1 <!DOCTYPE html> 2 <html lang="en"> 3 <head> 4 <title>Time zone and daylight saving time data</title> 5 <meta charset="UTF-8"> 6 <meta name="viewport" content="width=device-width, initial-scale=1"> 7 <style> 8 dd {margin-left: 1.3rem;} 9 pre {margin-left: 1.3rem; overflow: auto;} 10 ul {padding-left: 1.3rem;} 11 </style> 12 </head> 13 <body> 14 <main> 15 <h1>Time zone and daylight saving time data</h1> 16 <p> 17 <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and 18 <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a> 19 rules are controlled by individual 20 governments. They are sometimes changed with little notice, and their 21 histories and planned futures are often recorded only fitfully. Here 22 is a summary of attempts to organize and record relevant data in this 23 area. 24 </p> 25 <nav> 26 <ul> 27 <li>The <code><abbr>tz</abbr></code> database product and process 28 <ul> 29 <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li> 30 <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li> 31 <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li> 32 <li><a href="#coordinating">Coordinating with governments and distributors</a></li> 33 <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li> 34 </ul> 35 </li> 36 <li>Uses of the <code><abbr>tz</abbr></code> database 37 <ul> 38 <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li> 39 <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li> 40 <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li> 41 <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li> 42 <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li> 43 </ul> 44 </li> 45 <li>Related data 46 <ul> 47 <li><a href="#other-dbs">Other time zone databases</a></li> 48 <li><a href="#maps">Maps</a></li> 49 <li><a href="#boundaries">Time zone boundaries</a></li> 50 </ul> 51 </li> 52 <li>Timekeeping concepts 53 <ul> 54 <li><a href="#civil">Civil time concepts and history</a></li> 55 <li><a href="#national">National histories of legal time</a></li> 56 <li><a href="#costs">Costs and benefits of time shifts</a></li> 57 <li><a href="#precision">Precision timekeeping</a></li> 58 <li><a href="#notation">Time notation</a></li> 59 </ul> 60 </li> 61 <li><a href="#see-also">See also</a></li> 62 </ul> 63 </nav> 64 65 <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2> 66 <p> 67 The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a> 68 time zone database contains code and data 69 that represent the history of local time 70 for many representative locations around the globe. 71 It is updated periodically to reflect changes made by political bodies 72 to time zone boundaries and daylight saving rules. 73 This database (known as <code><abbr>tz</abbr></code>, 74 <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>) 75 is used by several implementations, 76 including 77 <a href="https://www.gnu.org/software/libc/">the 78 <abbr title="GNUs Not Unix">GNU</abbr> 79 C Library</a> (used in 80 <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>), 81 <a href="https://www.android.com">Android</a>, 82 <a href="https://www.freebsd.org">Free<abbr 83 title="Berkeley Software Distribution">BSD</abbr></a>, 84 <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>, 85 <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>, 86 <a href="https://www.chromium.org/chromium-os/">ChromiumOS</a>, 87 <a href="https://en.wikipedia.org/wiki/OpenHarmony">OpenHarmony</a>, 88 <a href="https://cygwin.com">Cygwin</a>, 89 <a href="https://mariadb.org">MariaDB</a>, 90 <a href="https://musl.libc.org">musl libc</a>, 91 <a href="https://www.mysql.com">MySQL</a>, 92 <a href="https://en.wikipedia.org/wiki/WebOS"><abbr 93 title="Web Operating System">webOS</abbr></a>, 94 <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr 95 title="Advanced Interactive eXecutive">AIX</abbr></a>, 96 <a href="https://www.apple.com/os/ios/"><abbr 97 title="iPhone OS">iOS</abbr></a>, 98 <a href="https://www.apple.com/os/macos/">macOS</a>, 99 <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>, 100 <a href="https://vmssoftware.com">Open<abbr 101 title="Virtual Memory System">VMS</abbr></a>, 102 <a href="https://www.oracle.com/database/">Oracle Database</a>, 103 <a href="https://www.oracle.com/solaris/solaris11/">Oracle Solaris</a>, 104 and <a href="https://qnx.software/en">QNX</a>.</p> 105 <p> 106 Each main entry in the database represents a <dfn>timezone</dfn> 107 for a set of civil-time clocks that have all agreed since 1970. 108 Timezones are typically identified by continent or ocean and then by the 109 name of the largest city within the region containing the clocks. 110 For example, <code>America/New_York</code> 111 represents most of the <abbr title="United States">US</abbr> eastern time zone; 112 <code>America/Phoenix</code> represents most of Arizona, which 113 uses mountain time without daylight saving time (<abbr>DST</abbr>); 114 <code>America/Detroit</code> represents most of Michigan, which uses 115 eastern time but with different <abbr>DST</abbr> rules in 1975; 116 and other entries represent smaller regions like Starke County, 117 Indiana, which switched from central to eastern time in 1991 118 and switched back in 2006. 119 To use the database on a <a 120 href="https://en.wikipedia.org/wiki/POSIX"><abbr 121 title="Portable Operating System Interface">POSIX</abbr>.1-2024</a> 122 implementation set the <code><abbr>TZ</abbr></code> 123 environment variable to the locations full name, 124 e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p> 125 <p> 126 Associated with each timezone is a history of offsets from 127 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal 128 Time</a> (<abbr>UT</abbr>), which is <a 129 href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean 130 Time</a> (<abbr>GMT</abbr>) with days beginning at midnight; 131 for timestamps after 1960 this is more precisely <a 132 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated 133 Universal Time</a> (<abbr>UTC</abbr>). 134 The database also records when daylight saving time was in use, 135 along with some time zone abbreviations such as <abbr>EST</abbr> 136 for Eastern Standard Time in the <abbr>US</abbr>.</p> 137 138 <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2> 139 <p> 140 The following <a 141 href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download 142 the latest releases two 143 <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a> 144 to a <abbr>GNU</abbr>/Linux or similar host.</p> 145 <pre><code>mkdir tzdb 146 cd tzdb 147 <a href="https://www.gnu.org/software/wget/">wget</a> https://data.iana.org/time-zones/tzcode-latest.tar.gz 148 wget https://data.iana.org/time-zones/tzdata-latest.tar.gz 149 <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 - 150 gzip -dc tzdata-latest.tar.gz | tar -xf - 151 </code></pre> 152 <p>Alternatively, the following shell commands download the same 153 release in a single-tarball format containing extra data 154 useful for regression testing:</p> 155 <pre><code>wget <a href="https://data.iana.org/time-zones/tzdb-latest.tar.lz">https://data.iana.org/time-zones/tzdb-latest.tar.lz</a> 156 <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf - 157 </code></pre> 158 <p>These commands use convenience links to the latest release 159 of the <code><abbr>tz</abbr></code> database hosted by the 160 <a href="https://www.iana.org/time-zones">Time Zone Database website</a> 161 of the <a href="https://www.iana.org">Internet Assigned Numbers 162 Authority (IANA)</a>. 163 Older releases are in files named 164 <code>tzcode<var>V</var>.tar.gz</code>, 165 <code>tzdata<var>V</var>.tar.gz</code>, and 166 <code>tzdb-<var>V</var>.tar.lz</code>, 167 where <code><var>V</var></code> is the version. 168 Since 1996, each version has been a four-digit year followed by 169 lower-case letter (<samp>a</samp> through <samp>z</samp>, 170 then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp> 171 through <samp>zzz</samp>, and so on). 172 Since version 2022a, each release has been distributed in 173 <a href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/pax.html#tag_20_94_13_06">POSIX 174 ustar interchange format</a>, compressed as described above; 175 older releases use a nearly compatible format. 