lib/libc/time/tz-link.htm

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   6 <title>Sources for Time Zone and Daylight Saving Time Data</title>
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   9 <meta name="DC.Creator" content="Eggert, Paul">
  10 <meta name="DC.Contributor" content="Olson, Arthur David">
  11 <meta name="DC.Date" content="2012-07-19">
  12 <meta name="DC.Description"
  13  content="Sources of information about time zones and daylight saving time">
  14 <meta name="DC.Identifier" content="http://www.twinsun.com/tz/tz-link.htm">
  15 <meta name="Keywords"
  16  content="database,daylight saving,DST,time zone,timezone,tz,zoneinfo">
  17 </head>
  18 <body>
  19 <h1>Sources for Time Zone and Daylight Saving Time Data</h1>
  20 <p>
  21 This file is in the public domain, so clarified as of
  22 2009-05-17 by Arthur David Olson.
  23 </p>
  24 <p>
  25 Please send corrections to this web page to the
  26 <a href="mailto:tz@iana.org">time zone mailing list</a>.</p>
  27 <h2>The <code>tz</code> database</h2>
  28 <p>
  29 The <a href="http://en.wikipedia.org/wiki/Public_domain">public-domain</a>
  30 time zone database contains code and data
  31 that represent the history of local time
  32 for many representative locations around the globe.
  33 It is updated periodically to reflect changes made by political bodies
  34 to <a href="http://en.wikipedia.org/wiki/Time_zone">time zone</a>
  35 boundaries, <a
  36 href="http://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr
  37 title="Coordinated Universal Time">UTC</abbr></a> offsets, and
  38 <a href="http://en.wikipedia.org/wiki/Daylight_saving">daylight-saving</a>
  39 rules.
  40 This database (often called <code>zoneinfo</code> or <a
  41 href="http://en.wikipedia.org/wiki/TZ_database"><code>tz</code></a>)
  42 is used by several implementations,
  43 including
  44 <a href="http://www.gnu.org/software/libc/">the
  45 <abbr title="GNU's Not Unix">GNU</abbr>
  46 C Library</a> used in
  47 <a href="http://www.linux.org/"><abbr>GNU</abbr>/Linux</a>,
  48 <a href="http://www.freebsd.org/">FreeBSD</a>,
  49 <a href="http://netbsd.org/">NetBSD</a>,
  50 <a href="http://openbsd.org/">OpenBSD</a>,
  51 <a href="http://cygwin.com/">Cygwin</a>,
  52 <a href="http://www.delorie.com/djgpp/"><abbr
  53 title="DJ's GNU Programming Platform">DJGPP</abbr></a>,
  54 <a href="http://ibm.com/aix">AIX</a>,
  55 <a href="http://www.apple.com/macosx/">Mac OS X</a>,
  56 <a href="http://h71000.www7.hp.com/">OpenVMS</a>,
  57 <a href="http://oracle.com/database">Oracle Database</a>,
  58 <a href="http://sun.com/software/solaris">Solaris</a>,
  59 <a href="http://h30097.www3.hp.com/">Tru64</a>, and
  60 <a href="http://sco.com/products/unixware">UnixWare</a>.</p>
  61 <p>
  62 Each location in the database represents a national region where all
  63 clocks keeping local time have agreed since 1970.
  64 Locations are identified by continent or ocean and then by the name of
  65 the location, which is typically the largest city within the region.
  66 For example, <code>America/New_York</code>
  67 represents most of the <abbr title="United States">US</abbr> eastern time zone;
  68 <code>America/Phoenix</code> represents most of Arizona, which
  69 uses mountain time without daylight saving time (<abbr
  70 title="daylight saving time">DST</abbr>);
  71 <code>America/Detroit</code> represents most of Michigan, which uses
  72 eastern time but with different <abbr>DST</abbr> rules in 1975;
  73 and other entries represent smaller regions like Starke County,
  74 Indiana, which switched from central to eastern time in 1991
  75 and switched back in 2006.
  76 To use the database on an extended <a
  77 href="http://en.wikipedia.org/wiki/POSIX"><abbr
  78 title="Portable Operating System Interface">POSIX</abbr></a>
  79 implementation set the <code>TZ</code> environment variable to
  80 the location's full name, e.g., <code>TZ="America/New_York"</code>.</p>
  81 <p>
  82 In the <code>tz</code> database's
  83 <a href="ftp://ftp.iana.org/tz/releases/"><abbr
  84 title="File Transfer Protocol">FTP</abbr> releases</a>
  85 the code is in the file <code>tzcode<var>C</var>.tar.gz</code>,
  86 where <code><var>C</var></code> is the code's version;
  87 similarly, the data are in <code>tzdata<var>D</var>.tar.gz</code>,
  88 where <code><var>D</var></code> is the data's version.
  89 Each version is a four-digit year followed by lower-case letters
  90 (a through z, then za through zz, then zza through zzz, and so on).
