1 1.1 christos <!DOCTYPE html> 2 1.1 christos <html lang="en"> 3 1.1 christos <head> 4 1.13 christos <title>Time zone and daylight saving time data</title> 5 1.1 christos <meta charset="UTF-8"> 6 1.21 christos <meta name="viewport" content="width=device-width, initial-scale=1"> 7 1.2 christos <style> 8 1.21 christos dd {margin-left: 1.3rem;} 9 1.21 christos pre {margin-left: 1.3rem; overflow: auto;} 10 1.21 christos ul {padding-left: 1.3rem;} 11 1.2 christos </style> 12 1.1 christos </head> 13 1.1 christos <body> 14 1.24 christos <main> 15 1.13 christos <h1>Time zone and daylight saving time data</h1> 16 1.1 christos <p> 17 1.1 christos <a href="https://en.wikipedia.org/wiki/Time_zone">Time zone</a> and 18 1.1 christos <a href="https://en.wikipedia.org/wiki/Daylight_saving_time">daylight-saving</a> 19 1.1 christos rules are controlled by individual 20 1.1 christos governments. They are sometimes changed with little notice, and their 21 1.1 christos histories and planned futures are often recorded only fitfully. Here 22 1.1 christos is a summary of attempts to organize and record relevant data in this 23 1.1 christos area. 24 1.1 christos </p> 25 1.6 christos <nav> 26 1.6 christos <ul> 27 1.6 christos <li>The <code><abbr>tz</abbr></code> database product and process 28 1.6 christos <ul> 29 1.6 christos <li><a href="#tzdb">The <code><abbr>tz</abbr></code> database</a></li> 30 1.6 christos <li><a href="#download">Downloading the <code><abbr>tz</abbr></code> database</a></li> 31 1.6 christos <li><a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a></li> 32 1.14 christos <li><a href="#coordinating">Coordinating with governments and distributors</a></li> 33 1.6 christos <li><a href="#commentary">Commentary on the <code><abbr>tz</abbr></code> database</a></li> 34 1.6 christos </ul> 35 1.6 christos </li> 36 1.6 christos <li>Uses of the <code><abbr>tz</abbr></code> database 37 1.6 christos <ul> 38 1.6 christos <li><a href="#web">Web sites using recent versions of the <code><abbr>tz</abbr></code> database</a></li> 39 1.6 christos <li><a href="#protocols">Network protocols for <code><abbr>tz</abbr></code> data</a></li> 40 1.6 christos <li><a href="#compilers">Other <code><abbr>tz</abbr></code> compilers</a></li> 41 1.6 christos <li><a href="#TZif">Other <abbr>TZif</abbr> readers</a></li> 42 1.6 christos <li><a href="#software">Other <code><abbr>tz</abbr></code>-based time zone software</a></li> 43 1.6 christos </ul> 44 1.6 christos </li> 45 1.6 christos <li>Related data 46 1.6 christos <ul> 47 1.6 christos <li><a href="#other-dbs">Other time zone databases</a></li> 48 1.6 christos <li><a href="#maps">Maps</a></li> 49 1.6 christos <li><a href="#boundaries">Time zone boundaries</a></li> 50 1.6 christos </ul> 51 1.6 christos </li> 52 1.6 christos <li>Timekeeping concepts 53 1.6 christos <ul> 54 1.6 christos <li><a href="#civil">Civil time concepts and history</a></li> 55 1.6 christos <li><a href="#national">National histories of legal time</a></li> 56 1.7 christos <li><a href="#costs">Costs and benefits of time shifts</a></li> 57 1.6 christos <li><a href="#precision">Precision timekeeping</a></li> 58 1.6 christos <li><a href="#notation">Time notation</a></li> 59 1.6 christos </ul> 60 1.6 christos </li> 61 1.13 christos <li><a href="#see-also">See also</a></li> 62 1.6 christos </ul> 63 1.6 christos </nav> 64 1.6 christos 65 1.1 christos <h2 id="tzdb">The <code><abbr title="time zone">tz</abbr></code> database</h2> 66 1.1 christos <p> 67 1.1 christos The <a href="https://en.wikipedia.org/wiki/Public_domain">public-domain</a> 68 1.1 christos time zone database contains code and data 69 1.1 christos that represent the history of local time 70 1.1 christos for many representative locations around the globe. 71 1.1 christos It is updated periodically to reflect changes made by political bodies 72 1.1 christos to time zone boundaries and daylight saving rules. 73 1.1 christos This database (known as <code><abbr>tz</abbr></code>, 74 1.1 christos <code><abbr>tzdb</abbr></code>, or <code>zoneinfo</code>) 75 1.1 christos is used by several implementations, 76 1.1 christos including 77 1.1 christos <a href="https://www.gnu.org/software/libc/">the 78 1.20 christos <abbr title="GNUs Not Unix">GNU</abbr> 79 1.1 christos C Library</a> (used in 80 1.1 christos <a href="https://en.wikipedia.org/wiki/Linux"><abbr>GNU</abbr>/Linux</a>), 81 1.1 christos <a href="https://www.android.com">Android</a>, 82 1.1 christos <a href="https://www.freebsd.org">Free<abbr 83 1.1 christos title="Berkeley Software Distribution">BSD</abbr></a>, 84 1.1 christos <a href="https://netbsd.org">Net<abbr>BSD</abbr></a>, 85 1.1 christos <a href="https://www.openbsd.org">Open<abbr>BSD</abbr></a>, 86 1.17 christos <a href="https://www.chromium.org/chromium-os/">ChromiumOS</a>, 87 1.24 christos <a href="https://en.wikipedia.org/wiki/OpenHarmony">OpenHarmony</a>, 88 1.1 christos <a href="https://cygwin.com">Cygwin</a>, 89 1.7 christos <a href="https://mariadb.org">MariaDB</a>, 90 1.17 christos <a href="https://musl.libc.org">musl libc</a>, 91 1.1 christos <a href="https://www.mysql.com">MySQL</a>, 92 1.1 christos <a href="https://en.wikipedia.org/wiki/WebOS"><abbr 93 1.1 christos title="Web Operating System">webOS</abbr></a>, 94 1.7 christos <a href="https://en.wikipedia.org/wiki/IBM_AIX"><abbr 95 1.1 christos title="Advanced Interactive eXecutive">AIX</abbr></a>, 96 1.21 christos <a href="https://www.apple.com/os/ios/"><abbr 97 1.1 christos title="iPhone OS">iOS</abbr></a>, 98 1.21 christos <a href="https://www.apple.com/os/macos/">macOS</a>, 99 1.1 christos <a href="https://www.microsoft.com/en-us/windows">Microsoft Windows</a>, 100 1.21 christos <a href="https://vmssoftware.com">Open<abbr 101 1.1 christos title="Virtual Memory System">VMS</abbr></a>, 102 1.20 christos <a href="https://www.oracle.com/database/">Oracle Database</a>, 103 1.21 christos <a href="https://www.oracle.com/solaris/solaris11/">Oracle Solaris</a>, 104 1.21 christos and <a href="https://qnx.software/en">QNX</a>.</p> 105 1.1 christos <p> 106 1.2 christos Each main entry in the database represents a <dfn>timezone</dfn> 107 1.2 christos for a set of civil-time clocks that have all agreed since 1970. 108 1.2 christos Timezones are typically identified by continent or ocean and then by the 109 1.2 christos name of the largest city within the region containing the clocks. 110 1.1 christos For example, <code>America/New_York</code> 111 1.1 christos represents most of the <abbr title="United States">US</abbr> eastern time zone; 112 1.1 christos <code>America/Phoenix</code> represents most of Arizona, which 113 1.2 christos uses mountain time without daylight saving time (<abbr>DST</abbr>); 114 1.1 christos <code>America/Detroit</code> represents most of Michigan, which uses 115 1.1 christos eastern time but with different <abbr>DST</abbr> rules in 1975; 116 1.1 christos and other entries represent smaller regions like Starke County, 117 1.1 christos Indiana, which switched from central to eastern time in 1991 118 1.1 christos and switched back in 2006. 119 1.17 christos To use the database on a <a 120 1.1 christos href="https://en.wikipedia.org/wiki/POSIX"><abbr 121 1.17 christos title="Portable Operating System Interface">POSIX</abbr>.1-2024</a> 122 1.1 christos implementation set the <code><abbr>TZ</abbr></code> 123 1.20 christos environment variable to the locations full name, 124 1.1 christos e.g., <code><abbr>TZ</abbr>="America/New_York"</code>.</p> 125 1.1 christos <p> 126 1.2 christos Associated with each timezone is a history of offsets from 127 1.1 christos <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal 128 1.1 christos Time</a> (<abbr>UT</abbr>), which is <a 129 1.1 christos href="https://en.wikipedia.org/wiki/Greenwich_Mean_Time">Greenwich Mean 130 1.1 christos Time</a> (<abbr>GMT</abbr>) with days beginning at midnight; 131 1.2 christos for timestamps after 1960 this is more precisely <a 132 1.1 christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time">Coordinated 133 1.1 christos Universal Time</a> (<abbr>UTC</abbr>). 134 1.1 christos The database also records when daylight saving time was in use, 135 1.1 christos along with some time zone abbreviations such as <abbr>EST</abbr> 136 1.1 christos for Eastern Standard Time in the <abbr>US</abbr>.</p> 137 1.6 christos 138 1.1 christos <h2 id="download">Downloading the <code><abbr>tz</abbr></code> database</h2> 139 1.1 christos <p> 140 1.1 christos The following <a 141 1.1 christos href="https://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download 142 1.20 christos the latest releases two 143 1.1 christos <a href="https://en.wikipedia.org/wiki/Tar_(computing)">tarballs</a> 144 1.1 christos to a <abbr>GNU</abbr>/Linux or similar host.</p> 145 1.2 christos <pre><code>mkdir tzdb 146 1.1 christos cd tzdb 147 1.24 christos <a href="https://www.gnu.org/software/wget/">wget</a> https://data.iana.org/time-zones/tzcode-latest.tar.gz 148 1.24 christos wget https://data.iana.org/time-zones/tzdata-latest.tar.gz 149 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 - 150 1.1 christos gzip -dc tzdata-latest.tar.gz | tar -xf - 151 1.1 christos </code></pre> 152 1.1 christos <p>Alternatively, the following shell commands download the same 153 1.1 christos release in a single-tarball format containing extra data 154 1.1 christos useful for regression testing:</p> 155 1.24 christos <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 1.1 christos <a href="https://www.nongnu.org/lzip/">lzip</a> -dc tzdb-latest.tar.lz | tar -xf - 157 1.1 christos </code></pre> 158 1.1 christos <p>These commands use convenience links to the latest release 159 1.1 christos of the <code><abbr>tz</abbr></code> database hosted by the 160 1.1 christos <a href="https://www.iana.org/time-zones">Time Zone Database website</a> 161 1.1 christos of the <a href="https://www.iana.org">Internet Assigned Numbers 162 1.1 christos Authority (IANA)</a>. 163 1.1 christos Older releases are in files named 164 1.1 christos <code>tzcode<var>V</var>.tar.gz</code>, 165 1.1 christos <code>tzdata<var>V</var>.tar.gz</code>, and 166 1.1 christos <code>tzdb-<var>V</var>.tar.lz</code>, 167 1.1 christos where <code><var>V</var></code> is the version. 168 1.1 christos Since 1996, each version has been a four-digit year followed by 169 1.1 christos lower-case letter (<samp>a</samp> through <samp>z</samp>, 170 1.1 christos then <samp>za</samp> through <samp>zz</samp>, then <samp>zza</samp> 171 1.1 christos through <samp>zzz</samp>, and so on). 172 1.13 christos Since version 2022a, each release has been distributed in 173 1.20 christos <a href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/pax.html#tag_20_94_13_06">POSIX 174 1.10 christos ustar interchange format</a>, compressed as described above; 175 1.14 christos older releases use a nearly compatible format. 