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="2007-12-26">
  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 <address>
  21 @(#)tz-link.htm 8.22
  22 </address>
  23 <p>
  24 This file is in the public domain, so clarified as of
  25 2009-05-17 by Arthur David Olson.
  26 </p>
  27 <p>
  28 Please send corrections to this web page to the
  29 <a href="mailto:tz@elsie.nci.nih.gov">time zone mailing list</a>.</p>
  30 <h2>The <code>tz</code> database</h2>
  31 <p>
  32 The <a href="http://en.wikipedia.org/wiki/Public_domain">public-domain</a>
  33 time zone database contains code and data
  34 that represent the history of local time
  35 for many representative locations around the globe.
  36 It is updated periodically to reflect changes made by political bodies
  37 to <a href="http://en.wikipedia.org/wiki/Time_zone">time zone</a>
  38 boundaries, <a
  39 href="http://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr
  40 title="Coordinated Universal Time">UTC</abbr></a> offsets, and
  41 <a href="http://en.wikipedia.org/wiki/Daylight_saving">daylight-saving</a>
  42 rules.
  43 This database (often called <code>tz</code> or <a
  44 href="http://en.wikipedia.org/wiki/Zoneinfo"><code>zoneinfo</code></a>)
  45 is used by several implementations,
  46 including
  47 <a href="http://www.gnu.org/software/libc/">the
  48 <abbr title="GNU's Not Unix">GNU</abbr>
  49 C Library</a> used in
  50 <a href="http://www.linux.org/"><abbr>GNU</abbr>/Linux</a>,
  51 <a href="http://www.freebsd.org/">FreeBSD</a>,
  52 <a href="http://netbsd.org/">NetBSD</a>,
  53 <a href="http://openbsd.org/">OpenBSD</a>,
  54 <a href="http://cygwin.com/">Cygwin</a>,
  55 <a href="http://www.delorie.com/djgpp/"><abbr
  56 title="DJ's GNU Programming Platform">DJGPP</abbr></a>,
  57 <a href="http://ibm.com/aix">AIX</a>,
  58 <a href="http://www.apple.com/macosx/">Mac OS X</a>,
  59 <a href="http://h71000.www7.hp.com/">OpenVMS</a>,
  60 <a href="http://oracle.com/database">Oracle Database</a>,
  61 <a href="http://sun.com/software/solaris">Solaris</a>,
  62 <a href="http://h30097.www3.hp.com/">Tru64</a>, and
  63 <a href="http://sco.com/products/unixware">UnixWare</a>.</p>
  64 <p>
  65 Each location in the database represents a national region where all
  66 clocks keeping local time have agreed since 1970.
  67 Locations are identified by continent or ocean and then by the name of
  68 the location, which is typically the largest city within the region.
  69 For example, <code>America/New_York</code>
  70 represents most of the <abbr title="United States">US</abbr> eastern time zone;
  71 <code>America/Phoenix</code> represents most of Arizona, which
  72 uses mountain time without daylight saving time (<abbr
  73 title="daylight saving time">DST</abbr>);
  74 <code>America/Detroit</code> represents most of Michigan, which uses
  75 eastern time but with different <abbr>DST</abbr> rules in 1975;
  76 and other entries represent smaller regions like Starke County,
  77 Indiana, which switched from central to eastern time in 1991
  78 and switched back in 2006.
  79 To use the database on an extended <a
  80 href="http://en.wikipedia.org/wiki/POSIX"><abbr
  81 title="Portable Operating System Interface">POSIX</abbr></a>
  82 implementation set the <code>TZ</code> environment variable to
  83 the location's full name, e.g., <code>TZ="America/New_York"</code>.</p>
  84 <p>
  85 In the <code>tz</code> database's
  86 <a href="ftp://elsie.nci.nih.gov/pub"><abbr
  87 title="File Transfer Protocol">FTP</abbr> distribution</a>
  88 the code is in the file <code>tzcode<var>C</var>.tar.gz</code>,
  89 where <code><var>C</var></code> is the code's version;
  90 similarly, the data are in <code>tzdata<var>D</var>.tar.gz</code>,
  91 where <code><var>D</var></code> is the data's version.