  91 Convenience links to
  92 the <a href="ftp://ftp.iana.org/tz/tzcode-latest.tar.gz">latest
  93 code</a> and
  94 <a href="ftp://ftp.iana.org/tz/tzdata-latest.tar.gz">latest data</a> revisions
  95 The following <a
  96 href="http://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
  97 these files to a <abbr>GNU</abbr>/Linux or similar host;
  98 see the downloaded
  99 <code>README</code> file for what to do next.</p>
 100 <pre style="margin-left: 2em"><code>mkdir tz
 101 cd tz
 102 <a href="http://www.gnu.org/software/wget/">wget</a> --retr-symlinks 'ftp://ftp.iana.org/tz/tz*-latest.tar.gz'
 103 <a href="http://www.gnu.org/software/gzip/">gzip</a> -dc tzcode-latest.tar.gz | <a href="http://www.gnu.org/software/tar/">tar</a> -xf -
 104 gzip -dc tzdata-latest.tar.gz | tar -xf -
 105 </code></pre>
 106 <p>
 107 The code and data files can also be obtained from the
 108 <a href="http://www.iana.org">IANA</a>
 109 <a href="http://www.iana.org/time-zones">timezone web page</a>.
 110 <p>
 111 The code lets you compile the <code>tz</code> source files into
 112 machine-readable binary files, one for each location. It also lets
 113 you read a <code>tz</code> binary file and interpret time stamps for that
 114 location.</p>
 115 <p>
 116 The data are by no means authoritative. If you find errors, please
 117 send changes to the <a href="mailto:tz@iana.org">time zone
 118 mailing list</a>. You can also <a
 119 href="http://news.gmane.org/gmane.comp.time.tz">browse recent
 120 messages</a> sent to the mailing list, <a
 121 href="https://mm.icann.org/mailman/listinfo/tz">subscribe</a> to it.
 122 browse the <a
 123 href="http://mm.icann.org/pipermail/tz/">archive of old
 124 messages</a> (message by message or in gzip compressed format),
 125 or retrieve <a
 126 href="ftp://ftp.iana.org/tz/releases/">archived older versions of code
 127 and data</a>.</p>
 128 <p>
 129 The Web has several other sources for time zone and daylight saving time data.
 130 Here are some recent links that may be of interest.
 131 </p>
 132 <h2>Web pages using recent versions of the <code>tz</code> database</h2>
 133 <p>
 134 These are listed roughly in ascending order of complexity and fanciness.
 135 </p>
 136 <ul>
 137 <li>
 138 <a href="http://permatime.com">
 139 Permatime</a>
 140 is a service for generating and viewing links that refer to a
 141 particular point in time and can be displayed in multiple timezones.
 142 It uses the ruby tzinfo gem.
 143 (From Tim Diggins, 2009-11-03.)
 144 </li>
 145 <li><a href="http://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
 146 lets you see the <code>TZ</code> values directly.</li>
 147 <li><a
 148 href="http://convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
 149 Time in 1000 Places</a> uses descriptions of the values.</li>
 150 <li><a href="http://www.timezoneconverter.com/">Time Zone Converter</a>
 151 uses a pulldown menu.</li>
 152 <li><a href="http://home.tiscali.nl/~t876506/TZworld.html">Complete
 153 timezone information for all countries</a> displays tables of DST rules.
 154 <li><a href="http://timeanddate.com/worldclock/">The World Clock -
 155 Time Zones</a> lets you sort zone names and convert times.</li>
 156 <li><a href="http://daylight-savings-time.info/">Graphical Display of
 157 Time Zones and Daylight Saving Times</a> shows a graph of time
 158 difference versus time for any pair of locations.</li>
 159 <li>The <a href="http://worldtimeengine.com/">World Time Engine</a>
 160 also contains data about time zone boundaries; it supports queries via place
 161 names and shows location maps.</li>
 162 <li><a href="http://simpletimerclocks.mozdev.org/">Simple Timer + Clocks</a>
 163 is a Firefox add-on which uses a timezone data file generated from the
 164 tz data files.</li>
 165 </ul>
 166 <h2>Other time zone database formats</h2>
 167 <ul>
 168 <li>The <a href="ftp://ftp.rfc-editor.org/in-notes/rfc2445.txt">
 169 Internet Calendaring and Scheduling Core Object Specification
 170 (iCalendar)</a>, Internet <abbr title="Request For
 171 Comments">RFC</abbr> 2445, published by the (now-concluded) <a
 172 href="http://ietf.org/html.charters/OLD/calsch-charter.html"><abbr
 173 title="Internet Engineering Task Force">IETF</abbr>
 174 Calendaring and Scheduling Working Group (<abbr
 175 title="Calendaring and Scheduling Working Group">calsch</abbr>)</a>
 176 covers time zone
 177 data; see its VTIMEZONE calendar component.
