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4 1.1 christos <title>Sources for time zone and daylight saving time data</title>
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10 1.1 christos <body>
11 1.1 christos <h1>Sources for 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 1.6 christos <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li>
30 1.6 christos </ul>
31 1.6 christos </li>
32 1.6 christos <li>Uses of the <code><abbr>tz</abbr></code> database
33 1.6 christos <ul>
34 1.6 christos <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li>
35 1.6 christos <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li>
36 1.6 christos <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li>
37 1.6 christos <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li>
38 1.6 christos <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li>
39 1.6 christos </ul>
40 1.6 christos </li>
41 1.6 christos <li>Related data
42 1.6 christos <ul>
43 1.6 christos <li><a href="#other-dbs">Other time zone databases</a></li>
44 1.6 christos <li><a href="#maps">Maps</a></li>
45 1.6 christos <li><a href="#boundaries">Time zone boundaries</a></li>
46 1.6 christos </ul>
47 1.6 christos </li>
48 1.6 christos <li>Timekeeping concepts
49 1.6 christos <ul>
50 1.6 christos <li><a href="#civil">Civil time concepts and history</a></li>
51 1.6 christos <li><a href="#national">National histories of legal time</a></li>
52 1.7 christos <li><a href="#costs">Costs and benefits of time shifts</a></li>
53 1.6 christos <li><a href="#precision">Precision timekeeping</a></li>
54 1.6 christos <li><a href="#notation">Time notation</a></li>
55 1.6 christos <li><a href="#see-also">See also</a></li>
56 1.6 christos </ul>
57 1.6 christos </li>
58 1.6 christos </ul>
59 1.6 christos </nav>
60 1.6 christos
61 1.6 christos <section>
62 1.1 christos <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2>
63 1.1 christos <p>
64 1.1 christos The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a>
65 1.1 christos time zone database contains code and data
66 1.1 christos that represent the history of local time
67 1.1 christos for many representative locations around the globe.
68 1.1 christos It is updated periodically to reflect changes made by political bodies
69 1.1 christos to time zone boundaries and daylight saving rules.
70 1.1 christos This database (known as <code><abbr>tz</abbr></code>,
71 1.1 christos <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>)
72 1.1 christos is used by several implementations,
73 1.1 christos including
74 1.1 christos <a href="https://www.gnu.org/software/libc/">the
75 1.1 christos <abbr title="GNU's Not Unix">GNU</abbr>
76 1.1 christos C Library</a> (used in
77 1.1 christos <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>),
78 1.1 christos <a href="https://www.android.com">Android</a>,
79 1.1 christos <a href="https://www.freebsd.org">Free<abbr
80 1.1 christos title="Berkeley Software Distribution">BSD</abbr></a>,
81 1.1 christos <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>,
82 1.1 christos <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>,
83 1.1 christos <a href="https://www.chromium.org/chromium-os">Chromium OS</a>,
84 1.1 christos <a href="https://cygwin.com">Cygwin</a>,
85 1.7 christos <a href="https://mariadb.org">MariaDB</a>,
86 1.1 christos <a href="https://en.wikipedia.org/wiki/MINIX">MINIX</a>,
87 1.1 christos <a href="https://www.mysql.com">MySQL</a>,
88 1.1 christos <a href="https://en.wikipedia.org/wiki/WebOS"><abbr
89 1.1 christos title="Web Operating System">webOS</abbr></a>,
90 1.7 christos <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr
91 1.1 christos title="Advanced Interactive eXecutive">AIX</abbr></a>,
92 1.6 christos <a href="https://www.apple.com/ios"><abbr
93 1.1 christos title="iPhone OS">iOS</abbr></a>,
94 1.6 christos <a href="https://www.apple.com/macos">macOS</a>,
95 1.1 christos <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>,
96 1.6 christos <a href="https://www.vmssoftware.com">Open<abbr
97 1.1 christos title="Virtual Memory System">VMS</abbr></a>,
98 1.6 christos <a href="https://www.oracle.com/database/">Oracle Database</a>, and
99 1.1 christos <a href="https://www.oracle.com/solaris">Oracle Solaris</a>.</p>
100 1.1 christos <p>
101 1.2 christos Each main entry in the database represents a <dfn>timezone</dfn>
102 1.2 christos for a set of civil-time clocks that have all agreed since 1970.
103 1.2 christos Timezones are typically identified by continent or ocean and then by the
104 1.2 christos name of the largest city within the region containing the clocks.
105 1.1 christos For example, <code>America/New_York</code>
106 1.1 christos represents most of the <abbr title="United States">US</abbr> eastern time zone;
107 1.1 christos <code>America/Phoenix</code> represents most of Arizona, which
108 1.2 christos uses mountain time without daylight saving time (<abbr>DST</abbr>);
109 1.1 christos <code>America/Detroit</code> represents most of Michigan, which uses
110 1.1 christos eastern time but with different <abbr>DST</abbr> rules in 1975;
111 1.1 christos and other entries represent smaller regions like Starke County,
112 1.1 christos Indiana, which switched from central to eastern time in 1991
113 1.1 christos and switched back in 2006.
114 1.1 christos To use the database on an extended <a
115 1.1 christos href="https://en.wikipedia.org/wiki/POSIX"><abbr
116 1.1 christos title="Portable Operating System Interface">POSIX</abbr></a>
117 1.1 christos implementation set the <code><abbr>TZ</abbr></code>
118 1.1 christos environment variable to the location's full name,
119 1.1 christos e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p>
120 1.1 christos <p>
121 1.2 christos Associated with each timezone is a history of offsets from
122 1.1 christos <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal
123 1.1 christos Time</a> (<abbr>UT</abbr>), which is <a
124 1.1 christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean
125 1.1 christos Time</a> (<abbr>GMT</abbr>) with days beginning at midnight;
126 1.2 christos for timestamps after 1960 this is more precisely <a
127 1.1 christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated
128 1.1 christos Universal Time</a> (<abbr>UTC</abbr>).
129 1.1 christos The database also records when daylight saving time was in use,
130 1.1 christos along with some time zone abbreviations such as <abbr>EST</abbr>
131 1.1 christos for Eastern Standard Time in the <abbr>US</abbr>.</p>
132 1.6 christos </section>
133 1.6 christos
134 1.6 christos <section>
135 1.1 christos <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2>
136 1.1 christos <p>
137 1.1 christos The following <a
138 1.1 christos href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
139 1.1 christos the latest release's two
140 1.1 christos <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a>
141 1.1 christos to a <abbr>GNU</abbr>/Linux or similar host.</p>
142 1.2 christos <pre><code>mkdir tzdb
143 1.1 christos cd tzdb
144 1.1 christos <a href="https://www.gnu.org/software/wget/">wget</a> https://www.iana.org/time-zones/repository/tzcode-latest.tar.gz
145 1.1 christos wget https://www.iana.org/time-zones/repository/tzdata-latest.tar.gz
146 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 -
147 1.1 christos gzip -dc tzdata-latest.tar.gz | tar -xf -
148 1.1 christos </code></pre>
149 1.1 christos <p>Alternatively, the following shell commands download the same
150 1.1 christos release in a single-tarball format containing extra data
151 1.1 christos useful for regression testing:</p>
152 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>
153 1.1 christos <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf -
154 1.1 christos </code></pre>
155 1.1 christos <p>These commands use convenience links to the latest release
156 1.1 christos of the <code><abbr>tz</abbr></code> database hosted by the
157 1.1 christos <a href="https://www.iana.org/time-zones">Time Zone Database website</a>
158 1.1 christos of the <a href="https://www.iana.org">Internet Assigned Numbers
159 1.1 christos Authority (IANA)</a>.
160 1.1 christos Older releases are in files named
161 1.1 christos <code>tzcode<var>V</var>.tar.gz</code>,
162 1.1 christos <code>tzdata<var>V</var>.tar.gz</code>, and
163 1.1 christos <code>tzdb-<var>V</var>.tar.lz</code>,
164 1.1 christos where <code><var>V</var></code> is the version.
165 1.1 christos Since 1996, each version has been a four-digit year followed by
166 1.1 christos lower-case letter (<samp>a</samp> through <samp>z</samp>,
167 1.1 christos then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp>
168 1.1 christos through <samp>zzz</samp>, and so on).
169 1.10 christos Since version 1999g, each release has been distributed in
170 1.10 christos <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06">POSIX
171 1.10 christos ustar interchange format</a>, compressed as described above;
172 1.10 christos older releases use a nearly-compatible format.
