rearguard_tarballs</code> and compiling
418 1.7 christos from the resulting tarballs instead.</p>
419 1.1 christos <ul>
420 1.17 christos <li><a href="https://github.com/libical/vzic">Vzic</a> is a <a
421 1.14 christos href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a>
422 1.1 christos program that compiles
423 1.1 christos <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
424 1.1 christos Vzic is freely
425 1.1 christos available under the <a
426 1.1 christos href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
427 1.1 christos General Public License (<abbr
428 1.1 christos title="General Public License">GPL</abbr>)</a>.</li>
429 1.1 christos <li><a
430 1.4 christos href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
431 1.1 christos contains a script <code>parse_olson</code> that compiles
432 1.1 christos <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
433 1.1 christos modules. It is part of the Perl <a
434 1.4 christos href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
435 1.4 christos which is freely
436 1.1 christos available under both the <abbr>GPL</abbr> and the Perl Artistic
437 1.1 christos License. DateTime::TimeZone also contains a script
438 1.1 christos <code>tests_from_zdump</code> that generates test cases for each clock
439 1.1 christos transition in the <code><abbr>tz</abbr></code> database.</li>
440 1.1 christos <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
441 1.1 christos Database Parser</a> is a
442 1.14 christos <a href="https://en.wikipedia.org/wiki/C++">C++</a> parser and
443 1.17 christos runtime library with a <a
444 1.22 christos href="https://en.cppreference.com/cpp/chrono"><code>std::chrono</code> API</a>
445 1.17 christos that is a standard part of C++.
446 1.1 christos It is freely available under the
447 1.1 christos <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
448 1.13 christos <li><a id="ICU" href="https://icu.unicode.org">International Components for
449 1.1 christos Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
450 1.14 christos href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a>
451 1.1 christos libraries for internationalization that
452 1.1 christos has a compiler from <code><abbr>tz</abbr></code> source
453 1.1 christos and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
454 1.1 christos (mentioned <a href="#CLDR">below</a>)
455 1.1 christos into an <abbr>ICU</abbr>-specific format.
456 1.1 christos <abbr>ICU</abbr> is freely available under a
457 1.1 christos <abbr>BSD</abbr>-style license.</li>
458 1.1 christos <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
459 1.1 christos the <a href="https://elixir-lang.org">Elixir</a> language downloads
460 1.7 christos and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
461 1.1 christos title="Application Program Interface">API</abbr>s for use. It is
462 1.1 christos freely available under the <abbr>MIT</abbr> license.</li>
463 1.1 christos <li>Java-based compilers and libraries include:
464 1.1 christos <ul>
465 1.1 christos <li>The <a
466 1.9 christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
467 1.1 christos tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
468 1.17 christos <a href="https://openjdk.org">OpenJDK</a> and
469 1.17 christos <a href="https://jdk.java.net">Oracle JDK</a>.
470 1.9 christos Although its source code is proprietary, its executable is available under the
471 1.9 christos <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
472 1.9 christos Timezone Updater License Agreement</a>.</li>
473 1.1 christos <li>The <a
474 1.21 christos href="https://www.oracle.com/technical-resources/articles/java/jf14-date-time.html">Java
475 1.6 christos SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
476 1.4 christos href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
477 1.1 christos which is freely available under a <abbr>BSD</abbr>-style license.</li>
478 1.20 christos <li><a href="https://www.joda.org/joda-time/">Joda-Time Java date
479 1.1 christos and time <abbr>API</abbr></a> contains a class
480 1.1 christos <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
481 1.1 christos <code><abbr>tz</abbr></code> source into a binary format. It inspired
482 1.1 christos Java 8 <code>java.time</code>, which its users should migrate to once
483 1.1 christos they can assume Java 8 or later. It is available under the <a
484 1.1 christos href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
485 1.9 christos <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
486 1.21 christos href="https://www.azul.com/products/components/ziupdater-time-zone-tool/">ZIUpdater</a>
487 1.20 christos are alternatives to TZUpdater. IANA Updaters license is unclear;
488 1.9 christos ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
489 1.4 christos <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
490 1.4 christos time library for Android</a> and
491 1.17 christos <a href="https://github.com/MenoData/Time4J">Time4J: Advanced date,
492 1.4 christos time and interval library for Java</a> compile
493 1.4 christos <code><abbr>tz</abbr></code> source into a binary format.
494 1.4 christos Time4A is available under the Apache License and Time4J is
495 1.1 christos available under the <a
496 1.1 christos href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
497 1.1 christos General Public License (<abbr title="Lesser General Public
498 1.1 christos License">LGPL</abbr>)</a>.</li>
499 1.1 christos <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
500 1.1 christos Java-based libraries.</li>
501 1.1 christos </ul>
502 1.20 christos <li><a href="https://nodatime.org">Noda Time Date and
503 1.1 christos time <abbr>API</abbr> for .NET</a>
504 1.4 christos is like Joda-Time and Time4J, but for the .NET framework instead of Java.
505 1.4 christos It is freely available under the Apache License.</li>
506 1.8 christos <li>Many modern
507 1.8 christos <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
508 1.8 christos runtimes support <code><abbr>tz</abbr></code> natively via the
509 1.16 christos <code>timeZone</code> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><code>Intl.DateTimeFormat</code></a>.
510 1.10 christos This can be used as-is or with most of the following libraries,
511 1.16 christos many of which also support runtimes lacking the <code>timeZone</code> option.
512 1.1 christos <ul>
513 1.8 christos <li>The <a
514 1.16 christos href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code>
515 1.8 christos timezone polyfill</a>
516 1.8 christos is freely available under a <abbr>BSD</abbr>-style license.</li>
517 1.17 christos <li>The <a href="https://date-fns.org">date-fns</a>
518 1.10 christos library manipulates timezone-aware timestamps in browsers and
519 1.10 christos in <a href="https://nodejs.org/en/">Node.js</a>.
520 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
521 1.10 christos <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
522 1.10 christos minimalist replacement for the date and time API of
523 1.10 christos the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
524 1.10 christos manipulation library.
525 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
526 1.10 christos <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
527 1.16 christos timezone support for the <code>Intl</code> API.
528 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
529 1.1 christos <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
530 1.10 christos Moment.js plugin.