  92 The following <a
  93 href="http://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download
  94 these files to a <abbr>GNU</abbr>/Linux or similar host;
  95 see the downloaded
  96 <code>README</code> file for what to do next.</p>
  97 <pre style="margin-left: 2em"><code><a href="http://www.gnu.org/software/wget/">wget</a> 'ftp://elsie.nci.nih.gov/pub/tz*.tar.gz'
  98 <a href="http://www.gnu.org/software/gzip/">gzip</a> -dc tzcode*.tar.gz | <a href="http://www.gnu.org/software/tar/">tar</a> -xf -
  99 gzip -dc tzdata*.tar.gz | tar -xf -
 100 </code></pre>
 101 <p>
 102 The code lets you compile the <code>tz</code> source files into
 103 machine-readable binary files, one for each location. It also lets
 104 you read a <code>tz</code> binary file and interpret time stamps for that
 105 location.</p>
 106 <p>
 107 The data are by no means authoritative. If you find errors, please
 108 send changes to the <a href="mailto:tz@elsie.nci.nih.gov">time zone
 109 mailing list</a>. You can also <a
 110 href="http://news.gmane.org/gmane.comp.time.tz">browse recent
 111 messages</a> sent to the mailing list, <a
 112 href="mailto:tz-request@elsie.nci.nih.gov">subscribe</a> to it,
 113 retrieve the <a
 114 href="ftp://elsie.nci.nih.gov/pub/tzarchive.gz">full archive of old
 115 messages</a> (in gzip compressed format), or retrieve <a
 116 href="ftp://munnari.oz.au/pub/oldtz">archived older versions of code
 117 and data</a>.</p>
 118 <p>
 119 The Web has several other sources for time zone and daylight saving time data.
 120 Here are some recent links that may be of interest.
 121 </p>
 122 <h2>Web pages using recent versions of the <code>tz</code> database</h2>
 123 <p>
 124 These are listed roughly in ascending order of complexity and fanciness.
 125 </p>
 126 <ul>
 127 <li><a href="http://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a>
 128 lets you see the <code>TZ</code> values directly.</li>
 129 <li><a
 130 href="http://convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current
 131 Time in 1000 Places</a> uses descriptions of the values.</li>
 132 <li><a href="http://www.timezoneconverter.com/">Time Zone Converter</a>
 133 uses a pulldown menu.</li>
 134 <li><a href="http://home.tiscali.nl/~t876506/TZworld.html">Complete
 135 timezone information for all countries</a> displays tables of DST rules.
 136 <li><a href="http://timeanddate.com/worldclock/">The World Clock -
 137 Time Zones</a> lets you sort zone names and convert times.</li>
 138 <li><a href="http://daylight-savings-time.info/">Graphical Display of
 139 Time Zones and Daylight Saving Times</a> shows a graph of time
 140 difference versus time for any pair of locations.</li>
 141 <li>The <a href="http://worldtimeengine.com/">World Time Engine</a>
 142 also contains data about time zone boundaries; it supports queries via place
 143 names and shows location maps.</li>
 144 </ul>
 145 <h2>Other time zone database formats</h2>
 146 <ul>
 147 <li>The <a href="ftp://ftp.rfc-editor.org/in-notes/rfc2445.txt">
 148 Internet Calendaring and Scheduling Core Object Specification
 149 (iCalendar)</a>, Internet <abbr title="Request For
 150 Comments">RFC</abbr> 2445, published by the (now-concluded) <a
 151 href="http://ietf.org/html.charters/OLD/calsch-charter.html"><abbr
 152 title="Internet Engineering Task Force">IETF</abbr>
 153 Calendaring and Scheduling Working Group (<abbr
 154 title="Calendaring and Scheduling Working Group">calsch</abbr>)</a>
 155 covers time zone
 156 data; see its VTIMEZONE calendar component.
 157 The <a href="http://calconnect.org/">Calendaring and Scheduling
 158 Consortium</a> is promoting further work in this area. <a
 159 href="http://calconnect.org/publications/icalendartimezoneproblemsandrecommendationsv1.0.pdf">iCalendar
 160 TIMEZONE Problems and Recommendations</a> offers guidelines and
 161 recommendations for the use of VTIMEZONE and <code>tz</code>.</li>
 162 <li><a href="http://calconnect.org/dstlinks.shtml">Extended Daylight
 163 Saving Time Links, Advisories and Changes</a> lists vendor material on recent
 164 daylight saving time changes.</li>
 165 <li><a
 166 href="http://calconnect.org/publications/timezoneregistryandservicerecommendationsv1.0.pdf">Timezone
 167 Registry and Service Recommendations</a> discusses a
 168 strategy for defining and deploying a time zone
 169 registration process that would establish unique names for each
 170 version of each <code>tz</code> zone, along with a polygonal
 171 representation of the geographical area corresponding to the
 172 zone.</li>
 173 <li>The <a
 174 href="http://lists.w3.org/Archives/Public/www-rdf-calendar/">www-rdf-calendar</a>
 175 list discusses <a
 176 href="http://www.w3.org/RDF/"><abbr
 177 title="Resource Description Framework">RDF</abbr></a>-based calendar
 178 and group scheduling systems, and has a <a
 179 href="http://www.w3.org/2002/12/cal/#tzd">workspace on time zone
 180 data</a> converted from <code>tz</code>. An earlier <a
 181 href="http://www.w3.org/2000/01/foo">schema</a> was sketched out.</li>
 182 </ul>
 183 <h2>Other <code>tz</code> compilers</h2>
 184 <ul>
 185 <li><a href="http://www.dachaplin.dsl.pipex.com/vzic/">Vzic iCalendar
 186 Timezone Converter</a> describes a <a
 187 href="http://en.wikipedia.org/wiki/C_%28programming_language%29">C</a>
 188 program that compiles
 189 <code>tz</code> source into iCalendar-compatible VTIMEZONE files.