173 1.1 christos Since version 2016h, each release has contained a text file named
174 1.1 christos "<samp>version</samp>" whose first (and currently only) line is the version.
175 1.7 christos Older releases are <a href="https://ftp.iana.org/tz/releases/">archived</a>,
176 1.7 christos and are also available in an
177 1.1 christos <a href="ftp://ftp.iana.org/tz/releases/"><abbr
178 1.1 christos title="File Transfer Protocol">FTP</abbr> directory</a> via a
179 1.1 christos less-secure protocol.</p>
180 1.1 christos <p>Alternatively, a development repository of code and data can be
181 1.1 christos retrieved from <a href="https://github.com">GitHub</a> via the shell
182 1.1 christos command:</p>
183 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>
184 1.1 christos </code></pre>
185 1.1 christos <p>
186 1.1 christos Since version 2012e, each release has been tagged in development repositories.
187 1.1 christos Untagged commits are less well tested and probably contain
188 1.1 christos more errors.</p>
189 1.1 christos <p>
190 1.1 christos After obtaining the code and data files, see the
191 1.1 christos <code>README</code> file for what to do next.
192 1.1 christos The code lets you compile the <code><abbr>tz</abbr></code> source files into
193 1.2 christos machine-readable binary files, one for each location. The binary files
194 1.5 christos are in a special timezone information format (<dfn><abbr>TZif</abbr></dfn>)
195 1.5 christos specified by <a href="https://tools.ietf.org/html/8536">Internet
196 1.5 christos <abbr>RFC</abbr> 8536</a>.
197 1.2 christos The code also lets
198 1.2 christos you read a <abbr>TZif</abbr> file and interpret timestamps for that
199 1.1 christos location.</p>
200 1.6 christos </section>
201 1.6 christos
202 1.6 christos <section>
203 1.1 christos <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2>
204 1.1 christos <p>
205 1.1 christos The <code><abbr>tz</abbr></code> code and data
206 1.1 christos are by no means authoritative. If you find errors, please
207 1.1 christos send changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>,
208 1.1 christos the time zone mailing list. You can also <a
209 1.1 christos href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it
210 1.1 christos and browse the <a
211 1.1 christos href="https://mm.icann.org/pipermail/tz/">archive of old
212 1.5 christos messages</a>.
213 1.5 christos <a href="https://tzdata-meta.timtimeonline.com/">Metadata for mailing list
214 1.5 christos discussions</a> and corresponding data changes can be
215 1.5 christos generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>.
216 1.5 christos </p>
217 1.1 christos <p>
218 1.1 christos If your government plans to change its time zone boundaries or
219 1.1 christos daylight saving rules, inform <code>tz (a] iana.org</code> well in
220 1.1 christos advance, as this will coordinate updates to many cell phones,
221 1.8 christos computers, and other devices around the world.
222 1.8 christos The change should be officially announced at least a year before it affects
223 1.8 christos how clocks operate; otherwise, there is a good chance that some
224 1.8 christos clocks will operate incorrectly after the change, due
225 1.1 christos to delays in propagating updates to software and data. The shorter
226 1.1 christos the notice, the more likely clock problems will arise; see "<a
227 1.1 christos href="https://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-changes/">On
228 1.1 christos the Timing of Time Zone Changes</a>" for examples.
229 1.8 christos The <code><abbr>tz</abbr></code> data can represent planned changes
230 1.8 christos far into the future, and a long-planned change can easily be reverted
231 1.8 christos or otherwise altered with a year's notice before the change would have
232 1.8 christos affected clocks.
233 1.1 christos </p>
234 1.1 christos <p>
235 1.1 christos Changes to the <code><abbr>tz</abbr></code> code and data are often
236 1.1 christos propagated to clients via operating system updates, so
237 1.1 christos client <code><abbr>tz</abbr></code> data can often be corrected by
238 1.1 christos applying these updates. With GNU/Linux and similar systems, if your
239 1.1 christos maintenance provider has not yet adopted the
240 1.1 christos latest <code><abbr>tz</abbr></code> data, you can often short-circuit
241 1.1 christos the process by tailoring the generic instructions in
242 1.1 christos the <code><abbr>tz</abbr> README</code> file and installing the latest
243 1.1 christos data yourself. System-specific instructions for installing the
244 1.1 christos latest <code><abbr>tz</abbr></code> data have also been published
245 1.11 christos for <a href="https://developer.ibm.com/articles/au-aix-olson-time-zone/"><abbr>AIX</abbr></a>,
246 1.1 christos <a
247 1.10 christos href="https://source.android.com/devices/tech/config/timezone-rules">Android</a>,
248 1.1 christos <a
249 1.11 christos href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr
250 1.1 christos title="International Components for Unicode">ICU</abbr></a>,
251 1.11 christos <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr>
252 1.9 christos JDK</a>,
253 1.9 christos <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a
254 1.1 christos href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>,
255 1.9 christos <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a
256 1.11 christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>.
257 1.1 christos </p>
258 1.12 christos <p>Since version 2013a,
259 1.12 christos sources for the <code><abbr>tz</abbr></code> database have been
260 1.1 christos <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr
261 1.1 christos title="Unicode Transformation Format 8-bit">UTF-8</abbr></a>
262 1.1 christos <a href="https://en.wikipedia.org/wiki/Text_file">text files</a>
263 1.1 christos with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr
264 1.1 christos title="linefeed">LF</abbr></a>,
265 1.1 christos which can be modified by common text editors such
266 1.1 christos as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>,
267 1.1 christos <a href="https://wiki.gnome.org/Apps/Gedit">gedit</a>, and
268 1.1 christos <a href="https://www.vim.org">vim</a>.
269 1.1 christos Specialized source-file editing can be done via the
270 1.1 christos <a href="https://packagecontrol.io/packages/zoneinfo">Sublime
271 1.1 christos zoneinfo</a> package for <a
272 1.1 christos href="https://www.sublimetext.com">Sublime Text</a> and the <a
273 1.1 christos href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode
274 1.1 christos zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual
275 1.1 christos Studio Code</a>.
276 1.1 christos </p>
277 1.1 christos <p>
278 1.1 christos For further information about updates, please see
279 1.1 christos <a href="https://tools.ietf.org/html/rfc6557">Procedures for
280 1.1 christos Maintaining the Time Zone Database</a> (Internet <abbr
281 1.1 christos title="Request For Comments">RFC</abbr> 6557). More detail can be
282 1.7 christos found in <a href="theory.html">Theory and pragmatics of the
283 1.7 christos <code><abbr>tz</abbr></code> code and data</a>.
284 1.2 christos <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a>
285 1.2 christos displays changes between recent <code><abbr>tzdb</abbr></code> versions.
286 1.1 christos </p>
287 1.6 christos </section>
288 1.6 christos
289 1.6 christos <section>
290 1.1 christos <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2>
291 1.1 christos <ul>
292 1.1 christos <li>The article
293 1.1 christos <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is
294 1.1 christos an encyclopedic summary.</li>
295 1.1 christos <li><a href="tz-how-to.html">How to Read the
296 1.1 christos tz Database Source Files</a> explains the <code><abbr>tz</abbr></code>
297 1.1 christos database format.</li>
298 1.1 christos <li><a
299 1.1 christos href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A
300 1.1 christos literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the
301 1.1 christos database's style.</li>
302 1.10 christos <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it:
303 1.10 christos managing time in the internet</a> analyzes the database longitudinally.</li>
304 1.1 christos </ul>
305 1.6 christos </section>
306 1.6 christos
307 1.6 christos <section>
308 1.1 christos <h2 id="web">Web sites using recent versions of the
309 1.1 christos <code><abbr>tz</abbr></code> database</h2>
310 1.1 christos <p>
311 1.1 christos These are listed roughly in ascending order of complexity and fanciness.
312 1.1 christos </p>
313 1.1 christos <ul>
314 1.1 christos <li><a href="https://time.is">Time.is</a> shows locations'
315 1.1 christos time and zones.</li>
316 1.1 christos <li><a href="https://www.timejones.com">TimeJones.com</a>,
317 1.1 christos <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and
318 1.7 christos <a href="https://www.worldclock.com">The World Clock</a>
319 1.1 christos are time zone converters.</li>
320 1.8 christos <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a>
321 1.8 christos publishes <code><abbr>tzdb</abbr></code>-derived data in
322 1.8 christos <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr
323 1.8 christos title="comma-separated values">CSV</abbr></a> and
324 1.8 christos in <a href="https://en.wikipedia.org/wiki/SQL"><abbr
325 1.8 christos title="Structured Query Language">SQL</abbr></a> form.</li>
326 1.1 christos <li><a
327 1.6 christos href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
328 1.1 christos lets you see the <code><abbr>TZ</abbr></code> values directly.</li>
329 1.1 christos <li><a
330 1.1 christos href="http://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
331 1.1 christos Time in 1000 Places</a> uses descriptions of the values.</li>
332 1.6 christos <li><a href="https://timezoneconverterapp.com/">Time Zone Converter</a>
333 1.1 christos uses a pulldown menu.</li>
334 1.7 christos <li><a href="https://home.kpn.nl/vanadovv/time/TZworld.html">Complete
335 1.10 christos timezone information for all countries</a>
336 1.10 christos displays tables of <abbr>DST</abbr> rules.
