<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> <title>Sources for Time Zone and Daylight Saving Time Data</title> <link rel="schema.DC" href="http://purl.org/DC/elements/1.1/"> <meta http-equiv="Content-type" content='text/html; charset="US-ASCII"'> <meta name="DC.Creator" content="Eggert, Paul"> <meta name="DC.Contributor" content="Olson, Arthur David"> <meta name="DC.Date" content="2007-12-26"> <meta name="DC.Description" content="Sources of information about time zones and daylight saving time"> <meta name="DC.Identifier" content="http://www.twinsun.com/tz/tz-link.htm"> <meta name="Keywords" content="database,daylight saving,DST,time zone,timezone,tz,zoneinfo"> </head> <body> <h1>Sources for Time Zone and Daylight Saving Time Data</h1> <address> @(#)tz-link.htm 8.32 </address> <p> This file is in the public domain, so clarified as of 2009-05-17 by Arthur David Olson. </p> <p> Please send corrections to this web page to the <a href="mailto:tz@elsie.nci.nih.gov">time zone mailing list</a>.</p> <h2>The <code>tz</code> database</h2> <p> The <a href="http://en.wikipedia.org/wiki/Public_domain">public-domain</a> time zone database contains code and data that represent the history of local time for many representative locations around the globe. It is updated periodically to reflect changes made by political bodies to <a href="http://en.wikipedia.org/wiki/Time_zone">time zone</a> boundaries, <a href="http://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr title="Coordinated Universal Time">UTC</abbr></a> offsets, and <a href="http://en.wikipedia.org/wiki/Daylight_saving">daylight-saving</a> rules. This database (often called <code>zoneinfo</code> or <a href="http://en.wikipedia.org/wiki/TZ_database"><code>tz</code></a>) is used by several implementations, including <a href="http://www.gnu.org/software/libc/">the <abbr title="GNU's Not Unix">GNU</abbr> C Library</a> used in <a href="http://www.linux.org/"><abbr>GNU</abbr>/Linux</a>, <a href="http://www.freebsd.org/">FreeBSD</a>, <a href="http://netbsd.org/">NetBSD</a>, <a href="http://openbsd.org/">OpenBSD</a>, <a href="http://cygwin.com/">Cygwin</a>, <a href="http://www.delorie.com/djgpp/"><abbr title="DJ's GNU Programming Platform">DJGPP</abbr></a>, <a href="http://ibm.com/aix">AIX</a>, <a href="http://www.apple.com/macosx/">Mac OS X</a>, <a href="http://h71000.www7.hp.com/">OpenVMS</a>, <a href="http://oracle.com/database">Oracle Database</a>, <a href="http://sun.com/software/solaris">Solaris</a>, <a href="http://h30097.www3.hp.com/">Tru64</a>, and <a href="http://sco.com/products/unixware">UnixWare</a>.</p> <p> Each location in the database represents a national region where all clocks keeping local time have agreed since 1970. Locations are identified by continent or ocean and then by the name of the location, which is typically the largest city within the region. For example, <code>America/New_York</code> represents most of the <abbr title="United States">US</abbr> eastern time zone; <code>America/Phoenix</code> represents most of Arizona, which uses mountain time without daylight saving time (<abbr title="daylight saving time">DST</abbr>); <code>America/Detroit</code> represents most of Michigan, which uses eastern time but with different <abbr>DST</abbr> rules in 1975; and other entries represent smaller regions like Starke County, Indiana, which switched from central to eastern time in 1991 and switched back in 2006. To use the database on an extended <a href="http://en.wikipedia.org/wiki/POSIX"><abbr title="Portable Operating System Interface">POSIX</abbr></a> implementation set the <code>TZ</code> environment variable to the location's full name, e.g., <code>TZ="America/New_York"</code>.