531 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
532 1.8 christos <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
533 1.8 christos JavaScript library that supports date arithmetic that is time zone
534 1.8 christos aware. It is freely available under the <abbr>MIT</abbr> license.</li>
535 1.10 christos <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
536 1.10 christos supports live <code><abbr>tzdb</abbr></code> updates,
537 1.10 christos astronomical and atomic time, a command-line interface,
538 1.10 christos and full <a
539 1.10 christos href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
540 1.10 christos Its companion <a
541 1.10 christos href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
542 1.10 christos can generate <abbr>TZif</abbr> and other files, and a companion website
543 1.10 christos <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
544 1.10 christos convert timestamps, view transition histories, and download code and data.
545 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
546 1.1 christos </ul>
547 1.10 christos The proposed <a
548 1.16 christos href="https://github.com/tc39/proposal-temporal"><code>Temporal</code>
549 1.10 christos objects</a> let programs access an abstract view of
550 1.10 christos <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
551 1.20 christos href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScripts
552 1.16 christos problematic <code>Date</code> objects</a> when working with dates and times.
553 1.17 christos <li><a href="https://github.com/JuliaTime">JuliaTime</a> contains a
554 1.1 christos compiler from <code><abbr>tz</abbr></code> source into
555 1.17 christos <a href="https://julialang.org">Julia</a>. It is freely available
556 1.1 christos under the <abbr>MIT</abbr> license.</li>
557 1.20 christos <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr>
558 1.6 christos <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
559 1.6 christos title="Free Pascal Compiler">FPC</abbr></a>
560 1.2 christos compiles from <code><abbr>tz</abbr></code> source into
561 1.2 christos <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
562 1.2 christos as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
563 1.2 christos and <a
564 1.2 christos href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
565 1.2 christos It is freely available under a <abbr>BSD</abbr>-style license.</li>
566 1.20 christos <li><a href="https://pythonhosted.org/pytz/">pytz World Timezone
567 1.1 christos Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
568 1.1 christos <a href="https://www.python.org">Python</a>.
569 1.9 christos It is freely available under a <abbr>BSD</abbr>-style license.
570 1.11 christos In code that can assume Python 3.6 or later it is largely superseded; see <a
571 1.11 christos href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
572 1.11 christos The Fastest Footgun in the West</a>.</li>
573 1.20 christos <li><a href="https://tzinfo.github.io">TZInfo
574 1.1 christos Ruby Timezone Library</a>
575 1.1 christos compiles <code><abbr>tz</abbr></code> source into
576 1.1 christos <a href="https://www.ruby-lang.org/en/">Ruby</a>.
577 1.1 christos It is freely available under the <abbr>MIT</abbr> license.</li>
578 1.13 christos <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time
579 1.1 christos Library</a> is
580 1.1 christos a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
581 1.1 christos library that compiles <code><abbr>tz</abbr></code> source into a time
582 1.1 christos zone repository whose format
583 1.1 christos is either proprietary or an <abbr>XML</abbr>-encoded
584 1.1 christos representation.</li>
585 1.21 christos <li><a id="Tcl" href="https://www.tcl-lang.org">Tcl</a>
586 1.1 christos contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
587 1.1 christos source into text files, along with a runtime that can read those
588 1.1 christos files. Tcl is freely available under a <abbr>BSD</abbr>-style
589 1.1 christos license.</li>
590 1.1 christos </ul>
591 1.6 christos </section>
592 1.6 christos
593 1.6 christos <section>
594 1.2 christos <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
595 1.1 christos <ul>
596 1.1 christos <li>The <a
597 1.1 christos href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
598 1.1 christos Library</a>
599 1.1 christos has an independent, thread-safe implementation of
600 1.2 christos a <abbr>TZif</abbr> file reader.
601 1.1 christos This library is freely available under the LGPL
602 1.1 christos and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
603 1.20 christos <li><a href="https://www.gnome.org">GNOME</a>s
604 1.21 christos <a href="https://docs.gtk.org/glib/">GLib</a> has
605 1.2 christos a <abbr>TZif</abbr> file reader written in C that
606 1.1 christos creates a <code>GTimeZone</code> object representing sets
607 1.1 christos of <abbr>UT</abbr> offsets.
608 1.1 christos It is freely available under the <abbr>LGPL</abbr>.</li>
609 1.1 christos <li>The
610 1.20 christos <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>s
611 1.1 christos <code>baltzo::TimeZoneUtil</code> component contains a C++
612 1.2 christos implementation of a <abbr>TZif</abbr> file reader. It is freely available under
613 1.1 christos the Apache License.</li>
614 1.1 christos <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
615 1.1 christos library that translates between <abbr>UT</abbr> and civil time and
616 1.2 christos can read <abbr>TZif</abbr> files. It is freely available under the Apache
617 1.1 christos License.</li>
618 1.21 christos <li>The <a href="https://go.dev">Go programming language</a>
619 1.20 christos has a <abbr>TZif</abbr> file reader <a
620 1.20 christos href="https://pkg.go.dev/time#LoadLocationFromTZData"><code>LoadLocationFromTZData</code></a>.</li>
621 1.16 christos <li>The
622 1.16 christos <a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code>
623 1.16 christos package</a> contains Python code
624 1.16 christos to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map
625 1.17 christos <code><abbr>tz</abbr></code> settings to proleptic TZ approximations.
626 1.16 christos For example, it maps <code>"Africa/Cairo"</code>
627 1.16 christos to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>,
628 1.16 christos an approximation valid for Cairo timestamps from 2023 on.
629 1.17 christos This can help porting to platforms that support only proleptic TZ.
630 1.16 christos The package is freely available under the MIT license.</li>
631 1.1 christos <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
632 1.2 christos library that reads <abbr>TZif</abbr> files and converts
633 1.2 christos timestamps from one time zone or format to another.
634 1.21 christos It is used by <a href="https://www.php.net"><abbr
635 1.1 christos title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
636 1.1 christos <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
637 1.1 christos and <a href="https://www.mongodb.com">MongoDB</a>.
638 1.1 christos It is freely available under the <abbr>MIT</abbr> license.</li>
639 1.1 christos <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
640 1.2 christos <abbr>TZif</abbr> file reader.</li>
641 1.4 christos <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
642 1.1 christos DateTime::TimeZone::Tzfile</a>
643 1.2 christos is a <abbr>TZif</abbr> file reader written in Perl.