337 1.1 christos <li><a href="https://www.timeanddate.com/worldclock/">The World Clock –
338 1.1 christos Worldwide</a> lets you sort zone names and convert times.</li>
339 1.1 christos <li><a href="https://24timezones.com">24TimeZones</a> has a world
340 1.1 christos time map and a time converter.</li>
341 1.1 christos <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a>
342 1.1 christos calculates the current time difference between locations.</li>
343 1.1 christos <li><a href="http://www.wx-now.com">Weather Now</a> and
344 1.1 christos <a href="http://www.thetimenow.com">The Time Now</a> list the weather too.</li>
345 1.1 christos </ul>
346 1.6 christos </section>
347 1.6 christos
348 1.6 christos <section>
349 1.1 christos <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2>
350 1.1 christos <ul>
351 1.1 christos <li>The <a href="https://www.ietf.org">Internet Engineering Task Force</a>'s
352 1.1 christos <a href="https://datatracker.ietf.org/wg/tzdist/charter/">Time Zone Data
353 1.1 christos Distribution Service (tzdist) working group</a> defined <a
354 1.1 christos href="https://tools.ietf.org/html/rfc7808">TZDIST</a>
355 1.1 christos (Internet <abbr>RFC</abbr> 7808), a time zone data distribution service,
356 1.1 christos along with <a href="https://tools.ietf.org/html/rfc7809">CalDAV</a>
357 1.1 christos (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for
358 1.2 christos transferring time zone data by reference.
359 1.8 christos <a href="https://devguide.calconnect.org/Time-Zones/TZDS/">TZDIST
360 1.8 christos implementations</a> are available.
361 1.2 christos The <a href="https://www.ietf.org/mailman/listinfo/tzdist-bis">tzdist-bis
362 1.9 christos mailing list</a> discusses possible extensions.</li>
363 1.1 christos <li>The <a href="https://tools.ietf.org/html/rfc5545">
364 1.1 christos Internet Calendaring and Scheduling Core Object Specification
365 1.1 christos (iCalendar)</a> (Internet <abbr>RFC</abbr> 5445)
366 1.1 christos covers time zone
367 1.1 christos data; see its VTIMEZONE calendar component.
368 1.1 christos The iCalendar format requires specialized parsers and generators; a
369 1.1 christos variant <a href="https://tools.ietf.org/html/rfc6321">xCal</a>
370 1.1 christos (Internet <abbr>RFC</abbr> 6321) uses
371 1.1 christos <a href="https://www.w3.org/XML/"><abbr
372 1.1 christos title="Extensible Markup Language">XML</abbr></a> format, and a variant
373 1.1 christos <a href="https://tools.ietf.org/html/rfc7265">jCal</a>
374 1.1 christos (Internet <abbr>RFC</abbr> 7265)
375 1.1 christos uses <a href="https://www.json.org"><abbr
376 1.1 christos title="JavaScript Object Notation">JSON</abbr></a> format.</li>
377 1.1 christos </ul>
378 1.6 christos </section>
379 1.6 christos
380 1.6 christos <section>
381 1.1 christos <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2>
382 1.7 christos <p>Although some of these do not fully support
383 1.7 christos <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code>
384 1.7 christos distributions you can generally work around compatibility problems by
385 1.7 christos running the command <code>make rearguard_tarballs</code> and compiling
386 1.7 christos from the resulting tarballs instead.</p>
387 1.1 christos <ul>
388 1.1 christos <li><a href="https://sourceforge.net/projects/vzic/">Vzic</a> is a <a
389 1.1 christos href="https://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
390 1.1 christos program that compiles
391 1.1 christos <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files.
392 1.1 christos Vzic is freely
393 1.1 christos available under the <a
394 1.1 christos href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
395 1.1 christos General Public License (<abbr
396 1.1 christos title="General Public License">GPL</abbr>)</a>.</li>
397 1.1 christos <li><a href="https://sourceforge.net/projects/tzical/">tziCal – tz
398 1.1 christos database conversion utility</a> is like Vzic, except for the <a
399 1.6 christos href="https://dotnet.microsoft.com">.NET framework</a>
400 1.1 christos and with a <abbr>BSD</abbr>-style license.</li>
401 1.1 christos <li><a
402 1.4 christos href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a>
403 1.1 christos contains a script <code>parse_olson</code> that compiles
404 1.1 christos <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a>
405 1.1 christos modules. It is part of the Perl <a
406 1.4 christos href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>,
407 1.4 christos which is freely
408 1.1 christos available under both the <abbr>GPL</abbr> and the Perl Artistic
409 1.1 christos License. DateTime::TimeZone also contains a script
410 1.1 christos <code>tests_from_zdump</code> that generates test cases for each clock
411 1.1 christos transition in the <code><abbr>tz</abbr></code> database.</li>
412 1.1 christos <li>The <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
413 1.1 christos Database Parser</a> is a
414 1.1 christos <a href="https://en.wikipedia.org/wiki/C%2B%2B">C++</a> parser and
415 1.9 christos runtime library with <a
416 1.9 christos href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0355r7.html">API</a>
417 1.9 christos adopted by
418 1.6 christos <a href="https://en.wikipedia.org/wiki/C++20">C++20</a>,
419 1.9 christos the current iteration of the C++ standard.
420 1.1 christos It is freely available under the
421 1.1 christos <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
422 1.1 christos <li><a id="ICU" href="http://site.icu-project.org">International Components for
423 1.1 christos Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
424 1.1 christos href="https://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
425 1.1 christos libraries for internationalization that
426 1.1 christos has a compiler from <code><abbr>tz</abbr></code> source
427 1.1 christos and from <abbr title="Common Locale Data Repository">CLDR</abbr> data
428 1.1 christos (mentioned <a href="#CLDR">below</a>)
429 1.1 christos into an <abbr>ICU</abbr>-specific format.
430 1.1 christos <abbr>ICU</abbr> is freely available under a
431 1.1 christos <abbr>BSD</abbr>-style license.</li>
432 1.1 christos <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for
433 1.1 christos the <a href="https://elixir-lang.org">Elixir</a> language downloads
434 1.7 christos and compiles <code><abbr>tz</abbr></code> source and exposes <abbr
435 1.1 christos title="Application Program Interface">API</abbr>s for use. It is
436 1.1 christos freely available under the <abbr>MIT</abbr> license.</li>
437 1.1 christos <li>Java-based compilers and libraries include:
438 1.1 christos <ul>
439 1.1 christos <li>The <a
440 1.9 christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater
441 1.1 christos tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by
442 1.9 christos <a href="https://openjdk.java.net/">OpenJDK</a> and
443 1.9 christos <a href="https://jdk.java.net/">Oracle JDK</a>.
444 1.9 christos Although its source code is proprietary, its executable is available under the
445 1.9 christos <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE
446 1.9 christos Timezone Updater License Agreement</a>.</li>
447 1.1 christos <li>The <a
448 1.4 christos href="https://www.oracle.com/technetwork/articles/java/jf14-date-time-2125367.html">Java
449 1.6 christos SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a
450 1.4 christos href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>,
451 1.1 christos which is freely available under a <abbr>BSD</abbr>-style license.</li>
452 1.4 christos <li><a href="https://www.joda.org/joda-time/">Joda-Time – Java date
453 1.1 christos and time <abbr>API</abbr></a> contains a class
454 1.1 christos <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
455 1.1 christos <code><abbr>tz</abbr></code> source into a binary format. It inspired
456 1.1 christos Java 8 <code>java.time</code>, which its users should migrate to once
457 1.1 christos they can assume Java 8 or later. It is available under the <a
458 1.1 christos href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li>
459 1.9 christos <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a
460 1.7 christos href="https://www.azul.com/products/open-source-tools/ziupdater-time-zone-tool/">ZIUpdater</a>
461 1.9 christos are alternatives to TZUpdater. IANA Updater's license is unclear;
462 1.9 christos ZIUpdater is licensed under the <abbr>GPL</abbr>.</li>
463 1.4 christos <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and
464 1.4 christos time library for Android</a> and
465 1.4 christos <a href="https://github.com/MenoData/Time4J/">Time4J: Advanced date,
466 1.4 christos time and interval library for Java</a> compile
467 1.4 christos <code><abbr>tz</abbr></code> source into a binary format.