176 Since version 2016h, each release has contained a text file named 177 <code>version</code> whose first (and currently only) line is the version. 178 <a href="https://ftp.iana.org/tz/releases/">Older archived releases are 179 available</a> via 180 <a href="https://en.wikipedia.org/wiki/HTTPS"><abbr 181 title="Hypertext Transfer Protocol Secure">HTTPS</abbr></a>, 182 <a href="https://en.wikipedia.org/wiki/Rsync"><abbr 183 title="remote sync">rsync</abbr></a>, and 184 <a href="https://en.wikipedia.org/wiki/FTP"><abbr 185 title="File Transfer Protocol">FTP</abbr></a>.</p> 186 <p>Alternatively, a development repository of code and data can be 187 retrieved from <a href="https://github.com">GitHub</a> via the shell 188 command:</p> 189 <pre><code><a href="https://git-scm.com">git</a> clone <a href="https://github.com/eggert/tz">https://github.com/eggert/tz</a> 190 </code></pre> 191 <p> 192 Since version 2012e, each release has been tagged in development repositories. 193 Untagged commits are less well tested and probably contain 194 more errors.</p> 195 <p> 196 After obtaining the code and data files, see the 197 <code>README</code> file for what to do next. 198 The code lets you compile the <code><abbr>tz</abbr></code> source files into 199 machine-readable binary files, one for each location. 200 The binary file format is specified by 201 <a href="https://www.rfc-editor.org/info/rfc9636/">The 202 Time Zone Information Format (<abbr>TZif</abbr>)</a> 203 (Internet <abbr title="Request For Comments">RFC</abbr> 9636). 204 The code also lets 205 you read a <abbr>TZif</abbr> file and interpret timestamps for that 206 location.</p> 207 208 <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2> 209 <p> 210 The <code><abbr>tz</abbr></code> code and data 211 are by no means authoritative. If you find errors, please 212 email changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>, 213 the time zone mailing list. 214 The mailing list and its archives are public, 215 so please do not send confidential information. 216 See 217 <a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing 218 lists main page</a> to subscribe or to browse its archive of old messages. 219 <a href="https://tzdata-meta.timtimeonline.com">Metadata for mailing list 220 discussions</a> and corresponding data changes can be 221 generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>. 222 </p> 223 <p> 224 Changes to the <code><abbr>tz</abbr></code> code and data are often 225 propagated to clients via operating system updates, so 226 client <code><abbr>tz</abbr></code> data can often be corrected by 227 applying these updates. With GNU/Linux and similar systems, if your 228 maintenance provider has not yet adopted the 229 latest <code><abbr>tz</abbr></code> data, you can often short-circuit 230 the process by tailoring the generic instructions in 231 the <code><abbr>tz</abbr> README</code> file and installing the latest 232 data yourself. System-specific instructions for installing the 233 latest <code><abbr>tz</abbr></code> data have also been published 234 for <a href="https://community.ibm.com/community/user/blogs/ravindra-shinde/2024/12/13/time-zone-update-tool-tz"><abbr>AIX</abbr></a> 235 (and 236 <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates">older 237 <abbr>AIX</abbr></a>), 238 <a 239 href="https://source.android.com/docs/core/permissions/timezone-rules">Android</a>, 240 <a 241 href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr 242 title="International Components for Unicode">ICU</abbr></a>, 243 <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr> 244 JDK</a>, 245 <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a 246 href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>, 247 <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a 248 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>. 249 </p> 250 <p>Since version 2013a, 251 sources for the <code><abbr>tz</abbr></code> database have been 252 <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr 253 title="Unicode Transformation Format 8-bit">UTF-8</abbr></a> 254 <a href="https://en.wikipedia.org/wiki/Text_file">text files</a> 255 with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr 256 title="linefeed">LF</abbr></a>, 257 which can be modified by common text editors such 258 as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>, 259 <a href="https://gedit-text-editor.org">gedit</a>, and 260 <a href="https://www.vim.org">vim</a>. 261 Specialized source-file editing can be done via the 262 <a href="https://packagecontrol.io/packages/zoneinfo">Sublime 263 zoneinfo</a> package for <a 264 href="https://www.sublimetext.com">Sublime Text</a> and the <a 265 href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode 266 zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual 267 Studio Code</a>. 268 </p> 269 <p> 270 For further information about updates, please see 271 <a href="https://www.rfc-editor.org/info/rfc6557/">Procedures for 272 Maintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557). 273 More detail can be 274 found in <a href="theory.html">Theory and pragmatics of the 275 <code><abbr>tz</abbr></code> code and data</a>. 276 <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a> 277 displays changes between recent <code><abbr>tzdb</abbr></code> versions. 278 </p> 279 280 <h2 id="coordinating">Coordinating with governments and distributors</h2> 281 <p> 282 As discussed in 283 <a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How 284 Time Zones Are Coordinated</a>, the time zone database relies on 285 collaboration among governments, the time zone database volunteer 286 community, and data distributors downstream.</p> 287 <p> 288 If your government plans to change its time zone boundaries or 289 daylight saving rules, please send email as described in 290 <a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a>. 291 Do this well in advance, 292 as this will lessen confusion and will coordinate updates to many cell phones, 293 computers, and other devices around the world. 294 In your email, please cite the legislation or regulation that specifies 295 the change, so that it can be checked for details such as the exact times 296 when clock transitions occur. 297 It is OK if a rule change is planned to affect clocks 298 far into the future, as a long-planned change can easily be reverted 299 or otherwise altered with a years notice before the change would have 300 affected clocks.</p> 301 <p> 302 There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases. 303 However, typically a release occurs every few months. 304 Many downstream timezone data distributors wait for 305 a <code><abbr>tzdb</abbr></code> release before they produce an update 306 to time zone behavior in consumer devices and software products. 307 After a release, various parties must integrate, test, 308 and roll out an update before <a 309 href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes. 310 These updates can be expensive, for both the <a 311 href="https://en.wikipedia.org/wiki/Quality_assurance">quality 312 assurance</a> process and the overall cost of shipping and installing 313 updates to each devices copy of <code><abbr>tzdb</abbr></code>. 314 Updates may be batched with other updates and may take substantial 315 time to reach end users after a release. 316 Older devices may no longer be supported and thus may never be updated, 317 which means they will continue to use out-of-date rules.</p> 318 <p> 319 For these reasons any rule change should be promulgated at least a 320 year before it affects how clocks operate; otherwise, there is a good 321 chance that many clocks will be wrong due to delays in propagating updates, 322 and that residents will be confused or even actively resist the change. 323 The shorter the notice, the more likely clock problems will arise; see <a 324 href="https://codeofmatt.com/on-the-timing-of-time-zone-changes/">On 325 the Timing of Time Zone Changes</a> for examples. 326 </p> 327 328 <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2> 329 <ul> 330 <li>The article 331 <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is 332 an encyclopedic summary.