</p> <p> In the <code>tz</code> database's <a href="ftp://elsie.nci.nih.gov/pub"><abbr title="File Transfer Protocol">FTP</abbr> distribution</a> the code is in the file <code>tzcode<var>C</var>.tar.gz</code>, where <code><var>C</var></code> is the code's version; similarly, the data are in <code>tzdata<var>D</var>.tar.gz</code>, where <code><var>D</var></code> is the data's version. Each version is a four-digit year followed by lower-case letters (a through z, then za through zz, then zza through zzz, and so on). The following <a href="http://en.wikipedia.org/wiki/Unix_shell">shell</a> commands download these files to a <abbr>GNU</abbr>/Linux or similar host; see the downloaded <code>README</code> file for what to do next.</p> <pre style="margin-left: 2em"><code>mkdir tz cd tz <a href="http://www.gnu.org/software/wget/">wget</a> 'ftp://elsie.nci.nih.gov/pub/tz*.tar.gz' <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 - gzip -dc tzdata*.tar.gz | tar -xf - </code></pre> <p> The code lets you compile the <code>tz</code> source files into machine-readable binary files, one for each location. It also lets you read a <code>tz</code> binary file and interpret time stamps for that location.</p> <p> The data are by no means authoritative. If you find errors, please send changes to the <a href="mailto:tz@elsie.nci.nih.gov">time zone mailing list</a>. You can also <a href="http://news.gmane.org/gmane.comp.time.tz">browse recent messages</a> sent to the mailing list, <a href="mailto:tz-request@elsie.nci.nih.gov">subscribe</a> to it, retrieve the <a href="ftp://elsie.nci.nih.gov/pub/tzarchive.gz">full archive of old messages</a> (in gzip compressed format), or retrieve <a href="ftp://munnari.oz.au/pub/oldtz">archived older versions of code and data</a>.</p> <p> The Web has several other sources for time zone and daylight saving time data. Here are some recent links that may be of interest. </p> <h2>Web pages using recent versions of the <code>tz</code> database</h2> <p> These are listed roughly in ascending order of complexity and fanciness. </p> <ul> <li> <a href="http://permatime.com"> Permatime</a> is a service for generating and viewing links that refer to a particular point in time and can be displayed in multiple timezones. It uses the ruby tzinfo gem. (From Tim Diggins, 2009-11-03.) </li> <li><a href="http://twiki.org/cgi-bin/xtra/tzdatepick.html">Date and Time Gateway</a> lets you see the <code>TZ</code> values directly.</li> <li><a href="http://convertit.com/Go/ConvertIt/World_Time/Current_Time.ASP">Current Time in 1000 Places</a> uses descriptions of the values.</li> <li><a href="http://www.timezoneconverter.com/">Time Zone Converter</a> uses a pulldown menu.</li> <li><a href="http://home.tiscali.nl/~t876506/TZworld.html">Complete timezone information for all countries</a> displays tables of DST rules. <li><a href="http://timeanddate.com/worldclock/">The World Clock - Time Zones</a> lets you sort zone names and convert times.</li> <li><a href="http://daylight-savings-time.info/">Graphical Display of Time Zones and Daylight Saving Times</a> shows a graph of time difference versus time for any pair of locations.</li> <li>The <a href="http://worldtimeengine.com/">World Time Engine</a> also contains data about time zone boundaries; it supports queries via place names and shows location maps.</li> <li><a href="http://simpletimerclocks.mozdev.org/">Simple Timer + Clocks</a> is a Firefox add-on which uses a timezone data file generated from the tz data files.</li> </ul> <h2>Other time zone database formats</h2> <ul> <li>The <a href="ftp://ftp.rfc-editor.org/in-notes/rfc2445.txt"> Internet Calendaring and Scheduling Core Object Specification (iCalendar)</a>, Internet <abbr title="Request For Comments">RFC</abbr> 2445, published by the (now-concluded) <a href="http://ietf.org/html.charters/OLD/calsch-charter.html"><abbr title="Internet Engineering Task Force">IETF</abbr> Calendaring and Scheduling Working Group (<abbr title="Calendaring and Scheduling Working Group">calsch</abbr>)</a> covers time zone data; see its VTIMEZONE calendar component. The <a href="http://calconnect.org/">Calendaring and Scheduling Consortium</a> is promoting further work in this area. <a href="http://calconnect.org/publications/icalendartimezoneproblemsandrecommendationsv1.0.pdf">iCalendar TIMEZONE Problems and Recommendations</a> offers guidelines and recommendations for the use of VTIMEZONE and <code>tz</code>.