644 1.1 christos It is freely available under the same terms as Perl
645 1.1 christos (dual <abbr>GPL</abbr> and Artistic license).</li>
646 1.9 christos <li>Python has a <a id="python-zoneinfo"
647 1.11 christos href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
648 1.8 christos class</a> that reads <abbr>TZif</abbr> data and creates objects
649 1.8 christos that represent <code><abbr>tzdb</abbr></code> timezones.
650 1.8 christos Python is freely available under the
651 1.11 christos <a href="https://docs.python.org/3/license.html">Python Software Foundation
652 1.8 christos License</a>.
653 1.17 christos A companion <a id="pypi-tzdata" href="https://pypi.org">PyPI</a> module
654 1.8 christos <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
655 1.8 christos supplies TZif data if the underlying system data cannot be found;
656 1.8 christos it is freely available under the Apache License.</li>
657 1.1 christos <li>The
658 1.1 christos public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
659 1.1 christos library contains a Python tool that
660 1.2 christos converts <abbr>TZif</abbr> data into
661 1.1 christos <abbr>JSON</abbr>-format data suitable for use
662 1.1 christos in its JavaScript library for time zone conversion. Dates before 1970
663 1.1 christos are not supported.</li>
664 1.1 christos <li>The <a
665 1.1 christos href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
666 1.1 christos package contains <a href="https://www.haskell.org">Haskell</a> code that
667 1.2 christos parses and uses <abbr>TZif</abbr> data. It is freely
668 1.1 christos available under a <abbr>BSD</abbr>-style license.</li>
669 1.1 christos </ul>
670 1.6 christos </section>
671 1.6 christos
672 1.6 christos <section>
673 1.1 christos <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
674 1.1 christos <ul>
675 1.1 christos <li><a href="https://foxclocks.org">FoxClocks</a>
676 1.1 christos is an extension for <a href="https://www.google.com/chrome/">Google
677 1.16 christos Chrome</a>, <a
678 1.21 christos href="https://www.firefox.com/en-US/">Firefox</a> and <a
679 1.21 christos href="https://www.thunderbird.net/en-US/">Thunderbird</a>.
680 1.1 christos It displays multiple clocks in the application window, and has a mapping
681 1.21 christos interface to <a href="https://earth.google.com/web/">Google Earth</a>.
682 1.1 christos It is freely available under the <abbr>GPL</abbr>.</li>
683 1.1 christos <li>Microsoft Windows 8.1
684 1.1 christos and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
685 1.3 christos data (mentioned <a href="#CLDR">below</a>) used by the
686 1.3 christos <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
687 1.3 christos <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
688 1.21 christos <a href="https://learn.microsoft.com/en-us/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
689 1.21 christos <a href="https://learn.microsoft.com/en-us/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
690 1.1 christos <a id="System.TimeZoneInfo"
691 1.21 christos href="https://learn.microsoft.com/en-us/archive/blogs/bclteam/exploring-windows-time-zones-with-system-timezoneinfo-josh-free">Exploring
692 1.1 christos Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
693 1.1 christos the older, proprietary method of Microsoft Windows 2000 and later,
694 1.1 christos which stores time zone data in the
695 1.1 christos <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
696 1.1 christos <a
697 1.20 christos href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone
698 1.20 christos Tzid table</a> or <a
699 1.21 christos href="https://github.com/unicode-org/cldr/blob/main/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
700 1.1 christos file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
701 1.1 christos to <code><abbr>tz</abbr></code> names.
702 1.21 christos These mappings can be performed programmatically via the
703 1.21 christos <a href="https://github.com/mattjohnsonpint/TimeZoneConverter">TimeZoneConverter</a>
704 1.21 christos .NET library,
705 1.3 christos or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
706 1.1 christos <li><a
707 1.21 christos href="https://www.oracle.com/java/">Oracle
708 1.1 christos Java</a> contains a copy of a subset of a recent
709 1.1 christos <code><abbr>tz</abbr></code> database in a
710 1.1 christos Java-specific format.</li>
711 1.1 christos </ul>
712 1.6 christos </section>
713 1.6 christos
714 1.6 christos <section>
715 1.1 christos <h2 id="other-dbs">Other time zone databases</h2>
716 1.1 christos <ul>
717 1.1 christos <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
718 1.20 christos is Astrodiensts Web version of Shanks and Pottengers out-of-print
719 1.4 christos time zone history atlases
720 1.21 christos <a href="https://search.worldcat.org/title/468828649">for the US</a> and
721 1.21 christos <a href="https://search.worldcat.org/title/76950459">for the world</a>.
722 1.4 christos Although these extensive atlases
723 1.4 christos <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
724 1.4 christos sources for much of the older <code><abbr>tz</abbr></code> data</a>,
725 1.4 christos they are unreliable as Shanks appears to have
726 1.1 christos guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
727 1.1 christos sources and do not indicate which entries are guesswork.</li>
728 1.1 christos <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
729 1.1 christos its own <code>tztab</code>(4) format.</li>
730 1.1 christos <li>Microsoft Windows has proprietary data mentioned
731 1.1 christos <a href="#System.TimeZoneInfo">above</a>.</li>
732 1.1 christos <li><a href="https://www.worldtimeserver.com">World Time Server</a>
733 1.1 christos is another time zone database.</li>
734 1.1 christos <li>The <a
735 1.21 christos href="https://www.iata.org/en/publications/manuals/standard-schedules-information">Standard
736 1.1 christos Schedules Information Manual</a> of the
737 1.1 christos International Air Transport Association
738 1.1 christos gives current time zone rules for airports served by commercial aviation.</li>
739 1.1 christos </ul>
740 1.6 christos </section>
741 1.6 christos
742 1.6 christos <section>
743 1.1 christos <h2 id="maps">Maps</h2>
744 1.1 christos <ul>
745 1.6 christos <li><a href="https://www.worldtimezone.com">World Time Zone Map
746 1.6 christos with current time</a>
747 1.21 christos has several fancy time zone maps; it covers Russia particularly well.</li>
748 1.1 christos <li><a
749 1.1 christos href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
750 1.1 christos much is time wrong around the world?</a> maps the difference between
751 1.1 christos mean solar and standard time, highlighting areas such as western China
752 1.20 christos where the two differ greatly. Its a bit out of date, unfortunately.</li>
753 1.21 christos <li>The
754 1.21 christos <a
755 1.21 christos href="https://maps.lib.utexas.edu/maps/world.html">PerryCastaeda
756 1.21 christos Library Map Collection</a> of the University of Texas at Austin has
757 1.21 christos copies of old maps taken from <a
758 1.21 christos href="https://en.wikipedia.org/wiki/The_World_Factbook"><em>The
759 1.21 christos World Factbook</em></a>, formerly published by the
760 1.21 christos <a href="https://www.cia.gov">US Central Intelligence Agency</a>.