468 1.4 christos Time4A is available under the Apache License and Time4J is
469 1.1 christos available under the <a
470 1.1 christos href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser
471 1.1 christos General Public License (<abbr title="Lesser General Public
472 1.1 christos License">LGPL</abbr>)</a>.</li>
473 1.1 christos <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and
474 1.1 christos Java-based libraries.</li>
475 1.1 christos </ul>
476 1.1 christos <li><a href="https://nodatime.org">Noda Time – Date and
477 1.1 christos time <abbr>API</abbr> for .NET</a>
478 1.4 christos is like Joda-Time and Time4J, but for the .NET framework instead of Java.
479 1.4 christos It is freely available under the Apache License.</li>
480 1.8 christos <li>Many modern
481 1.8 christos <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a>
482 1.8 christos runtimes support <code><abbr>tz</abbr></code> natively via the
483 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>.
484 1.10 christos This can be used as-is or with most of the following libraries,
485 1.10 christos many of which also support runtimes lacking the <samp>timeZone</samp> option.
486 1.1 christos <ul>
487 1.8 christos <li>The <a
488 1.8 christos href="https://github.com/formatjs/date-time-format-timezone"><samp>Intl.DateTimeFormat</samp>
489 1.8 christos timezone polyfill</a>
490 1.8 christos is freely available under a <abbr>BSD</abbr>-style license.</li>
491 1.10 christos <li>The <a href="https://date-fns.org/">date-fns</a>
492 1.10 christos library manipulates timezone-aware timestamps in browsers and
493 1.10 christos in <a href="https://nodejs.org/en/">Node.js</a>.
494 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
495 1.10 christos <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a
496 1.10 christos minimalist replacement for the date and time API of
497 1.10 christos the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date
498 1.10 christos manipulation library.
499 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
500 1.10 christos <li><a href="https://moment.github.io/luxon/">Luxon</a> improves
501 1.10 christos timezone support for the <samp>Intl</samp> API.
502 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
503 1.1 christos <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a
504 1.10 christos Moment.js plugin.
505 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
506 1.8 christos <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a
507 1.8 christos JavaScript library that supports date arithmetic that is time zone
508 1.8 christos aware. It is freely available under the <abbr>MIT</abbr> license.</li>
509 1.10 christos <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a>
510 1.10 christos supports live <code><abbr>tzdb</abbr></code> updates,
511 1.10 christos astronomical and atomic time, a command-line interface,
512 1.10 christos and full <a
513 1.10 christos href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>.
514 1.10 christos Its companion <a
515 1.10 christos href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a>
516 1.10 christos can generate <abbr>TZif</abbr> and other files, and a companion website
517 1.10 christos <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you
518 1.10 christos convert timestamps, view transition histories, and download code and data.
519 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li>
520 1.1 christos </ul>
521 1.10 christos The proposed <a
522 1.10 christos href="https://github.com/tc39/proposal-temporal"><samp>Temporal</samp>
523 1.10 christos objects</a> let programs access an abstract view of
524 1.10 christos <code><abbr>tzdb</abbr></code> data, and are designed to replace <a
525 1.10 christos href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScript's
526 1.10 christos problematic <samp>Date</samp> objects</a> when working with dates and times.
527 1.1 christos <li><a href="https://github.com/JuliaTime/">JuliaTime</a> contains a
528 1.1 christos compiler from <code><abbr>tz</abbr></code> source into
529 1.1 christos <a href="https://julialang.org/">Julia</a>. It is freely available
530 1.1 christos under the <abbr>MIT</abbr> license.</li>
531 1.6 christos <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> –
532 1.6 christos <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr
533 1.6 christos title="Free Pascal Compiler">FPC</abbr></a>
534 1.2 christos compiles from <code><abbr>tz</abbr></code> source into
535 1.2 christos <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a>
536 1.2 christos as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a>
537 1.2 christos and <a
538 1.2 christos href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>.
539 1.2 christos It is freely available under a <abbr>BSD</abbr>-style license.</li>
540 1.1 christos <li><a href="http://pytz.sourceforge.net">pytz – World Timezone
541 1.1 christos Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into
542 1.1 christos <a href="https://www.python.org">Python</a>.
543 1.9 christos It is freely available under a <abbr>BSD</abbr>-style license.
544 1.11 christos In code that can assume Python 3.6 or later it is largely superseded; see <a
545 1.11 christos href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz:
546 1.11 christos The Fastest Footgun in the West</a>.</li>
547 1.1 christos <li><a href="https://tzinfo.github.io">TZInfo –
548 1.1 christos Ruby Timezone Library</a>
549 1.1 christos compiles <code><abbr>tz</abbr></code> source into
550 1.1 christos <a href="https://www.ruby-lang.org/en/">Ruby</a>.
551 1.1 christos It is freely available under the <abbr>MIT</abbr> license.</li>
552 1.1 christos <li>The <a href="http://www.squeaksource.com/Chronos/">Chronos Date/Time
553 1.1 christos Library</a> is
554 1.1 christos a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class
555 1.1 christos library that compiles <code><abbr>tz</abbr></code> source into a time
556 1.1 christos zone repository whose format
557 1.1 christos is either proprietary or an <abbr>XML</abbr>-encoded
558 1.1 christos representation.</li>
559 1.1 christos <li><a id="Tcl" href="https://tcl.tk">Tcl</a>
560 1.1 christos contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code>
561 1.1 christos source into text files, along with a runtime that can read those
562 1.1 christos files. Tcl is freely available under a <abbr>BSD</abbr>-style
563 1.1 christos license.</li>
564 1.1 christos </ul>
565 1.6 christos </section>
566 1.6 christos
567 1.6 christos <section>
568 1.2 christos <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2>
569 1.1 christos <ul>
570 1.1 christos <li>The <a
571 1.1 christos href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
572 1.1 christos Library</a>
573 1.1 christos has an independent, thread-safe implementation of
574 1.2 christos a <abbr>TZif</abbr> file reader.
575 1.1 christos This library is freely available under the LGPL
576 1.1 christos and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
577 1.1 christos <li><a href="https://www.gnome.org">GNOME</a>'s
578 1.1 christos <a href="https://developer.gnome.org/glib/">GLib</a> has
579 1.2 christos a <abbr>TZif</abbr> file reader written in C that
580 1.1 christos creates a <code>GTimeZone</code> object representing sets
581 1.1 christos of <abbr>UT</abbr> offsets.
582 1.1 christos It is freely available under the <abbr>LGPL</abbr>.</li>
583 1.1 christos <li>The
584 1.1 christos <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>'s
585 1.1 christos <code>baltzo::TimeZoneUtil</code> component contains a C++
586 1.2 christos implementation of a <abbr>TZif</abbr> file reader. It is freely available under
587 1.1 christos the Apache License.</li>
588 1.1 christos <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++
589 1.1 christos library that translates between <abbr>UT</abbr> and civil time and
590 1.2 christos can read <abbr>TZif</abbr> files. It is freely available under the Apache
591 1.1 christos License.</li>
592 1.1 christos <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C
593 1.2 christos library that reads <abbr>TZif</abbr> files and converts
594 1.2 christos timestamps from one time zone or format to another.
595 1.1 christos It is used by <a href="https://secure.php.net"><abbr
596 1.1 christos title="PHP: Hypertext Preprocessor">PHP</abbr></a>,
597 1.1 christos <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>,
598 1.1 christos and <a href="https://www.mongodb.com">MongoDB</a>.
599 1.1 christos It is freely available under the <abbr>MIT</abbr> license.</li>
600 1.1 christos <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a
601 1.2 christos <abbr>TZif</abbr> file reader.</li>
602 1.4 christos <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile">
603 1.1 christos DateTime::TimeZone::Tzfile</a>
604 1.2 christos is a <abbr>TZif</abbr> file reader written in Perl.
605 1.1 christos It is freely available under the same terms as Perl
606 1.1 christos (dual <abbr>GPL</abbr> and Artistic license).</li>
607 1.9 christos <li>Python has a <a id="python-zoneinfo"
608 1.11 christos href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code>
609 1.8 christos class</a> that reads <abbr>TZif</abbr> data and creates objects
610 1.8 christos that represent <code><abbr>tzdb</abbr></code> timezones.