</li> 333 <li><a href="tz-how-to.html">How to Read the 334 tz Database Source Files</a> explains the <code><abbr>tz</abbr></code> 335 database source format.</li> 336 <li><a 337 href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A 338 literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the 339 databases style.</li> 340 <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it: 341 managing time in the internet</a> analyzes the database longitudinally.</li> 342 </ul> 343 344 <h2 id="web">Web sites using recent versions of the 345 <code><abbr>tz</abbr></code> database</h2> 346 <p> 347 These are listed roughly in ascending order of complexity and fanciness. 348 </p> 349 <ul> 350 <li><a href="https://time.is">Time.is</a> shows locations 351 time and zones.</li> 352 <li><a href="https://www.timejones.com">TimeJones.com</a>, 353 <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and 354 <a href="https://www.worldclock.com">The World Clock</a> 355 are time zone converters.</li> 356 <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a> 357 publishes <code><abbr>tzdb</abbr></code>-derived data in 358 <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr 359 title="comma-separated values">CSV</abbr></a> and 360 in <a href="https://en.wikipedia.org/wiki/SQL"><abbr 361 title="Structured Query Language">SQL</abbr></a> form.</li> 362 <li><a 363 href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a> 364 lets you see the <code><abbr>TZ</abbr></code> values directly.</li> 365 <li><a 366 href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current 367 Time in 1000 Places</a> uses descriptions of the values.</li> 368 <li><a href="https://www.timeanddate.com/worldclock/">The World Clock 369 Worldwide</a> lets you sort zone names and convert times.</li> 370 <li><a href="https://24timezones.com">24TimeZones</a> has a world 371 time map and a time converter.</li> 372 <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a> 373 calculates the current time difference between locations.</li> 374 <li><a href="https://www.wx-now.com">Weather Now</a> and 375 <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li> 376 </ul> 377 378 <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2> 379 <ul> 380 <li><a href="https://www.rfc-editor.org/info/rfc7808/">Time Zone 381 Data Distribution Service</a> (Internet <abbr>RFC</abbr> 7808) 382 is associated with 383 <a href="https://www.rfc-editor.org/info/rfc7809/">CalDAV</a> 384 (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for 385 transferring time zone data by reference.</li> 386 <li>The <a href="https://www.rfc-editor.org/info/rfc5545/">iCalendar format</a> 387 (Internet <abbr>RFC</abbr> 5545) 388 covers time zone 389 data; see its VTIMEZONE calendar component. 390 The iCalendar format requires specialized parsers and generators; a 391 variant <a href="https://www.rfc-editor.org/info/rfc6321/">xCal</a> 392 (Internet <abbr>RFC</abbr> 6321) uses 393 <a href="https://www.w3.org/XML/"><abbr 394 title="Extensible Markup Language">XML</abbr></a> format, and a variant 395 <a href="https://www.rfc-editor.org/info/rfc7265/">jCal</a> 396 (Internet <abbr>RFC</abbr> 7265) 397 uses <a href="https://www.json.org/json-en.html"><abbr 398 title="JavaScript Object Notation">JSON</abbr></a> format.</li> 399 </ul> 400 401 <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2> 402 <p>Although some of these do not fully support 403 <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code> 404 distributions you can generally work around compatibility problems by 405 running the command <code>make rearguard_tarballs</code> and compiling 406 from the resulting tarballs instead.</p> 407 <ul> 408 <li><a href="https://github.com/libical">Libical</a> 409 contains <code>vzic</code>, a <a 410 href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a> 411 program that compiles 412 <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files. 413 Vzic is freely 414 available under the <a 415 href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr> 416 General Public License (<abbr 417 title="General Public License">GPL</abbr>)</a>.</li> 418 <li>The <a id="TZDP" 419 href="https://howardhinnant.github.io/date/tz.html">Time Zone 420 Database Parser</a> contains a 421 <a href="https://en.wikipedia.org/wiki/C++">C++</a> runtime parser 422 supporting the <a 423 href="https://en.cppreference.com/cpp/chrono#Time_zone"><code>std::chrono::time_zone</code> API</a> 424 that is standardized in 425 <a href="https://en.wikipedia.org/wiki/C%2B%2B20">C++20</a> and later. 426 It is freely available under the 427 <a href="https://en.wikipedia.org/wiki/MIT_License"><abbr 428 title="Massachusetts Institute of Technology">MIT</abbr> license</a>. 429 This approach is used by the <code>std::chrono::time_zone</code> implementation 430 of <a href="https://gcc.gnu.org/"><abbr 431 title="the GNU Compiler Collection">GCC</abbr></a> 432 <code>libstdc++</code>, which is available under the GPL combined 433 with the <a href="https://www.gnu.org/licenses/gcc-exception.html">GCC 434 Runtime Library Exception</a>.</li> 435 <li><a 436 href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a> 437 contains a script <code>parse_olson</code> that compiles 438 <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a> 439 modules. It is part of the Perl <a 440 href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>, 441 which is freely 442 available under both the <abbr>GPL</abbr> and the Perl Artistic 443 License. DateTime::TimeZone also contains a script 444 <code>tests_from_zdump</code> that generates test cases for each clock 445 transition in the <code><abbr>tz</abbr></code> database.</li> 446 <li><a id="ICU" href="https://icu.unicode.org">International Components for 447 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a 448 href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a> 449 libraries for internationalization that 450 has a compiler from <code><abbr>tz</abbr></code> source 451 and from <abbr title="Common Locale Data Repository">CLDR</abbr> data 452 (mentioned <a href="#CLDR">below</a>) 453 into an <abbr>ICU</abbr>-specific format. 454 <abbr>ICU</abbr> is freely available under a 455 <abbr>BSD</abbr>-style license.</li> 456 <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for 457 the <a href="https://elixir-lang.org">Elixir</a> language downloads 458 and compiles <code><abbr>tz</abbr></code> source and exposes <abbr 459 title="Application Program Interface">API</abbr>s for use. It is 460 freely available under the <abbr>MIT</abbr> license.</li> 461 <li>Java-based compilers and libraries include: 462 <ul> 463 <li>The <a 464 href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater 465 tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by 466 <a href="https://openjdk.org">OpenJDK</a> and 467 <a href="https://jdk.java.net">Oracle JDK</a>. 468 Although its source code is proprietary, its executable is available under the 469 <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE 470 Timezone Updater License Agreement</a>.</li> 471 <li>The <a 472 href="https://www.oracle.com/technical-resources/articles/java/jf14-date-time.html">Java 473 SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a 474 href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>, 475 which is freely available under a <abbr>BSD</abbr>-style license.</li> 476 <li><a href="https://www.joda.org/joda-time/">Joda-Time Java date 477 and time <abbr>API</abbr></a> contains a class 478 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles 479 <code><abbr>tz</abbr></code> source into a binary format. It inspired 480 Java 8 <code>java.time</code>, which its users should migrate to once 481 they can assume Java 8 or later. It is available under the <a 482 href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li> 483 <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a 484 href="https://www.azul.com/products/components/ziupdater-time-zone-tool/">ZIUpdater</a> 485 are alternatives to TZUpdater. IANA Updaters license is unclear; 486 ZIUpdater is licensed under the <abbr>GPL</abbr>.</li> 487 <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and 488 time library for Android</a> and 489 <a href="https://github.com/MenoData/Time4J">Time4J: Advanced date, 490 time and interval library for Java</a> compile 491 <code><abbr>tz</abbr></code> source into a binary format. 