761 1.21 christos Although the maps pictorial quality is good,
762 1.21 christos the maps do not indicate daylight saving time.</li>
763 1.1 christos </ul>
764 1.6 christos </section>
765 1.6 christos
766 1.6 christos <section>
767 1.1 christos <h2 id="boundaries">Time zone boundaries</h2>
768 1.2 christos <p>Geographical boundaries between timezones are available
769 1.9 christos from several <a
770 1.9 christos href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
771 1.9 christos geolocation</a>
772 1.1 christos services and other sources.</p>
773 1.1 christos <ul>
774 1.1 christos <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
775 1.1 christos Boundary Builder</a> extracts
776 1.1 christos <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
777 1.2 christos boundaries of <code><abbr>tzdb</abbr></code> timezones.
778 1.1 christos Its code is freely available under the <abbr>MIT</abbr> license, and
779 1.1 christos its data entries are freely available under the
780 1.1 christos <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
781 1.16 christos Open Database License</a>. The borders appear to be quite accurate.
782 1.16 christos Its main web page lists more than twenty libraries
783 1.16 christos for looking up a timezone name from a GPS coordinate.</li>
784 1.1 christos <li>Free access via a network API, if you register a key, is provided by
785 1.4 christos the <a
786 1.4 christos href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
787 1.4 christos Timezone web service</a>, the <a
788 1.21 christos href="https://developers.google.com/maps/documentation/timezone/overview">Google
789 1.4 christos Maps Time Zone API</a>, and
790 1.8 christos the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
791 1.1 christos Commercial network API access is provided
792 1.1 christos by <a href="https://askgeo.com">AskGeo</a>
793 1.1 christos and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
794 1.1 christos </li>
795 1.20 christos <li><a
796 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
797 1.1 christos to get a time zone from a location using latitude and longitude
798 1.20 christos coordinates?</a> discusses other geolocation possibilities.</li>
799 1.21 christos <li><a href="https://statoids.com/statoids.html">Administrative
800 1.20 christos Divisions of Countries (Statoids)</a> lists
801 1.1 christos political subdivision data related to time zones.</li>
802 1.13 christos <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software
803 1.20 christos GIS and Database Tools</a> includes a Manifold-format map of
804 1.16 christos world time zone boundaries circa 2007, distributed under the
805 1.1 christos <abbr>GPL</abbr>.</li>
806 1.1 christos <li>A ship within the <a
807 1.1 christos href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
808 1.20 christos waters</a> of any nation uses that nations time. In international
809 1.20 christos waters, time zone boundaries are meridians 15 apart, except that
810 1.20 christos <abbr>UT</abbr>12 and <abbr>UT</abbr>+12 are each 7.5
811 1.1 christos wide and are separated by
812 1.20 christos the 180 meridian (not by the International Date Line, which is
813 1.20 christos for land and territorial waters only). A captain can change ships
814 1.1 christos clocks any time after entering a new time zone; midnight changes are
815 1.1 christos common.</li>
816 1.1 christos </ul>
817 1.6 christos </section>
818 1.6 christos
819 1.6 christos <section>
820 1.1 christos <h2 id="civil">Civil time concepts and history</h2>
821 1.1 christos <ul>
822 1.1 christos <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
823 1.1 christos Walk through Time</a>
824 1.1 christos surveys the evolution of timekeeping.</li>
825 1.4 christos <li>The history of daylight saving time is surveyed in <a
826 1.21 christos href="https://www.webexhibits.org/daylightsaving/">About Daylight
827 1.20 christos Saving Time History, rationale, laws & dates</a> and summarized in
828 1.4 christos <a href="http://seizethedaylight.com/dst/">A Brief
829 1.4 christos History of Daylight Saving Time</a>.</li>
830 1.4 christos <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
831 1.4 christos Lords</a> discusses how authoritarians manipulate civil time.</li>
832 1.20 christos <li><a href="https://www.w3.org/TR/timezone/">Working with Time
833 1.20 christos and Time Zones</a>
834 1.1 christos contains guidelines and best practices for software applications that
835 1.1 christos deal with civil time.</li>
836 1.16 christos <li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of
837 1.1 christos the International Date Line</a> tells the story of the most important
838 1.1 christos time zone boundary.</li>
839 1.21 christos <li><a href="https://statoids.com/tconcept.html">Basic Time
840 1.1 christos Zone Concepts</a> discusses terminological issues behind time zones.</li>
841 1.1 christos </ul>
842 1.6 christos </section>
843 1.6 christos
844 1.6 christos <section>
845 1.1 christos <h2 id="national">National histories of legal time</h2>
846 1.1 christos <dl>
847 1.1 christos <dt>Australia</dt>
848 1.20 christos <dd>The Bureau of Meteorology publishes a list of <a
849 1.1 christos href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
850 1.1 christos Dates of Daylight Savings Time within Australia</a>.</dd>
851 1.1 christos <dt>Belgium</dt>
852 1.7 christos <dd>The Royal Observatory of Belgium maintains a table of time in
853 1.7 christos Belgium (in
854 1.16 christos <a href="https://robinfo.oma.be/nl/astro-info/tijd/"
855 1.7 christos hreflang="nl">Dutch</a> and <a
856 1.16 christos href="https://robinfo.oma.be/fr/astro-info/heure/"
857 1.7 christos hreflang="fr">French</a>).</dd>
858 1.1 christos <dt>Brazil</dt>
859 1.1 christos <dd>The Time Service Department of the National Observatory
860 1.1 christos records <a href="http://pcdsh01.on.br/DecHV.html"
861 1.20 christos hreflang="pt-BR">Brazils daylight saving time decrees (in
862 1.1 christos Portuguese)</a>.</dd>
863 1.1 christos <dt>Canada</dt>
864 1.1 christos <dd>National Research Council Canada publishes current
865 1.1 christos and some older information about <a
866 1.6 christos href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
867 1.6 christos zones and daylight saving time</a>.</dd>
868 1.1 christos <dt>Chile</dt>
869 1.1 christos <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
870 1.11 christos <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
871 1.20 christos Chiles official time (in Spanish)</a>.</dd>
872 1.3 christos <dt>China</dt>
873 1.3 christos <dd>The Hong Kong Observatory maintains a
874 1.7 christos <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
875 1.3 christos summer time in Hong Kong</a>,
876 1.20 christos and Macaus Meteorological and Geophysical Bureau maintains a <a
877 1.7 christos href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
878 1.3 christos history for Macau</a>.