 178 The <a href="http://calconnect.org/">Calendaring and Scheduling
 179 Consortium</a> is promoting further work in this area. <a
 180 href="http://calconnect.org/publications/icalendartimezoneproblemsandrecommendationsv1.0.pdf">iCalendar
 181 TIMEZONE Problems and Recommendations</a> offers guidelines and
 182 recommendations for the use of VTIMEZONE and <code>tz</code>.</li>
 183 <li><a href="http://calconnect.org/dstlinks.shtml">Extended Daylight
 184 Saving Time Links, Advisories and Changes</a> lists vendor material on recent
 185 daylight saving time changes.</li>
 186 <li><a
 187 href="http://calconnect.org/publications/timezoneregistryandservicerecommendationsv1.0.pdf">Timezone
 188 Registry and Service Recommendations</a> discusses a
 189 strategy for defining and deploying a time zone
 190 registration process that would establish unique names for each
 191 version of each <code>tz</code> zone, along with a polygonal
 192 representation of the geographical area corresponding to the
 193 zone.</li>
 194 <li>The <a
 195 href="http://lists.w3.org/Archives/Public/www-rdf-calendar/">www-rdf-calendar</a>
 196 list discusses <a
 197 href="http://www.w3.org/RDF/"><abbr
 198 title="Resource Description Framework">RDF</abbr></a>-based calendar
 199 and group scheduling systems, and has a <a
 200 href="http://www.w3.org/2002/12/cal/#tzd">workspace on time zone
 201 data</a> converted from <code>tz</code>. An earlier <a
 202 href="http://www.w3.org/2000/01/foo">schema</a> was sketched out.</li>
 203 </ul>
 204 <h2>Other <code>tz</code> compilers</h2>
 205 <ul>
 206 <li><a href="http://www.dachaplin.dsl.pipex.com/vzic/">Vzic iCalendar
 207 Timezone Converter</a> describes a <a
 208 href="http://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
 209 program that compiles
 210 <code>tz</code> source into iCalendar-compatible VTIMEZONE files.
 211 Vzic is freely
 212 available under the <a
 213 href="http://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
 214 General Public License (<abbr
 215 title="General Public License">GPL</abbr>)</a>.</li>
 216 <li><a href="http://sourceforge.net/projects/tzical">tziCal - tz
 217 database conversion utility</a> is like Vzic, except for the <a
 218 href="http://msdn.microsoft.com/netframework">.NET framework</a>.</li>
 219 <li><a
 220 href="http://search.cpan.org/dist/DateTime-TimeZone/">DateTime::TimeZone</a>
 221 contains a script <code>parse_olson</code> that compiles
 222 <code>tz</code> source into <a href="http://www.perl.org/">Perl</a>
 223 modules. It is part of the Perl <a
 224 href="http://datetime.perl.org/">DateTime Project</a>, which is freely
 225 available under both the <abbr>GPL</abbr> and the Perl Artistic
 226 License. DateTime::TimeZone also contains a script
 227 <code>tests_from_zdump</code> that generates test cases for each clock
 228 transition in the <code>tz</code> database.</li>
 229 <li><a href="http://icu-project.org/">International Components for
 230 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
 231 href="http://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
 232 libraries for internationalization that
 233 has a compiler from <code>tz</code> source
 234 into an <abbr>ICU</abbr>-specific format.
 235 <abbr>ICU</abbr> is freely available under a
 236 <abbr title="Berkeley Software Distribution">BSD</abbr>-style license.</li>
 237 <li><a href="http://joda-time.sourceforge.net/">Joda Time - Java date
 238 and time <abbr title="Application Program Interface">API</abbr></a>
 239 contains a class
 240 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
 241 <code>tz</code> source into a Joda-specific binary format. Joda Time
 242 is freely available under a <abbr>BSD</abbr>-style license.</li>
 243 <li><a href="http://pytz.sourceforge.net">PyTZ - Python Time
 244 Zone Library</a> compiles <code>tz</code> source into
 245 <a href="http://python.org/">Python</a>.
 246 It is freely available under a <abbr>BSD</abbr>-style license.</li>
 247 <li><a href="http://tzinfo.rubyforge.org/">TZInfo - Ruby Timezone Library</a>
 248 compiles <code>tz</code> source into
 249 <a href="http://ruby-lang.org">Ruby</a>.
 250 It is freely available under the <abbr
 251 title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
 252 <li>The <a href="http://chronos-st.org/">Chronos Date/Time
 253 Library</a> is a <a href="http://smalltalk.org">Smalltalk</a> class
 254 library that compiles <code>tz</code> source into a <a
 255 href="http://date-time-zone.com/">time zone repository</a> whose format
 256 is either proprietary or an <a href="http://www.w3.org/XML/"><abbr
 257 title="Extensible Markup Language">XML</abbr></a>-encoded
 258 representation.</li>
 259 <li>Starting with version 8.5, <a href="http://tcl.tk/">Tcl</a>
 260 contains a developer-oriented parser that compiles <samp>tz</samp>
 261 source into text files, along with a runtime that can read those
 262 files. Tcl is freely available under a <abbr>BSD</abbr>-style
 263 license.</li>
 264 </ul>
 265 <h2>Other <code>tz</code> binary file readers</h2>
 266 <li><a href="http://search.cpan.org/perldoc?DateTime::TimeZone::Tzfile">
 267 DateTime::TimeZone::Tzfile</a>
 268 is a <code>tz</code> binary file reader written in Perl.