</li> <li><a href="http://calconnect.org/dstlinks.shtml">Extended Daylight Saving Time Links, Advisories and Changes</a> lists vendor material on recent daylight saving time changes.</li> <li><a href="http://calconnect.org/publications/timezoneregistryandservicerecommendationsv1.0.pdf">Timezone Registry and Service Recommendations</a> discusses a strategy for defining and deploying a time zone registration process that would establish unique names for each version of each <code>tz</code> zone, along with a polygonal representation of the geographical area corresponding to the zone.</li> <li>The <a href="http://lists.w3.org/Archives/Public/www-rdf-calendar/">www-rdf-calendar</a> list discusses <a href="http://www.w3.org/RDF/"><abbr title="Resource Description Framework">RDF</abbr></a>-based calendar and group scheduling systems, and has a <a href="http://www.w3.org/2002/12/cal/#tzd">workspace on time zone data</a> converted from <code>tz</code>. An earlier <a href="http://www.w3.org/2000/01/foo">schema</a> was sketched out.</li> </ul> <h2>Other <code>tz</code> compilers</h2> <ul> <li><a href="http://www.dachaplin.dsl.pipex.com/vzic/">Vzic iCalendar Timezone Converter</a> describes a <a href="http://en.wikipedia.org/wiki/C_%28programming_language%29">C</a> program that compiles <code>tz</code> source into iCalendar-compatible VTIMEZONE files. Vzic is freely available under the <a href="http://www.gnu.org/copyleft/gpl.html"><abbr>GNU</abbr> General Public License (<abbr title="General Public License">GPL</abbr>)</a>.</li> <li><a href="http://sourceforge.net/projects/tzical">tziCal - tz database conversion utility</a> is like Vzic, except for the <a href="http://msdn.microsoft.com/netframework">.NET framework</a>.</li> <li><a href="http://search.cpan.org/dist/DateTime-TimeZone/">DateTime::TimeZone</a> contains a script <code>parse_olson</code> that compiles <code>tz</code> source into <a href="http://www.perl.org/">Perl</a> modules. It is part of the Perl <a href="http://datetime.perl.org/">DateTime Project</a>, which is freely available under both the <abbr>GPL</abbr> and the Perl Artistic License. DateTime::TimeZone also contains a script <code>tests_from_zdump</code> that generates test cases for each clock transition in the <code>tz</code> database.</li> <li><a href="http://icu-project.org/">International Components for Unicode (<abbr>ICU</abbr>)</a> contains C/C++ and <a href="http://en.wikipedia.org/wiki/Java_%28programming_language%29">Java</a> libraries for internationalization that has a compiler from <code>tz</code> source into an <abbr>ICU</abbr>-specific format. <abbr>ICU</abbr> is freely available under a <abbr title="Berkeley Software Distribution">BSD</abbr>-style license.</li> <li><a href="http://joda-time.sourceforge.net/">Joda Time - Java date and time <abbr title="Application Program Interface">API</abbr></a> contains a class <code>org.joda.time.tz.ZoneInfoCompiler</code> that compiles <code>tz</code> source into a Joda-specific binary format. Joda Time is freely available under a <abbr>BSD</abbr>-style license.</li> <li><a href="http://pytz.sourceforge.net">PyTZ - Python Time Zone Library</a> compiles <code>tz</code> source into <a href="http://python.org/">Python</a>. It is freely available under a <abbr>BSD</abbr>-style license.</li> <li><a href="http://tzinfo.rubyforge.org/">TZInfo - Ruby Timezone Library</a> compiles <code>tz</code> source into <a href="http://ruby-lang.org">Ruby</a>. It is freely available under the <abbr title="Massachusetts Institute of Technology">MIT</abbr> license.