879 1.3 christos Unfortunately the latter is incomplete and has errors.</dd>
880 1.1 christos <dt>Czech Republic</dt>
881 1.1 christos <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
882 1.1 christos hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
883 1.10 christos summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
884 1.1 christos <dt>Germany</dt>
885 1.1 christos <dd>The National Institute for Science and Technology maintains the <a
886 1.21 christos href="https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
887 1.1 christos of Legal Time in Germany</a>.</dd>
888 1.1 christos <dt>Israel</dt>
889 1.17 christos <dd><a
890 1.17 christos href="https://tz.cs.huji.ac.il">Israel Timezone Files</a>
891 1.17 christos lists official time-change announcements and laws since 1940,
892 1.17 christos almost all in Hebrew.</dd>
893 1.2 christos <dt>Malaysia</dt>
894 1.2 christos <dd>See Singapore <a href="#Singapore">below</a>.</dd>
895 1.1 christos <dt>Mexico</dt>
896 1.1 christos <dd>The Investigation and Analysis Service of the Mexican Library of
897 1.1 christos Congress has published a <a
898 1.13 christos href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm"
899 1.1 christos hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
900 1.1 christos <dt>Netherlands</dt>
901 1.16 christos <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm"
902 1.1 christos hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
903 1.1 christos covers the history of local time in the Netherlands from ancient times.</dd>
904 1.1 christos <dt>New Zealand</dt>
905 1.1 christos <dd>The Department of Internal Affairs maintains a brief <a
906 1.21 christos href="https://www.govt.nz/browse/recreation-and-the-environment/daylight-saving/history-of-daylight-saving-in-nz/">History
907 1.21 christos of Daylight Saving in NZ</a>.</dd>
908 1.14 christos <dt>Palestine</dt>
909 1.20 christos <dd>The Ministry of Telecom and Digital Economy publishes a <a
910 1.20 christos href="https://mtde.gov.ps/home/TimeZone"
911 1.14 christos hreflang="ar">history of clock changes (in Arabic)</a>.</dd>
912 1.10 christos <dt>Portugal</dt>
913 1.10 christos <dd>The Lisbon Astronomical Observatory publishes a
914 1.10 christos <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
915 1.10 christos legal time (in Portuguese)</a>.</dd>
916 1.1 christos <dt>Singapore</dt>
917 1.1 christos <dd><a id="Singapore"
918 1.7 christos href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
919 1.20 christos is Singapore in the Wrong Time Zone?</a> details the
920 1.1 christos history of legal time in Singapore and Malaysia.</dd>
921 1.1 christos <dt>United Kingdom</dt>
922 1.1 christos <dd><a
923 1.1 christos href="https://www.polyomino.org.uk/british-time/">History of
924 1.1 christos legal time in Britain</a> discusses in detail the country
925 1.6 christos with perhaps the best-documented history of clock adjustments.</dd>
926 1.1 christos <dt>United States</dt>
927 1.20 christos <dd>The Department of Transportations <a
928 1.1 christos href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
929 1.18 christos Time Zone Proceedings</a> lists changes to
930 1.18 christos official written time zone boundaries, and its <a
931 1.18 christos href="https://geodata.bts.gov/datasets/usdot::time-zones/about">Time
932 1.18 christos Zones dataset</a> maps current boundaries.
933 1.18 christos These boundaries are only for standard time, so the current map puts
934 1.18 christos all of Arizona in one time zone even though part of Arizona
935 1.18 christos observes <abbr>DST</abbr> and part does not.</dd>
936 1.1 christos <dt>Uruguay</dt>
937 1.1 christos <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
938 1.1 christos Navy (SOHMA) publishes an annual <a
939 1.13 christos href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
940 1.1 christos (in Spanish)</a>.</dd>
941 1.1 christos </dl>
942 1.6 christos </section>
943 1.6 christos
944 1.6 christos <section>
945 1.7 christos <h2 id="costs">Costs and benefits of time shifts</h2>
946 1.7 christos <p>Various sources argue for and against daylight saving time and time
947 1.7 christos zone shifts, and many scientific studies have been conducted. This
948 1.8 christos section summarizes reviews and position statements based on
949 1.8 christos scientific literature in the area.</p>
950 1.7 christos <ul>
951 1.7 christos <li>Carey RN, Sarma KM.
952 1.7 christos <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
953 1.7 christos daylight saving time on road traffic collision risk: a systematic
954 1.7 christos review</a>.
955 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>.
956 1.7 christos This reviews research literature and concludes that the evidence
957 1.7 christos neither supports nor refutes road safety benefits from
958 1.7 christos shifts in time zones.</li>
959 1.7 christos <li>Havranek T, Herman D, Irsova D.
960 1.20 christos Does daylight saving save electricity? A meta-analysis.
961 1.20 christos <em>Energy J.</em> 2018;39(2):3561.
962 1.7 christos doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
963 1.20 christos This analyzes research literature and concludes, Electricity savings
964 1.7 christos are larger for countries farther away from the equator, while
965 1.20 christos subtropical regions consume more electricity because of <abbr>DST</abbr>.</li>
966 1.20 christos <li>Neumann P, von Blanckenburg K. <a
967 1.20 christos href="https://journals.sagepub.com/doi/full/10.1177/0961463X241310562">What
968 1.20 christos time will it be? A comprehensive literature review on daylight saving time</a>.
969 1.21 christos <em>Time Soc</em>. 2025;34(4):684745.
970 1.20 christos doi:<a href="https://doi.org/10.1177/0961463X241310562">10.1177/0961463X241310562</a>.