492 Time4A is available under the Apache License and Time4J is 493 available under the <a 494 href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser 495 General Public License (<abbr title="Lesser General Public 496 License">LGPL</abbr>)</a>.</li> 497 <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and 498 Java-based libraries.</li> 499 </ul> 500 </li> 501 <li><a href="https://nodatime.org">Noda Time Date and 502 time <abbr>API</abbr> for .NET</a> 503 is like Joda-Time and Time4J, but for the .NET framework instead of Java. 504 It is freely available under the Apache License.</li> 505 <li>Many modern 506 <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a> 507 runtimes support <code><abbr>tz</abbr></code> natively via the 508 <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>. 509 This can be used as-is or with most of the following libraries, 510 many of which also support runtimes lacking the <code>timeZone</code> option. 511 <ul> 512 <li>The <a 513 href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code> 514 timezone polyfill</a> 515 is freely available under a <abbr>BSD</abbr>-style license.</li> 516 <li>The <a href="https://date-fns.org">date-fns</a> 517 library manipulates timezone-aware timestamps in browsers and 518 in <a href="https://nodejs.org/en/">Node.js</a>. 519 It is freely available under the <abbr>MIT</abbr> license.</li> 520 <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a 521 minimalist replacement for the date and time API of 522 the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date 523 manipulation library. 524 It is freely available under the <abbr>MIT</abbr> license.</li> 525 <li><a href="https://moment.github.io/luxon/">Luxon</a> improves 526 timezone support for the <code>Intl</code> API. 527 It is freely available under the <abbr>MIT</abbr> license.</li> 528 <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a 529 Moment.js plugin. 530 It is freely available under the <abbr>MIT</abbr> license.</li> 531 <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a 532 JavaScript library that supports date arithmetic that is time zone 533 aware. It is freely available under the <abbr>MIT</abbr> license.</li> 534 <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a> 535 supports live <code><abbr>tzdb</abbr></code> updates, 536 astronomical and atomic time, a command-line interface, 537 and full <a 538 href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>. 539 Its companion <a 540 href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a> 541 can generate <abbr>TZif</abbr> and other files, and a companion website 542 <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you 543 convert timestamps, view transition histories, and download code and data. 544 It is freely available under the <abbr>MIT</abbr> license.</li> 545 </ul> 546 The proposed <a 547 href="https://github.com/tc39/proposal-temporal"><code>Temporal</code> 548 objects</a> let programs access an abstract view of 549 <code><abbr>tzdb</abbr></code> data, and are designed to replace <a 550 href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScripts 551 problematic <code>Date</code> objects</a> when working with dates and times. 552 </li> 553 <li><a href="https://github.com/JuliaTime">JuliaTime</a> contains a 554 compiler from <code><abbr>tz</abbr></code> source into 555 <a href="https://julialang.org">Julia</a>. It is freely available 556 under the <abbr>MIT</abbr> license.</li> 557 <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> 558 <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr 559 title="Free Pascal Compiler">FPC</abbr></a> 560 compiles from <code><abbr>tz</abbr></code> source into 561 <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a> 562 as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a> 563 and <a 564 href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>. 565 It is freely available under a <abbr>BSD</abbr>-style license.</li> 566 <li><a href="https://pythonhosted.org/pytz/">pytz World Timezone 567 Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into 568 <a href="https://www.python.org">Python</a>. 569 It is freely available under a <abbr>BSD</abbr>-style license. 570 In code that can assume Python 3.6 or later it is largely superseded; see <a 571 href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz: 572 The Fastest Footgun in the West</a>.</li> 573 <li><a href="https://tzinfo.github.io">TZInfo 574 Ruby Timezone Library</a> 575 compiles <code><abbr>tz</abbr></code> source into 576 <a href="https://www.ruby-lang.org/en/">Ruby</a>. 577 It is freely available under the <abbr>MIT</abbr> license.</li> 578 <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time 579 Library</a> is 580 a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class 581 library that compiles <code><abbr>tz</abbr></code> source into a time 582 zone repository whose format 583 is either proprietary or an <abbr>XML</abbr>-encoded 584 representation.</li> 585 <li><a id="Tcl" href="https://www.tcl-lang.org">Tcl</a> 586 contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code> 587 source into text files, along with a runtime that can read those 588 files. Tcl is freely available under a <abbr>BSD</abbr>-style 589 license.</li> 590 </ul> 591 592 <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2> 593 <ul> 594 <li>The <a 595 href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C 596 Library</a> 597 has a thread-safe <abbr>TZif</abbr> file reader. 598 This library is freely available under the LGPL 599 and is widely used in <abbr>GNU</abbr>/Linux systems.</li> 600 <li><a href="https://www.gnome.org">GNOME</a>s 601 <a href="https://docs.gtk.org/glib/">GLib</a> has 602 a <abbr>TZif</abbr> file reader written in C that 603 creates a <code>GTimeZone</code> object representing sets 604 of <abbr>UT</abbr> offsets. 605 It is freely available under the <abbr>LGPL</abbr>.</li> 606 <li>The <a href="https://en.wikipedia.org/wiki/Musl">musl</a> C library 607 also has an <abbr>TZif</abbr> reader. 608 It is freely available under the <abbr>MIT</abbr> license.</li> 609 <li>The Time Zone Database Parser for C++, mentioned <a href="#TZDP">above,</a> 610 can be configured to read TZif binary files. 611 A similar approach is taken in 612 <a href="https://www.llvm.org/"><abbr>LLVM</abbr></a> 613 <code>std::chrono::time_zone</code>, which is available 614 under the Apache License with 615 <a href="https://llvm.org/docs/DeveloperPolicy.html#open-source-licensing-terms">LLVM 616 Exceptions</a>.</li> 617 <li>The 618 <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>s 619 <code>baltzo::TimeZoneUtil</code> component contains a C++ 620 implementation of a <abbr>TZif</abbr> file reader. It is freely available under 621 the Apache License.</li> 622 <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++ 623 library that translates between <abbr>UT</abbr> and civil time and 624 can read <abbr>TZif</abbr> files. It is freely available under the Apache 625 License.</li> 626 <li>The <a href="https://go.dev">Go programming language</a> 627 has a <abbr>TZif</abbr> file reader <a 628 href="https://pkg.go.dev/time#LoadLocationFromTZData"><code>LoadLocationFromTZData</code></a>.</li> 629 <li>The 630 <a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code> 631 package</a> contains Python code 632 to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map 633 <code><abbr>tz</abbr></code> settings to proleptic TZ approximations. 634 For example, it maps <code>"Africa/Cairo"</code> 635 to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>, 636 an approximation valid for Cairo timestamps from 2023 on. 637 This can help porting to platforms that support only proleptic TZ. 638 The package is freely available under the MIT license.</li> 639 <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C 640 library that reads <abbr>TZif</abbr> files and converts 641 timestamps from one time zone or format to another. 642 It is used by <a href="https://www.php.net"><abbr 643 title="PHP: Hypertext Preprocessor">PHP</abbr></a>, 644 <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>, 645 and <a href="https://www.mongodb.com">MongoDB</a>. 