 190 Vzic is freely
 191 available under the <a
 192 href="http://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr>
 193 General Public License (<abbr
 194 title="General Public License">GPL</abbr>)</a>.</li>
 195 <li><a href="http://sourceforge.net/projects/tzical">tziCal - tz
 196 database conversion utility</a> is like Vzic, except for the <a
 197 href="http://msdn.microsoft.com/netframework">.NET framework</a>.</li>
 198 <li><a
 199 href="http://search.cpan.org/dist/DateTime-TimeZone/">DateTime::TimeZone</a>
 200 contains a script <code>parse_olson</code> that compiles
 201 <code>tz</code> source into <a href="http://www.perl.org/">Perl</a>
 202 modules. It is part of the Perl <a
 203 href="http://datetime.perl.org/">DateTime Project</a>, which is freely
 204 available under both the <abbr>GPL</abbr> and the Perl Artistic
 205 License. DateTime::TimeZone also contains a script
 206 <code>tests_from_zdump</code> that generates test cases for each clock
 207 transition in the <code>tz</code> database.</li>
 208 <li><a href="http://icu-project.org/">International Components for
 209 Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a
 210 href="http://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a>
 211 libraries for internationalization that
 212 has a compiler from <code>tz</code> source
 213 into an <abbr>ICU</abbr>-specific format.
 214 <abbr>ICU</abbr> is freely available under a
 215 <abbr title="Berkeley Software Distribution">BSD</abbr>-style license.</li>
 216 <li><a href="http://joda-time.sourceforge.net/">Joda Time - Java date
 217 and time <abbr title="Application Program Interface">API</abbr></a>
 218 contains a class
 219 <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles
 220 <code>tz</code> source into a Joda-specific binary format. Joda Time
 221 is freely available under a <abbr>BSD</abbr>-style license.</li>
 222 <li><a href="http://pytz.sourceforge.net">PyTZ - Python Time
 223 Zone Library</a> compiles <code>tz</code> source into
 224 <a href="http://python.org/">Python</a>.
 225 It is freely available under a <abbr>BSD</abbr>-style license.</li>
 226 <li><a href="http://tzinfo.rubyforge.org/">TZInfo - Ruby Timezone Library</a>
 227 compiles <code>tz</code> source into
 228 <a href="http://ruby-lang.org">Ruby</a>.
 229 It is freely available under the <abbr
 230 title="Massachusetts Institute of Technology">MIT</abbr> license.</li>
 231 <li>The <a href="http://chronos-st.org/">Chronos Date/Time
 232 Library</a> is a <a href="http://smalltalk.org">Smalltalk</a> class
 233 library that compiles <code>tz</code> source into a <a
 234 href="http://date-time-zone.com/">time zone repository</a> whose format
 235 is either proprietary or an <a href="http://www.w3.org/XML/"><abbr
 236 title="Extensible Markup Language">XML</abbr></a>-encoded
 237 representation.</li>
 238 <li>Starting with version 8.5, <a href="http://tcl.tk/">Tcl</a>
 239 contains a developer-oriented parser that compiles <samp>tz</samp>
 240 source into text files, along with a runtime that can read those
 241 files. Tcl is freely available under a <abbr>BSD</abbr>-style
 242 license.</li>
 243 </ul>
 244 <h2>Other <code>tz</code> binary file readers</h2>
 245 <ul>
 246 <li>The <a
 247 href="http://www.gnu.org/software/libc/"><abbr>GNU</abbr> C
 248 Library</a>
 249 has an independent, thread-safe implementation of
 250 a <code>tz</code> binary file reader.
 251 This library is freely available under the
 252 <a href="http://www.gnu.org/copyleft/lesser.html">
 253 <abbr>GNU</abbr> Lesser General Public License
 254 (<abbr title="Lesser General Public License">LGPL</abbr>)</a>,
 255 and is widely used in <abbr>GNU</abbr>/Linux systems.</li>
 256 <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a>
 257 is a <code>tz</code> binary file reader written in Java.