611 1.8 christos Python is freely available under the
612 1.11 christos <a href="https://docs.python.org/3/license.html">Python Software Foundation
613 1.8 christos License</a>.
614 1.9 christos A companion <a id="pypi-tzdata" href="https://pypi.org/">PyPI</a> module
615 1.8 christos <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a>
616 1.8 christos supplies TZif data if the underlying system data cannot be found;
617 1.8 christos it is freely available under the Apache License.</li>
618 1.1 christos <li>The
619 1.1 christos public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a>
620 1.1 christos library contains a Python tool that
621 1.2 christos converts <abbr>TZif</abbr> data into
622 1.1 christos <abbr>JSON</abbr>-format data suitable for use
623 1.1 christos in its JavaScript library for time zone conversion. Dates before 1970
624 1.1 christos are not supported.</li>
625 1.1 christos <li>The <a
626 1.1 christos href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a>
627 1.1 christos package contains <a href="https://www.haskell.org">Haskell</a> code that
628 1.2 christos parses and uses <abbr>TZif</abbr> data. It is freely
629 1.1 christos available under a <abbr>BSD</abbr>-style license.</li>
630 1.1 christos </ul>
631 1.6 christos </section>
632 1.6 christos
633 1.6 christos <section>
634 1.1 christos <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2>
635 1.1 christos <ul>
636 1.1 christos <li><a href="https://foxclocks.org">FoxClocks</a>
637 1.1 christos is an extension for <a href="https://www.google.com/chrome/">Google
638 1.1 christos Chrome</a> and for <a
639 1.11 christos href="https://wiki.mozilla.org/Modules/Toolkit">Mozilla
640 1.1 christos Toolkit</a> applications like <a
641 1.1 christos href="https://www.mozilla.org/en-US/firefox/new/">Firefox</a> and <a
642 1.1 christos href="https://www.mozilla.org/en-US/thunderbird/">Thunderbird</a>.
643 1.1 christos It displays multiple clocks in the application window, and has a mapping
644 1.1 christos interface to <a href="https://www.google.com/earth/">Google Earth</a>.
645 1.1 christos It is freely available under the <abbr>GPL</abbr>.</li>
646 1.1 christos <li><a href="https://golang.org">Go programming language</a>
647 1.1 christos implementations contain a copy of a 32-bit subset of a recent
648 1.1 christos <code><abbr>tz</abbr></code> database in a
649 1.1 christos Go-specific format.</li>
650 1.1 christos <li>Microsoft Windows 8.1
651 1.1 christos and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr>
652 1.3 christos data (mentioned <a href="#CLDR">below</a>) used by the
653 1.3 christos <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> /
654 1.3 christos <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes
655 1.3 christos <a href="https://docs.microsoft.com/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and
656 1.3 christos <a href="https://docs.microsoft.com/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>.
657 1.1 christos <a id="System.TimeZoneInfo"
658 1.1 christos href="https://blogs.msdn.microsoft.com/bclteam/2007/06/07/exploring-windows-time-zones-with-system-timezoneinfo-josh-free/">Exploring
659 1.1 christos Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes
660 1.1 christos the older, proprietary method of Microsoft Windows 2000 and later,
661 1.1 christos which stores time zone data in the
662 1.1 christos <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The
663 1.1 christos <a
664 1.1 christos href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone →
665 1.1 christos Tzid table</a> or <a
666 1.6 christos href="https://github.com/unicode-org/cldr/blob/master/common/supplemental/windowsZones.xml"><abbr>XML</abbr>
667 1.1 christos file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs
668 1.1 christos to <code><abbr>tz</abbr></code> names.
669 1.3 christos These mappings can be performed programmatically via the <a href="https://github.com/mj1856/TimeZoneConverter">TimeZoneConverter</a> .NET library,
670 1.3 christos or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.
671 1.1 christos <li><a
672 1.1 christos href="https://www.oracle.com/java/index.html">Oracle
673 1.1 christos Java</a> contains a copy of a subset of a recent
674 1.1 christos <code><abbr>tz</abbr></code> database in a
675 1.1 christos Java-specific format.</li>
676 1.1 christos </ul>
677 1.6 christos </section>
678 1.6 christos
679 1.6 christos <section>
680 1.1 christos <h2 id="other-dbs">Other time zone databases</h2>
681 1.1 christos <ul>
682 1.1 christos <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a>
683 1.4 christos is Astrodienst's Web version of Shanks and Pottenger's out-of-print
684 1.4 christos time zone history atlases
685 1.4 christos <a href="https://www.worldcat.org/oclc/468828649">for the US</a> and
686 1.9 christos <a href="https://www.worldcat.org/oclc/76950459">for the world</a>.
687 1.4 christos Although these extensive atlases
688 1.4 christos <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were
689 1.4 christos sources for much of the older <code><abbr>tz</abbr></code> data</a>,
690 1.4 christos they are unreliable as Shanks appears to have
691 1.1 christos guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no
692 1.1 christos sources and do not indicate which entries are guesswork.</li>
693 1.1 christos <li><a href="https://en.wikipedia.org/wiki/HP-UX">HP-UX</a> has a database in
694 1.1 christos its own <code>tztab</code>(4) format.</li>
695 1.1 christos <li>Microsoft Windows has proprietary data mentioned
696 1.1 christos <a href="#System.TimeZoneInfo">above</a>.</li>
697 1.1 christos <li><a href="https://www.worldtimeserver.com">World Time Server</a>
698 1.1 christos is another time zone database.</li>
699 1.1 christos <li>The <a
700 1.1 christos href="https://www.iata.org/publications/store/Pages/standard-schedules-information.aspx">Standard
701 1.1 christos Schedules Information Manual</a> of the
702 1.1 christos International Air Transport Association
703 1.1 christos gives current time zone rules for airports served by commercial aviation.</li>
704 1.1 christos </ul>
705 1.6 christos </section>
706 1.6 christos
707 1.6 christos <section>
708 1.1 christos <h2 id="maps">Maps</h2>
709 1.1 christos <ul>
710 1.9 christos <li>The <a
711 1.9 christos href="https://www.cia.gov/the-world-factbook/maps/world-regional/">World
712 1.9 christos and Regional Maps section</a> of <em>The World Factbook</em>, published by the
713 1.9 christos <a href="https://www.cia.gov">US Central Intelligence
714 1.9 christos Agency (<abbr
715 1.9 christos title="Central Intelligence Agency">CIA</abbr>)</a>, contains a time
716 1.9 christos zone map; the
717 1.1 christos <a
718 1.6 christos href="https://legacy.lib.utexas.edu/maps/world.html">Perry–Castañeda
719 1.1 christos Library Map Collection</a>
720 1.1 christos of the University of Texas at Austin has copies of
721 1.1 christos recent editions.
722 1.1 christos The pictorial quality is good,
723 1.1 christos but the maps do not indicate daylight saving time,
724 1.1 christos and parts of the data are a few years out of date.</li>
725 1.6 christos <li><a href="https://www.worldtimezone.com">World Time Zone Map
726 1.6 christos with current time</a>
727 1.1 christos has several fancy time zone maps; it covers Russia particularly well.
728 1.1 christos The maps' pictorial quality is not quite as good as the
729 1.1 christos <abbr>CIA</abbr>'s
730 1.1 christos but the maps are more up to date.</li>
731 1.1 christos <li><a
732 1.1 christos href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How
733 1.1 christos much is time wrong around the world?</a> maps the difference between
734 1.1 christos mean solar and standard time, highlighting areas such as western China
735 1.1 christos where the two differ greatly. It's a bit out of date, unfortunately.</li>
736 1.1 christos </ul>
737 1.6 christos </section>
738 1.6 christos
739 1.6 christos <section>
740 1.1 christos <h2 id="boundaries">Time zone boundaries</h2>
741 1.2 christos <p>Geographical boundaries between timezones are available
742 1.9 christos from several <a
743 1.9 christos href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet
744 1.9 christos geolocation</a>
745 1.1 christos services and other sources.</p>
746 1.1 christos <ul>
747 1.1 christos <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone
748 1.1 christos Boundary Builder</a> extracts
749 1.1 christos <a href="https://www.openstreetmap.org">Open Street Map</a> data to build
750 1.2 christos boundaries of <code><abbr>tzdb</abbr></code> timezones.