646 It is freely available under the <abbr>MIT</abbr> license.</li> 647 <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a 648 <abbr>TZif</abbr> file reader.</li> 649 <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile"> 650 DateTime::TimeZone::Tzfile</a> 651 is a <abbr>TZif</abbr> file reader written in Perl. 652 It is freely available under the same terms as Perl 653 (dual <abbr>GPL</abbr> and Artistic license).</li> 654 <li>Python has a <a id="python-zoneinfo" 655 href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code> 656 class</a> that reads <abbr>TZif</abbr> data and creates objects 657 that represent <code><abbr>tzdb</abbr></code> timezones. 658 Python is freely available under the 659 <a href="https://docs.python.org/3/license.html">Python Software Foundation 660 License</a>. 661 A companion <a id="pypi-tzdata" href="https://pypi.org">PyPI</a> module 662 <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a> 663 supplies TZif data if the underlying system data cannot be found; 664 it is freely available under the Apache License.</li> 665 <li>The 666 public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a> 667 library contains a Python tool that 668 converts <abbr>TZif</abbr> data into 669 <abbr>JSON</abbr>-format data suitable for use 670 in its JavaScript library for time zone conversion. Dates before 1970 671 are not supported.</li> 672 <li>The <a href="https://crates.io/crates/jiff">jiff</a> datetime library 673 for <a href="https://rust-lang.org/">Rust</a> either compiles 674 <code><abbr>tz</abbr></code> source into Rust, 675 or consults <abbr>TZif</abbr> data at runtime. 676 It is in the public domain. 677 It intends to supplant the soft-deprecated 678 <a href="https://crates.io/crates/chrono-tz">Chrono-TZ</a>.</li> 679 <li>The <a 680 href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a> 681 package contains <a href="https://www.haskell.org">Haskell</a> code that 682 parses and uses <abbr>TZif</abbr> data. It is freely 683 available under a <abbr>BSD</abbr>-style license.</li> 684 </ul> 685 686 <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2> 687 <ul> 688 <li><a href="https://foxclocks.org">FoxClocks</a> 689 is an extension for <a href="https://www.google.com/chrome/">Google 690 Chrome</a>, <a 691 href="https://www.firefox.com/en-US/">Firefox</a> and <a 692 href="https://www.thunderbird.net/en-US/">Thunderbird</a>. 693 It displays multiple clocks in the application window, and has a mapping 694 interface to <a href="https://earth.google.com/web/">Google Earth</a>. 695 It is freely available under the <abbr>GPL</abbr>.</li> 696 <li>Microsoft Windows 8.1 697 and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr> 698 data (mentioned <a href="#CLDR">below</a>) used by the 699 <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> / 700 <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes 701 <a href="https://learn.microsoft.com/en-us/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and 702 <a href="https://learn.microsoft.com/en-us/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>, 703 and with updates discussed in the 704 <a href="https://techcommunity.microsoft.com/category/windows/blog/dstblog">Daylight 705 Saving Time & Time Zone Blog</a>. 706 <a id="System.TimeZoneInfo" 707 href="https://learn.microsoft.com/en-us/archive/blogs/bclteam/exploring-windows-time-zones-with-system-timezoneinfo-josh-free">Exploring 708 Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes 709 the older, proprietary method of Microsoft Windows 2000 and later, 710 which stores time zone data in the 711 <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The 712 <a 713 href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone 714 Tzid table</a> or <a 715 href="https://github.com/unicode-org/cldr/blob/main/common/supplemental/windowsZones.xml"><abbr>XML</abbr> 716 file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs 717 to <code><abbr>tz</abbr></code> names. 718 These mappings can be performed programmatically via the 719 <a href="https://github.com/mattjohnsonpint/TimeZoneConverter">TimeZoneConverter</a> 720 .NET library, 721 or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.</li> 722 <li><a 723 href="https://www.oracle.com/java/">Oracle 724 Java</a> contains a copy of a subset of a recent 725 <code><abbr>tz</abbr></code> database in a 726 Java-specific format.</li> 727 </ul> 728 729 <h2 id="other-dbs">Other time zone databases</h2> 730 <ul> 731 <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a> 732 is Astrodiensts Web version of Shanks and Pottengers out-of-print 733 time zone history atlases 734 <a href="https://search.worldcat.org/title/468828649">for the US</a> and 735 <a href="https://search.worldcat.org/title/76950459">for the world</a>. 736 Although these extensive atlases 737 <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were 738 sources for much of the older <code><abbr>tz</abbr></code> data</a>, 739 they are unreliable as Shanks appears to have 740 guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no 741 sources and do not indicate which entries are guesswork.</li> 742 <li>Microsoft Windows has proprietary data mentioned 743 <a href="#System.TimeZoneInfo">above</a>.</li> 744 <li><a href="https://www.worldtimeserver.com">World Time Server</a> 745 is another time zone database.</li> 746 <li>The <a 747 href="https://www.iata.org/en/publications/manuals/standard-schedules-information">Standard 748 Schedules Information Manual</a> of the 749 International Air Transport Association 750 gives current time zone rules for airports served by commercial aviation.</li> 751 </ul> 752 753 <h2 id="maps">Maps</h2> 754 <ul> 755 <li><a href="https://www.worldtimezone.com">World Time Zone Map 756 with current time</a> 757 has several fancy time zone maps; it covers Russia particularly well.</li> 758 <li><a 759 href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How 760 much is time wrong around the world?</a> maps the difference between 761 mean solar and standard time, highlighting areas such as western China 762 where the two differ greatly. Its a bit out of date, unfortunately.</li> 763 <li>The 764 <a 765 href="https://maps.lib.utexas.edu/maps/world.html">PerryCastaeda 766 Library Map Collection</a> of the University of Texas at Austin has 767 copies of old maps taken from <a 768 href="https://en.wikipedia.org/wiki/The_World_Factbook"><em>The 769 World Factbook</em></a>, formerly published by the 770 <a href="https://www.cia.gov">US Central Intelligence Agency</a>. 771 Although the maps pictorial quality is good, 772 the maps do not indicate daylight saving time.</li> 773 </ul> 774 775 <h2 id="boundaries">Time zone boundaries</h2> 776 <p>Geographical boundaries between timezones are available 777 from several <a 778 href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet 779 geolocation</a> 780 services and other sources.</p> 781 <ul> 782 <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone 783 Boundary Builder</a> extracts 784 <a href="https://www.openstreetmap.org">Open Street Map</a> data to build 785 boundaries of <code><abbr>tzdb</abbr></code> timezones. 786 Its code is freely available under the <abbr>MIT</abbr> license, and 787 its data entries are freely available under the 788 <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons 789 Open Database License</a>. The borders appear to be quite accurate. 790 Its main web page lists more than twenty libraries 791 for looking up a timezone name from a GPS coordinate.</li> 792 <li>Free access via a network API, if you register a key, is provided by 793 the <a 794 href="https://www.geonames.org/export/web-services.html#timezone">GeoNames 795 Timezone web service</a>, the <a 796 href="https://developers.google.com/maps/documentation/timezone/overview">Google 797 Maps Time Zone API</a>, and 798 the <a href="https://timezonedb.com/api">TimeZoneDB API</a>. 799 Commercial network API access is provided 800 by <a href="https://askgeo.com">AskGeo</a> 801 and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>. 802 </li> 803 <li><a 804 href="https://stackoverflow.com/questions/16086962/how-to-get-a-time-zone-from-a-location-using-latitude-and-longitude-coordinates/16086964">How 805 to get a time zone from a location using latitude and longitude 806 coordinates?