176 1.1 christos Since version 2016h, each release has contained a text file named 177 1.20 christos <code>version</code> whose first (and currently only) line is the version. 178 1.20 christos <a href="https://ftp.iana.org/tz/releases/">Older archived releases are 179 1.20 christos available</a> via 180 1.20 christos <a href="https://en.wikipedia.org/wiki/HTTPS"><abbr 181 1.20 christos title="Hypertext Transfer Protocol Secure">HTTPS</abbr></a>, 182 1.20 christos <a href="https://en.wikipedia.org/wiki/Rsync"><abbr 183 1.20 christos title="remote sync">rsync</abbr></a>, and 184 1.20 christos <a href="https://en.wikipedia.org/wiki/FTP"><abbr 185 1.24 christos title="File Transfer Protocol">FTP</abbr></a>.</p> 186 1.21 christos <p>Alternatively, a development repository of code and data can be 187 1.1 christos retrieved from <a href="https://github.com">GitHub</a> via the shell 188 1.1 christos command:</p> 189 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> 190 1.1 christos </code></pre> 191 1.1 christos <p> 192 1.1 christos Since version 2012e, each release has been tagged in development repositories. 193 1.1 christos Untagged commits are less well tested and probably contain 194 1.1 christos more errors.</p> 195 1.1 christos <p> 196 1.1 christos After obtaining the code and data files, see the 197 1.1 christos <code>README</code> file for what to do next. 198 1.1 christos The code lets you compile the <code><abbr>tz</abbr></code> source files into 199 1.23 christos machine-readable binary files, one for each location. 200 1.23 christos The binary file format is specified by 201 1.23 christos <a href="https://www.rfc-editor.org/info/rfc9636/">The 202 1.17 christos Time Zone Information Format (<abbr>TZif</abbr>)</a> 203 1.18 christos (Internet <abbr title="Request For Comments">RFC</abbr> 9636). 204 1.2 christos The code also lets 205 1.2 christos you read a <abbr>TZif</abbr> file and interpret timestamps for that 206 1.1 christos location.</p> 207 1.6 christos 208 1.1 christos <h2 id="changes">Changes to the <code><abbr>tz</abbr></code> database</h2> 209 1.1 christos <p> 210 1.1 christos The <code><abbr>tz</abbr></code> code and data 211 1.1 christos are by no means authoritative. If you find errors, please 212 1.17 christos email changes to <a href="mailto:tz (a] iana.org"><code>tz (a] iana.org</code></a>, 213 1.20 christos the time zone mailing list. 214 1.20 christos The mailing list and its archives are public, 215 1.20 christos so please do not send confidential information. 216 1.20 christos See 217 1.17 christos <a href="https://lists.iana.org/postorius/lists/tz.iana.org/">the mailing 218 1.20 christos lists main page</a> to subscribe or to browse its archive of old messages. 219 1.17 christos <a href="https://tzdata-meta.timtimeonline.com">Metadata for mailing list 220 1.5 christos discussions</a> and corresponding data changes can be 221 1.5 christos generated <a href="https://github.com/timparenti/tzdata-meta">automatically</a>. 222 1.5 christos </p> 223 1.1 christos <p> 224 1.1 christos Changes to the <code><abbr>tz</abbr></code> code and data are often 225 1.1 christos propagated to clients via operating system updates, so 226 1.1 christos client <code><abbr>tz</abbr></code> data can often be corrected by 227 1.1 christos applying these updates. With GNU/Linux and similar systems, if your 228 1.1 christos maintenance provider has not yet adopted the 229 1.1 christos latest <code><abbr>tz</abbr></code> data, you can often short-circuit 230 1.1 christos the process by tailoring the generic instructions in 231 1.1 christos the <code><abbr>tz</abbr> README</code> file and installing the latest 232 1.1 christos data yourself. System-specific instructions for installing the 233 1.1 christos latest <code><abbr>tz</abbr></code> data have also been published 234 1.24 christos 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 1.24 christos (and 236 1.24 christos <a href="https://www.ibm.com/support/pages/aix-time-zone-olson-tzdata-updates">older 237 1.24 christos <abbr>AIX</abbr></a>), 238 1.1 christos <a 239 1.21 christos href="https://source.android.com/docs/core/permissions/timezone-rules">Android</a>, 240 1.1 christos <a 241 1.11 christos href="https://unicode-org.github.io/icu/userguide/datetime/timezone/"><abbr 242 1.1 christos title="International Components for Unicode">ICU</abbr></a>, 243 1.11 christos <a href="https://www.ibm.com/support/pages/java-sdk-time-zone-update-utility"><abbr>IBM</abbr> 244 1.9 christos JDK</a>, 245 1.9 christos <a href="https://www.joda.org/joda-time/tz_update.html">Joda-Time</a>, <a 246 1.1 christos href="https://dev.mysql.com/doc/refman/en/time-zone-support.html">MySQL</a>, 247 1.9 christos <a href="https://nodatime.org/userguide/tzdb">Noda Time</a>, and <a 248 1.11 christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">OpenJDK/Oracle JDK</a>. 249 1.1 christos </p> 250 1.12 christos <p>Since version 2013a, 251 1.12 christos sources for the <code><abbr>tz</abbr></code> database have been 252 1.1 christos <a href="https://en.wikipedia.org/wiki/UTF-8"><abbr 253 1.1 christos title="Unicode Transformation Format 8-bit">UTF-8</abbr></a> 254 1.1 christos <a href="https://en.wikipedia.org/wiki/Text_file">text files</a> 255 1.1 christos with lines terminated by <a href="https://en.wikipedia.org/wiki/Newline"><abbr 256 1.1 christos title="linefeed">LF</abbr></a>, 257 1.1 christos which can be modified by common text editors such 258 1.1 christos as <a href="https://www.gnu.org/software/emacs/">GNU Emacs</a>, 259 1.20 christos <a href="https://gedit-text-editor.org">gedit</a>, and 260 1.1 christos <a href="https://www.vim.org">vim</a>. 261 1.1 christos Specialized source-file editing can be done via the 262 1.1 christos <a href="https://packagecontrol.io/packages/zoneinfo">Sublime 263 1.1 christos zoneinfo</a> package for <a 264 1.1 christos href="https://www.sublimetext.com">Sublime Text</a> and the <a 265 1.1 christos href="https://marketplace.visualstudio.com/items?itemName=gilmoreorless.vscode-zoneinfo">VSCode 266 1.1 christos zoneinfo</a> extension for <a href="https://code.visualstudio.com">Visual 267 1.1 christos Studio Code</a>. 268 1.1 christos </p> 269 1.1 christos <p> 270 1.1 christos For further information about updates, please see 271 1.23 christos <a href="https://www.rfc-editor.org/info/rfc6557/">Procedures for 272 1.17 christos Maintaining the Time Zone Database</a> (Internet <abbr>RFC</abbr> 6557). 273 1.17 christos More detail can be 274 1.7 christos found in <a href="theory.html">Theory and pragmatics of the 275 1.7 christos <code><abbr>tz</abbr></code> code and data</a>. 276 1.2 christos <a href="https://a0.github.io/a0-tzmigration/">A0 TimeZone Migration</a> 277 1.2 christos displays changes between recent <code><abbr>tzdb</abbr></code> versions. 278 1.1 christos </p> 279 1.6 christos 280 1.14 christos <h2 id="coordinating">Coordinating with governments and distributors</h2> 281 1.14 christos <p> 282 1.14 christos As discussed in 283 1.20 christos <a href="https://www.icann.org/en/blogs/details/how-time-zones-are-coordinated-13-03-2023-en">How 284 1.20 christos Time Zones Are Coordinated</a>, the time zone database relies on 285 1.14 christos collaboration among governments, the time zone database volunteer 286 1.24 christos community, and data distributors downstream.</p> 287 1.14 christos <p> 288 1.14 christos If your government plans to change its time zone boundaries or 289 1.20 christos daylight saving rules, please send email as described in 290 1.21 christos <a href="#changes">Changes to the <code><abbr>tz</abbr></code> database</a>. 291 1.20 christos Do this well in advance, 292 1.14 christos as this will lessen confusion and will coordinate updates to many cell phones, 293 1.14 christos computers, and other devices around the world. 294 1.14 christos In your email, please cite the legislation or regulation that specifies 295 1.14 christos the change, so that it can be checked for details such as the exact times 296 1.14 christos when clock transitions occur. 297 1.14 christos It is OK if a rule change is planned to affect clocks 298 1.14 christos far into the future, as a long-planned change can easily be reverted 299 1.20 christos or otherwise altered with a years notice before the change would have 300 1.14 christos affected clocks.</p> 301 1.14 christos <p> 302 1.14 christos There is no fixed schedule for <code><abbr>tzdb</abbr></code> releases. 303 1.14 christos However, typically a release occurs every few months. 304 1.14 christos Many downstream timezone data distributors wait for 305 1.14 christos a <code><abbr>tzdb</abbr></code> release before they produce an update 306 1.14 christos to time zone behavior in consumer devices and software products. 307 1.14 christos After a release, various parties must integrate, test, 308 1.14 christos and roll out an update before <a 309 1.14 christos href="https://en.wikipedia.org/wiki/End_user">end users</a> see changes. 310 1.14 christos These updates can be expensive, for both the <a 311 1.14 christos href="https://en.wikipedia.org/wiki/Quality_assurance">quality 312 1.14 christos assurance</a> process and the overall cost of shipping and installing 313 1.20 christos updates to each devices copy of <code><abbr>tzdb</abbr></code>. 314 1.14 christos Updates may be batched with other updates and may take substantial 315 1.14 christos time to reach end users after a release. 316 1.14 christos Older devices may no longer be supported and thus may never be updated, 317 1.14 christos which means they will continue to use out-of-date rules.</p> 318 1.14 christos <p> 319 1.14 christos For these reasons any rule change should be promulgated at least a 320 1.14 christos year before it affects how clocks operate; otherwise, there is a good 321 1.14 christos chance that many clocks will be wrong due to delays in propagating updates, 322 1.14 christos and that residents will be confused or even actively resist the change. 323 1.20 christos The shorter the notice, the more likely clock problems will arise; see <a 324 1.21 christos href="https://codeofmatt.com/on-the-timing-of-time-zone-changes/">On 325 1.20 christos the Timing of Time Zone Changes</a> for examples. 326 1.14 christos </p> 327 1.14 christos 328 1.1 christos <h2 id="commentary">Commentary on the <code><abbr>tz</abbr></code> database</h2> 329 1.1 christos <ul> 330 1.1 christos <li>The article 331 1.1 christos <a href="https://en.wikipedia.org/wiki/Tz_database">tz database</a> is 332 1.1 christos an encyclopedic summary.