971 1.21 christos This reviews <abbr>DST</abbr>s effects on electricity, health, crime, road
972 1.21 christos safety, and the economy, focusing on research since 2010, and concludes that
973 1.21 christos year-round standard time is preferable overall.</li>
974 1.21 christos <li>Romigi A, Franco V, Scoditti E <em>et al</em>.
975 1.21 christos The effects of daylight saving time and clock time transitions on sleep and
976 1.21 christos sleepiness: a systematic review. <em>Sleep Med Rev.</em> 2025;84:102161. doi:<a
977 1.21 christos href="https://doi.org/10.1016/j.smrv.2025.102161">10.1016/j.smrv.2025.102161</a>.
978 1.21 christos This reviews <abbr>DST</abbr> and <abbr>DST</abbr> transitions,
979 1.21 christos and concludes that they both harm sleep, health and behavior.</li>
980 1.20 christos </ul>
981 1.20 christos
982 1.20 christos <p>The following medical societies have taken positions on the
983 1.20 christos advisability of clock shifts:</p>
984 1.20 christos
985 1.20 christos <ul>
986 1.20 christos <li>In 2022 the American Medical Association issued a
987 1.21 christos <a href="https://www.ama-assn.org/press-center/ama-press-releases/ama-calls-permanent-standard-time">statement
988 1.20 christos supporting permanent standard time</a> on health grounds.</li>
989 1.20 christos <li>Crawford MR, Winnebeck EC, von Schantz M <em>et al</em>.
990 1.20 christos <a href="https://onlinelibrary.wiley.com/doi/10.1111/jsr.14352">The
991 1.20 christos British Sleep Society position statement on Daylight Saving Time in the UK</a>.
992 1.20 christos <em>J Sleep Res.</em> 2025;34(3):e14352.
993 1.20 christos doi:<a href="https://doi.org/10.1111/jsr.14352">10.1111/jsr.14352</a>.
994 1.21 christos This recommends that the UK abolish <abbr>DST</abbr> for health reasons.</li>
995 1.16 christos <li>Malow BA. <a
996 1.16 christos href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time
997 1.16 christos to abolish the clock change and adopt permanent
998 1.16 christos standard time in the United States:
999 1.16 christos a Sleep Research Society position statement</a>.
1000 1.15 christos <em>Sleep.</em> 2022;45(12):zsac236.
1001 1.15 christos doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>.
1002 1.21 christos The Sleep Research Society
1003 1.20 christos advocates permanent standard time due to its health benefits.</li>
1004 1.15 christos <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>.
1005 1.15 christos <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time
1006 1.15 christos is the optimal choice for health and safety:
1007 1.8 christos an American Academy of Sleep Medicine position statement</a>.
1008 1.20 christos <em>J Clin Sleep Med.</em> 2024;20(1):121125.
1009 1.15 christos doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>.
1010 1.15 christos The AASM argues for permanent standard time due to health and safety risks
1011 1.15 christos and economic costs of both <abbr>DST</abbr> transitions and
1012 1.15 christos permanent <abbr>DST</abbr>.</li>
1013 1.13 christos <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>.
1014 1.16 christos <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why
1015 1.13 christos should we abolish Daylight Saving Time?</a>
1016 1.20 christos <em>J Biol Rhythms.</em> 2019;34(3):227230.
1017 1.13 christos doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>.
1018 1.15 christos The Society for Research on Biological Rhythms
1019 1.18 christos opposes <abbr>DST</abbr> changes and permanent <abbr>DST</abbr>,
1020 1.18 christos and advocates that governments adopt
1021 1.20 christos permanent Standard Time for the health and safety of their citizens.</li>
1022 1.7 christos </ul>
1023 1.7 christos </section>
1024 1.7 christos
1025 1.7 christos <section>
1026 1.1 christos <h2 id="precision">Precision timekeeping</h2>
1027 1.1 christos <ul>
1028 1.1 christos <li><a
1029 1.1 christos href="http://leapsecond.com/hpan/an1289.pdf">The
1030 1.1 christos Science of Timekeeping</a> is a thorough introduction
1031 1.1 christos to the theory and practice of precision timekeeping.</li>
1032 1.1 christos <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
1033 1.14 christos Time 2016</a> contains several freely readable papers.</li>
1034 1.13 christos <li><a href="https://www.ntp.org"><abbr
1035 1.1 christos title="Network Time Protocol">NTP</abbr>: The Network
1036 1.1 christos Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
1037 1.1 christos discusses how to synchronize clocks of
1038 1.1 christos Internet hosts.</li>
1039 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>
1040 1.2 christos family of software algorithms can achieve accuracy to a few tens of
1041 1.2 christos nanoseconds in scalable server farms without special hardware.</li>
1042 1.1 christos <li>The <a
1043 1.21 christos href="https://www.nist.gov/el/intelligent-systems-division-73500/ieee-1588">Precision
1044 1.1 christos Time Protocol</a> (<abbr
1045 1.1 christos title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
1046 1.2 christos can achieve submicrosecond clock accuracy on a local area network
1047 1.2 christos with special-purpose hardware.</li>
1048 1.1 christos <li><a
1049 1.18 christos href="https://www.rfc-editor.org/rfc/rfc4833">Timezone
1050 1.1 christos Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
1051 1.1 christos (Internet <abbr>RFC</abbr> 4833)
1052 1.1 christos specifies a <a
1053 1.1 christos href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
1054 1.1 christos option for a server to configure
1055 1.20 christos a clients time zone and daylight saving settings automatically.</li>
1056 1.11 christos <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
1057 1.11 christos Scales</a> describes astronomical time scales like
1058 1.1 christos <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
1059 1.1 christos <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
1060 1.1 christos <abbr title="Barycentric Dynamic Time">TDB</abbr>.