</a> discusses other geolocation possibilities.</li> 807 <li><a href="https://statoids.com/statoids.html">Administrative 808 Divisions of Countries (Statoids)</a> lists 809 political subdivision data related to time zones.</li> 810 <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software 811 GIS and Database Tools</a> includes a Manifold-format map of 812 world time zone boundaries circa 2007, distributed under the 813 <abbr>GPL</abbr>.</li> 814 <li>A ship within the <a 815 href="https://en.wikipedia.org/wiki/Territorial_waters">territorial 816 waters</a> of any nation uses that nations time. In international 817 waters, time zone boundaries are meridians 15 apart, except that 818 <abbr>UT</abbr>12 and <abbr>UT</abbr>+12 are each 7.5 819 wide and are separated by 820 the 180 meridian (not by the International Date Line, which is 821 for land and territorial waters only). A captain can change ships 822 clocks any time after entering a new time zone; midnight changes are 823 common.</li> 824 </ul> 825 826 <h2 id="civil">Civil time concepts and history</h2> 827 <ul> 828 <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A 829 Walk through Time</a> 830 surveys the evolution of timekeeping.</li> 831 <li>The history of daylight saving time is surveyed in <a 832 href="https://www.webexhibits.org/daylightsaving/">About Daylight 833 Saving Time History, rationale, laws & dates</a> and summarized in 834 <a href="http://seizethedaylight.com/dst/">A Brief 835 History of Daylight Saving Time</a>.</li> 836 <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time 837 Lords</a> discusses how authoritarians manipulate civil time.</li> 838 <li><a href="https://www.w3.org/TR/timezone/">Working with Time 839 and Time Zones</a> 840 contains guidelines and best practices for software applications that 841 deal with civil time.</li> 842 <li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of 843 the International Date Line</a> tells the story of the most important 844 time zone boundary.</li> 845 <li><a href="https://statoids.com/tconcept.html">Basic Time 846 Zone Concepts</a> discusses terminological issues behind time zones.</li> 847 </ul> 848 849 <h2 id="national">National histories of legal time</h2> 850 <dl> 851 <dt>Australia</dt> 852 <dd>The Bureau of Meteorology publishes a list of <a 853 href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation 854 Dates of Daylight Savings Time within Australia</a>.</dd> 855 <dt>Belgium</dt> 856 <dd>The Royal Observatory of Belgium maintains a table of time in 857 Belgium (in 858 <a href="https://robinfo.oma.be/nl/astro-info/tijd/" 859 hreflang="nl">Dutch</a> and <a 860 href="https://robinfo.oma.be/fr/astro-info/heure/" 861 hreflang="fr">French</a>).</dd> 862 <dt>Brazil</dt> 863 <dd>The Time Service Department of the National Observatory 864 records <a href="http://pcdsh01.on.br/DecHV.html" 865 hreflang="pt-BR">Brazils daylight saving time decrees (in 866 Portuguese)</a>.</dd> 867 <dt>Canada</dt> 868 <dd>National Research Council Canada publishes current 869 and some older information about <a 870 href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time 871 zones and daylight saving time</a>.</dd> 872 <dt>Chile</dt> 873 <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a 874 <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of 875 Chiles official time (in Spanish)</a>.</dd> 876 <dt>China</dt> 877 <dd>The Hong Kong Observatory maintains a 878 <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of 879 summer time in Hong Kong</a>, 880 and Macaus Meteorological and Geophysical Bureau maintains a <a 881 href="https://www.smg.gov.mo/en/subpage/224/page/174">similar 882 history for Macau</a>. 883 Unfortunately the latter is incomplete and has errors.</dd> 884 <dt>Czech Republic</dt> 885 <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas" 886 hreflang="cs">When daylight saving time starts and ends (in Czech)</a> 887 summarizes and cites historical <abbr>DST</abbr> regulations.</dd> 888 <dt>Germany</dt> 889 <dd>The National Institute for Science and Technology maintains the <a 890 href="https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation 891 of Legal Time in Germany</a>.</dd> 892 <dt>Israel</dt> 893 <dd><a 894 href="https://tz.cs.huji.ac.il">Israel Timezone Files</a> 895 lists official time-change announcements and laws since 1940, 896 almost all in Hebrew.</dd> 897 <dt>Malaysia</dt> 898 <dd>See Singapore <a href="#Singapore">below</a>.</dd> 899 <dt>Mexico</dt> 900 <dd>The Investigation and Analysis Service of the Mexican Library of 901 Congress has published a <a 902 href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm" 903 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd> 904 <dt>Netherlands</dt> 905 <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm" 906 hreflang="nl">Legal time in the Netherlands (in Dutch)</a> 907 covers the history of local time in the Netherlands from ancient times.</dd> 908 <dt>New Zealand</dt> 909 <dd>The Department of Internal Affairs maintains a brief <a 910 href="https://www.govt.nz/browse/recreation-and-the-environment/daylight-saving/history-of-daylight-saving-in-nz/">History 911 of Daylight Saving in NZ</a>.</dd> 912 <dt>Palestine</dt> 913 <dd>The Ministry of Telecom and Digital Economy publishes a <a 914 href="https://mtde.gov.ps/home/TimeZone" 915 hreflang="ar">history of clock changes (in Arabic)</a>.</dd> 916 <dt>Portugal</dt> 917 <dd>The Lisbon Astronomical Observatory publishes a 918 <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of 919 legal time (in Portuguese)</a>.</dd> 920 <dt>Singapore</dt> 921 <dd><a id="Singapore" 922 href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why 923 is Singapore in the Wrong Time Zone?</a> details the 924 history of legal time in Singapore and Malaysia.</dd> 925 <dt>United Kingdom</dt> 926 <dd><a 927 href="https://www.polyomino.org.uk/british-time/">History of 928 legal time in Britain</a> discusses in detail the country 929 with perhaps the best-documented history of clock adjustments.</dd> 930 <dt>United States</dt> 931 <dd>The Department of Transportations <a 932 href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent 933 Time Zone Proceedings</a> lists changes to 934 official written time zone boundaries, and its <a 935 href="https://geodata.bts.gov/datasets/usdot::time-zones/about">Time 936 Zones dataset</a> maps current boundaries. 937 These boundaries are only for standard time, so the current map puts 938 all of Arizona in one time zone even though part of Arizona 939 observes <abbr>DST</abbr> and part does not.</dd> 940 <dt>Uruguay</dt> 941 <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan 942 Navy (SOHMA) publishes an annual <a 943 href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac 944 (in Spanish)</a>.</dd> 945 </dl> 946 947 <h2 id="costs">Costs and benefits of time shifts</h2> 948 <p>Various sources argue for and against daylight saving time and time 949 zone shifts, and many scientific studies have been conducted. This 950 section summarizes reviews and position statements based on 951 scientific literature in the area.</p> 952 <ul> 953 <li>Carey RN, Sarma KM. 954 <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of 955 daylight saving time on road traffic collision risk: a systematic 956 review</a>. 957 <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>. 958 This reviews research literature and concludes that the evidence 959 neither supports nor refutes road safety benefits from 960 shifts in time zones.</li> 961 <li>Havranek T, Herman D, Irsova D. 962 Does daylight saving save electricity? A meta-analysis. 963 <em>Energy J.</em> 2018;39(2):3561. 964 doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>. 965 This analyzes research literature and concludes, Electricity savings 966 are larger for countries farther away from the equator, while 967 subtropical regions consume more electricity because of <abbr>DST</abbr>.</li> 968 <li>Neumann P, von Blanckenburg K. <a 969 href="https://journals.sagepub.com/doi/full/10.1177/0961463X241310562">What 970 time will it be? A comprehensive literature review on daylight saving time</a>. 