</li> 333 1.1 christos <li><a href="tz-how-to.html">How to Read the 334 1.1 christos tz Database Source Files</a> explains the <code><abbr>tz</abbr></code> 335 1.23 christos database source format.</li> 336 1.1 christos <li><a 337 1.1 christos href="https://blog.jonudell.net/2009/10/23/a-literary-appreciation-of-the-olsonzoneinfotz-database/">A 338 1.1 christos literary appreciation of the Olson/Zoneinfo/tz database</a> comments on the 339 1.20 christos databases style.</li> 340 1.10 christos <li><a href="https://doi.org/10.1145/3340301.3341125">What time is it: 341 1.10 christos managing time in the internet</a> analyzes the database longitudinally.</li> 342 1.1 christos </ul> 343 1.6 christos 344 1.1 christos <h2 id="web">Web sites using recent versions of the 345 1.1 christos <code><abbr>tz</abbr></code> database</h2> 346 1.1 christos <p> 347 1.1 christos These are listed roughly in ascending order of complexity and fanciness. 348 1.1 christos </p> 349 1.1 christos <ul> 350 1.20 christos <li><a href="https://time.is">Time.is</a> shows locations 351 1.1 christos time and zones.</li> 352 1.1 christos <li><a href="https://www.timejones.com">TimeJones.com</a>, 353 1.1 christos <a href="https://timezoneconverterapp.com">Time Zone Converter</a> and 354 1.7 christos <a href="https://www.worldclock.com">The World Clock</a> 355 1.1 christos are time zone converters.</li> 356 1.8 christos <li><a href="https://timezonedb.com/download">TimeZoneDB Database</a> 357 1.8 christos publishes <code><abbr>tzdb</abbr></code>-derived data in 358 1.8 christos <a href="https://en.wikipedia.org/wiki/Comma-separated_values"><abbr 359 1.8 christos title="comma-separated values">CSV</abbr></a> and 360 1.8 christos in <a href="https://en.wikipedia.org/wiki/SQL"><abbr 361 1.8 christos title="Structured Query Language">SQL</abbr></a> form.</li> 362 1.1 christos <li><a 363 1.6 christos href="https://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a> 364 1.1 christos lets you see the <code><abbr>TZ</abbr></code> values directly.</li> 365 1.1 christos <li><a 366 1.13 christos href="https://www.convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current 367 1.1 christos Time in 1000 Places</a> uses descriptions of the values.</li> 368 1.20 christos <li><a href="https://www.timeanddate.com/worldclock/">The World Clock 369 1.1 christos Worldwide</a> lets you sort zone names and convert times.</li> 370 1.1 christos <li><a href="https://24timezones.com">24TimeZones</a> has a world 371 1.1 christos time map and a time converter.</li> 372 1.1 christos <li><a href="https://www.zeitverschiebung.net/en/">Time Difference</a> 373 1.1 christos calculates the current time difference between locations.</li> 374 1.13 christos <li><a href="https://www.wx-now.com">Weather Now</a> and 375 1.13 christos <a href="https://www.thetimenow.com">The Time Now</a> list the weather too.</li> 376 1.1 christos </ul> 377 1.6 christos 378 1.1 christos <h2 id="protocols">Network protocols for <code><abbr>tz</abbr></code> data</h2> 379 1.1 christos <ul> 380 1.23 christos <li><a href="https://www.rfc-editor.org/info/rfc7808/">Time Zone 381 1.23 christos Data Distribution Service</a> (Internet <abbr>RFC</abbr> 7808) 382 1.20 christos is associated with 383 1.23 christos <a href="https://www.rfc-editor.org/info/rfc7809/">CalDAV</a> 384 1.1 christos (Internet <abbr>RFC</abbr> 7809), a calendar access protocol for 385 1.23 christos transferring time zone data by reference.</li> 386 1.23 christos <li>The <a href="https://www.rfc-editor.org/info/rfc5545/">iCalendar format</a> 387 1.23 christos (Internet <abbr>RFC</abbr> 5545) 388 1.1 christos covers time zone 389 1.1 christos data; see its VTIMEZONE calendar component. 390 1.1 christos The iCalendar format requires specialized parsers and generators; a 391 1.23 christos variant <a href="https://www.rfc-editor.org/info/rfc6321/">xCal</a> 392 1.1 christos (Internet <abbr>RFC</abbr> 6321) uses 393 1.1 christos <a href="https://www.w3.org/XML/"><abbr 394 1.1 christos title="Extensible Markup Language">XML</abbr></a> format, and a variant 395 1.23 christos <a href="https://www.rfc-editor.org/info/rfc7265/">jCal</a> 396 1.1 christos (Internet <abbr>RFC</abbr> 7265) 397 1.17 christos uses <a href="https://www.json.org/json-en.html"><abbr 398 1.1 christos title="JavaScript Object Notation">JSON</abbr></a> format.</li> 399 1.1 christos </ul> 400 1.6 christos 401 1.1 christos <h2 id="compilers">Other <code><abbr>tz</abbr></code> compilers</h2> 402 1.7 christos <p>Although some of these do not fully support 403 1.7 christos <code><abbr>tz</abbr></code> data, in recent <code><abbr>tzdb</abbr></code> 404 1.7 christos distributions you can generally work around compatibility problems by 405 1.7 christos running the command <code>make rearguard_tarballs</code> and compiling 406 1.7 christos from the resulting tarballs instead.</p> 407 1.1 christos <ul> 408 1.23 christos <li><a href="https://github.com/libical">Libical</a> 409 1.24 christos contains <code>vzic</code>, a <a 410 1.14 christos href="https://en.wikipedia.org/wiki/C_(programming_language)">C</a> 411 1.1 christos program that compiles 412 1.1 christos <code><abbr>tz</abbr></code> source into iCalendar-compatible VTIMEZONE files. 413 1.1 christos Vzic is freely 414 1.1 christos available under the <a 415 1.1 christos href="https://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr> 416 1.1 christos General Public License (<abbr 417 1.1 christos title="General Public License">GPL</abbr>)</a>.</li> 418 1.24 christos <li>The <a id="TZDP" 419 1.24 christos href="https://howardhinnant.github.io/date/tz.html">Time Zone 420 1.24 christos Database Parser</a> contains a 421 1.24 christos <a href="https://en.wikipedia.org/wiki/C++">C++</a> runtime parser 422 1.24 christos supporting the <a 423 1.24 christos href="https://en.cppreference.com/cpp/chrono#Time_zone"><code>std::chrono::time_zone</code> API</a> 424 1.24 christos that is standardized in 425 1.24 christos <a href="https://en.wikipedia.org/wiki/C%2B%2B20">C++20</a> and later. 426 1.24 christos It is freely available under the 427 1.24 christos <a href="https://en.wikipedia.org/wiki/MIT_License"><abbr 428 1.24 christos title="Massachusetts Institute of Technology">MIT</abbr> license</a>. 429 1.24 christos This approach is used by the <code>std::chrono::time_zone</code> implementation 430 1.24 christos of <a href="https://gcc.gnu.org/"><abbr 431 1.24 christos title="the GNU Compiler Collection">GCC</abbr></a> 432 1.24 christos <code>libstdc++</code>, which is available under the GPL combined 433 1.24 christos with the <a href="https://www.gnu.org/licenses/gcc-exception.html">GCC 434 1.24 christos Runtime Library Exception</a>.</li> 435 1.1 christos <li><a 436 1.4 christos href="https://metacpan.org/release/DateTime-TimeZone">DateTime::TimeZone</a> 437 1.1 christos contains a script <code>parse_olson</code> that compiles 438 1.1 christos <code><abbr>tz</abbr></code> source into <a href="https://www.perl.org">Perl</a> 439 1.1 christos modules. It is part of the Perl <a 440 1.4 christos href="https://github.com/houseabsolute/DateTime.pm/wiki">DateTime Project</a>, 441 1.4 christos which is freely 442 1.1 christos available under both the <abbr>GPL</abbr> and the Perl Artistic 443 1.1 christos License. DateTime::TimeZone also contains a script 444 1.1 christos <code>tests_from_zdump</code> that generates test cases for each clock 445 1.1 christos transition in the <code><abbr>tz</abbr></code> database.</li> 446 1.13 christos <li><a id="ICU" href="https://icu.unicode.org">International Components for 447 1.1 christos Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a 448 1.14 christos href="https://en.wikipedia.org/wiki/Java_(programming_language)">Java</a> 449 1.1 christos libraries for internationalization that 450 1.1 christos has a compiler from <code><abbr>tz</abbr></code> source 451 1.1 christos and from <abbr title="Common Locale Data Repository">CLDR</abbr> data 452 1.1 christos (mentioned <a href="#CLDR">below</a>) 453 1.1 christos into an <abbr>ICU</abbr>-specific format. 454 1.1 christos <abbr>ICU</abbr> is freely available under a 455 1.1 christos <abbr>BSD</abbr>-style license.</li> 456 1.1 christos <li>The <a href="https://github.com/lau/tzdata">Tzdata</a> package for 457 1.1 christos the <a href="https://elixir-lang.org">Elixir</a> language downloads 458 1.7 christos and compiles <code><abbr>tz</abbr></code> source and exposes <abbr 459 1.1 christos title="Application Program Interface">API</abbr>s for use. It is 460 1.1 christos freely available under the <abbr>MIT</abbr> license.</li> 461 1.1 christos <li>Java-based compilers and libraries include: 462 1.1 christos <ul> 463 1.1 christos <li>The <a 464 1.9 christos href="https://www.oracle.com/java/technologies/javase/tzupdater-readme.html">TZUpdater 465 1.1 christos tool</a> compiles <code><abbr>tz</abbr></code> source into the format used by 466 1.17 christos <a href="https://openjdk.org">OpenJDK</a> and 467 1.17 christos <a href="https://jdk.java.net">Oracle JDK</a>. 468 1.9 christos Although its source code is proprietary, its executable is available under the 469 1.9 christos <a href="https://www.oracle.com/a/tech/docs/tzupdater-lic.html">Java SE 470 1.9 christos Timezone Updater License Agreement</a>.</li> 471 1.1 christos <li>The <a 472 1.21 christos href="https://www.oracle.com/technical-resources/articles/java/jf14-date-time.html">Java 473 1.6 christos SE 8 Date and Time</a> <abbr>API</abbr> can be supplemented by <a 474 1.4 christos href="https://www.threeten.org/threeten-extra/">ThreeTen-Extra</a>, 475 1.1 christos which is freely available under a <abbr>BSD</abbr>-style license.</li> 476 1.20 christos <li><a href="https://www.joda.org/joda-time/">Joda-Time Java date 477 1.1 christos and time <abbr>API</abbr></a> contains a class 478 1.1 christos <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles 479 1.1 christos <code><abbr>tz</abbr></code> source into a binary format. It inspired 480 1.1 christos Java 8 <code>java.time</code>, which its users should migrate to once 481 1.1 christos they can assume Java 8 or later. It is available under the <a 482 1.1 christos href="https://www.apache.org/licenses/LICENSE-2.0">Apache License</a>.</li> 483 1.9 christos <li><a href="https://bell-sw.com/pages/iana-updater/">IANA Updater</a> and <a 484 1.21 christos href="https://www.azul.com/products/components/ziupdater-time-zone-tool/">ZIUpdater</a> 485 1.20 christos are alternatives to TZUpdater. IANA Updaters license is unclear; 486 1.9 christos ZIUpdater is licensed under the <abbr>GPL</abbr>.</li> 487 1.4 christos <li><a href="https://github.com/MenoData/Time4A">Time4A: Advanced date and 488 1.4 christos time library for Android</a> and 489 1.17 christos <a href="https://github.com/MenoData/Time4J">Time4J: Advanced date, 490 1.4 christos time and interval library for Java</a> compile 491 1.4 christos <code><abbr>tz</abbr></code> source into a binary format. 