 269 It is freely available under the same terms as Perl
 270 (dual <abbr>GPL</abbr> and Artistic license).</li>
 271 <ul>
 272 <li>The <a
 273 href="http://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
 274 Library</a>
 275 has an independent, thread-safe implementation of
 276 a <code>tz</code> binary file reader.
 277 This library is freely available under the
 278 <a href="http://www.gnu.org/copyleft/lesser.html">
 279 <abbr>GNU</abbr> Lesser General Public License
 280 (<abbr title="Lesser General Public License">LGPL</abbr>)</a>,
 281 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
 282 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
 283 is a <code>tz</code> binary file reader written in Java.
 284 It is freely available under the <abbr>LGPL</abbr>.</li>
 285 <li>Tcl, mentioned above, also contains a
 286 <code>tz</code> binary file reader.</li>
 287 </ul>
 288 <h2>Other <code>tz</code>-based time zone software</h2>
 289 <ul>
 290 <li><a href="http://stemhaus.com/firefox/foxclocks/">FoxClocks</a>
 291 is an extension for <a
 292 href="http://developer.mozilla.org/en/docs/Toolkit_API">Mozilla
 293 Toolkit</a> applications like <a
 294 href="http://mozilla.com/firefox">Firefox</a>, <a
 295 href="http://mozilla.com/thunderbird">Thunderbird</a>, and
 296 <a
 297 href="http://www.mozilla.org/projects/calendar/sunbird/">Sunbird</a>.
 298 It displays multiple clocks in the application window, and has a mapping
 299 interface to <a href="http://earth.google.com/">Google Earth</a>.
 300 It is freely available under the <abbr>GPL</abbr>.</li>
 301 <li><a
 302 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
 303 clock (intclock)</a> is a multi-timezone clock for
 304 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
 305 under the <abbr>GPL</abbr>.</li>
 306 <li><a href="http://codeplex.com/publicdomain">PublicDomain</a>
 307 has a copy of a recent <code>tz</code> database, accessed via a <a
 308 href="http://en.wikipedia.org/wiki/C_Sharp">C#</a> library. As its
 309 name suggests, it is in the public domain. Only current time stamps
 310 are well supported; historical data are compiled into the runtime but
 311 are not easily accessible.</li>
 312 <li><a href="http://java.sun.com/">Sun Java</a> releases since 1.4
 313 contain a copy of a subset of a recent <code>tz</code> database in a
 314 Java-specific format.</li>
 315 <li><a href="http://kimmo.suominen.com/sw/timezone/">Time Zone</a> is
 316 a <a href="http://wordpress.org/">WordPress</a> plugin. It is freely
 317 available under a <abbr>BSD</abbr>-style license.</li>
 318 <li><a
 319 href="http://veladg.com/velaterra.html">VelaTerra</a> is
 320 a Mac OS X program. Its developers
 321 <a href="http://veladg.com/tzoffer.html">offer free
 322 licenses</a> to <code>tz</code> contributors.</li>
 323 <li><a
 324 href="http://worldtimeexplorer.com/">World Time Explorer</a> is a
 325 Microsoft Windows program.</li>
 326 <li>
 327 <a href="http://www.toriasoft.com">
 328 WorldClock for Windows and Windows Mobile</a>
 329 lets users "see the time in up to 25 locations in the world at once."
 330 (From Hans Nieuwenhuis, 2009-11-02.)