 258 It is freely available under the <abbr>LGPL</abbr>.</li>
 259 <li>Tcl, mentioned above, also contains a
 260 <code>tz</code> binary file reader.</li>
 261 </ul>
 262 <h2>Other <code>tz</code>-based time zone software</h2>
 263 <ul>
 264 <li><a href="http://stemhaus.com/firefox/foxclocks/">FoxClocks</a>
 265 is an extension for <a
 266 href="http://developer.mozilla.org/en/docs/Toolkit_API">Mozilla
 267 Toolkit</a> applications like <a
 268 href="http://mozilla.com/firefox">Firefox</a>, <a
 269 href="http://mozilla.com/thunderbird">Thunderbird</a>, and
 270 <a
 271 href="http://www.mozilla.org/projects/calendar/sunbird/">Sunbird</a>.
 272 It displays multiple clocks in the application window, and has a mapping
 273 interface to <a href="http://earth.google.com/">Google Earth</a>.
 274 It is freely available under the <abbr>GPL</abbr>.</li>
 275 <li><a
 276 href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International
 277 clock (intclock)</a> is a multi-timezone clock for
 278 <abbr>GNU</abbr>/Linux and similar systems. It is freely available
 279 under the <abbr>GPL</abbr>.</li>
 280 <li><a href="http://codeplex.com/publicdomain">PublicDomain</a>
 281 has a copy of a recent <code>tz</code> database, accessed via a <a
 282 href="http://en.wikipedia.org/wiki/C_Sharp">C#</a> library. As its
 283 name suggests, it is in the public domain. Only current time stamps
 284 are well supported; historical data are compiled into the runtime but
 285 are not easily accessible.</li>
 286 <li><a href="http://java.sun.com/">Sun Java</a> releases since 1.4
 287 contain a copy of a subset of a recent <code>tz</code> database in a
 288 Java-specific format.</li>
 289 <li><a href="http://kimmo.suominen.com/sw/timezone/">Time Zone</a> is
 290 a <a href="http://wordpress.org/">WordPress</a> plugin. It is freely
 291 available under a <abbr>BSD</abbr>-style license.</li>
 292 <li><a
 293 href="http://veladg.com/velaterra.html">VelaTerra</a> is
 294 a Mac OS X program. Its developers
 295 <a href="http://veladg.com/tzoffer.html">offer free
 296 licenses</a> to <code>tz</code> contributors.</li>
 297 <li><a
 298 href="http://worldtimeexplorer.com/">World Time Explorer</a> is a
 299 Microsoft Windows program.</li>
 300 </ul>
 301 <h2>Other time zone databases</h2>
 302 <ul>
 303 <li><a href="http://www.astro.com/cgi/aq.cgi">Atlas Query</a>
 304 is Astrodienst's Web version of Shanks's
 305 excellent time zone history atlases published in both <a
 306 href="http://astrocom.com/products/software.php?software_id=ibmwboth">computer</a>
 307 and book form (<a
 308 href="http://astrocom.com/products/book.php?book_id=b110x">one volume
 309 for the USA</a>, and <a
 310 href="http://astrocom.com/products/book.php?book_id=b112x">one for
 311 other locations</a>) by <a
 312 href="http://astrocom.com/">Astro Communications Services</a>.</li>
 313 <li><a href="http://worldtime.com/">WORLDTIME: interactive atlas,
 314 time info, public holidays</a>
 315 contains information on local time, sunrise and sunset,
 316 and public holidays in several hundred cities around the world.</li>
 317 <li><a href="http://worldtimeserver.com/">World Time Server</a>
 318 is another time zone database.</li>
 319 <li><a href="http://tycho.usno.navy.mil/tzones.html">World Time Zones</a>
 320 contains data from the Time Service Department of the
 321 <abbr>US</abbr> Naval Observatory, used as the source
 322 for the <code>usno*</code> files in the <code>tz</code> distribution.</li>
 323 <li>The <a href="http://iata.org/ps/publications/SSIM.htm">Standard
 324 Schedules Information Manual</a> of the
 325 <a href="http://iata.org/index.htm">International Air Transport
 326 Association</a>
 327 gives current time zone rules for airports served by commercial aviation.</li>
 328 <li>Some Microsoft Windows versions contain time zone information in
 329 an undocumented format, with IDs that can be mapped to <code>TZ</code>
 330 values using the <a
 331 href="http://unicode.org/cldr/data/diff/supplemental/windows_tzid.html">Windows
 332 &rarr; Tzid table</a> maintained by the <abbr
 333 title="Common Locale Data Repository">CLDR</abbr> data mentioned
 334 below.</li>
 335 </ul>
 336 <h2>Maps</h2>
 337 <ul>
 338 <li>The <a href="https://www.cia.gov/">United States Central
 339 Intelligence Agency (<abbr
 340 title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a
 341 href="https://www.cia.gov/library/publications/the-world-factbook/reference_maps/pdf/time_zones.pdf">time
 342 zone map</a>; the
 343 <a
 344 href="http://www.lib.utexas.edu/maps/world.html">Perry-Casta&ntilde;eda
 345 Library Map Collection</a>
 346 of the University of Texas at Austin has copies of
 347 recent editions.