1061 1.1 christos <li>The <a href="https://www.iau.org"><abbr
1062 1.20 christos title="International Astronomical Union">IAU</abbr></a>s <a
1063 1.13 christos href="https://www.iausofa.org"><abbr
1064 1.1 christos title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
1065 1.1 christos collection contains C and <a
1066 1.1 christos href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
1067 1.1 christos code for converting among time scales like
1068 1.1 christos <abbr title="International Atomic Time">TAI</abbr>,
1069 1.1 christos <abbr>TDB</abbr>, <abbr>TDT</abbr> and
1070 1.13 christos <abbr>UTC</abbr>. It is freely available under the
1071 1.21 christos <a href="https://www.iausofa.org/terms-and-conditions">SOFA license</a>.</li>
1072 1.1 christos <li><a
1073 1.1 christos href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
1074 1.20 christos Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
1075 1.1 christos diverse local time
1076 1.1 christos scales used by each landed mission on Mars.</li>
1077 1.1 christos <li><a href="http://leapsecond.com">LeapSecond.com</a> is
1078 1.1 christos dedicated not only to leap seconds but to precise time and frequency
1079 1.1 christos in general. It covers the state of the art in amateur timekeeping, and
1080 1.1 christos how the art has progressed over the past few decades.</li>
1081 1.7 christos <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
1082 1.7 christos href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
1083 1.20 christos and time-signal emissions</a>, International Telecommunication Union
1084 1.7 christos Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
1085 1.1 christos <li><a
1086 1.1 christos href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
1087 1.1 christos title="International Earth Rotation and Reference Systems Service">IERS</abbr>
1088 1.1 christos Bulletins</a> contains official publications of the International
1089 1.1 christos Earth Rotation and Reference Systems Service, which decides when leap
1090 1.20 christos seconds occur.
1091 1.20 christos The <code><abbr>tz</abbr></code> code and data support leap seconds
1092 1.20 christos via an optional <code>"right"</code> configuration where a computers internal
1093 1.9 christos <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
1094 1.20 christos as opposed to the default <code>"posix"</code> configuration
1095 1.9 christos where the internal clock ignores leap seconds.
1096 1.9 christos The two configurations agree for timestamps starting with 1972-01-01 00:00:00
1097 1.9 christos <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for
1098 1.9 christos timestamps starting with <code>time_t</code> 78 796 800,
1099 1.9 christos which corresponds to the first leap second
1100 1.20 christos 1972-06-30 23:59:60 <abbr>UTC</abbr> in the <code>"right"</code> configuration,
1101 1.9 christos and to
1102 1.20 christos 1972-07-01 00:00:00 <abbr>UTC</abbr> in the <code>"posix"</code> configuration.
1103 1.9 christos In practice the two configurations also agree for timestamps before
1104 1.9 christos 1972 even though the historical situation is messy, partly because
1105 1.9 christos neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
1106 1.14 christos is well-defined for sufficiently old timestamps.</li>
1107 1.17 christos <li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The
1108 1.17 christos <abbr>NTP</abbr> Leap Second File</a> covers the text file
1109 1.17 christos <code>leap-seconds.list</code>, which lists the currently known leap seconds.
1110 1.17 christos The <abbr>IERS</abbr> maintains this file, and a copy is distributed by
1111 1.17 christos <code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like
1112 1.17 christos <a href="https://www.ntp.org">classic
1113 1.17 christos <code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a>
1114 1.17 christos and <a href="https://ntpsec.org">NTPsec</a>.
1115 1.17 christos The <code><abbr>tz</abbr></code> database also distributes leap second
1116 1.17 christos information in a differently-formatted <code>leapseconds</code> text file,
1117 1.20 christos as well as in the <code>"right"</code> configuration in binary form; for
1118 1.17 christos example, <code>right/UTC</code> can be used
1119 1.17 christos by <a href="https://chrony-project.org"><code>chrony</code></a>,
1120 1.17 christos another <abbr>NTP</abbr> implementation.</li>
1121 1.1 christos <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1122 1.1 christos discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1123 1.1 christos leap second so that they disagree with <abbr>UTC</abbr> by at most a
1124 1.1 christos half second, even though every <abbr>POSIX</abbr> minute has exactly
1125 1.4 christos sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1126 1.20 christos <code>"posix"</code> configuration, is <a
1127 1.21 christos href="https://gitlab.com/NTPsec/ntpsec/-/blob/master/docs/leapsmear.adoc">supported</a>
1128 1.21 christos by the abovementioned <abbr>NTP</abbr> implementations, <a
1129 1.12 christos href="https://github.com/google/unsmear">supports</a> conversion between
1130 1.12 christos <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
1131 1.7 christos cloud service providers. However, according to
1132 1.20 christos <a href="https://www.rfc-editor.org/rfc/rfc8633#section-3.7.1">3.7.1 of
1133 1.7 christos Network Time Protocol Best Current Practices</a>
1134 1.7 christos (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1135 1.9 christos applications requiring accurate <abbr>UTC</abbr> or civil time,
1136 1.7 christos and is intended for use only in single, well-controlled environments.</li>
1137 1.1 christos <li>The <a
1138 1.22 christos href="https://groups.io/g/LEAPSECS">LEAPSECS List</a> covers <a
1139 1.21 christos href="https://gge.ext.unb.ca/Resources/gpsworld.november99.pdf">McCarthy
1140 1.20 christos and Klepczynskis 1999 proposal to discontinue leap seconds</a>,
1141 1.1 christos discussed further in
1142 1.1 christos <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1143 1.1 christos leap second: its history and possible future</a>.
1144 1.1 christos <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1145 1.1 christos might be redefined
1146 1.1 christos without Leap Seconds</a> gives pointers on this
1147 1.14 christos contentious issue.
1148 1.22 christos The General Conference on Weights and Measures (CGPM)
1149 1.16 christos <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a>
1150 1.17 christos to discontinue the use of leap seconds by 2035, and requested that no
1151 1.17 christos discontinuous adjustments be made to UTC for at least a century.
1152 1.16 christos The World Radiocommunication Conference <a
1153 1.16 christos href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved
1154 1.22 christos in 2023</a> to cooperate with this process. A draft <a
1155 1.22 christos href="https://www.bipm.org/documents/d/guest/cgpm-2026-draft-resolutions">Resolution
1156 1.22 christos C to make continuous UTC effective on 2027-05-20</a>,
1157 1.22 christos and thereby discontinue leap seconds,
1158 1.22 christos has been scheduled for the 28th CGPM starting 2026-10-13 in Paris.
1159 1.22 christos One proposal to implement this
1160 1.20 christos would replace leap seconds with seven 13-second leap smears occurring once per
1161 1.17 christos decade until 2100, with leap smears after that gradually increasing in size.