 331 </li>
 332 <li>
 333 <a href="http://www.relativedata.com/time-zone-master">
 334 Time Zone Master Basic
 335 </a> "allows people to display multiple desktop clocks, and to
 336 research current and historical time information, as well as times of
 337 astronomical events (sunrise/transit/set, moonrise/transit/set, phases,
 338 season starts) for user-selected dates in the past and future. It can
 339 automatically download, compile and use the tzdata**.gz database files
 340 as they are released to keep the data up to date. The software is
 341 free." (Davie Patte)
 342 </li>
 343 </ul>
 344 <h2>Other time zone databases</h2>
 345 <ul>
 346 <li><a href="http://www.astro.com/cgi/aq.cgi">Atlas Query</a>
 347 is Astrodienst's Web version of Shanks's
 348 excellent time zone history atlases published in both <a
 349 href="http://astrocom.com/products/software.php?software_id=ibmwboth">computer</a>
 350 and book form (<a
 351 href="http://astrocom.com/products/book.php?book_id=b110x">one volume
 352 for the USA</a>, and <a
 353 href="http://astrocom.com/products/book.php?book_id=b112x">one for
 354 other locations</a>) by <a
 355 href="http://astrocom.com/">Astro Communications Services</a>.</li>
 356 <li><a href="http://worldtime.com/">WORLDTIME: interactive atlas,
 357 time info, public holidays</a>
 358 contains information on local time, sunrise and sunset,
 359 and public holidays in several hundred cities around the world.</li>
 360 <li><a href="http://worldtimeserver.com/">World Time Server</a>
 361 is another time zone database.</li>
 362 <li><a href="http://tycho.usno.navy.mil/tzones.html">World Time Zones</a>
 363 contains data from the Time Service Department of the
 364 <abbr>US</abbr> Naval Observatory, used as the source
 365 for the <code>usno*</code> files in the <code>tz</code> distribution.</li>
 366 <li>The <a href="http://iata.org/ps/publications/SSIM.htm">Standard
 367 Schedules Information Manual</a> of the
 368 <a href="http://iata.org/index.htm">International Air Transport
 369 Association</a>
 370 gives current time zone rules for airports served by commercial aviation.</li>
 371 <li>Some Microsoft Windows versions contain time zone information in
 372 an undocumented format, with IDs that can be mapped to <code>TZ</code>
 373 values using the <a
 374 href="http://unicode.org/cldr/data/diff/supplemental/windows_tzid.html">Windows
 375 &rarr; Tzid table</a> maintained by the <abbr
 376 title="Common Locale Data Repository">CLDR</abbr> data mentioned
 377 below.</li>
 378 <li>
 379 <a href="http://code.google.com/p/tzdata/">
 380 http://code.google.com/p/tzdata/
 381 </a>
 382 provides programming-language-specific representations of timezone
 383 data. Currently this includes XML, PHP, Ruby, Javascript, JSON and CSV
 384 formatted data. The repository is updated as soon as the FTP
 385 distribution is updated. All data can be downloaded as a zip and/or it
 386 can be obtained/synced via anonymous SVN. Data is made available under
 387 the MIT license. (From Rich Tibbett.)
 388 </li>
 389 </ul>
 390 <h2>Maps</h2>
 391 <ul>
 392 <li>The <a href="https://www.cia.gov/">United States Central
 393 Intelligence Agency (<abbr
 394 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
 395 href="https://www.cia.gov/library/publications/the-world-factbook/reference_maps/pdf/time_zones.pdf">time
 396 zone map</a>; the
 397 <a
 398 href="http://www.lib.utexas.edu/maps/world.html">Perry-Casta&ntilde;eda
 399 Library Map Collection</a>
 400 of the University of Texas at Austin has copies of
 401 recent editions.
 402 The pictorial quality is good,
 403 but the maps do not indicate summer time,
 404 and parts of the data are a few years out of date.</li>
 405 <li><a href="http://worldtimezone.com/">Current time around the world
 406 and standard time zones map of the world</a>
 407 has several fancy time zone maps; it covers Russia particularly well.
 408 The maps' pictorial quality is not quite as good as the
 409 <abbr>CIA</abbr>'s
 410 but the maps are more up to date.</li>
 411 </ul>
 412 <h2>Time zone boundaries</h2>
 413 <ul>
 414 <li><a href="http://efele.net/maps/tz/">TZ timezone maps</a> contains a <a
 415 href="http://en.wikipedia.org/wiki/Shapefile">shapefile</a> of the
 416 <code>tz</code> regions in the world.</li>
 417 <li><a href="http://statoids.com/statoids.html">Administrative Divisions
 418 of Countries ("Statoids")</a> contains detailed lists of
 419 <code>tz</code>-related zone subdivision data.</li>
 420 <li><a href="http://home.tiscali.nl/~t876506/Multizones.html">Time
 421 zone boundaries for multizone countries</a> summarizes legal
 422 boundaries between time zones within countries.</li>
 423 <li>Manifold.net's <a
 424 href="http://www.manifold.net/download/freemaps.html">Free Maps and
 425 <abbr title="Geographic Information Systems">GIS</abbr>
 426 Data</a> includes a Manifold-format map of
 427 world time zone boundaries distributed under the
 428 <abbr>GPL</abbr>.</li>
 429 <li>The <abbr>US</abbr> Geological Survey's National Atlas of
 430 the United States
 431 publishes the <a href="http://nationalatlas.gov/mld/timeznp.html">Time
 432 Zones of the United States</a> in the public domain.</li>
 433 <li>The GeoCommunity lists several commercial sources for <a
 434 href="http://spatialnews.geocomm.com/features/timezones/">International
 435 Time Zones and Time Zone Data</a>.</li>
 436 <li>A ship within the <a
 437 href="http://en.wikipedia.org/wiki/Territorial_waters">territorial
 438 waters</a> of any nation uses that nation's time. In international
 439 waters, time zone boundaries are meridians 15&deg; apart, except that
 440 UTC&minus;12 and UTC+12 are each 7.5&deg; wide and are separated by
 441 the 180&deg; meridian (not by the International Date Line, which is
 442 for land and territorial waters only). A captain can change ship's
 443 clocks any time after entering a new time zone; midnight changes are
 444 common.</li>
 445 </ul>
 446 <h2>Civil time concepts and history</h2>
 447 <ul>
 448 <li><a href="http://physics.nist.gov/GenInt/Time/time.html">A
 449 Walk through Time</a>
 450 surveys the evolution of timekeeping.</li>
 451 <li><a href="http://webexhibits.org/daylightsaving/">About Daylight
 452 Saving Time - History, rationale, laws &amp; dates</a>
 453 is an overall history of <abbr>DST</abbr>.</li>
 454 <li><a href="http://energy.ca.gov/daylightsaving.html">Saving Time,
 455 Saving Energy</a> discusses a primary justification for <abbr>DST</abbr>.</li>
 456 <li><a href="http://seizethedaylight.com/dst/">Who Knew? A Brief
 457 History of Daylight Saving Time</a> summarizes some of the contentious
 458 history of <abbr>DST</abbr>.</li>
 459 <li><a href="http://toi.iriti.cnr.it/">The
 460 Time of Internet</a>
 461 describes time zones and daylight saving time,
 462 with diagrams.