971 <em>Time Soc</em>. 2025;34(4):684745. 972 doi:<a href="https://doi.org/10.1177/0961463X241310562">10.1177/0961463X241310562</a>. 973 This reviews <abbr>DST</abbr>s effects on electricity, health, crime, road 974 safety, and the economy, focusing on research since 2010, and concludes that 975 year-round standard time is preferable overall.</li> 976 <li>Romigi A, Franco V, Scoditti E <em>et al</em>. 977 The effects of daylight saving time and clock time transitions on sleep and 978 sleepiness: a systematic review. <em>Sleep Med Rev.</em> 2025;84:102161. doi:<a 979 href="https://doi.org/10.1016/j.smrv.2025.102161">10.1016/j.smrv.2025.102161</a>. 980 This reviews <abbr>DST</abbr> and <abbr>DST</abbr> transitions, 981 and concludes that they both harm sleep, health and behavior.</li> 982 </ul> 983 984 <p>The following medical societies have taken positions on the 985 advisability of clock shifts:</p> 986 987 <ul> 988 <li>In 2022 the American Medical Association issued a 989 <a href="https://www.ama-assn.org/press-center/ama-press-releases/ama-calls-permanent-standard-time">statement 990 supporting permanent standard time</a> on health grounds.</li> 991 <li>Crawford MR, Winnebeck EC, von Schantz M <em>et al</em>. 992 <a href="https://onlinelibrary.wiley.com/doi/10.1111/jsr.14352">The 993 British Sleep Society position statement on Daylight Saving Time in the UK</a>. 994 <em>J Sleep Res.</em> 2025;34(3):e14352. 995 doi:<a href="https://doi.org/10.1111/jsr.14352">10.1111/jsr.14352</a>. 996 This recommends that the UK abolish <abbr>DST</abbr> for health reasons.</li> 997 <li>Malow BA. <a 998 href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time 999 to abolish the clock change and adopt permanent 1000 standard time in the United States: 1001 a Sleep Research Society position statement</a>. 1002 <em>Sleep.</em> 2022;45(12):zsac236. 1003 doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>. 1004 The Sleep Research Society 1005 advocates permanent standard time due to its health benefits.</li> 1006 <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>. 1007 <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time 1008 is the optimal choice for health and safety: 1009 an American Academy of Sleep Medicine position statement</a>. 1010 <em>J Clin Sleep Med.</em> 2024;20(1):121125. 1011 doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>. 1012 The AASM argues for permanent standard time due to health and safety risks 1013 and economic costs of both <abbr>DST</abbr> transitions and 1014 permanent <abbr>DST</abbr>.</li> 1015 <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>. 1016 <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why 1017 should we abolish Daylight Saving Time?</a> 1018 <em>J Biol Rhythms.</em> 2019;34(3):227230. 1019 doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>. 1020 The Society for Research on Biological Rhythms 1021 opposes <abbr>DST</abbr> changes and permanent <abbr>DST</abbr>, 1022 and advocates that governments adopt 1023 permanent Standard Time for the health and safety of their citizens.</li> 1024 </ul> 1025 1026 <h2 id="precision">Precision timekeeping</h2> 1027 <ul> 1028 <li><a 1029 href="http://leapsecond.com/hpan/an1289.pdf">The 1030 Science of Timekeeping</a> is a thorough introduction 1031 to the theory and practice of precision timekeeping.</li> 1032 <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of 1033 Time 2016</a> contains several freely readable papers.</li> 1034 <li><a href="https://www.ntp.org"><abbr 1035 title="Network Time Protocol">NTP</abbr>: The Network 1036 Time Protocol</a> (Internet <abbr>RFC</abbr> 5905) 1037 discusses how to synchronize clocks of 1038 Internet hosts.</li> 1039 <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 family of software algorithms can achieve accuracy to a few tens of 1041 nanoseconds in scalable server farms without special hardware.</li> 1042 <li>The <a 1043 href="https://www.nist.gov/el/intelligent-systems-division-73500/ieee-1588">Precision 1044 Time Protocol</a> (<abbr 1045 title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588) 1046 can achieve submicrosecond clock accuracy on a local area network 1047 with special-purpose hardware.</li> 1048 <li><a 1049 href="https://www.rfc-editor.org/info/rfc4833/">Timezone 1050 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a> 1051 (Internet <abbr>RFC</abbr> 4833) 1052 specifies a <a 1053 href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a> 1054 option for a server to configure 1055 a clients time zone and daylight saving settings automatically.</li> 1056 <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time 1057 Scales</a> describes astronomical time scales like 1058 <abbr title="Terrestrial Dynamic Time">TDT</abbr>, 1059 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and 1060 <abbr title="Barycentric Dynamic Time">TDB</abbr>.</li> 1061 <li>The <a href="https://www.iau.org"><abbr 1062 title="International Astronomical Union">IAU</abbr></a>s <a 1063 href="https://www.iausofa.org"><abbr 1064 title="Standards Of Fundamental Astronomy">SOFA</abbr></a> 1065 collection contains C and <a 1066 href="https://en.wikipedia.org/wiki/Fortran">Fortran</a> 1067 code for converting among time scales like 1068 <abbr title="International Atomic Time">TAI</abbr>, 1069 <abbr>TDB</abbr>, <abbr>TDT</abbr> and 1070 <abbr>UTC</abbr>. It is freely available under the 1071 <a href="https://www.iausofa.org/terms-and-conditions">SOFA license</a>.</li> 1072 <li><a 1073 href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock 1074 Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the 1075 diverse local time 1076 scales used by each landed mission on Mars.</li> 1077 <li><a href="http://leapsecond.com">LeapSecond.com</a> is 1078 dedicated not only to leap seconds but to precise time and frequency 1079 in general. It covers the state of the art in amateur timekeeping, and 1080 how the art has progressed over the past few decades.</li> 1081 <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a 1082 href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency 1083 and time-signal emissions</a>, International Telecommunication Union 1084 Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li> 1085 <li><a 1086 href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr 1087 title="International Earth Rotation and Reference Systems Service">IERS</abbr> 1088 Bulletins</a> contains official publications of the International 1089 Earth Rotation and Reference Systems Service, which decides when leap 1090 seconds occur. 1091 The <code><abbr>tz</abbr></code> code and data support leap seconds 1092 via an optional <code>"right"</code> configuration where a computers internal 1093 <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second, 1094 as opposed to the default <code>"posix"</code> configuration 1095 where the internal clock ignores leap seconds. 1096 The two configurations agree for timestamps starting with 1972-01-01 00:00:00 1097 <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for 1098 timestamps starting with <code>time_t</code> 78 796 800, 1099 which corresponds to the first leap second 1100 1972-06-30 23:59:60 <abbr>UTC</abbr> in the <code>"right"</code> configuration, 1101 and to 1102 1972-07-01 00:00:00 <abbr>UTC</abbr> in the <code>"posix"</code> configuration. 1103 In practice the two configurations also agree for timestamps before 1104 1972 even though the historical situation is messy, partly because 1105 neither <abbr>UTC</abbr> nor <abbr>TAI</abbr> 1106 is well-defined for sufficiently old timestamps.</li> 1107 <li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The 1108 <abbr>NTP</abbr> Leap Second File</a> covers the text file 1109 <code>leap-seconds.list</code>, which lists the currently known leap seconds. 1110 The <abbr>IERS</abbr> maintains this file, and a copy is distributed by 1111 <code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like 1112 <a href="https://www.ntp.org">classic 1113 <code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a> 1114 and <a href="https://ntpsec.org">NTPsec</a>. 