</li> <li>The <a href="http://chronos-st.org/">Chronos Date/Time Library</a> is a <a href="http://smalltalk.org">Smalltalk</a> class library that compiles <code>tz</code> source into a <a href="http://date-time-zone.com/">time zone repository</a> whose format is either proprietary or an <a href="http://www.w3.org/XML/"><abbr title="Extensible Markup Language">XML</abbr></a>-encoded representation.</li> <li>Starting with version 8.5, <a href="http://tcl.tk/">Tcl</a> contains a developer-oriented parser that compiles <samp>tz</samp> source into text files, along with a runtime that can read those files. Tcl is freely available under a <abbr>BSD</abbr>-style license.</li> </ul> <h2>Other <code>tz</code> binary file readers</h2> <ul> <li>The <a href="http://www.gnu.org/software/libc/"><abbr>GNU</abbr> C Library</a> has an independent, thread-safe implementation of a <code>tz</code> binary file reader. This library is freely available under the <a href="http://www.gnu.org/copyleft/lesser.html"> <abbr>GNU</abbr> Lesser General Public License (<abbr title="Lesser General Public License">LGPL</abbr>)</a>, and is widely used in <abbr>GNU</abbr>/Linux systems.</li> <li><a href="http://bmsi.com/java/#TZ">ZoneInfo.java</a> is a <code>tz</code> binary file reader written in Java. It is freely available under the <abbr>LGPL</abbr>.</li> <li>Tcl, mentioned above, also contains a <code>tz</code> binary file reader.</li> </ul> <h2>Other <code>tz</code>-based time zone software</h2> <ul> <li><a href="http://stemhaus.com/firefox/foxclocks/">FoxClocks</a> is an extension for <a href="http://developer.mozilla.org/en/docs/Toolkit_API">Mozilla Toolkit</a> applications like <a href="http://mozilla.com/firefox">Firefox</a>, <a href="http://mozilla.com/thunderbird">Thunderbird</a>, and <a href="http://www.mozilla.org/projects/calendar/sunbird/">Sunbird</a>. It displays multiple clocks in the application window, and has a mapping interface to <a href="http://earth.google.com/">Google Earth</a>. It is freely available under the <abbr>GPL</abbr>.</li> <li><a href="http://users.skynet.be/Peter.Verthez/projects/intclock/">International clock (intclock)</a> is a multi-timezone clock for <abbr>GNU</abbr>/Linux and similar systems. It is freely available under the <abbr>GPL</abbr>.</li> <li><a href="http://codeplex.com/publicdomain">PublicDomain</a> has a copy of a recent <code>tz</code> database, accessed via a <a href="http://en.wikipedia.org/wiki/C_Sharp">C#</a> library. As its name suggests, it is in the public domain. Only current time stamps are well supported; historical data are compiled into the runtime but are not easily accessible.</li> <li><a href="http://java.sun.com/">Sun Java</a> releases since 1.4 contain a copy of a subset of a recent <code>tz</code> database in a Java-specific format.</li> <li><a href="http://kimmo.suominen.com/sw/timezone/">Time Zone</a> is a <a href="http://wordpress.org/">WordPress</a> plugin. It is freely available under a <abbr>BSD</abbr>-style license.</li> <li><a href="http://veladg.com/velaterra.html">VelaTerra</a> is a Mac OS X program. Its developers <a href="http://veladg.com/tzoffer.html">offer free licenses</a> to <code>tz</code> contributors.</li> <li><a href="http://worldtimeexplorer.com/">World Time Explorer</a> is a Microsoft Windows program.</li> <li> <a href="http://www.toriasoft.com"> WorldClock for Windows and Windows Mobile</a> lets users "see the time in up to 25 locations in the world at once." (From Hans Nieuwenhuis, 2009-11-02.) </li> <li> <a href="http://www.relativedata.com/time-zone-master"> Time Zone Master Basic </a> "allows people to display multiple desktop clocks, and to research current and historical time information, as well as times of astronomical events (sunrise/transit/set, moonrise/transit/set, phases, season starts) for user-selected dates in the past and future. It can automatically download, compile and use the tzdata**.gz database files as they are released to keep the data up to date. The software is free." (Davie Patte) </li> </ul> <h2>Other time zone databases</h2> <ul> <li><a href="http://www.astro.com/cgi/aq.cgi">Atlas Query</a> is Astrodienst's Web version of Shanks's excellent time zone history atlases published in both <a href="http://astrocom.com/products/software.php?software_id=ibmwboth">computer</a> and book form (<a href="http://astrocom.com/products/book.php?book_id=b110x">one volume for the USA</a>, and <a href="http://astrocom.com/products/book.php?book_id=b112x">one for other locations</a>) by <a href="http://astrocom.com/">Astro Communications Services</a>.