751 1.1 christos Its code is freely available under the <abbr>MIT</abbr> license, and
752 1.1 christos its data entries are freely available under the
753 1.1 christos <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons
754 1.1 christos Open Database License</a>. The maps' borders appear to be quite accurate.</li>
755 1.1 christos <li>Programmatic interfaces that map geographical coordinates via tz_world to
756 1.2 christos <code><abbr>tzdb</abbr></code> timezones include:
757 1.1 christos <ul>
758 1.1 christos <li><a href="https://github.com/mj1856/GeoTimeZone">GeoTimeZone</a> is
759 1.1 christos written in <a
760 1.1 christos href="https://en.wikipedia.org/wiki/C_Sharp_(programming_language)">C#</a>
761 1.1 christos and is freely available under the <abbr>MIT</abbr> license.</li>
762 1.1 christos <li>The <a href="https://github.com/bradfitz/latlong">latlong package</a>
763 1.1 christos is written in Go and is freely available under the Apache License.</li>
764 1.1 christos <li><a href="https://github.com/drtimcooper/LatLongToTimezone">LatLongToTimezone</a>,
765 1.1 christos in both Java and
766 1.1 christos <a href="https://en.wikipedia.org/wiki/Swift_(programming_language)">Swift</a>
767 1.1 christos form, is freely available under the MIT license.</li>
768 1.10 christos <li>For Node.js,
769 1.1 christos the <a href="https://www.npmjs.com/package/geo-tz">geo-tz module</a>
770 1.1 christos is freely available under the MIT license, and
771 1.1 christos the <a href="https://www.npmjs.com/package/tz-lookup">tz-lookup module</a>
772 1.1 christos is in the public domain.</li>
773 1.1 christos <li>The <a
774 1.1 christos href="https://github.com/MrMinimal64/timezonefinder">timezonefinder</a>
775 1.1 christos library for Python is freely available under the MIT license.
776 1.1 christos <li>The <a
777 1.1 christos href="https://github.com/gunyarakun/timezone_finder">timezone_finder</a>
778 1.1 christos library for Ruby is freely available under the MIT license.</li>
779 1.1 christos </ul></li>
780 1.1 christos <li>Free access via a network API, if you register a key, is provided by
781 1.4 christos the <a
782 1.4 christos href="https://www.geonames.org/export/web-services.html#timezone">GeoNames
783 1.4 christos Timezone web service</a>, the <a
784 1.4 christos href="https://developers.google.com/maps/documentation/timezone/intro">Google
785 1.4 christos Maps Time Zone API</a>, and
786 1.8 christos the <a href="https://timezonedb.com/api">TimeZoneDB API</a>.
787 1.1 christos Commercial network API access is provided
788 1.1 christos by <a href="https://askgeo.com">AskGeo</a>
789 1.1 christos and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>.
790 1.1 christos </li>
791 1.1 christos <li>"<a
792 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
793 1.1 christos to get a time zone from a location using latitude and longitude
794 1.1 christos coordinates?</a>" discusses other geolocation possibilities.</li>
795 1.1 christos <li><a href="http://statoids.com/statoids.html">Administrative
796 1.1 christos Divisions of Countries ("Statoids")</a> lists
797 1.1 christos political subdivision data related to time zones.</li>
798 1.7 christos <li><a href="https://home.kpn.nl/vanadovv/time/Multizones.html">Time
799 1.1 christos zone boundaries for multizone countries</a> summarizes legal
800 1.1 christos boundaries between time zones within countries.</li>
801 1.1 christos <li><a href="http://manifold.net/info/freestuff.shtml">Manifold Software
802 1.1 christos – GIS and Database Tools</a> includes a Manifold-format map of
803 1.1 christos world time zone boundaries distributed under the
804 1.1 christos <abbr>GPL</abbr>.</li>
805 1.1 christos <li>A ship within the <a
806 1.1 christos href="https://en.wikipedia.org/wiki/Territorial_waters">territorial
807 1.1 christos waters</a> of any nation uses that nation's time. In international
808 1.1 christos waters, time zone boundaries are meridians 15° apart, except that
809 1.1 christos <abbr>UT</abbr>−12 and <abbr>UT</abbr>+12 are each 7.5°
810 1.1 christos wide and are separated by
811 1.1 christos the 180° meridian (not by the International Date Line, which is
812 1.1 christos for land and territorial waters only). A captain can change ship's
813 1.1 christos clocks any time after entering a new time zone; midnight changes are
814 1.1 christos common.</li>
815 1.1 christos </ul>
816 1.6 christos </section>
817 1.6 christos
818 1.6 christos <section>
819 1.1 christos <h2 id="civil">Civil time concepts and history</h2>
820 1.1 christos <ul>
821 1.1 christos <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A
822 1.1 christos Walk through Time</a>
823 1.1 christos surveys the evolution of timekeeping.</li>
824 1.4 christos <li>The history of daylight saving time is surveyed in <a
825 1.4 christos href="http://www.webexhibits.org/daylightsaving/">About Daylight
826 1.4 christos Saving Time – History, rationale, laws & dates</a> and summarized in
827 1.4 christos <a href="http://seizethedaylight.com/dst/">A Brief
828 1.4 christos History of Daylight Saving Time</a>.</li>
829 1.4 christos <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time
830 1.4 christos Lords</a> discusses how authoritarians manipulate civil time.</li>
831 1.1 christos <li><a href="https://www.w3.org/TR/timezone/">Working with Time Zones</a>
832 1.1 christos contains guidelines and best practices for software applications that
833 1.1 christos deal with civil time.</li>
834 1.1 christos <li><a href="https://www.staff.science.uu.nl/~gent0113/idl/idl.htm">A History of
835 1.1 christos the International Date Line</a> tells the story of the most important
836 1.1 christos time zone boundary.</li>
837 1.1 christos <li><a href="http://statoids.com/tconcept.html">Basic Time
838 1.1 christos Zone Concepts</a> discusses terminological issues behind time zones.</li>
839 1.1 christos </ul>
840 1.6 christos </section>
841 1.6 christos
842 1.6 christos <section>
843 1.1 christos <h2 id="national">National histories of legal time</h2>
844 1.1 christos <dl>
845 1.1 christos <dt>Australia</dt>
846 1.6 christos <dd>The Parliamentary Library commissioned a <a
847 1.1 christos href="https://www.aph.gov.au/binaries/library/pubs/rp/2009-10/10rp10.pdf">research
848 1.1 christos paper on daylight saving time in Australia</a>.
849 1.1 christos The Bureau of Meteorology publishes a list of <a
850 1.1 christos href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation
851 1.1 christos Dates of Daylight Savings Time within Australia</a>.</dd>
852 1.1 christos <dt>Belgium</dt>
853 1.7 christos <dd>The Royal Observatory of Belgium maintains a table of time in
854 1.7 christos Belgium (in
855 1.7 christos <a href="https://www.astro.oma.be/GENERAL/INFO/nli001a.html"
856 1.7 christos hreflang="nl">Dutch</a> and <a
857 1.7 christos href="https://www.astro.oma.be/GENERAL/INFO/fri001a.html"
858 1.7 christos hreflang="fr">French</a>).</dd>
859 1.1 christos <dt>Brazil</dt>
860 1.1 christos <dd>The Time Service Department of the National Observatory
861 1.1 christos records <a href="http://pcdsh01.on.br/DecHV.html"
862 1.1 christos hreflang="pt-BR">Brazil's daylight saving time decrees (in
863 1.1 christos Portuguese)</a>.</dd>
864 1.1 christos <dt>Canada</dt>
865 1.1 christos <dd>National Research Council Canada publishes current
866 1.1 christos and some older information about <a
867 1.6 christos href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time
868 1.6 christos zones and daylight saving time</a>.</dd>
869 1.1 christos <dt>Chile</dt>
870 1.1 christos <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a
871 1.11 christos <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of
872 1.1 christos Chile's official time (in Spanish)</a>.</dd>
873 1.3 christos <dt>China</dt>
874 1.3 christos <dd>The Hong Kong Observatory maintains a
875 1.7 christos <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of
876 1.3 christos summer time in Hong Kong</a>,
877 1.3 christos and Macau's Meteorological and Geophysical Bureau maintains a <a
878 1.7 christos href="https://www.smg.gov.mo/en/subpage/224/page/174">similar
879 1.3 christos history for Macau</a>.