492 1.4 christos Time4A is available under the Apache License and Time4J is 493 1.1 christos available under the <a 494 1.1 christos href="https://www.gnu.org/copyleft/lesser.html"><abbr>GNU</abbr> Lesser 495 1.1 christos General Public License (<abbr title="Lesser General Public 496 1.1 christos License">LGPL</abbr>)</a>.</li> 497 1.1 christos <li><abbr>ICU</abbr> (mentioned <a href="#ICU">above</a>) contains compilers and 498 1.1 christos Java-based libraries.</li> 499 1.1 christos </ul> 500 1.24 christos </li> 501 1.20 christos <li><a href="https://nodatime.org">Noda Time Date and 502 1.1 christos time <abbr>API</abbr> for .NET</a> 503 1.4 christos is like Joda-Time and Time4J, but for the .NET framework instead of Java. 504 1.4 christos It is freely available under the Apache License.</li> 505 1.8 christos <li>Many modern 506 1.8 christos <a href="https://en.wikipedia.org/wiki/JavaScript">JavaScript</a> 507 1.8 christos runtimes support <code><abbr>tz</abbr></code> natively via the 508 1.16 christos <code>timeZone</code> option of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat"><code>Intl.DateTimeFormat</code></a>. 509 1.10 christos This can be used as-is or with most of the following libraries, 510 1.16 christos many of which also support runtimes lacking the <code>timeZone</code> option. 511 1.1 christos <ul> 512 1.8 christos <li>The <a 513 1.16 christos href="https://github.com/formatjs/date-time-format-timezone"><code>Intl.DateTimeFormat</code> 514 1.8 christos timezone polyfill</a> 515 1.8 christos is freely available under a <abbr>BSD</abbr>-style license.</li> 516 1.17 christos <li>The <a href="https://date-fns.org">date-fns</a> 517 1.10 christos library manipulates timezone-aware timestamps in browsers and 518 1.10 christos in <a href="https://nodejs.org/en/">Node.js</a>. 519 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li> 520 1.10 christos <li><a href="https://github.com/iamkun/dayjs">Day.js</a> is a 521 1.10 christos minimalist replacement for the date and time API of 522 1.10 christos the <a href="https://momentjs.com/docs/">now-legacy Moment.js</a> date 523 1.10 christos manipulation library. 524 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li> 525 1.10 christos <li><a href="https://moment.github.io/luxon/">Luxon</a> improves 526 1.16 christos timezone support for the <code>Intl</code> API. 527 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li> 528 1.1 christos <li><a href="https://momentjs.com/timezone/">Moment Timezone</a> is a 529 1.10 christos Moment.js plugin. 530 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li> 531 1.8 christos <li><a href="https://github.com/bigeasy/timezone">Timezone</a> is a 532 1.8 christos JavaScript library that supports date arithmetic that is time zone 533 1.8 christos aware. It is freely available under the <abbr>MIT</abbr> license.</li> 534 1.10 christos <li><a href="https://www.npmjs.com/package/@tubular/time">@tubular/time</a> 535 1.10 christos supports live <code><abbr>tzdb</abbr></code> updates, 536 1.10 christos astronomical and atomic time, a command-line interface, 537 1.10 christos and full <a 538 1.10 christos href="https://en.wikipedia.org/wiki/TypeScript">TypeScript</a>. 539 1.10 christos Its companion <a 540 1.10 christos href="https://www.npmjs.com/package/@tubular/time-tzdb">@tubular/time-tzdb</a> 541 1.10 christos can generate <abbr>TZif</abbr> and other files, and a companion website 542 1.10 christos <a href="https://tzexplorer.org">Timezone Database Explorer</a> lets you 543 1.10 christos convert timestamps, view transition histories, and download code and data. 544 1.10 christos It is freely available under the <abbr>MIT</abbr> license.</li> 545 1.1 christos </ul> 546 1.10 christos The proposed <a 547 1.16 christos href="https://github.com/tc39/proposal-temporal"><code>Temporal</code> 548 1.10 christos objects</a> let programs access an abstract view of 549 1.10 christos <code><abbr>tzdb</abbr></code> data, and are designed to replace <a 550 1.20 christos href="https://codeofmatt.com/javascript-date-type-is-horribly-broken/">JavaScripts 551 1.16 christos problematic <code>Date</code> objects</a> when working with dates and times. 552 1.24 christos </li> 553 1.17 christos <li><a href="https://github.com/JuliaTime">JuliaTime</a> contains a 554 1.1 christos compiler from <code><abbr>tz</abbr></code> source into 555 1.17 christos <a href="https://julialang.org">Julia</a>. It is freely available 556 1.1 christos under the <abbr>MIT</abbr> license.</li> 557 1.20 christos <li><a href="https://github.com/pavkam/tzdb"><abbr>TZDB</abbr> 558 1.6 christos <abbr>IANA</abbr> Time Zone Database for Delphi/<abbr 559 1.6 christos title="Free Pascal Compiler">FPC</abbr></a> 560 1.2 christos compiles from <code><abbr>tz</abbr></code> source into 561 1.2 christos <a href="https://en.wikipedia.org/wiki/Object_Pascal">Object Pascal</a> 562 1.2 christos as compiled by <a href="https://en.wikipedia.org/wiki/Delphi_(IDE)">Delphi</a> 563 1.2 christos and <a 564 1.2 christos href="https://en.wikipedia.org/wiki/Free_Pascal"><abbr>FPC</abbr></a>. 565 1.2 christos It is freely available under a <abbr>BSD</abbr>-style license.</li> 566 1.20 christos <li><a href="https://pythonhosted.org/pytz/">pytz World Timezone 567 1.1 christos Definitions for Python</a> compiles <code><abbr>tz</abbr></code> source into 568 1.1 christos <a href="https://www.python.org">Python</a>. 569 1.9 christos It is freely available under a <abbr>BSD</abbr>-style license. 570 1.11 christos In code that can assume Python 3.6 or later it is largely superseded; see <a 571 1.11 christos href="https://blog.ganssle.io/articles/2018/03/pytz-fastest-footgun.html">pytz: 572 1.11 christos The Fastest Footgun in the West</a>.</li> 573 1.20 christos <li><a href="https://tzinfo.github.io">TZInfo 574 1.1 christos Ruby Timezone Library</a> 575 1.1 christos compiles <code><abbr>tz</abbr></code> source into 576 1.1 christos <a href="https://www.ruby-lang.org/en/">Ruby</a>. 577 1.1 christos It is freely available under the <abbr>MIT</abbr> license.</li> 578 1.13 christos <li>The <a href="https://www.squeaksource.com/Chronos/">Chronos Date/Time 579 1.1 christos Library</a> is 580 1.1 christos a <a href="https://en.wikipedia.org/wiki/Smalltalk">Smalltalk</a> class 581 1.1 christos library that compiles <code><abbr>tz</abbr></code> source into a time 582 1.1 christos zone repository whose format 583 1.1 christos is either proprietary or an <abbr>XML</abbr>-encoded 584 1.1 christos representation.</li> 585 1.21 christos <li><a id="Tcl" href="https://www.tcl-lang.org">Tcl</a> 586 1.1 christos contains a developer-oriented parser that compiles <code><abbr>tz</abbr></code> 587 1.1 christos source into text files, along with a runtime that can read those 588 1.1 christos files. Tcl is freely available under a <abbr>BSD</abbr>-style 589 1.1 christos license.</li> 590 1.1 christos </ul> 591 1.6 christos 592 1.2 christos <h2 id="TZif">Other <abbr>TZif</abbr> readers</h2> 593 1.1 christos <ul> 594 1.1 christos <li>The <a 595 1.1 christos href="https://www.gnu.org/software/libc/"><abbr>GNU</abbr> C 596 1.1 christos Library</a> 597 1.23 christos has a thread-safe <abbr>TZif</abbr> file reader. 598 1.1 christos This library is freely available under the LGPL 599 1.1 christos and is widely used in <abbr>GNU</abbr>/Linux systems.</li> 600 1.20 christos <li><a href="https://www.gnome.org">GNOME</a>s 601 1.21 christos <a href="https://docs.gtk.org/glib/">GLib</a> has 602 1.2 christos a <abbr>TZif</abbr> file reader written in C that 603 1.1 christos creates a <code>GTimeZone</code> object representing sets 604 1.1 christos of <abbr>UT</abbr> offsets. 605 1.1 christos It is freely available under the <abbr>LGPL</abbr>.</li> 606 1.24 christos <li>The <a href="https://en.wikipedia.org/wiki/Musl">musl</a> C library 607 1.24 christos also has an <abbr>TZif</abbr> reader. 608 1.24 christos It is freely available under the <abbr>MIT</abbr> license.</li> 609 1.24 christos <li>The Time Zone Database Parser for C++, mentioned <a href="#TZDP">above,</a> 610 1.24 christos can be configured to read TZif binary files. 611 1.24 christos A similar approach is taken in 612 1.24 christos <a href="https://www.llvm.org/"><abbr>LLVM</abbr></a> 613 1.24 christos <code>std::chrono::time_zone</code>, which is available 614 1.24 christos under the Apache License with 615 1.24 christos <a href="https://llvm.org/docs/DeveloperPolicy.html#open-source-licensing-terms">LLVM 616 1.24 christos Exceptions</a>.</li> 617 1.1 christos <li>The 618 1.20 christos <a href="https://github.com/bloomberg/bde/wiki">BDE Standard Library</a>s 619 1.1 christos <code>baltzo::TimeZoneUtil</code> component contains a C++ 620 1.2 christos implementation of a <abbr>TZif</abbr> file reader. It is freely available under 621 1.1 christos the Apache License.</li> 622 1.1 christos <li><a href="https://github.com/google/cctz">CCTZ</a> is a simple C++ 623 1.1 christos library that translates between <abbr>UT</abbr> and civil time and 624 1.2 christos can read <abbr>TZif</abbr> files. It is freely available under the Apache 625 1.1 christos License.</li> 626 1.21 christos <li>The <a href="https://go.dev">Go programming language</a> 627 1.20 christos has a <abbr>TZif</abbr> file reader <a 628 1.20 christos href="https://pkg.go.dev/time#LoadLocationFromTZData"><code>LoadLocationFromTZData</code></a>.</li> 629 1.16 christos <li>The 630 1.16 christos <a href="https://github.com/nayarsystems/posix_tz_db"><code>posix_tz_db</code> 631 1.16 christos package</a> contains Python code 632 1.16 christos to generate <abbr>CSV</abbr> and <abbr>JSON</abbr> tables that map 633 1.17 christos <code><abbr>tz</abbr></code> settings to proleptic TZ approximations. 634 1.16 christos For example, it maps <code>"Africa/Cairo"</code> 635 1.16 christos to <code>"EET-2EEST,M4.5.5/0,M10.5.4/24"</code>, 636 1.16 christos an approximation valid for Cairo timestamps from 2023 on. 637 1.17 christos This can help porting to platforms that support only proleptic TZ. 638 1.16 christos The package is freely available under the MIT license.</li> 639 1.1 christos <li><a href="https://github.com/derickr/timelib">Timelib</a> is a C 640 1.2 christos library that reads <abbr>TZif</abbr> files and converts 641 1.2 christos timestamps from one time zone or format to another. 642 1.21 christos It is used by <a href="https://www.php.net"><abbr 643 1.1 christos title="PHP: Hypertext Preprocessor">PHP</abbr></a>, 644 1.1 christos <a href="https://hhvm.com"><abbr title="HipHop Virtual Machine">HHVM</abbr></a>, 645 1.1 christos and <a href="https://www.mongodb.com">MongoDB</a>. 