</li> <li><a href="http://worldtime.com/">WORLDTIME: interactive atlas, time info, public holidays</a> contains information on local time, sunrise and sunset, and public holidays in several hundred cities around the world.</li> <li><a href="http://worldtimeserver.com/">World Time Server</a> is another time zone database.</li> <li><a href="http://tycho.usno.navy.mil/tzones.html">World Time Zones</a> contains data from the Time Service Department of the <abbr>US</abbr> Naval Observatory, used as the source for the <code>usno*</code> files in the <code>tz</code> distribution.</li> <li>The <a href="http://iata.org/ps/publications/SSIM.htm">Standard Schedules Information Manual</a> of the <a href="http://iata.org/index.htm">International Air Transport Association</a> gives current time zone rules for airports served by commercial aviation.</li> <li>Some Microsoft Windows versions contain time zone information in an undocumented format, with IDs that can be mapped to <code>TZ</code> values using the <a href="http://unicode.org/cldr/data/diff/supplemental/windows_tzid.html">Windows → Tzid table</a> maintained by the <abbr title="Common Locale Data Repository">CLDR</abbr> data mentioned below.</li> <li> <a href="http://code.google.com/p/tzdata/"> http://code.google.com/p/tzdata/ </a> provides programming-language-specific representations of timezone data. Currently this includes XML, PHP, Ruby, Javascript, JSON and CSV formatted data. The repository is updated as soon as the FTP distribution is updated. All data can be downloaded as a zip and/or it can be obtained/synced via anonymous SVN. Data is made available under the MIT license. (From Rich Tibbett.) </li> </ul> <h2>Maps</h2> <ul> <li>The <a href="https://www.cia.gov/">United States Central Intelligence Agency (<abbr title="Central Intelligence Agency">CIA</abbr>)</a> publishes a <a href="https://www.cia.gov/library/publications/the-world-factbook/reference_maps/pdf/time_zones.pdf">time zone map</a>; the <a href="http://www.lib.utexas.edu/maps/world.html">Perry-Castañeda Library Map Collection</a> of the University of Texas at Austin has copies of recent editions. The pictorial quality is good, but the maps do not indicate summer time, and parts of the data are a few years out of date.</li> <li><a href="http://worldtimezone.com/">Current time around the world and standard time zones map of the world</a> has several fancy time zone maps; it covers Russia particularly well. The maps' pictorial quality is not quite as good as the <abbr>CIA</abbr>'s but the maps are more up to date.</li> </ul> <h2>Time zone boundaries</h2> <ul> <li><a href="http://efele.net/maps/tz/">TZ timezone maps</a> contains a <a href="http://en.wikipedia.org/wiki/Shapefile">shapefile</a> of the <code>tz</code> regions in the world.</li> <li><a href="http://statoids.com/statoids.html">Administrative Divisions of Countries ("Statoids")</a> contains detailed lists of <code>tz</code>-related zone subdivision data.</li> <li><a href="http://home.tiscali.nl/~t876506/Multizones.html">Time zone boundaries for multizone countries</a> summarizes legal boundaries between time zones within countries.</li> <li>Manifold.net's <a href="http://www.manifold.net/download/freemaps.html">Free Maps and <abbr title="Geographic Information Systems">GIS</abbr> Data</a> includes a Manifold-format map of world time zone boundaries distributed under the <abbr>GPL</abbr>.