 348 The pictorial quality is good,
 349 but the maps do not indicate summer time,
 350 and parts of the data are a few years out of date.</li>
 351 <li><a href="http://worldtimezone.com/">Current time around the world
 352 and standard time zones map of the world</a>
 353 has several fancy time zone maps; it covers Russia particularly well.
 354 The maps' pictorial quality is not quite as good as the
 355 <abbr>CIA</abbr>'s
 356 but the maps are more up to date.</li>
 357 </ul>
 358 <h2>Time zone boundaries</h2>
 359 <ul>
 360 <li><a href="http://efele.net/maps/tz/">TZ timezone maps</a> contains a <a
 361 href="http://en.wikipedia.org/wiki/Shapefile">shapefile</a> of the
 362 <code>tz</code> regions in the world.</li>
 363 <li><a href="http://statoids.com/statoids.html">Administrative Divisions
 364 of Countries ("Statoids")</a> contains detailed lists of
 365 <code>tz</code>-related zone subdivision data.</li>
 366 <li><a href="http://home.tiscali.nl/~t876506/Multizones.html">Time
 367 zone boundaries for multizone countries</a> summarizes legal
 368 boundaries between time zones within countries.</li>
 369 <li>Manifold.net's <a
 370 href="http://www.manifold.net/download/freemaps.html">Free Maps and
 371 <abbr title="Geographic Information Systems">GIS</abbr>
 372 Data</a> includes a Manifold-format map of
 373 world time zone boundaries distributed under the
 374 <abbr>GPL</abbr>.</li>
 375 <li>The <abbr>US</abbr> Geological Survey's National Atlas of
 376 the United States
 377 publishes the <a href="http://nationalatlas.gov/mld/timeznp.html">Time
 378 Zones of the United States</a> in the public domain.</li>
 379 <li>The GeoCommunity lists several commercial sources for <a
 380 href="http://spatialnews.geocomm.com/features/timezones/">International
 381 Time Zones and Time Zone Data</a>.</li>
 382 <li>A ship within the <a
 383 href="http://en.wikipedia.org/wiki/Territorial_waters">territorial
 384 waters</a> of any nation uses that nation's time. In international
 385 waters, time zone boundaries are meridians 15&deg; apart, except that
 386 UTC&minus;12 and UTC+12 are each 7.5&deg; wide and are separated by
 387 the 180&deg; meridian (not by the International Date Line, which is
 388 for land and territorial waters only). A captain can change ship's
 389 clocks any time after entering a new time zone; midnight changes are
 390 common.</li>
 391 </ul>
 392 <h2>Civil time concepts and history</h2>
 393 <ul>
 394 <li><a href="http://physics.nist.gov/GenInt/Time/time.html">A
 395 Walk through Time</a>
 396 surveys the evolution of timekeeping.</li>
 397 <li><a href="http://webexhibits.org/daylightsaving/">About Daylight
 398 Saving Time - History, rationale, laws &amp; dates</a>
 399 is an overall history of <abbr>DST</abbr>.</li>
 400 <li><a href="http://energy.ca.gov/daylightsaving.html">Saving Time,
 401 Saving Energy</a> discusses a primary justification for <abbr>DST</abbr>.</li>
 402 <li><a href="http://seizethedaylight.com/dst/">Who Knew? A Brief
 403 History of Daylight Saving Time</a> summarizes some of the contentious
 404 history of <abbr>DST</abbr>.</li>
 405 <li><a href="http://toi.iriti.cnr.it/">The
 406 Time of Internet</a>
 407 describes time zones and daylight saving time,
 408 with diagrams.