646 1.1 christos It is freely available under the <abbr>MIT</abbr> license.</li> 647 1.1 christos <li>Tcl, mentioned <a href="#Tcl">above</a>, also contains a 648 1.2 christos <abbr>TZif</abbr> file reader.</li> 649 1.4 christos <li><a href="https://metacpan.org/pod/DateTime::TimeZone::Tzfile"> 650 1.1 christos DateTime::TimeZone::Tzfile</a> 651 1.2 christos is a <abbr>TZif</abbr> file reader written in Perl. 652 1.1 christos It is freely available under the same terms as Perl 653 1.1 christos (dual <abbr>GPL</abbr> and Artistic license).</li> 654 1.9 christos <li>Python has a <a id="python-zoneinfo" 655 1.11 christos href="https://docs.python.org/3/library/zoneinfo.html"><code>zoneinfo.ZoneInfo</code> 656 1.8 christos class</a> that reads <abbr>TZif</abbr> data and creates objects 657 1.8 christos that represent <code><abbr>tzdb</abbr></code> timezones. 658 1.8 christos Python is freely available under the 659 1.11 christos <a href="https://docs.python.org/3/license.html">Python Software Foundation 660 1.8 christos License</a>. 661 1.17 christos A companion <a id="pypi-tzdata" href="https://pypi.org">PyPI</a> module 662 1.8 christos <a href="https://pypi.org/project/tzdata/"><code>tzdata</code></a> 663 1.8 christos supplies TZif data if the underlying system data cannot be found; 664 1.8 christos it is freely available under the Apache License.</li> 665 1.1 christos <li>The 666 1.1 christos public-domain <a href="https://github.com/dbaron/tz.js">tz.js</a> 667 1.1 christos library contains a Python tool that 668 1.2 christos converts <abbr>TZif</abbr> data into 669 1.1 christos <abbr>JSON</abbr>-format data suitable for use 670 1.1 christos in its JavaScript library for time zone conversion. Dates before 1970 671 1.1 christos are not supported.</li> 672 1.24 christos <li>The <a href="https://crates.io/crates/jiff">jiff</a> datetime library 673 1.24 christos for <a href="https://rust-lang.org/">Rust</a> either compiles 674 1.24 christos <code><abbr>tz</abbr></code> source into Rust, 675 1.24 christos or consults <abbr>TZif</abbr> data at runtime. 676 1.24 christos It is in the public domain. 677 1.24 christos It intends to supplant the soft-deprecated 678 1.24 christos <a href="https://crates.io/crates/chrono-tz">Chrono-TZ</a>.</li> 679 1.1 christos <li>The <a 680 1.1 christos href="https://hackage.haskell.org/package/timezone-olson">timezone-olson</a> 681 1.1 christos package contains <a href="https://www.haskell.org">Haskell</a> code that 682 1.2 christos parses and uses <abbr>TZif</abbr> data. It is freely 683 1.1 christos available under a <abbr>BSD</abbr>-style license.</li> 684 1.1 christos </ul> 685 1.6 christos 686 1.1 christos <h2 id="software">Other <code><abbr>tz</abbr></code>-based time zone software</h2> 687 1.1 christos <ul> 688 1.1 christos <li><a href="https://foxclocks.org">FoxClocks</a> 689 1.1 christos is an extension for <a href="https://www.google.com/chrome/">Google 690 1.16 christos Chrome</a>, <a 691 1.21 christos href="https://www.firefox.com/en-US/">Firefox</a> and <a 692 1.21 christos href="https://www.thunderbird.net/en-US/">Thunderbird</a>. 693 1.1 christos It displays multiple clocks in the application window, and has a mapping 694 1.21 christos interface to <a href="https://earth.google.com/web/">Google Earth</a>. 695 1.1 christos It is freely available under the <abbr>GPL</abbr>.</li> 696 1.1 christos <li>Microsoft Windows 8.1 697 1.1 christos and later has <code><abbr>tz</abbr></code> data and <abbr>CLDR</abbr> 698 1.3 christos data (mentioned <a href="#CLDR">below</a>) used by the 699 1.3 christos <a href="https://en.wikipedia.org/wiki/Windows_Runtime">Windows Runtime</a> / 700 1.3 christos <a href="https://en.wikipedia.org/wiki/Universal_Windows_Platform">Universal Windows Platform</a> classes 701 1.21 christos <a href="https://learn.microsoft.com/en-us/uwp/api/Windows.Globalization.DateTimeFormatting.DateTimeFormatter"><code>DateTimeFormatter</code></a> and 702 1.24 christos <a href="https://learn.microsoft.com/en-us/uwp/api/windows.globalization.calendar"><code>Calendar</code></a>, 703 1.24 christos and with updates discussed in the 704 1.24 christos <a href="https://techcommunity.microsoft.com/category/windows/blog/dstblog">Daylight 705 1.24 christos Saving Time & Time Zone Blog</a>. 706 1.1 christos <a id="System.TimeZoneInfo" 707 1.21 christos href="https://learn.microsoft.com/en-us/archive/blogs/bclteam/exploring-windows-time-zones-with-system-timezoneinfo-josh-free">Exploring 708 1.1 christos Windows Time Zones with <code>System.TimeZoneInfo</code></a> describes 709 1.1 christos the older, proprietary method of Microsoft Windows 2000 and later, 710 1.1 christos which stores time zone data in the 711 1.1 christos <a href="https://en.wikipedia.org/wiki/Windows_Registry">Windows Registry</a>. The 712 1.1 christos <a 713 1.20 christos href="https://unicode.org/cldr/charts/latest/supplemental/zone_tzid.html">Zone 714 1.20 christos Tzid table</a> or <a 715 1.21 christos href="https://github.com/unicode-org/cldr/blob/main/common/supplemental/windowsZones.xml"><abbr>XML</abbr> 716 1.1 christos file</a> of the <abbr>CLDR</abbr> data maps proprietary zone IDs 717 1.1 christos to <code><abbr>tz</abbr></code> names. 718 1.21 christos These mappings can be performed programmatically via the 719 1.21 christos <a href="https://github.com/mattjohnsonpint/TimeZoneConverter">TimeZoneConverter</a> 720 1.21 christos .NET library, 721 1.24 christos or the ICU Java and C++ libraries mentioned <a href="#ICU">above</a>.</li> 722 1.1 christos <li><a 723 1.21 christos href="https://www.oracle.com/java/">Oracle 724 1.1 christos Java</a> contains a copy of a subset of a recent 725 1.1 christos <code><abbr>tz</abbr></code> database in a 726 1.1 christos Java-specific format.</li> 727 1.1 christos </ul> 728 1.6 christos 729 1.1 christos <h2 id="other-dbs">Other time zone databases</h2> 730 1.1 christos <ul> 731 1.1 christos <li><a href="https://www.astro.com/atlas">Time-zone Atlas</a> 732 1.20 christos is Astrodiensts Web version of Shanks and Pottengers out-of-print 733 1.4 christos time zone history atlases 734 1.21 christos <a href="https://search.worldcat.org/title/468828649">for the US</a> and 735 1.21 christos <a href="https://search.worldcat.org/title/76950459">for the world</a>. 736 1.4 christos Although these extensive atlases 737 1.4 christos <a href="https://astrologynewsservice.com/opinion/how-astrologers-contributed-to-the-information-age-a-brief-history-of-time/">were 738 1.4 christos sources for much of the older <code><abbr>tz</abbr></code> data</a>, 739 1.4 christos they are unreliable as Shanks appears to have 740 1.1 christos guessed many <abbr>UT</abbr> offsets and transitions. The atlases cite no 741 1.1 christos sources and do not indicate which entries are guesswork.</li> 742 1.1 christos <li>Microsoft Windows has proprietary data mentioned 743 1.1 christos <a href="#System.TimeZoneInfo">above</a>.</li> 744 1.1 christos <li><a href="https://www.worldtimeserver.com">World Time Server</a> 745 1.1 christos is another time zone database.</li> 746 1.1 christos <li>The <a 747 1.21 christos href="https://www.iata.org/en/publications/manuals/standard-schedules-information">Standard 748 1.1 christos Schedules Information Manual</a> of the 749 1.1 christos International Air Transport Association 750 1.1 christos gives current time zone rules for airports served by commercial aviation.</li> 751 1.1 christos </ul> 752 1.6 christos 753 1.1 christos <h2 id="maps">Maps</h2> 754 1.1 christos <ul> 755 1.6 christos <li><a href="https://www.worldtimezone.com">World Time Zone Map 756 1.6 christos with current time</a> 757 1.21 christos has several fancy time zone maps; it covers Russia particularly well.</li> 758 1.1 christos <li><a 759 1.1 christos href="https://blog.poormansmath.net/how-much-is-time-wrong-around-the-world/">How 760 1.1 christos much is time wrong around the world?</a> maps the difference between 761 1.1 christos mean solar and standard time, highlighting areas such as western China 762 1.20 christos where the two differ greatly. Its a bit out of date, unfortunately.</li> 763 1.21 christos <li>The 764 1.21 christos <a 765 1.21 christos href="https://maps.lib.utexas.edu/maps/world.html">PerryCastaeda 766 1.21 christos Library Map Collection</a> of the University of Texas at Austin has 767 1.21 christos copies of old maps taken from <a 768 1.21 christos href="https://en.wikipedia.org/wiki/The_World_Factbook"><em>The 769 1.21 christos World Factbook</em></a>, formerly published by the 770 1.21 christos <a href="https://www.cia.gov">US Central Intelligence Agency</a>. 771 1.21 christos Although the maps pictorial quality is good, 772 1.21 christos the maps do not indicate daylight saving time.</li> 773 1.1 christos </ul> 774 1.6 christos 775 1.1 christos <h2 id="boundaries">Time zone boundaries</h2> 776 1.2 christos <p>Geographical boundaries between timezones are available 777 1.9 christos from several <a 778 1.9 christos href="https://en.wikipedia.org/wiki/Internet_geolocation">Internet 779 1.9 christos geolocation</a> 780 1.1 christos services and other sources.</p> 781 1.1 christos <ul> 782 1.1 christos <li><a href="https://github.com/evansiroky/timezone-boundary-builder">Timezone 783 1.1 christos Boundary Builder</a> extracts 784 1.1 christos <a href="https://www.openstreetmap.org">Open Street Map</a> data to build 785 1.2 christos boundaries of <code><abbr>tzdb</abbr></code> timezones. 786 1.1 christos Its code is freely available under the <abbr>MIT</abbr> license, and 787 1.1 christos its data entries are freely available under the 788 1.1 christos <a href="https://opendatacommons.org/licenses/odbl/">Open Data Commons 789 1.16 christos Open Database License</a>. The borders appear to be quite accurate. 790 1.16 christos Its main web page lists more than twenty libraries 791 1.16 christos for looking up a timezone name from a GPS coordinate.</li> 792 1.1 christos <li>Free access via a network API, if you register a key, is provided by 793 1.4 christos the <a 794 1.4 christos href="https://www.geonames.org/export/web-services.html#timezone">GeoNames 795 1.4 christos Timezone web service</a>, the <a 796 1.21 christos href="https://developers.google.com/maps/documentation/timezone/overview">Google 797 1.4 christos Maps Time Zone API</a>, and 798 1.8 christos the <a href="https://timezonedb.com/api">TimeZoneDB API</a>. 799 1.1 christos Commercial network API access is provided 800 1.1 christos by <a href="https://askgeo.com">AskGeo</a> 801 1.1 christos and <a href="https://www.geogarage.com/blog/news-1/post/geogarage-time-zone-api-31">GeoGarage</a>. 