</li> <li>The <abbr>US</abbr> Geological Survey's National Atlas of the United States publishes the <a href="http://nationalatlas.gov/mld/timeznp.html">Time Zones of the United States</a> in the public domain.</li> <li>The GeoCommunity lists several commercial sources for <a href="http://spatialnews.geocomm.com/features/timezones/">International Time Zones and Time Zone Data</a>.</li> <li>A ship within the <a href="http://en.wikipedia.org/wiki/Territorial_waters">territorial waters</a> of any nation uses that nation's time. In international waters, time zone boundaries are meridians 15° apart, except that UTC−12 and UTC+12 are each 7.5° wide and are separated by the 180° meridian (not by the International Date Line, which is for land and territorial waters only). A captain can change ship's clocks any time after entering a new time zone; midnight changes are common.</li> </ul> <h2>Civil time concepts and history</h2> <ul> <li><a href="http://physics.nist.gov/GenInt/Time/time.html">A Walk through Time</a> surveys the evolution of timekeeping.</li> <li><a href="http://webexhibits.org/daylightsaving/">About Daylight Saving Time - History, rationale, laws & dates</a> is an overall history of <abbr>DST</abbr>.</li> <li><a href="http://energy.ca.gov/daylightsaving.html">Saving Time, Saving Energy</a> discusses a primary justification for <abbr>DST</abbr>.</li> <li><a href="http://seizethedaylight.com/dst/">Who Knew? A Brief History of Daylight Saving Time</a> summarizes some of the contentious history of <abbr>DST</abbr>.</li> <li><a href="http://toi.iriti.cnr.it/">The Time of Internet</a> describes time zones and daylight saving time, with diagrams. The time zone map is out of date, however.</li> <li><a href="http://www.phys.uu.nl/~vgent/idl/idl.htm">A History of the International Date Line</a> tells the story of the most important time zone boundary.</li> <li><a href="http://statoids.com/tconcept.html">Basic Time Zone Concepts</a> discusses terminological issues behind time zones.</li> </ul> <h2>National histories of legal time</h2> <dl> <dt>Australia</dt> <dd>The Parliamentary Library has commissioned <a href="http://www.aph.gov.au/LIBRARY/Pubs/rn/2006-07/07rn13.pdf">research note on daylight saving time in Australia</a>. The Bureau of Meteorology publishes a list of <a href="http://www.bom.gov.au/climate/averages/tables/dst_times.shtml">Implementation Dates of Daylight Savings Time within Australia</a>.</dd> <dt>Belgium</dt> <dd>The Royal Observatory of Belgium maintains a table of <a href="http://www.astro.oma.be/GENERAL/INFO/nli001a.html" hreflang="nl">time in Belgium (in Dutch)</a>.</dd> <dt>Brazil</dt> <dd>The Time Service Department of the National Observatory records <a href="http://pcdsh01.on.br/DecHV.html" hreflang="pt-BR">Brazil's daylight saving time decrees (in Portuguese)</a>.</dd> <dt>Canada</dt> <dd>The Institute for National Measurement Standards publishes current and some older information about <a href="http://inms-ienm.nrc-cnrc.gc.ca/time_services/daylight_saving_e.html">Time Zones & Daylight Saving Time</a>.</dd> <dt>Chile</dt> <dd>The Chilean Hydrographic and Oceanographic Service publishes a <a href="http://www.horaoficial.cl/horaof.htm" hreflang="es"> history of official time (in Spanish)</a>.</dd> <dt>Germany</dt> <dd>The National Institute for Science and Technology maintains the <a href="http://www.ptb.de/en/org/4/44/441/dars_e.htm">Realisation of Legal Time in Germany</a>.</dd> <dt>Israel</dt> <dd>The Interior Ministry periodically issues <a href="ftp://ftp.cs.huji.ac.il/pub/tz/announcements" hreflang="he">announcements (in Hebrew)</a>.</dd> <dt>Mexico</dt> <dd>The Investigation and Analysis Service of the Mexican Library of Congress has published a <a href="http://www.cddhcu.gob.mx/bibliot/publica/inveyana/polisoc/horver/" hreflang="es">history of Mexican local time (in Spanish)</a>.</dd> <dt>Malaysia</dt> <dd>See Singapore below.