880 1.3 christos Unfortunately the latter is incomplete and has errors.</dd>
881 1.1 christos <dt>Czech Republic</dt>
882 1.1 christos <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas"
883 1.1 christos hreflang="cs">When daylight saving time starts and ends (in Czech)</a>
884 1.10 christos summarizes and cites historical <abbr>DST</abbr> regulations.</dd>
885 1.1 christos <dt>Germany</dt>
886 1.1 christos <dd>The National Institute for Science and Technology maintains the <a
887 1.1 christos href="https://www.ptb.de/cms/en/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation
888 1.1 christos of Legal Time in Germany</a>.</dd>
889 1.1 christos <dt>Israel</dt>
890 1.1 christos <dd>The Interior Ministry periodically issues <a
891 1.1 christos href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
892 1.1 christos hreflang="he">announcements (in Hebrew)</a>.</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 1.1 christos href="http://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 1.1 christos <dd><a href="https://www.staff.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 1.1 christos href="https://www.dia.govt.nz/Daylight-Saving-History">History of
907 1.6 christos Daylight Saving</a>.</dd>
908 1.10 christos <dt>Portugal</dt>
909 1.10 christos <dd>The Lisbon Astronomical Observatory publishes a
910 1.10 christos <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of
911 1.10 christos legal time (in Portuguese)</a>.</dd>
912 1.1 christos <dt>Singapore</dt>
913 1.1 christos <dd><a id="Singapore"
914 1.7 christos href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why
915 1.1 christos is Singapore in the "Wrong" Time Zone?</a> details the
916 1.1 christos history of legal time in Singapore and Malaysia.</dd>
917 1.1 christos <dt>United Kingdom</dt>
918 1.1 christos <dd><a
919 1.1 christos href="https://www.polyomino.org.uk/british-time/">History of
920 1.1 christos legal time in Britain</a> discusses in detail the country
921 1.6 christos with perhaps the best-documented history of clock adjustments.</dd>
922 1.1 christos <dt>United States</dt>
923 1.1 christos <dd>The Department of Transportation's <a
924 1.1 christos href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent
925 1.1 christos Time Zone Proceedings</a> lists changes to time zone boundaries.</dd>
926 1.1 christos <dt>Uruguay</dt>
927 1.1 christos <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan
928 1.1 christos Navy (SOHMA) publishes an annual <a
929 1.7 christos href="http://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac
930 1.1 christos (in Spanish)</a>.</dd>
931 1.1 christos </dl>
932 1.6 christos </section>
933 1.6 christos
934 1.6 christos <section>
935 1.7 christos <h2 id="costs">Costs and benefits of time shifts</h2>
936 1.7 christos <p>Various sources argue for and against daylight saving time and time
937 1.7 christos zone shifts, and many scientific studies have been conducted. This
938 1.8 christos section summarizes reviews and position statements based on
939 1.8 christos scientific literature in the area.</p>
940 1.7 christos <ul>
941 1.7 christos <li>Carey RN, Sarma KM.
942 1.7 christos <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of
943 1.7 christos daylight saving time on road traffic collision risk: a systematic
944 1.7 christos review</a>.
945 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>.
946 1.7 christos This reviews research literature and concludes that the evidence
947 1.7 christos neither supports nor refutes road safety benefits from
948 1.7 christos shifts in time zones.</li>
949 1.7 christos <li>Havranek T, Herman D, Irsova D.
950 1.7 christos <a href="https://www.econstor.eu/handle/10419/174191">Does daylight
951 1.7 christos saving save electricity? A meta-analysis</a>. <em>Energy J.</em>
952 1.7 christos 2018;39(2).
953 1.7 christos doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>.
954 1.7 christos This analyzes research literature and concludes, "Electricity savings
955 1.7 christos are larger for countries farther away from the equator, while
956 1.10 christos subtropical regions consume more electricity because of <abbr>DST</abbr>."</li>
957 1.8 christos <li>Rishi MA, Ahmed O, Barrantes Perez JH <em>et al</em>.
958 1.8 christos <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.8780">Daylight saving time:
959 1.8 christos an American Academy of Sleep Medicine position statement</a>.
960 1.8 christos <em>J Clin Sleep Med.</em>
961 1.8 christos 2020;<a href="https://doi.org/10.5664/jcsm.8780">10.5664/jcsm.8780</a>.
962 1.8 christos This argues for permanent standard time due to health risks of both
963 1.10 christos <abbr>DST</abbr> transitions and permanent <abbr>DST</abbr>.</li>
964 1.7 christos <li>Roenneberg T, Winnebeck EC, Klerman EB.
965 1.7 christos <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6692659/">Daylight
966 1.7 christos saving time and artificial time zones – a battle between
967 1.7 christos biological and social times</a>. <em>Front Physiol.</em> 2019;10:944.
968 1.7 christos doi:<a href="https://doi.org/10.3389/fphys.2019.00944">10.3389/fphys.2019.00944</a>.
969 1.10 christos This reviews evidence about the health effects of <abbr>DST</abbr>
970 1.10 christos and concludes,
971 1.7 christos "In summary, the scientific literature strongly argues against the
972 1.10 christos switching between <abbr>DST</abbr> and Standard Time and even more so against
973 1.10 christos adopting <abbr>DST</abbr> permanently."</li>
974 1.7 christos </ul>
975 1.7 christos </section>
976 1.7 christos
977 1.7 christos <section>
978 1.1 christos <h2 id="precision">Precision timekeeping</h2>
979 1.1 christos <ul>
980 1.1 christos <li><a
981 1.1 christos href="http://leapsecond.com/hpan/an1289.pdf">The
982 1.1 christos Science of Timekeeping</a> is a thorough introduction
983 1.1 christos to the theory and practice of precision timekeeping.</li>
984 1.1 christos <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of
985 1.1 christos Time 2016</a> contains several freely-readable papers.</li>
986 1.1 christos <li><a href="http://www.ntp.org"><abbr
987 1.1 christos title="Network Time Protocol">NTP</abbr>: The Network
988 1.1 christos Time Protocol</a> (Internet <abbr>RFC</abbr> 5905)
989 1.1 christos discusses how to synchronize clocks of
990 1.1 christos Internet hosts.</li>
991 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>
992 1.2 christos family of software algorithms can achieve accuracy to a few tens of
993 1.2 christos nanoseconds in scalable server farms without special hardware.</li>
994 1.1 christos <li>The <a
995 1.1 christos href="https://www.nist.gov/intelligent-systems-division/ieee-1588">Precision
996 1.1 christos Time Protocol</a> (<abbr
997 1.1 christos title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588)
998 1.2 christos can achieve submicrosecond clock accuracy on a local area network
999 1.2 christos with special-purpose hardware.</li>
1000 1.1 christos <li><a
1001 1.1 christos href="https://tools.ietf.org/html/rfc4833">Timezone
1002 1.1 christos Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
1003 1.1 christos (Internet <abbr>RFC</abbr> 4833)
1004 1.1 christos specifies a <a
1005 1.1 christos href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a>
1006 1.1 christos option for a server to configure
1007 1.1 christos a client's time zone and daylight saving settings automatically.</li>
1008 1.11 christos <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time
1009 1.11 christos Scales</a> describes astronomical time scales like
1010 1.1 christos <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
1011 1.1 christos <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
1012 1.1 christos <abbr title="Barycentric Dynamic Time">TDB</abbr>.
1013 1.1 christos <li>The <a href="https://www.iau.org"><abbr
1014 1.1 christos title="International Astronomical Union">IAU</abbr></a>'s <a
1015 1.1 christos href="http://www.iausofa.org"><abbr
1016 1.1 christos title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
1017 1.1 christos collection contains C and <a
1018 1.1 christos href="https://en.wikipedia.org/wiki/Fortran">Fortran</a>
1019 1.1 christos code for converting among time scales like
1020 1.1 christos <abbr title="International Atomic Time">TAI</abbr>,
1021 1.1 christos <abbr>TDB</abbr>, <abbr>TDT</abbr> and
1022 1.1 christos <abbr>UTC</abbr>.</li>
1023 1.1 christos <li><a
1024 1.1 christos href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock
1025 1.1 christos – Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the
1026 1.1 christos diverse local time
1027 1.1 christos scales used by each landed mission on Mars.</li>
1028 1.1 christos <li><a href="http://leapsecond.com">LeapSecond.com</a> is
1029 1.1 christos dedicated not only to leap seconds but to precise time and frequency
1030 1.1 christos in general. It covers the state of the art in amateur timekeeping, and
1031 1.1 christos how the art has progressed over the past few decades.</li>
1032 1.7 christos <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a
1033 1.7 christos href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency
1034 1.7 christos and time-signal emissions</a>, International Telecommunication Union –
1035 1.7 christos Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li>
1036 1.1 christos <li><a
1037 1.1 christos href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr
1038 1.1 christos title="International Earth Rotation and Reference Systems Service">IERS</abbr>
1039 1.1 christos Bulletins</a> contains official publications of the International
1040 1.1 christos Earth Rotation and Reference Systems Service, which decides when leap
1041 1.4 christos seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
1042 1.9 christos via an optional "<code>right</code>" configuration where a computer's internal
1043 1.9 christos <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
1044 1.9 christos as opposed to the default "<code>posix</code>" configuration
1045 1.9 christos where the internal clock ignores leap seconds.