802 1.1 christos </li> 803 1.20 christos <li><a 804 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 805 1.1 christos to get a time zone from a location using latitude and longitude 806 1.20 christos coordinates?</a> discusses other geolocation possibilities.</li> 807 1.21 christos <li><a href="https://statoids.com/statoids.html">Administrative 808 1.20 christos Divisions of Countries (Statoids)</a> lists 809 1.1 christos political subdivision data related to time zones.</li> 810 1.13 christos <li><a href="https://manifold.net/info/freestuff.shtml">Manifold Software 811 1.20 christos GIS and Database Tools</a> includes a Manifold-format map of 812 1.16 christos world time zone boundaries circa 2007, distributed under the 813 1.1 christos <abbr>GPL</abbr>.</li> 814 1.1 christos <li>A ship within the <a 815 1.1 christos href="https://en.wikipedia.org/wiki/Territorial_waters">territorial 816 1.20 christos waters</a> of any nation uses that nations time. In international 817 1.20 christos waters, time zone boundaries are meridians 15 apart, except that 818 1.20 christos <abbr>UT</abbr>12 and <abbr>UT</abbr>+12 are each 7.5 819 1.1 christos wide and are separated by 820 1.20 christos the 180 meridian (not by the International Date Line, which is 821 1.20 christos for land and territorial waters only). A captain can change ships 822 1.1 christos clocks any time after entering a new time zone; midnight changes are 823 1.1 christos common.</li> 824 1.1 christos </ul> 825 1.6 christos 826 1.1 christos <h2 id="civil">Civil time concepts and history</h2> 827 1.1 christos <ul> 828 1.1 christos <li><a href="https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time">A 829 1.1 christos Walk through Time</a> 830 1.1 christos surveys the evolution of timekeeping.</li> 831 1.4 christos <li>The history of daylight saving time is surveyed in <a 832 1.21 christos href="https://www.webexhibits.org/daylightsaving/">About Daylight 833 1.20 christos Saving Time History, rationale, laws & dates</a> and summarized in 834 1.4 christos <a href="http://seizethedaylight.com/dst/">A Brief 835 1.4 christos History of Daylight Saving Time</a>.</li> 836 1.4 christos <li><a href="https://www.laphamsquarterly.org/roundtable/time-lords">Time 837 1.4 christos Lords</a> discusses how authoritarians manipulate civil time.</li> 838 1.20 christos <li><a href="https://www.w3.org/TR/timezone/">Working with Time 839 1.20 christos and Time Zones</a> 840 1.1 christos contains guidelines and best practices for software applications that 841 1.1 christos deal with civil time.</li> 842 1.16 christos <li><a href="https://webspace.science.uu.nl/~gent0113/idl/idl.htm">A History of 843 1.1 christos the International Date Line</a> tells the story of the most important 844 1.1 christos time zone boundary.</li> 845 1.21 christos <li><a href="https://statoids.com/tconcept.html">Basic Time 846 1.1 christos Zone Concepts</a> discusses terminological issues behind time zones.</li> 847 1.1 christos </ul> 848 1.6 christos 849 1.1 christos <h2 id="national">National histories of legal time</h2> 850 1.1 christos <dl> 851 1.1 christos <dt>Australia</dt> 852 1.20 christos <dd>The Bureau of Meteorology publishes a list of <a 853 1.1 christos href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation 854 1.1 christos Dates of Daylight Savings Time within Australia</a>.</dd> 855 1.1 christos <dt>Belgium</dt> 856 1.7 christos <dd>The Royal Observatory of Belgium maintains a table of time in 857 1.7 christos Belgium (in 858 1.16 christos <a href="https://robinfo.oma.be/nl/astro-info/tijd/" 859 1.7 christos hreflang="nl">Dutch</a> and <a 860 1.16 christos href="https://robinfo.oma.be/fr/astro-info/heure/" 861 1.7 christos hreflang="fr">French</a>).</dd> 862 1.1 christos <dt>Brazil</dt> 863 1.1 christos <dd>The Time Service Department of the National Observatory 864 1.1 christos records <a href="http://pcdsh01.on.br/DecHV.html" 865 1.20 christos hreflang="pt-BR">Brazils daylight saving time decrees (in 866 1.1 christos Portuguese)</a>.</dd> 867 1.1 christos <dt>Canada</dt> 868 1.1 christos <dd>National Research Council Canada publishes current 869 1.1 christos and some older information about <a 870 1.6 christos href="https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/time-zones-daylight-saving-time">time 871 1.6 christos zones and daylight saving time</a>.</dd> 872 1.1 christos <dt>Chile</dt> 873 1.1 christos <dd>The Hydrographic and Oceanographic Service of the Chilean Navy publishes a 874 1.11 christos <a href="https://www.horaoficial.cl/historia_hora.php" hreflang="es">history of 875 1.20 christos Chiles official time (in Spanish)</a>.</dd> 876 1.3 christos <dt>China</dt> 877 1.3 christos <dd>The Hong Kong Observatory maintains a 878 1.7 christos <a href="https://www.hko.gov.hk/en/gts/time/Summertime.htm">history of 879 1.3 christos summer time in Hong Kong</a>, 880 1.20 christos and Macaus Meteorological and Geophysical Bureau maintains a <a 881 1.7 christos href="https://www.smg.gov.mo/en/subpage/224/page/174">similar 882 1.3 christos history for Macau</a>. 883 1.3 christos Unfortunately the latter is incomplete and has errors.</dd> 884 1.1 christos <dt>Czech Republic</dt> 885 1.1 christos <dd><a href="https://kalendar.beda.cz/kdy-zacina-a-konci-letni-cas" 886 1.1 christos hreflang="cs">When daylight saving time starts and ends (in Czech)</a> 887 1.10 christos summarizes and cites historical <abbr>DST</abbr> regulations.</dd> 888 1.1 christos <dt>Germany</dt> 889 1.1 christos <dd>The National Institute for Science and Technology maintains the <a 890 1.21 christos href="https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-441/realisation-of-legal-time-in-germany.html">Realisation 891 1.1 christos of Legal Time in Germany</a>.</dd> 892 1.1 christos <dt>Israel</dt> 893 1.17 christos <dd><a 894 1.17 christos href="https://tz.cs.huji.ac.il">Israel Timezone Files</a> 895 1.17 christos lists official time-change announcements and laws since 1940, 896 1.17 christos almost all in Hebrew.</dd> 897 1.2 christos <dt>Malaysia</dt> 898 1.2 christos <dd>See Singapore <a href="#Singapore">below</a>.</dd> 899 1.1 christos <dt>Mexico</dt> 900 1.1 christos <dd>The Investigation and Analysis Service of the Mexican Library of 901 1.1 christos Congress has published a <a 902 1.13 christos href="https://www.diputados.gob.mx/bibliot/publica/inveyana/polisoc/horver/index.htm" 903 1.1 christos hreflang="es">history of Mexican local time (in Spanish)</a>.</dd> 904 1.1 christos <dt>Netherlands</dt> 905 1.16 christos <dd><a href="https://webspace.science.uu.nl/~gent0113/wettijd/wettijd.htm" 906 1.1 christos hreflang="nl">Legal time in the Netherlands (in Dutch)</a> 907 1.1 christos covers the history of local time in the Netherlands from ancient times.</dd> 908 1.1 christos <dt>New Zealand</dt> 909 1.1 christos <dd>The Department of Internal Affairs maintains a brief <a 910 1.21 christos href="https://www.govt.nz/browse/recreation-and-the-environment/daylight-saving/history-of-daylight-saving-in-nz/">History 911 1.21 christos of Daylight Saving in NZ</a>.</dd> 912 1.14 christos <dt>Palestine</dt> 913 1.20 christos <dd>The Ministry of Telecom and Digital Economy publishes a <a 914 1.20 christos href="https://mtde.gov.ps/home/TimeZone" 915 1.14 christos hreflang="ar">history of clock changes (in Arabic)</a>.</dd> 916 1.10 christos <dt>Portugal</dt> 917 1.10 christos <dd>The Lisbon Astronomical Observatory publishes a 918 1.10 christos <a href="https://oal.ul.pt/hora-legal/" hreflang="pt">history of 919 1.10 christos legal time (in Portuguese)</a>.</dd> 920 1.1 christos <dt>Singapore</dt> 921 1.1 christos <dd><a id="Singapore" 922 1.7 christos href="https://web.archive.org/web/20190822231045/http://www.math.nus.edu.sg/~mathelmr/teaching/timezone.html">Why 923 1.20 christos is Singapore in the Wrong Time Zone?</a> details the 924 1.1 christos history of legal time in Singapore and Malaysia.</dd> 925 1.1 christos <dt>United Kingdom</dt> 926 1.1 christos <dd><a 927 1.1 christos href="https://www.polyomino.org.uk/british-time/">History of 928 1.1 christos legal time in Britain</a> discusses in detail the country 929 1.6 christos with perhaps the best-documented history of clock adjustments.</dd> 930 1.1 christos <dt>United States</dt> 931 1.20 christos <dd>The Department of Transportations <a 932 1.1 christos href="https://www.transportation.gov/regulations/recent-time-zone-proceedings">Recent 933 1.18 christos Time Zone Proceedings</a> lists changes to 934 1.18 christos official written time zone boundaries, and its <a 935 1.18 christos href="https://geodata.bts.gov/datasets/usdot::time-zones/about">Time 936 1.18 christos Zones dataset</a> maps current boundaries. 937 1.18 christos These boundaries are only for standard time, so the current map puts 938 1.18 christos all of Arizona in one time zone even though part of Arizona 939 1.18 christos observes <abbr>DST</abbr> and part does not.</dd> 940 1.1 christos <dt>Uruguay</dt> 941 1.1 christos <dd>The Oceanography, Hydrography, and Meteorology Service of the Uruguayan 942 1.1 christos Navy (SOHMA) publishes an annual <a 943 1.13 christos href="https://sohma.armada.mil.uy/index.php/servicios/datos-astronomicos" hreflang="es">almanac 944 1.1 christos (in Spanish)</a>.</dd> 945 1.1 christos </dl> 946 1.6 christos 947 1.7 christos <h2 id="costs">Costs and benefits of time shifts</h2> 948 1.7 christos <p>Various sources argue for and against daylight saving time and time 949 1.7 christos zone shifts, and many scientific studies have been conducted. This 950 1.8 christos section summarizes reviews and position statements based on 951 1.8 christos scientific literature in the area.</p> 952 1.7 christos <ul> 953 1.7 christos <li>Carey RN, Sarma KM. 954 1.7 christos <a href="https://bmjopen.bmj.com/content/7/6/e014319.long">Impact of 955 1.7 christos daylight saving time on road traffic collision risk: a systematic 956 1.7 christos review</a>. 957 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>. 958 1.7 christos This reviews research literature and concludes that the evidence 959 1.7 christos neither supports nor refutes road safety benefits from 960 1.7 christos shifts in time zones.</li> 961 1.7 christos <li>Havranek T, Herman D, Irsova D. 962 1.20 christos Does daylight saving save electricity? A meta-analysis. 963 1.20 christos <em>Energy J.</em> 2018;39(2):3561. 964 1.7 christos doi:<a href="https://doi.org/10.5547/01956574.39.2.thav">10.5547/01956574.39.2.thav</a>. 965 1.20 christos This analyzes research literature and concludes, Electricity savings 966 1.7 christos are larger for countries farther away from the equator, while 967 1.20 christos subtropical regions consume more electricity because of <abbr>DST</abbr>.