1162 1.20 christos See:
1163 1.20 christos <ul>
1164 1.22 christos <li>Levine J. <a href="https://tf.nist.gov/general/pdf/3242.pdf">A
1165 1.20 christos proposal to change the leap-second adjustments to
1166 1.20 christos coordinated universal time</a>. <em>Metrologia.</em> 2024;61(5):055002. doi:<a
1167 1.22 christos href="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a>
1168 1.22 christos with followups in doi:<a
1169 1.22 christos href="https://doi.org/10.1088/1681-7575/ade314">10.1088/1681-7575/ade314</a>
1170 1.22 christos and doi:<a
1171 1.22 christos href="https://doi.org/10.1088/1681-7575/ade315">10.1088/1681-7575/ade315</a>.</li>
1172 1.20 christos </ul>
1173 1.17 christos However, there is still no consensus on whether this is the best way
1174 1.17 christos to replace leap seconds.
1175 1.14 christos </li>
1176 1.1 christos </ul>
1177 1.6 christos </section>
1178 1.6 christos
1179 1.6 christos <section>
1180 1.1 christos <h2 id="notation">Time notation</h2>
1181 1.1 christos <ul>
1182 1.13 christos <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data
1183 1.1 christos Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1184 1.1 christos zone names, abbreviations, identifiers, and formats. For example, it
1185 1.20 christos contains French translations for Eastern European Summer Time,
1186 1.20 christos <abbr title="Eastern European Summer Time">EEST</abbr>, and Bucharest. Its
1187 1.1 christos <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1188 1.1 christos charts</a> show these values for many locales. Data values are available in
1189 1.1 christos both <abbr title="Locale Data Markup Language">LDML</abbr>
1190 1.1 christos (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1191 1.1 christos <li>
1192 1.1 christos <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1193 1.6 christos the international standard date and time notation</a> covers
1194 1.1 christos <a
1195 1.6 christos href="https://www.iso.org/standard/70907.html"><em><abbr
1196 1.6 christos title="International Organization for Standardization">ISO</abbr>
1197 1.20 christos 8601-1:2019 Date and time Representations for information
1198 1.20 christos interchange Part 1: Basic rules</em></a>.</li>
1199 1.1 christos <li>
1200 1.6 christos <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1201 1.20 christos Schema: Datatypes dateTime</a> specifies a format inspired by
1202 1.1 christos <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1203 1.20 christos <li><a href="https://www.rfc-editor.org/rfc/rfc5322#section-3.3">3.3 of
1204 1.1 christos Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1205 1.1 christos specifies the time notation used in email and <a
1206 1.1 christos href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1207 1.1 christos headers.</li>
1208 1.1 christos <li>
1209 1.18 christos <a href="https://www.rfc-editor.org/rfc/rfc3339">Date and Time
1210 1.1 christos on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1211 1.17 christos specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.
1212 1.18 christos An extension, <a href="https://www.rfc-editor.org/rfc/rfc9557">Date
1213 1.17 christos and Time on the Internet: Timestamps with Additional Information</a>
1214 1.17 christos (Internet <abbr>RFC</abbr> 9557) extends this profile
1215 1.17 christos to let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp
1216 1.20 christos via suffixes like <code>[Asia/Tokyo]</code>.
1217 1.1 christos <li>
1218 1.7 christos <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1219 1.1 christos Formats on the Web</a> surveys web- and Internet-oriented date and time
1220 1.1 christos formats.</li>
1221 1.1 christos <li>Alphabetic time zone abbreviations should not be used as unique
1222 1.1 christos identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1223 1.1 christos practice. For example, in English-speaking North America
1224 1.20 christos <abbr>CST</abbr> denotes 6 hours behind <abbr>UT</abbr>,
1225 1.1 christos but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1226 1.1 christos and French-speaking North Americans prefer
1227 1.20 christos <abbr title="Heure Normale du Centre">HNC</abbr> to
1228 1.20 christos <abbr>CST</abbr>. The <code><abbr>tz</abbr></code>
1229 1.2 christos database contains English abbreviations for many timestamps;
1230 1.20 christos unfortunately some of these abbreviations were merely the database maintainers
1231 1.1 christos inventions, and these have been removed when possible.</li>
1232 1.1 christos <li>Numeric time zone abbreviations typically count hours east of
1233 1.1 christos <abbr>UT</abbr>, e.g., +09 for Japan and
1234 1.20 christos 10 for Hawaii. However, <abbr>POSIX</abbr> proleptic
1235 1.17 christos <code><abbr>TZ</abbr></code> settings use the opposite convention.
1236 1.1 christos For example, one might use <code><abbr>TZ</abbr>="<abbr
1237 1.1 christos title="Japan Standard Time">JST</abbr>-9"</code> and
1238 1.1 christos <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1239 1.1 christos for Japan and Hawaii, respectively. If the
1240 1.1 christos <code><abbr>tz</abbr></code> database is available, it is usually better to use
1241 1.1 christos settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1242 1.1 christos <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1243 1.2 christos confusion, handle old timestamps better, and insulate you better from
1244 1.1 christos any future changes to the rules. One should never set
1245 1.1 christos <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1246 1.1 christos <code>"GMT-9"</code>, though, since this would incorrectly imply that
1247 1.1 christos local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1248 1.20 christos is called <abbr>GMT</abbr>.</li>
1249 1.1 christos </ul>
1250 1.6 christos </section>
1251 1.6 christos
1252 1.6 christos <section>
1253 1.1 christos <h2 id="see-also">See also</h2>
1254 1.1 christos <ul>
1255 1.7 christos <li><a href="theory.html">Theory and pragmatics of the
1256 1.7 christos <code><abbr>tz</abbr></code> code and data</a></li>
1257 1.1 christos <li><a href="tz-art.html">Time and the Arts</a></li>
1258 1.1 christos </ul>
1259 1.6 christos </section>
1260 1.6 christos
1261 1.6 christos <footer>
1262 1.1 christos <hr>
1263 1.1 christos This web page is in the public domain, so clarified as of
1264 1.1 christos 2009-05-17 by Arthur David Olson.
1265 1.1 christos <br>
1266 1.1 christos Please send corrections to this web page to the
1267 1.1 christos <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1268 1.20 christos The mailing list and its archives are public,
1269 1.20 christos so please do not send confidential information.
1270 1.6 christos </footer>
1271 1.1 christos </body>
1272 1.1 christos </html>
1273