 463 The time zone map is out of date, however.</li>
 464 <li><a href="http://www.phys.uu.nl/~vgent/idl/idl.htm">A History of
 465 the International Date Line</a> tells the story of the most important
 466 time zone boundary.</li>
 467 <li><a href="http://statoids.com/tconcept.html">Basic Time
 468 Zone Concepts</a> discusses terminological issues behind time zones.</li>
 469 </ul>
 470 <h2>National histories of legal time</h2>
 471 <dl>
 472 <dt>Australia</dt>
 473 <dd>The Parliamentary Library has commissioned <a
 474 href="http://www.aph.gov.au/LIBRARY/Pubs/rn/2006-07/07rn13.pdf">research
 475 note on daylight saving time in Australia</a>.
 476 The Bureau of Meteorology publishes a list of
 477 <a href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation Dates of Daylight Savings Time within Australia</a>.</dd>
 478 <dt>Belgium</dt>
 479 <dd>The Royal Observatory of Belgium maintains a table of <a
 480 href="http://www.astro.oma.be/GENERAL/INFO/nli001a.html"
 481 hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
 482 <dt>Brazil</dt>
 483 <dd>The Time Service Department of the National Observatory
 484 records <a href="http://pcdsh01.on.br/DecHV.html"
 485 hreflang="pt-BR">Brazil's daylight saving time decrees (in
 486 Portuguese)</a>.</dd>
 487 <dt>Canada</dt>
 488 <dd>The Institute for National Measurement Standards publishes current
 489 and some older information about <a
 490 href="http://inms-ienm.nrc-cnrc.gc.ca/time_services/daylight_saving_e.html">Time
 491 Zones &amp; Daylight Saving Time</a>.</dd>
 492 <dt>Chile</dt>
 493 <dd>The Chilean Hydrographic and Oceanographic Service publishes a <a
 494 href="http://www.horaoficial.cl/horaof.htm" hreflang="es"> history of
 495 official time (in Spanish)</a>.</dd>
 496 <dt>Germany</dt>
 497 <dd>The National Institute for Science and Technology maintains the <a
 498 href="http://www.ptb.de/en/org/4/44/441/dars_e.htm">Realisation of
 499 Legal Time in Germany</a>.</dd>
 500 <dt>Israel</dt>
 501 <dd>The Interior Ministry periodically issues <a
 502 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
 503 hreflang="he">announcements (in Hebrew)</a>.</dd>
 504 <dt>Mexico</dt>
 505 <dd>The Investigation and Analysis Service of the Mexican Library of
 506 Congress has published a <a
 507 href="http://www.cddhcu.gob.mx/bibliot/publica/inveyana/polisoc/horver/"
 508 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
 509 <dt>Malaysia</dt>
 510 <dd>See Singapore below.</dd>
 511 <dt>Netherlands</dt>
 512 <dd><a href="http://www.phys.uu.nl/~vgent/wettijd/wettijd.htm"
 513 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
 514 covers the history of local time in the Netherlands from ancient times.</dd>
 515 <dt>New Zealand</dt>
 516 <dd>The Department of Internal Affairs maintains a brief history <a
 517 href="http://dia.govt.nz/diawebsite.nsf/wpg_URL/Resource-material-Information-We-Provide-About-Daylight-Saving">About
 518 Daylight Saving</a>. The privately-maintained <a
 519 href="http://astrologyschool.com/nztime.html">History of New Zealand
 520 time</a> has more details.</dd>
 521 <dt>Norway</dt>
 522 <dd>The Norwegian Meteorological Institute lists
 523 <a href="http://met.no/met/met_lex/q_u/sommertid.html" hreflang="no">Summer
 524 time in Norway (in Norwegian)</a>, citing the
 525 Institute of Theoretical Astrophysics, Oslo.</dd>
 526 <dt>Singapore</dt>
 527 <dd><a
 528 href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why
 529 is Singapore in the "Wrong" Time Zone?</a> details the
 530 history of legal time in Singapore and Malaysia.</dd>
 531 <dt>United Kingdom</dt>
 532 <dd><a
 533 href="http://www.srcf.ucam.org/~jsm28/british-time/">History of
 534 legal time in Britain</a> discusses in detail the country
 535 with perhaps the best-documented history of clock adjustments.