</li> 968 1.20 christos <li>Neumann P, von Blanckenburg K. <a 969 1.20 christos href="https://journals.sagepub.com/doi/full/10.1177/0961463X241310562">What 970 1.20 christos time will it be? A comprehensive literature review on daylight saving time</a>. 971 1.21 christos <em>Time Soc</em>. 2025;34(4):684745. 972 1.20 christos doi:<a href="https://doi.org/10.1177/0961463X241310562">10.1177/0961463X241310562</a>. 973 1.21 christos This reviews <abbr>DST</abbr>s effects on electricity, health, crime, road 974 1.21 christos safety, and the economy, focusing on research since 2010, and concludes that 975 1.21 christos year-round standard time is preferable overall.</li> 976 1.21 christos <li>Romigi A, Franco V, Scoditti E <em>et al</em>. 977 1.21 christos The effects of daylight saving time and clock time transitions on sleep and 978 1.21 christos sleepiness: a systematic review. <em>Sleep Med Rev.</em> 2025;84:102161. doi:<a 979 1.21 christos href="https://doi.org/10.1016/j.smrv.2025.102161">10.1016/j.smrv.2025.102161</a>. 980 1.21 christos This reviews <abbr>DST</abbr> and <abbr>DST</abbr> transitions, 981 1.21 christos and concludes that they both harm sleep, health and behavior.</li> 982 1.20 christos </ul> 983 1.20 christos 984 1.20 christos <p>The following medical societies have taken positions on the 985 1.20 christos advisability of clock shifts:</p> 986 1.20 christos 987 1.20 christos <ul> 988 1.20 christos <li>In 2022 the American Medical Association issued a 989 1.21 christos <a href="https://www.ama-assn.org/press-center/ama-press-releases/ama-calls-permanent-standard-time">statement 990 1.20 christos supporting permanent standard time</a> on health grounds.</li> 991 1.20 christos <li>Crawford MR, Winnebeck EC, von Schantz M <em>et al</em>. 992 1.20 christos <a href="https://onlinelibrary.wiley.com/doi/10.1111/jsr.14352">The 993 1.20 christos British Sleep Society position statement on Daylight Saving Time in the UK</a>. 994 1.20 christos <em>J Sleep Res.</em> 2025;34(3):e14352. 995 1.20 christos doi:<a href="https://doi.org/10.1111/jsr.14352">10.1111/jsr.14352</a>. 996 1.21 christos This recommends that the UK abolish <abbr>DST</abbr> for health reasons.</li> 997 1.16 christos <li>Malow BA. <a 998 1.16 christos href="https://academic.oup.com/sleep/article/45/12/zsac236/6717940">It is time 999 1.16 christos to abolish the clock change and adopt permanent 1000 1.16 christos standard time in the United States: 1001 1.16 christos a Sleep Research Society position statement</a>. 1002 1.15 christos <em>Sleep.</em> 2022;45(12):zsac236. 1003 1.15 christos doi:<a href="https://doi.org/10.1093/sleep/zsac236">10.1093/sleep/zsac236</a>. 1004 1.21 christos The Sleep Research Society 1005 1.20 christos advocates permanent standard time due to its health benefits.</li> 1006 1.15 christos <li>Rishi MA, Cheng JY, Strang AR <em>et al</em>. 1007 1.15 christos <a href="https://jcsm.aasm.org/doi/10.5664/jcsm.10898">Permanent standard time 1008 1.15 christos is the optimal choice for health and safety: 1009 1.8 christos an American Academy of Sleep Medicine position statement</a>. 1010 1.20 christos <em>J Clin Sleep Med.</em> 2024;20(1):121125. 1011 1.15 christos doi:<a href="https://doi.org/10.5664/jcsm.10898">10.5664/jcsm.10898</a>. 1012 1.15 christos The AASM argues for permanent standard time due to health and safety risks 1013 1.15 christos and economic costs of both <abbr>DST</abbr> transitions and 1014 1.15 christos permanent <abbr>DST</abbr>.</li> 1015 1.13 christos <li>Roenneberg T, Wirz-Justice A, Skene DJ <em>et al</em>. 1016 1.16 christos <a href="https://journals.sagepub.com/doi/10.1177/0748730419854197">Why 1017 1.13 christos should we abolish Daylight Saving Time?</a> 1018 1.20 christos <em>J Biol Rhythms.</em> 2019;34(3):227230. 1019 1.13 christos doi:<a href="https://doi.org/10.1177/0748730419854197">10.1177/0748730419854197</a>. 1020 1.15 christos The Society for Research on Biological Rhythms 1021 1.18 christos opposes <abbr>DST</abbr> changes and permanent <abbr>DST</abbr>, 1022 1.18 christos and advocates that governments adopt 1023 1.20 christos permanent Standard Time for the health and safety of their citizens.</li> 1024 1.7 christos </ul> 1025 1.7 christos 1026 1.1 christos <h2 id="precision">Precision timekeeping</h2> 1027 1.1 christos <ul> 1028 1.1 christos <li><a 1029 1.1 christos href="http://leapsecond.com/hpan/an1289.pdf">The 1030 1.1 christos Science of Timekeeping</a> is a thorough introduction 1031 1.1 christos to the theory and practice of precision timekeeping.</li> 1032 1.1 christos <li><a href="https://doi.org/10.1007/978-3-319-59909-0">The Science of 1033 1.14 christos Time 2016</a> contains several freely readable papers.</li> 1034 1.13 christos <li><a href="https://www.ntp.org"><abbr 1035 1.1 christos title="Network Time Protocol">NTP</abbr>: The Network 1036 1.1 christos Time Protocol</a> (Internet <abbr>RFC</abbr> 5905) 1037 1.1 christos discusses how to synchronize clocks of 1038 1.1 christos Internet hosts.</li> 1039 1.2 christos <li>The <a href="https://www.usenix.org/system/files/conference/nsdi18/nsdi18-geng.pdf"><span style="font-variant: small-caps">Huygens</span></a> 1040 1.2 christos family of software algorithms can achieve accuracy to a few tens of 1041 1.2 christos nanoseconds in scalable server farms without special hardware.</li> 1042 1.1 christos <li>The <a 1043 1.21 christos href="https://www.nist.gov/el/intelligent-systems-division-73500/ieee-1588">Precision 1044 1.1 christos Time Protocol</a> (<abbr 1045 1.1 christos title="Institute of Electrical and Electronics Engineers">IEEE</abbr> 1588) 1046 1.2 christos can achieve submicrosecond clock accuracy on a local area network 1047 1.2 christos with special-purpose hardware.</li> 1048 1.1 christos <li><a 1049 1.23 christos href="https://www.rfc-editor.org/info/rfc4833/">Timezone 1050 1.1 christos Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a> 1051 1.1 christos (Internet <abbr>RFC</abbr> 4833) 1052 1.1 christos specifies a <a 1053 1.1 christos href="https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol"><abbr>DHCP</abbr></a> 1054 1.1 christos option for a server to configure 1055 1.20 christos a clients time zone and daylight saving settings automatically.</li> 1056 1.11 christos <li><a href="https://www.ucolick.org/~sla/leapsecs/timescales.html">Time 1057 1.11 christos Scales</a> describes astronomical time scales like 1058 1.1 christos <abbr title="Terrestrial Dynamic Time">TDT</abbr>, 1059 1.1 christos <abbr title="Geocentric Coordinate Time">TCG</abbr>, and 1060 1.24 christos <abbr title="Barycentric Dynamic Time">TDB</abbr>.</li> 1061 1.1 christos <li>The <a href="https://www.iau.org"><abbr 1062 1.20 christos title="International Astronomical Union">IAU</abbr></a>s <a 1063 1.13 christos href="https://www.iausofa.org"><abbr 1064 1.1 christos title="Standards Of Fundamental Astronomy">SOFA</abbr></a> 1065 1.1 christos collection contains C and <a 1066 1.1 christos href="https://en.wikipedia.org/wiki/Fortran">Fortran</a> 1067 1.1 christos code for converting among time scales like 1068 1.1 christos <abbr title="International Atomic Time">TAI</abbr>, 1069 1.1 christos <abbr>TDB</abbr>, <abbr>TDT</abbr> and 1070 1.13 christos <abbr>UTC</abbr>. It is freely available under the 1071 1.21 christos <a href="https://www.iausofa.org/terms-and-conditions">SOFA license</a>.</li> 1072 1.1 christos <li><a 1073 1.1 christos href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Mars24 Sunclock 1074 1.20 christos Time on Mars</a> describes Airy Mean Time (<abbr>AMT</abbr>) and the 1075 1.1 christos diverse local time 1076 1.1 christos scales used by each landed mission on Mars.</li> 1077 1.1 christos <li><a href="http://leapsecond.com">LeapSecond.com</a> is 1078 1.1 christos dedicated not only to leap seconds but to precise time and frequency 1079 1.1 christos in general. It covers the state of the art in amateur timekeeping, and 1080 1.1 christos how the art has progressed over the past few decades.</li> 1081 1.7 christos <li>The rules for leap seconds are specified in Annex 1 (Time scales) of <a 1082 1.7 christos href="https://www.itu.int/rec/R-REC-TF.460-6-200202-I/">Standard-frequency 1083 1.20 christos and time-signal emissions</a>, International Telecommunication Union 1084 1.7 christos Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).</li> 1085 1.1 christos <li><a 1086 1.1 christos href="https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html"><abbr 1087 1.1 christos title="International Earth Rotation and Reference Systems Service">IERS</abbr> 1088 1.1 christos Bulletins</a> contains official publications of the International 1089 1.1 christos Earth Rotation and Reference Systems Service, which decides when leap 1090 1.20 christos seconds occur. 1091 1.20 christos The <code><abbr>tz</abbr></code> code and data support leap seconds 1092 1.20 christos via an optional <code>"right"</code> configuration where a computers internal 1093 1.9 christos <code>time_t</code> integer clock counts every <abbr>TAI</abbr> second, 1094 1.20 christos as opposed to the default <code>"posix"</code> configuration 1095 1.9 christos where the internal clock ignores leap seconds. 1096 1.9 christos The two configurations agree for timestamps starting with 1972-01-01 00:00:00 1097 1.9 christos <abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for 1098 1.9 christos timestamps starting with <code>time_t</code> 78 796 800, 1099 1.9 christos which corresponds to the first leap second 1100 1.20 christos 1972-06-30 23:59:60 <abbr>UTC</abbr> in the <code>"right"</code> configuration, 1101 1.9 christos and to 1102 1.20 christos 1972-07-01 00:00:00 <abbr>UTC</abbr> in the <code>"posix"</code> configuration. 1103 1.9 christos In practice the two configurations also agree for timestamps before 1104 1.9 christos 1972 even though the historical situation is messy, partly because 1105 1.9 christos neither <abbr>UTC</abbr> nor <abbr>TAI</abbr> 1106 1.14 christos is well-defined for sufficiently old timestamps.</li> 1107 1.17 christos <li><a href="https://kb.meinbergglobal.com/kb/time_sync/ntp/configuration/ntp_leap_second_file">The 1108 1.17 christos <abbr>NTP</abbr> Leap Second File</a> covers the text file 1109 1.17 christos <code>leap-seconds.list</code>, which lists the currently known leap seconds. 1110 1.17 christos The <abbr>IERS</abbr> maintains this file, and a copy is distributed by 1111 1.17 christos <code><abbr>tzdb</abbr></code> for use by <abbr>NTP</abbr> implementations like 1112 1.17 christos <a href="https://www.ntp.org">classic 1113 1.17 christos <code><abbr title="Network Time Protocol Daemon">ntpd</abbr></code></a> 1114 1.17 christos and <a href="https://ntpsec.org">NTPsec</a>. 