1046 1.9 christos The two configurations agree for timestamps starting with 1972-01-01 00:00:00
1047 1.9 christos <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for
1048 1.9 christos timestamps starting with <code>time_t</code> 78 796 800,
1049 1.9 christos which corresponds to the first leap second
1050 1.9 christos 1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration,
1051 1.9 christos and to
1052 1.9 christos 1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration.
1053 1.9 christos In practice the two configurations also agree for timestamps before
1054 1.9 christos 1972 even though the historical situation is messy, partly because
1055 1.9 christos neither <abbr>UTC</abbr> nor <abbr>TAI</abbr>
1056 1.9 christos is well-defined for sufficiently-old timestamps.</li>
1057 1.1 christos <li><a href="https://developers.google.com/time/smear">Leap Smear</a>
1058 1.1 christos discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
1059 1.1 christos leap second so that they disagree with <abbr>UTC</abbr> by at most a
1060 1.1 christos half second, even though every <abbr>POSIX</abbr> minute has exactly
1061 1.4 christos sixty seconds. This approach works with the default <code><abbr>tz</abbr></code>
1062 1.1 christos "<code>posix</code>" configuration, is <a
1063 1.1 christos href="http://bk1.ntp.org/ntp-stable/README.leapsmear">supported</a> by
1064 1.12 christos the <abbr>NTP</abbr> reference implementation, <a
1065 1.12 christos href="https://github.com/google/unsmear">supports</a> conversion between
1066 1.12 christos <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major
1067 1.7 christos cloud service providers. However, according to
1068 1.7 christos <a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">§3.7.1 of
1069 1.7 christos Network Time Protocol Best Current Practices</a>
1070 1.7 christos (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for
1071 1.9 christos applications requiring accurate <abbr>UTC</abbr> or civil time,
1072 1.7 christos and is intended for use only in single, well-controlled environments.</li>
1073 1.1 christos <li>The <a
1074 1.1 christos href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap
1075 1.1 christos Second Discussion List</a> covers <a
1076 1.1 christos href="https://www2.unb.ca/gge/Resources/gpsworld.november99.pdf">McCarthy
1077 1.1 christos and Klepczynski's 1999 proposal to discontinue leap seconds</a>,
1078 1.1 christos discussed further in
1079 1.1 christos <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
1080 1.1 christos leap second: its history and possible future</a>.
1081 1.1 christos <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr>
1082 1.1 christos might be redefined
1083 1.1 christos without Leap Seconds</a> gives pointers on this
1084 1.1 christos contentious issue, which was active until 2015 and could become active
1085 1.1 christos again.</li>
1086 1.1 christos </ul>
1087 1.6 christos </section>
1088 1.6 christos
1089 1.6 christos <section>
1090 1.1 christos <h2 id="notation">Time notation</h2>
1091 1.1 christos <ul>
1092 1.1 christos <li>The <a id="CLDR" href="http://cldr.unicode.org">Unicode Common Locale Data
1093 1.1 christos Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time
1094 1.1 christos zone names, abbreviations, identifiers, and formats. For example, it
1095 1.1 christos contains French translations for "Eastern European Summer Time",
1096 1.1 christos "<abbr title="Eastern European Summer Time">EEST</abbr>", and
1097 1.1 christos "Bucharest". Its
1098 1.1 christos <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type
1099 1.1 christos charts</a> show these values for many locales. Data values are available in
1100 1.1 christos both <abbr title="Locale Data Markup Language">LDML</abbr>
1101 1.1 christos (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.
1102 1.1 christos <li>
1103 1.1 christos <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of
1104 1.6 christos the international standard date and time notation</a> covers
1105 1.1 christos <a
1106 1.6 christos href="https://www.iso.org/standard/70907.html"><em><abbr
1107 1.6 christos title="International Organization for Standardization">ISO</abbr>
1108 1.6 christos 8601-1:2019 – Date and time – Representations for information
1109 1.6 christos interchange – Part 1: Basic rules</em></a>.</li>
1110 1.1 christos <li>
1111 1.6 christos <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr>
1112 1.1 christos Schema: Datatypes – dateTime</a> specifies a format inspired by
1113 1.1 christos <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li>
1114 1.1 christos <li><a href="https://tools.ietf.org/html/rfc5322#section-3.3">§3.3 of
1115 1.1 christos Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322)
1116 1.1 christos specifies the time notation used in email and <a
1117 1.1 christos href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a>
1118 1.1 christos headers.</li>
1119 1.1 christos <li>
1120 1.1 christos <a href="https://tools.ietf.org/html/rfc3339">Date and Time
1121 1.1 christos on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
1122 1.1 christos specifies an <abbr>ISO</abbr> 8601
1123 1.1 christos profile for use in new Internet
1124 1.1 christos protocols.</li>
1125 1.1 christos <li>
1126 1.7 christos <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time
1127 1.1 christos Formats on the Web</a> surveys web- and Internet-oriented date and time
1128 1.1 christos formats.</li>
1129 1.1 christos <li>Alphabetic time zone abbreviations should not be used as unique
1130 1.1 christos identifiers for <abbr>UT</abbr> offsets as they are ambiguous in
1131 1.1 christos practice. For example, in English-speaking North America
1132 1.1 christos "<abbr>CST</abbr>" denotes 6 hours behind <abbr>UT</abbr>,
1133 1.1 christos but in China it denotes 8 hours ahead of <abbr>UT</abbr>,
1134 1.1 christos and French-speaking North Americans prefer
1135 1.1 christos "<abbr title="Heure Normale du Centre">HNC</abbr>" to
1136 1.1 christos "<abbr>CST</abbr>". The <code><abbr>tz</abbr></code>
1137 1.2 christos database contains English abbreviations for many timestamps;
1138 1.1 christos unfortunately some of these abbreviations were merely the database maintainers'
1139 1.1 christos inventions, and these have been removed when possible.</li>
1140 1.1 christos <li>Numeric time zone abbreviations typically count hours east of
1141 1.1 christos <abbr>UT</abbr>, e.g., +09 for Japan and
1142 1.1 christos −10 for Hawaii. However, the <abbr>POSIX</abbr>
1143 1.1 christos <code><abbr>TZ</abbr></code> environment variable uses the opposite convention.
1144 1.1 christos For example, one might use <code><abbr>TZ</abbr>="<abbr
1145 1.1 christos title="Japan Standard Time">JST</abbr>-9"</code> and
1146 1.1 christos <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code>
1147 1.1 christos for Japan and Hawaii, respectively. If the
1148 1.1 christos <code><abbr>tz</abbr></code> database is available, it is usually better to use
1149 1.1 christos settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and
1150 1.1 christos <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid
1151 1.2 christos confusion, handle old timestamps better, and insulate you better from
1152 1.1 christos any future changes to the rules. One should never set
1153 1.1 christos <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like
1154 1.1 christos <code>"GMT-9"</code>, though, since this would incorrectly imply that
1155 1.1 christos local time is nine hours ahead of <abbr>UT</abbr> and the time zone
1156 1.1 christos is called "<abbr>GMT</abbr>".</li>
1157 1.1 christos </ul>
1158 1.6 christos </section>
1159 1.6 christos
1160 1.6 christos <section>
1161 1.1 christos <h2 id="see-also">See also</h2>
1162 1.1 christos <ul>
1163 1.7 christos <li><a href="theory.html">Theory and pragmatics of the
1164 1.7 christos <code><abbr>tz</abbr></code> code and data</a></li>
1165 1.1 christos <li><a href="tz-art.html">Time and the Arts</a></li>
1166 1.1 christos </ul>
1167 1.6 christos </section>
1168 1.6 christos
1169 1.6 christos <footer>
1170 1.1 christos <hr>
1171 1.1 christos This web page is in the public domain, so clarified as of
1172 1.1 christos 2009-05-17 by Arthur David Olson.
1173 1.1 christos <br>
1174 1.1 christos Please send corrections to this web page to the
1175 1.1 christos <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
1176 1.6 christos </footer>
1177 1.1 christos </body>
1178 1.1 christos </html>
1179