 536 The National Physical Laboratory also maintains an <a
 537 href="http://www.npl.co.uk/server.php?show=ConWebDoc.2714">Archive
 538 of Summer time dates</a>.</dd>
 539 </dl>
 540 <h2>Precision timekeeping</h2>
 541 <ul>
 542 <li><a
 543 href="http://literature.agilent.com/litweb/pdf/5965-7984E.pdf">The
 544 Science of Timekeeping</a> is a thorough introduction
 545 to the theory and practice of precision timekeeping.</li>
 546 <li><a href="http://www.ntp.org/"><abbr
 547 title="Network Time Protocol">NTP</abbr>: The Network
 548 Time Protocol</a>
 549 discusses how to synchronize clocks of
 550 Internet hosts.</li>
 551 <li><a
 552 href="ftp://ftp.rfc-editor.org/in-notes/rfc4833.txt">Timezone
 553 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
 554 (Internet <abbr>RFC</abbr> 4833)
 555 specifies a <a
 556 href="http://www.dhcp.org/">DHCP</a> option for a server to configure
 557 a client's time zone and daylight saving settings automatically.</li>
 558 <li><a href="http://gauss.gge.unb.ca/GMT.UT.and.the.RGO.html">A Few
 559 Facts Concerning <abbr title="Greenwich Mean Time">GMT</abbr>, <abbr
 560 title="Universal Time">UT</abbr>, and
 561 the <abbr title="Royal Greenwich Observatory">RGO</abbr></a>
 562 answers questions like "What is the
 563 difference between <abbr>GMT</abbr> and <abbr>UTC</abbr>?"</li>
 564 <li><a
 565 href="http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
 566 Times</a> explains more abstruse astronomical time scales like
 567 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
 568 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
 569 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
 570 <a href="http://www.ucolick.org/~sla/leapsecs/timescales.html">Time
 571 Scales</a> goes into more detail, particularly for historical variants.</li>
 572 <li>The <a href="http://iau.org/"><abbr
 573 title="International Astronomical Union">IAU</abbr></a>'s <a
 574 href="http://www.iau-sofa.rl.ac.uk/"><abbr
 575 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
 576 initiative publishes Fortran
 577 code for converting among time scales like
 578 <abbr title="International Atomic Time">TAI</abbr>,
 579 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
 580 <abbr>UTC</abbr>.</li>
 581 <li><a href="http://jpl.nasa.gov/basics/bsf2-3.htm">Basics of
 582 Space Flight - Reference Systems - Time Conventions</a>
 583 briefly explains interplanetary space flight timekeeping.</li>
 584 <li><a
 585 href="http://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical
 586 Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a> briefly
 587 describes Mars Coordinated Time (<abbr
 588 title="Mars Coordinated Time">MTC</abbr>) and the
 589 diverse local time
 590 scales used by each landed mission on Mars.</li>
 591 <li><a href="http://leapsecond.com/">LeapSecond.com</a> is
 592 dedicated not only to leap seconds but to precise time and frequency
 593 in general. It covers the state of the art in amateur timekeeping, and
 594 how the art has progressed over the past few decades.</li>
 595 <li><a
 596 href="http://hpiers.obspm.fr/eop-pc/products/bulletins/bulletins.html">Bulletins
 597 maintained by the
 598 <abbr title="International Earth Rotation Service">IERS</abbr>
 599 <abbr title="Earth Orientation Parameters">EOP</abbr>
 600 (<abbr title="Product Center">PC</abbr>)</a> contains official publications of
 601 the Earth Orientation Parameters Product Center of the
 602 International Earth Rotation Service, the committee that decides
 603 when leap seconds occur.</li>
 604 <li>The <a
 605 href="http://six.pairlist.net/mailman/listinfo/leapsecs">Leap
 606 Second Discussion List</a> covers <a
 607 href="http://gauss.gge.unb.ca/papers.pdf/gpsworld.november99.pdf">McCarthy
 608 and Klepczynski's proposal to discontinue leap seconds</a>,
 609 discussed further in
 610 <a href="http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
 611 leap second: its history and possible future</a>.