1115 1.17 christos The <code><abbr>tz</abbr></code> database also distributes leap second 1116 1.17 christos information in a differently-formatted <code>leapseconds</code> text file, 1117 1.20 christos as well as in the <code>"right"</code> configuration in binary form; for 1118 1.17 christos example, <code>right/UTC</code> can be used 1119 1.17 christos by <a href="https://chrony-project.org"><code>chrony</code></a>, 1120 1.17 christos another <abbr>NTP</abbr> implementation.</li> 1121 1.1 christos <li><a href="https://developers.google.com/time/smear">Leap Smear</a> 1122 1.1 christos discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a 1123 1.1 christos leap second so that they disagree with <abbr>UTC</abbr> by at most a 1124 1.1 christos half second, even though every <abbr>POSIX</abbr> minute has exactly 1125 1.4 christos sixty seconds. This approach works with the default <code><abbr>tz</abbr></code> 1126 1.20 christos <code>"posix"</code> configuration, is <a 1127 1.21 christos href="https://gitlab.com/NTPsec/ntpsec/-/blob/master/docs/leapsmear.adoc">supported</a> 1128 1.23 christos by the <abbr>NTP</abbr> implementations mentioned above, <a 1129 1.12 christos href="https://github.com/google/unsmear">supports</a> conversion between 1130 1.12 christos <abbr>UTC</abbr> and smeared <abbr>POSIX</abbr> timestamps, and is used by major 1131 1.7 christos cloud service providers. However, according to 1132 1.23 christos <a href="https://www.rfc-editor.org/info/rfc8633#section-3.7.1">3.7.1 of 1133 1.7 christos Network Time Protocol Best Current Practices</a> 1134 1.7 christos (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for 1135 1.9 christos applications requiring accurate <abbr>UTC</abbr> or civil time, 1136 1.7 christos and is intended for use only in single, well-controlled environments.</li> 1137 1.1 christos <li>The <a 1138 1.22 christos href="https://groups.io/g/LEAPSECS">LEAPSECS List</a> covers <a 1139 1.21 christos href="https://gge.ext.unb.ca/Resources/gpsworld.november99.pdf">McCarthy 1140 1.20 christos and Klepczynskis 1999 proposal to discontinue leap seconds</a>, 1141 1.1 christos discussed further in 1142 1.1 christos <a href="https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The 1143 1.1 christos leap second: its history and possible future</a>. 1144 1.1 christos <a href="https://www.ucolick.org/~sla/leapsecs/"><abbr>UTC</abbr> 1145 1.1 christos might be redefined 1146 1.1 christos without Leap Seconds</a> gives pointers on this 1147 1.14 christos contentious issue. 1148 1.22 christos The General Conference on Weights and Measures (CGPM) 1149 1.16 christos <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">decided in 2022</a> 1150 1.17 christos to discontinue the use of leap seconds by 2035, and requested that no 1151 1.17 christos discontinuous adjustments be made to UTC for at least a century. 1152 1.16 christos The World Radiocommunication Conference <a 1153 1.16 christos href="https://www.itu.int/dms_pub/itu-r/opb/act/R-ACT-WRC.15-2023-PDF-E.pdf">resolved 1154 1.22 christos in 2023</a> to cooperate with this process. A draft <a 1155 1.22 christos href="https://www.bipm.org/documents/d/guest/cgpm-2026-draft-resolutions">Resolution 1156 1.22 christos C to make continuous UTC effective on 2027-05-20</a>, 1157 1.22 christos and thereby discontinue leap seconds, 1158 1.22 christos has been scheduled for the 28th CGPM starting 2026-10-13 in Paris. 1159 1.22 christos One proposal to implement this 1160 1.20 christos would replace leap seconds with seven 13-second leap smears occurring once per 1161 1.17 christos decade until 2100, with leap smears after that gradually increasing in size. 1162 1.20 christos See: 1163 1.20 christos <ul> 1164 1.22 christos <li>Levine J. <a href="https://tf.nist.gov/general/pdf/3242.pdf">A 1165 1.20 christos proposal to change the leap-second adjustments to 1166 1.20 christos coordinated universal time</a>. <em>Metrologia.</em> 2024;61(5):055002. doi:<a 1167 1.22 christos href="https://doi.org/10.1088/1681-7575/ad6266">10.1088/1681-7575/ad6266</a> 1168 1.22 christos with followups in doi:<a 1169 1.22 christos href="https://doi.org/10.1088/1681-7575/ade314">10.1088/1681-7575/ade314</a> 1170 1.22 christos and doi:<a 1171 1.22 christos href="https://doi.org/10.1088/1681-7575/ade315">10.1088/1681-7575/ade315</a>.</li> 1172 1.20 christos </ul> 1173 1.17 christos However, there is still no consensus on whether this is the best way 1174 1.17 christos to replace leap seconds. 1175 1.14 christos </li> 1176 1.1 christos </ul> 1177 1.6 christos 1178 1.1 christos <h2 id="notation">Time notation</h2> 1179 1.1 christos <ul> 1180 1.13 christos <li>The <a id="CLDR" href="https://cldr.unicode.org">Unicode Common Locale Data 1181 1.1 christos Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time 1182 1.1 christos zone names, abbreviations, identifiers, and formats. For example, it 1183 1.20 christos contains French translations for Eastern European Summer Time, 1184 1.20 christos <abbr title="Eastern European Summer Time">EEST</abbr>, and Bucharest. Its 1185 1.1 christos <a href="https://unicode.org/cldr/charts/latest/by_type/">by-type 1186 1.1 christos charts</a> show these values for many locales. Data values are available in 1187 1.1 christos both <abbr title="Locale Data Markup Language">LDML</abbr> 1188 1.24 christos (an <abbr>XML</abbr> format) and <abbr>JSON</abbr>.</li> 1189 1.1 christos <li> 1190 1.1 christos <a href="https://www.cl.cam.ac.uk/~mgk25/iso-time.html">A summary of 1191 1.6 christos the international standard date and time notation</a> covers 1192 1.1 christos <a 1193 1.6 christos href="https://www.iso.org/standard/70907.html"><em><abbr 1194 1.6 christos title="International Organization for Standardization">ISO</abbr> 1195 1.20 christos 8601-1:2019 Date and time Representations for information 1196 1.20 christos interchange Part 1: Basic rules</em></a>.</li> 1197 1.1 christos <li> 1198 1.6 christos <a href="https://www.w3.org/TR/xmlschema/#dateTime"><abbr>XML</abbr> 1199 1.20 christos Schema: Datatypes dateTime</a> specifies a format inspired by 1200 1.1 christos <abbr>ISO</abbr> 8601 that is in common use in <abbr>XML</abbr> data.</li> 1201 1.23 christos <li><a href="https://www.rfc-editor.org/info/rfc5322#section-3.3">3.3 of 1202 1.1 christos Internet Message Format</a> (Internet <abbr>RFC</abbr> 5322) 1203 1.1 christos specifies the time notation used in email and <a 1204 1.1 christos href="https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol"><abbr>HTTP</abbr></a> 1205 1.1 christos headers.</li> 1206 1.1 christos <li> 1207 1.23 christos <a href="https://www.rfc-editor.org/info/rfc3339/">Date and Time 1208 1.1 christos on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339) 1209 1.17 christos specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols. 1210 1.23 christos An extension, <a href="https://www.rfc-editor.org/info/rfc9557/">Date 1211 1.17 christos and Time on the Internet: Timestamps with Additional Information</a> 1212 1.17 christos (Internet <abbr>RFC</abbr> 9557) extends this profile 1213 1.17 christos to let you specify the <code><abbr>tzdb</abbr></code> timezone of a timestamp 1214 1.24 christos via suffixes like <code>[Asia/Tokyo]</code>.</li> 1215 1.1 christos <li> 1216 1.7 christos <a href="https://web.archive.org/web/20190130042457/https://www.hackcraft.net/web/datetime/">Date & Time 1217 1.1 christos Formats on the Web</a> surveys web- and Internet-oriented date and time 1218 1.1 christos formats.</li> 1219 1.1 christos <li>Alphabetic time zone abbreviations should not be used as unique 1220 1.1 christos identifiers for <abbr>UT</abbr> offsets as they are ambiguous in 1221 1.1 christos practice. For example, in English-speaking North America 1222 1.20 christos <abbr>CST</abbr> denotes 6 hours behind <abbr>UT</abbr>, 1223 1.1 christos but in China it denotes 8 hours ahead of <abbr>UT</abbr>, 1224 1.1 christos and French-speaking North Americans prefer 1225 1.20 christos <abbr title="Heure Normale du Centre">HNC</abbr> to 1226 1.20 christos <abbr>CST</abbr>. The <code><abbr>tz</abbr></code> 1227 1.2 christos database contains English abbreviations for many timestamps; 1228 1.20 christos unfortunately some of these abbreviations were merely the database maintainers 1229 1.1 christos inventions, and these have been removed when possible.</li> 1230 1.1 christos <li>Numeric time zone abbreviations typically count hours east of 1231 1.1 christos <abbr>UT</abbr>, e.g., +09 for Japan and 1232 1.20 christos 10 for Hawaii. However, <abbr>POSIX</abbr> proleptic 1233 1.17 christos <code><abbr>TZ</abbr></code> settings use the opposite convention. 1234 1.1 christos For example, one might use <code><abbr>TZ</abbr>="<abbr 1235 1.1 christos title="Japan Standard Time">JST</abbr>-9"</code> and 1236 1.1 christos <code><abbr>TZ</abbr>="<abbr title="Hawaii Standard Time">HST</abbr>10"</code> 1237 1.1 christos for Japan and Hawaii, respectively. If the 1238 1.1 christos <code><abbr>tz</abbr></code> database is available, it is usually better to use 1239 1.1 christos settings like <code><abbr>TZ</abbr>="Asia/Tokyo"</code> and 1240 1.1 christos <code><abbr>TZ</abbr>="Pacific/Honolulu"</code> instead, as this should avoid 1241 1.2 christos confusion, handle old timestamps better, and insulate you better from 1242 1.1 christos any future changes to the rules. One should never set 1243 1.1 christos <abbr>POSIX</abbr> <code><abbr>TZ</abbr></code> to a value like 1244 1.1 christos <code>"GMT-9"</code>, though, since this would incorrectly imply that 1245 1.1 christos local time is nine hours ahead of <abbr>UT</abbr> and the time zone 1246 1.20 christos is called <abbr>GMT</abbr>.</li> 1247 1.1 christos </ul> 1248 1.24 christos </main> 1249 1.6 christos 1250 1.24 christos <aside> 1251 1.1 christos <h2 id="see-also">See also</h2> 1252 1.1 christos <ul> 1253 1.7 christos <li><a href="theory.html">Theory and pragmatics of the 1254 1.7 christos <code><abbr>tz</abbr></code> code and data</a></li> 1255 1.23 christos <li><a href="tz-art.html">Time and the arts</a></li> 1256 1.1 christos </ul> 1257 1.24 christos </aside> 1258 1.6 christos 1259 1.6 christos <footer> 1260 1.1 christos <hr> 1261 1.1 christos This web page is in the public domain, so clarified as of 1262 1.1 christos 2009-05-17 by Arthur David Olson. 1263 1.1 christos <br> 1264 1.1 christos Please send corrections to this web page to the 1265 1.1 christos <a href="mailto:tz (a] iana.org">time zone mailing list</a>. 1266 1.20 christos The mailing list and its archives are public, 1267 1.20 christos so please do not send confidential information. 1268 1.6 christos </footer> 1269 1.1 christos </body> 1270 1.1 christos </html> 1271