 409 The time zone map is out of date, however.</li>
 410 <li><a href="http://www.phys.uu.nl/~vgent/idl/idl.htm">A History of
 411 the International Date Line</a> tells the story of the most important
 412 time zone boundary.</li>
 413 <li><a href="http://statoids.com/tconcept.html">Basic Time
 414 Zone Concepts</a> discusses terminological issues behind time zones.</li>
 415 </ul>
 416 <h2>National histories of legal time</h2>
 417 <dl>
 418 <dt>Australia</dt>
 419 <dd>The Parliamentary Library has commissioned <a
 420 href="http://www.aph.gov.au/LIBRARY/Pubs/rn/2006-07/07rn13.pdf">research
 421 note on daylight saving time in Australia</a>.
 422 The Bureau of Meteorology publishes a list of
 423 <a href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation Dates of Daylight Savings Time within Australia</a>.</dd>
 424 <dt>Belgium</dt>
 425 <dd>The Royal Observatory of Belgium maintains a table of <a
 426 href="http://www.astro.oma.be/GENERAL/INFO/nli001a.html"
 427 hreflang="nl">time in Belgium (in Dutch)</a>.</dd>
 428 <dt>Brazil</dt>
 429 <dd>The Time Service Department of the National Observatory
 430 records <a href="http://pcdsh01.on.br/DecHV.html"
 431 hreflang="pt-BR">Brazil's daylight saving time decrees (in
 432 Portuguese)</a>.</dd>
 433 <dt>Canada</dt>
 434 <dd>The Institute for National Measurement Standards publishes current
 435 and some older information about <a
 436 href="http://inms-ienm.nrc-cnrc.gc.ca/time_services/daylight_saving_e.html">Time
 437 Zones &amp; Daylight Saving Time</a>.</dd>
 438 <dt>Chile</dt>
 439 <dd>The Chilean Hydrographic and Oceanographic Service publishes a <a
 440 href="http://www.horaoficial.cl/horaof.htm" hreflang="es"> history of
 441 official time (in Spanish)</a>.</dd>
 442 <dt>Germany</dt>
 443 <dd>The National Institute for Science and Technology maintains the <a
 444 href="http://www.ptb.de/en/org/4/44/441/dars_e.htm">Realisation of
 445 Legal Time in Germany</a>.</dd>
 446 <dt>Israel</dt>
 447 <dd>The Interior Ministry periodically issues <a
 448 href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements"
 449 hreflang="he">announcements (in Hebrew)</a>.</dd>
 450 <dt>Mexico</dt>
 451 <dd>The Investigation and Analysis Service of the Mexican Library of
 452 Congress has published a <a
 453 href="http://www.cddhcu.gob.mx/bibliot/publica/inveyana/polisoc/horver/"
 454 hreflang="es">history of Mexican local time (in Spanish)</a>.</dd>
 455 <dt>Malaysia</dt>
 456 <dd>See Singapore below.</dd>
 457 <dt>Netherlands</dt>
 458 <dd><a href="http://www.phys.uu.nl/~vgent/wettijd/wettijd.htm"
 459 hreflang="nl">Legal time in the Netherlands (in Dutch)</a>
 460 covers the history of local time in the Netherlands from ancient times.</dd>
 461 <dt>New Zealand</dt>
 462 <dd>The Department of Internal Affairs maintains a brief history <a
 463 href="http://dia.govt.nz/diawebsite.nsf/wpg_URL/Resource-material-Information-We-Provide-About-Daylight-Saving">About
 464 Daylight Saving</a>. The privately-maintained <a
 465 href="http://astrologyschool.com/nztime.html">History of New Zealand
 466 time</a> has more details.</dd>
 467 <dt>Norway</dt>
 468 <dd>The Norwegian Meteorological Institute lists
 469 <a href="http://met.no/met/met_lex/q_u/sommertid.html" hreflang="no">Summer
 470 time in Norway (in Norwegian)</a>, citing the
 471 Institute of Theoretical Astrophysics, Oslo.</dd>
 472 <dt>Singapore</dt>
 473 <dd><a
 474 href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why
 475 is Singapore in the "Wrong" Time Zone?</a> details the
 476 history of legal time in Singapore and Malaysia.</dd>
 477 <dt>United Kingdom</dt>
 478 <dd><a
 479 href="http://www.srcf.ucam.org/~jsm28/british-time/">History of
 480 legal time in Britain</a> discusses in detail the country
 481 with perhaps the best-documented history of clock adjustments.