1115 The <code><abbr>tz</abbr></code> database also distributes leap second 1116 information in a differently-formatted <code>leapseconds</code> text file, 1117 as well as in the <code>"right"</code> configuration in binary form; for 1118 example, <code>right/UTC</code> can be used 1119 by <a href="https://chrony-project.org"><code>chrony</code></a>, 1120 another <abbr>NTP</abbr> implementation.</li> 1121 <li><a href="https://developers.google.com/time/smear">Leap Smear</a> 1122 discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a 1123 leap second so that they disagree with <abbr>UTC</abbr> by at most a 1124 half second, even though every <abbr>POSIX</abbr> minute has exactly 1125 sixty seconds. This approach works with the default <code><abbr>tz</abbr></code> 1126 <code>"posix"</code> configuration, is <a 1127 href="https://gitlab.com/NTPsec/ntpsec/-/blob/master/docs/leapsmear.adoc">supported</a> 1128 by the <abbr>NTP</abbr> implementations mentioned above, <a 1129 href="https://github.com/google/unsmear">supports</a> conversion between 1130 <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major 1131 cloud service providers. However, according to 1132 <a href="https://www.rfc-editor.org/info/rfc8633#section-3.7.1">3.7.1 of 1133 Network Time Protocol Best Current Practices</a> 1134 (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for 1135 applications requiring accurate <abbr>UTC</abbr> or civil time, 1136 and is intended for use only in single, well-controlled environments.</li> 1137 <li>The <a 1138 href="https://groups.io/g/LEAPSECS">LEAPSECS List</a> covers <a 1139 href="https://gge.ext.unb.ca/Resources/gpsworld.november99.pdf">McCarthy 1140 and Klepczynskis 1999 proposal to discontinue leap seconds</a>, 1141 discussed further in 1142 <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The 1143 leap second: its history and possible future</a>. 1144 <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr> 1145 might be redefined 1146 without Leap Seconds</a> gives pointers on this 1147 contentious issue. 1148 The General Conference on Weights and Measures (CGPM) 1149 <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a> 1150 to discontinue the use of leap seconds by 2035, and requested that no 1151 discontinuous adjustments be made to UTC for at least a century. 1152 The World Radiocommunication Conference <a 1153 href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved 1154 in 2023</a> to cooperate with this process. A draft <a 1155 href="https://www.bipm.org/documents/d/guest/cgpm-2026-draft-resolutions">Resolution 1156 C to make continuous UTC effective on 2027-05-20</a>, 1157 and thereby discontinue leap seconds, 1158 has been scheduled for the 28th CGPM starting 2026-10-13 in Paris. 1159 One proposal to implement this 1160 would replace leap seconds with seven 13-second leap smears occurring once per 1161 decade until 2100, with leap smears after that gradually increasing in size. 1162 See: 1163 <ul> 1164 <li>Levine J. <a href="https://tf.nist.gov/general/pdf/3242.pdf">A 1165 proposal to change the leap-second adjustments to 1166 coordinated universal time</a>. <em>Metrologia.</em> 2024;61(5):055002. doi:<a 1167 href="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a> 1168 with followups in doi:<a 1169 href="https://doi.org/10.1088/1681-7575/ade314">10.1088/1681-7575/ade314</a> 1170 and doi:<a 1171 href="https://doi.org/10.1088/1681-7575/ade315">10.1088/1681-7575/ade315</a>.</li> 1172 </ul> 1173 However, there is still no consensus on whether this is the best way 1174 to replace leap seconds. 1175 </li> 1176 </ul> 1177 1178 <h2 id="notation">Time notation</h2> 1179 <ul> 1180 <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data 1181 Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time 1182 zone names, abbreviations, identifiers, and formats. For example, it 1183 contains French translations for Eastern European Summer Time, 1184 <abbr title="Eastern European Summer Time">EEST</abbr>, and Bucharest. Its 1185 <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type 1186 charts</a> show these values for many locales. Data values are available in 1187 both <abbr title="Locale Data Markup Language">LDML</abbr> 1188 (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.</li> 1189 <li> 1190 <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of 1191 the international standard date and time notation</a> covers 1192 <a 1193 href="https://www.iso.org/standard/70907.html"><em><abbr 1194 title="International Organization for Standardization">ISO</abbr> 1195 8601-1:2019 Date and time Representations for information 1196 interchange Part 1: Basic rules</em></a>.</li> 1197 <li> 1198 <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr> 1199 Schema: Datatypes dateTime</a> specifies a format inspired by 1200 <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li> 1201 <li><a href="https://www.rfc-editor.org/info/rfc5322#section-3.3">3.3 of 1202 Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322) 1203 specifies the time notation used in email and <a 1204 href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a> 1205 headers.</li> 1206 <li> 1207 <a href="https://www.rfc-editor.org/info/rfc3339/">Date and Time 1208 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339) 1209 specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols. 1210 An extension, <a href="https://www.rfc-editor.org/info/rfc9557/">Date 1211 and Time on the Internet: Timestamps with Additional Information</a> 1212 (Internet <abbr>RFC</abbr> 9557) extends this profile 1213 to let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp 1214 via suffixes like <code>[Asia/Tokyo]</code>.</li> 1215 <li> 1216 <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time 1217 Formats on the Web</a> surveys web- and Internet-oriented date and time 1218 formats.</li> 1219 <li>Alphabetic time zone abbreviations should not be used as unique 1220 identifiers for <abbr>UT</abbr> offsets as they are ambiguous in 1221 practice. For example, in English-speaking North America 1222 <abbr>CST</abbr> denotes 6 hours behind <abbr>UT</abbr>, 1223 but in China it denotes 8 hours ahead of <abbr>UT</abbr>, 1224 and French-speaking North Americans prefer 1225 <abbr title="Heure Normale du Centre">HNC</abbr> to 1226 <abbr>CST</abbr>. The <code><abbr>tz</abbr></code> 1227 database contains English abbreviations for many timestamps; 1228 unfortunately some of these abbreviations were merely the database maintainers 1229 inventions, and these have been removed when possible.</li> 1230 <li>Numeric time zone abbreviations typically count hours east of 1231 <abbr>UT</abbr>, e.g., +09 for Japan and 1232 10 for Hawaii. However, <abbr>POSIX</abbr> proleptic 1233 <code><abbr>TZ</abbr></code> settings use the opposite convention. 1234 For example, one might use <code><abbr>TZ</abbr>="<abbr 1235 title="Japan Standard Time">JST</abbr>-9"</code> and 1236 <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code> 1237 for Japan and Hawaii, respectively. If the 1238 <code><abbr>tz</abbr></code> database is available, it is usually better to use 1239 settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and 1240 <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid 1241 confusion, handle old timestamps better, and insulate you better from 1242 any future changes to the rules. One should never set 1243 <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like 1244 <code>"GMT-9"</code>, though, since this would incorrectly imply that 1245 local time is nine hours ahead of <abbr>UT</abbr> and the time zone 1246 is called <abbr>GMT</abbr>.</li> 1247 </ul> 1248 </main> 1249 1250 <aside> 1251 <h2 id="see-also">See also</h2> 1252 <ul> 1253 <li><a href="theory.html">Theory and pragmatics of the 1254 <code><abbr>tz</abbr></code> code and data</a></li> 1255 <li><a href="tz-art.html">Time and the arts</a></li> 1256 </ul> 1257 </aside> 1258 1259 <footer> 1260 <hr> 1261 This web page is in the public domain, so clarified as of 1262 2009-05-17 by Arthur David Olson. 1263 <br> 1264 Please send corrections to this web page to the 1265 <a href="mailto:tz (a] iana.org">time zone mailing list</a>. 1266 The mailing list and its archives are public, 1267 so please do not send confidential information. 1268 </footer> 1269 </body> 1270 </html> 1271