 612 The (now disbanded) <a href="http://members.aas.org/comms/leap.cfm"><abbr
 613 title="American Astronomical Society">AAS</abbr> Leap Second
 614 Committee</a> has solicited input on this proposal.
 615 <a href="http://www.ucolick.org/~sla/leapsecs/">The
 616 Future of Leap Seconds</a> covers this
 617 contentious issue.</li>
 618 </ul>
 619 <h2>Time notation</h2>
 620 <ul>
 621 <li>
 622 <a href="http://www.cl.cam.ac.uk/~mgk25/iso-time.html">A Summary of
 623 the International Standard Date and Time Notation</a> is a good
 624 summary of
 625 <a
 626 href="http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=40874"><abbr
 627 title="International Organization for Standardization">ISO</abbr>
 628 8601:2004 -- Data elements and interchange formats -- Information
 629 interchange -- Representation of dates and times</a>.</li>
 630 <li>
 631 <a href="http://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr>
 632 Schema: Datatypes - dateTime</a> specifies a format inspired by
 633 <abbr>ISO</abbr> 8601 that is in common use in XML data.</li>
 634 <li>
 635 <a href="ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt">Internet
 636 Message Format</a> (Internet <abbr>RFC</abbr> 2822) &sect;3.3
 637 specifies the time notation used in email and <a
 638 href="ftp://ftp.rfc-editor.org/in-notes/rfc2616.txt"><abbr>HTTP</abbr></a>
 639 headers.</li>
 640 <li>
 641 <a href="ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt">Date and Time
 642 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
 643 specifies an <abbr>ISO</abbr> 8601
 644 profile for use in new Internet
 645 protocols.</li>
 646 <li>
 647 <a href="http://www.hackcraft.net/web/datetime/">Date &amp; Time
 648 Formats on the Web</a> surveys web- and Internet-oriented date and time
 649 formats.</li>
 650 <li>
 651 <a href="http://exit109.com/~ghealton/y2k/yrexamples.html">The
 652 Best of Dates, the Worst of Dates</a> covers many problems encountered
 653 by software developers when handling dates and time stamps.</li>
 654 <li>The <a
 655 href="http://unicode.org/cldr/">Unicode Common Locale Data Repository
 656 (<abbr>CLDR</abbr>) Project</a> has localizations for time zone names,
 657 abbreviations, identifiers, and formats. For example, it contains
 658 French translations for "Eastern European Summer Time", "<abbr
 659 title="Eastern European Summer Time">EEST</abbr>", and
 660 "Bucharest". <a
 661 href="http://unicode.org/cldr/data/charts/by_type/names.metazone.html">By-Type
 662 Chart: names.metazone</a> shows these values for many locales.
 663 <abbr>ICU</abbr> contains a mechanism for using this data.</li>
 664 <li>Alphabetic time zone abbreviations should not be used as unique
 665 identifiers for <abbr>UTC</abbr> offsets as they are ambiguous in
 666 practice. For example, "<abbr>EST</abbr>" denotes 5 hours behind
 667 <abbr>UTC</abbr> in English-speaking North America, but it denotes 10
 668 or 11 hours ahead of <abbr>UTC</abbr> in Australia; and
 669 French-speaking North Americans prefer
 670 "<abbr title="Heure Normale de l'Est">HNE</abbr>" to
 671 "<abbr>EST</abbr>". For <abbr>POSIX</abbr> the <code>tz</code>
 672 database contains English abbreviations for all time stamps but in
 673 many cases these are merely inventions of the database
 674 maintainers.</li>
 675 <li>Numeric time zone abbreviations typically count hours east of
 676 <abbr>UTC</abbr>, e.g., <code>+09</code> for Japan and
 677 <code>-10</code> for Hawaii. However, the <abbr>POSIX</abbr>
 678 <code>TZ</code> environment variable uses the opposite convention. For
 679 example, one might use <code>TZ="JST-9"</code> and
 680 <code>TZ="HST10"</code> for Japan and Hawaii, respectively. If the
 681 <code>tz</code> database is available, it is usually better to use
 682 settings like <code>TZ="Asia/Tokyo"</code> and
 683 <code>TZ="Pacific/Honolulu"</code> instead, as this should avoid
 684 confusion, handle old time stamps better, and insulate you better from
 685 any future changes to the rules. One should never set
 686 <abbr>POSIX</abbr> <code>TZ</code> to a value like
 687 <code>"GMT-9"</code>, though, since this would falsely claim that
 688 local time is nine hours ahead of <abbr>UTC</abbr> and the time zone
 689 is called "<abbr>GMT</abbr>".</li>
 690 </ul>
 691 <h2>Related indexes</h2>
 692 <ul>
 693 <li><a href="tz-art.htm">Time and the Arts</a></li>
 694 <li><a href="http://www.dmoz.org/Reference/Time/">Open Directory -
 695 Reference: Time</a></li>
 696 <li><a href="http://directory.google.com/Top/Reference/Time/">Google Directory
 697 - Reference &gt; Time</a></li>
 698 <li><a href="http://dir.yahoo.com/Science/Measurements_and_Units/Time">Yahoo!
 699 Directory &gt; Science &gt; Measurements and Units &gt; Time</a></li>
 700 </ul>
 701 </body>
 702 </html>