 482 The National Physical Laboratory also maintains an <a
 483 href="http://www.npl.co.uk/server.php?show=ConWebDoc.2714">Archive
 484 of Summer time dates</a>.</dd>
 485 </dl>
 486 <h2>Precision timekeeping</h2>
 487 <ul>
 488 <li><a
 489 href="http://literature.agilent.com/litweb/pdf/5965-7984E.pdf">The
 490 Science of Timekeeping</a> is a thorough introduction
 491 to the theory and practice of precision timekeeping.</li>
 492 <li><a href="http://www.ntp.org/"><abbr
 493 title="Network Time Protocol">NTP</abbr>: The Network
 494 Time Protocol</a>
 495 discusses how to synchronize clocks of
 496 Internet hosts.</li>
 497 <li><a
 498 href="ftp://ftp.rfc-editor.org/in-notes/rfc4833.txt">Timezone
 499 Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a>
 500 (Internet <abbr>RFC</abbr> 4833)
 501 specifies a <a
 502 href="http://www.dhcp.org/">DHCP</a> option for a server to configure
 503 a client's time zone and daylight saving settings automatically.</li>
 504 <li><a href="http://gauss.gge.unb.ca/GMT.UT.and.the.RGO.html">A Few
 505 Facts Concerning <abbr title="Greenwich Mean Time">GMT</abbr>, <abbr
 506 title="Universal Time">UT</abbr>, and
 507 the <abbr title="Royal Greenwich Observatory">RGO</abbr></a>
 508 answers questions like "What is the
 509 difference between <abbr>GMT</abbr> and <abbr>UTC</abbr>?"</li>
 510 <li><a
 511 href="http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical
 512 Times</a> explains more abstruse astronomical time scales like
 513 <abbr title="Terrestrial Dynamic Time">TDT</abbr>,
 514 <abbr title="Geocentric Coordinate Time">TCG</abbr>, and
 515 <abbr title="Barycentric Dynamic Time">TDB</abbr>.
 516 <a href="http://www.ucolick.org/~sla/leapsecs/timescales.html">Time
 517 Scales</a> goes into more detail, particularly for historical variants.</li>
 518 <li>The <a href="http://iau.org/"><abbr
 519 title="International Astronomical Union">IAU</abbr></a>'s <a
 520 href="http://www.iau-sofa.rl.ac.uk/"><abbr
 521 title="Standards Of Fundamental Astronomy">SOFA</abbr></a>
 522 initiative publishes Fortran
 523 code for converting among time scales like
 524 <abbr title="International Atomic Time">TAI</abbr>,
 525 <abbr>TDB</abbr>, <abbr>TDT</abbr> and
 526 <abbr>UTC</abbr>.</li>
 527 <li><a href="http://jpl.nasa.gov/basics/bsf2-3.htm">Basics of
 528 Space Flight - Reference Systems - Time Conventions</a>
 529 briefly explains interplanetary space flight timekeeping.</li>
 530 <li><a
 531 href="http://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical
 532 Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a> briefly
 533 describes Mars Coordinated Time (<abbr
 534 title="Mars Coordinated Time">MTC</abbr>) and the
 535 diverse local time
 536 scales used by each landed mission on Mars.</li>
 537 <li><a href="http://leapsecond.com/">LeapSecond.com</a> is
 538 dedicated not only to leap seconds but to precise time and frequency
 539 in general. It covers the state of the art in amateur timekeeping, and
 540 how the art has progressed over the past few decades.</li>
 541 <li><a
 542 href="http://hpiers.obspm.fr/eop-pc/products/bulletins/bulletins.html">Bulletins
 543 maintained by the
 544 <abbr title="International Earth Rotation Service">IERS</abbr>
 545 <abbr title="Earth Orientation Parameters">EOP</abbr>
 546 (<abbr title="Product Center">PC</abbr>)</a> contains official publications of
 547 the Earth Orientation Parameters Product Center of the
 548 International Earth Rotation Service, the committee that decides
 549 when leap seconds occur.</li>
 550 <li>The <a
 551 href="http://six.pairlist.net/mailman/listinfo/leapsecs">Leap
 552 Second Discussion List</a> covers <a
 553 href="http://gauss.gge.unb.ca/papers.pdf/gpsworld.november99.pdf">McCarthy
 554 and Klepczynski's proposal to discontinue leap seconds</a>,
 555 discussed further in
 556 <a href="http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The
 557 leap second: its history and possible future</a>.
 558 The (now disbanded) <a href="http://members.aas.org/comms/leap.cfm"><abbr
 559 title="American Astronomical Society">AAS</abbr> Leap Second
 560 Committee</a> has solicited input on this proposal.
 561 <a href="http://www.ucolick.org/~sla/leapsecs/">The
 562 Future of Leap Seconds</a> covers this
 563 contentious issue.</li>
 564 </ul>
 565 <h2>Time notation</h2>
 566 <ul>
 567 <li>
 568 <a href="http://www.cl.cam.ac.uk/~mgk25/iso-time.html">A Summary of
 569 the International Standard Date and Time Notation</a> is a good
 570 summary of
 571 <a
 572 href="http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=40874"><abbr
 573 title="International Organization for Standardization">ISO</abbr>
 574 8601:2004 -- Data elements and interchange formats -- Information
 575 interchange -- Representation of dates and times</a>.</li>
 576 <li>
 577 <a href="http://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr>
 578 Schema: Datatypes - dateTime</a> specifies a format inspired by
 579 <abbr>ISO</abbr> 8601 that is in common use in XML data.</li>
 580 <li>
 581 <a href="ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt">Internet
 582 Message Format</a> (Internet <abbr>RFC</abbr> 2822) &sect;3.3
 583 specifies the time notation used in email and <a
 584 href="ftp://ftp.rfc-editor.org/in-notes/rfc2616.txt"><abbr>HTTP</abbr></a>
 585 headers.</li>
 586 <li>
 587 <a href="ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt">Date and Time
 588 on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339)
 589 specifies an <abbr>ISO</abbr> 8601
 590 profile for use in new Internet
 591 protocols.</li>
 592 <li>
 593 <a href="http://www.hackcraft.net/web/datetime/">Date &amp; Time
 594 Formats on the Web</a> surveys web- and Internet-oriented date and time
 595 formats.</li>
 596 <li>
 597 <a href="http://exit109.com/~ghealton/y2k/yrexamples.html">The
 598 Best of Dates, the Worst of Dates</a> covers many problems encountered
 599 by software developers when handling dates and time stamps.</li>
 600 <li>The <a
 601 href="http://unicode.org/cldr/">Unicode Common Locale Data Repository
 602 (<abbr>CLDR</abbr>) Project</a> has localizations for time zone names,
 603 abbreviations, identifiers, and formats. For example, it contains
 604 French translations for "Eastern European Summer Time", "<abbr
 605 title="Eastern European Summer Time">EEST</abbr>", and
 606 "Bucharest". <a
 607 href="http://unicode.org/cldr/data/charts/by_type/names.metazone.html">By-Type
 608 Chart: names.metazone</a> shows these values for many locales.
 609 <abbr>ICU</abbr> contains a mechanism for using this data.</li>
 610 <li>Alphabetic time zone abbreviations should not be used as unique
 611 identifiers for <abbr>UTC</abbr> offsets as they are ambiguous in
 612 practice. For example, "<abbr>EST</abbr>" denotes 5 hours behind
 613 <abbr>UTC</abbr> in English-speaking North America, but it denotes 10
 614 or 11 hours ahead of <abbr>UTC</abbr> in Australia; and
 615 French-speaking North Americans prefer
 616 "<abbr title="Heure Normale de l'Est">HNE</abbr>" to
 617 "<abbr>EST</abbr>". For <abbr>POSIX</abbr> the <code>tz</code>
 618 database contains English abbreviations for all time stamps but in
 619 many cases these are merely inventions of the database
 620 maintainers.</li>
 621 <li>Numeric time zone abbreviations typically count hours east of
 622 <abbr>UTC</abbr>, e.g., <code>+09</code> for Japan and
 623 <code>-10</code> for Hawaii. However, the <abbr>POSIX</abbr>
 624 <code>TZ</code> environment variable uses the opposite convention. For
 625 example, one might use <code>TZ="JST-9"</code> and
 626 <code>TZ="HST10"</code> for Japan and Hawaii, respectively. If the
 627 <code>tz</code> database is available, it is usually better to use
 628 settings like <code>TZ="Asia/Tokyo"</code> and
 629 <code>TZ="Pacific/Honolulu"</code> instead, as this should avoid
 630 confusion, handle old time stamps better, and insulate you better from
 631 any future changes to the rules. One should never set
 632 <abbr>POSIX</abbr> <code>TZ</code> to a value like
 633 <code>"GMT-9"</code>, though, since this would falsely claim that
 634 local time is nine hours ahead of <abbr>UTC</abbr> and the time zone
 635 is called "<abbr>GMT</abbr>".</li>
 636 </ul>
 637 <h2>Related indexes</h2>
 638 <ul>
 639 <li><a href="tz-art.htm">Time and the Arts</a></li>
 640 <li><a href="http://www.dmoz.org/Reference/Time/">Open Directory -
 641 Reference: Time</a></li>
 642 <li><a href="http://directory.google.com/Top/Reference/Time/">Google Directory
 643 - Reference &gt; Time</a></li>
 644 <li><a href="http://dir.yahoo.com/Science/Measurements_and_Units/Time">Yahoo!
 645 Directory &gt; Science &gt; Measurements and Units &gt; Time</a></li>
 646 </ul>
 647 </body>
 648 </html>