</dd> <dt>Netherlands</dt> <dd><a href="http://www.phys.uu.nl/~vgent/wettijd/wettijd.htm" hreflang="nl">Legal time in the Netherlands (in Dutch)</a> covers the history of local time in the Netherlands from ancient times.</dd> <dt>New Zealand</dt> <dd>The Department of Internal Affairs maintains a brief history <a href="http://dia.govt.nz/diawebsite.nsf/wpg_URL/Resource-material-Information-We-Provide-About-Daylight-Saving">About Daylight Saving</a>. The privately-maintained <a href="http://astrologyschool.com/nztime.html">History of New Zealand time</a> has more details.</dd> <dt>Norway</dt> <dd>The Norwegian Meteorological Institute lists <a href="http://met.no/met/met_lex/q_u/sommertid.html" hreflang="no">Summer time in Norway (in Norwegian)</a>, citing the Institute of Theoretical Astrophysics, Oslo.</dd> <dt>Singapore</dt> <dd><a href="http://www.math.nus.edu.sg/aslaksen/teaching/timezone.html">Why is Singapore in the "Wrong" Time Zone?</a> details the history of legal time in Singapore and Malaysia.</dd> <dt>United Kingdom</dt> <dd><a href="http://www.srcf.ucam.org/~jsm28/british-time/">History of legal time in Britain</a> discusses in detail the country with perhaps the best-documented history of clock adjustments. The National Physical Laboratory also maintains an <a href="http://www.npl.co.uk/server.php?show=ConWebDoc.2714">Archive of Summer time dates</a>.</dd> </dl> <h2>Precision timekeeping</h2> <ul> <li><a href="http://literature.agilent.com/litweb/pdf/5965-7984E.pdf">The Science of Timekeeping</a> is a thorough introduction to the theory and practice of precision timekeeping.</li> <li><a href="http://www.ntp.org/"><abbr title="Network Time Protocol">NTP</abbr>: The Network Time Protocol</a> discusses how to synchronize clocks of Internet hosts.</li> <li><a href="ftp://ftp.rfc-editor.org/in-notes/rfc4833.txt">Timezone Options for <abbr title="Dynamic Host Configuration Protocol">DHCP</abbr></a> (Internet <abbr>RFC</abbr> 4833) specifies a <a href="http://www.dhcp.org/">DHCP</a> option for a server to configure a client's time zone and daylight saving settings automatically.</li> <li><a href="http://gauss.gge.unb.ca/GMT.UT.and.the.RGO.html">A Few Facts Concerning <abbr title="Greenwich Mean Time">GMT</abbr>, <abbr title="Universal Time">UT</abbr>, and the <abbr title="Royal Greenwich Observatory">RGO</abbr></a> answers questions like "What is the difference between <abbr>GMT</abbr> and <abbr>UTC</abbr>?"</li> <li><a href="http://www.cv.nrao.edu/~rfisher/Ephemerides/times.html">Astronomical Times</a> explains more abstruse astronomical time scales like <abbr title="Terrestrial Dynamic Time">TDT</abbr>, <abbr title="Geocentric Coordinate Time">TCG</abbr>, and <abbr title="Barycentric Dynamic Time">TDB</abbr>. <a href="http://www.ucolick.org/~sla/leapsecs/timescales.html">Time Scales</a> goes into more detail, particularly for historical variants.</li> <li>The <a href="http://iau.org/"><abbr title="International Astronomical Union">IAU</abbr></a>'s <a href="http://www.iau-sofa.rl.ac.uk/"><abbr title="Standards Of Fundamental Astronomy">SOFA</abbr></a> initiative publishes Fortran code for converting among time scales like <abbr title="International Atomic Time">TAI</abbr>, <abbr>TDB</abbr>, <abbr>TDT</abbr> and <abbr>UTC</abbr>.</li> <li><a href="http://jpl.nasa.gov/basics/bsf2-3.htm">Basics of Space Flight - Reference Systems - Time Conventions</a> briefly explains interplanetary space flight timekeeping.</li> <li><a href="http://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a> briefly describes Mars Coordinated Time (<abbr title="Mars Coordinated Time">MTC</abbr>) and the diverse local time scales used by each landed mission on Mars.</li> <li><a href="http://leapsecond.com/">LeapSecond.com</a> is dedicated not only to leap seconds but to precise time and frequency in general. It covers the state of the art in amateur timekeeping, and how the art has progressed over the past few decades.</li> <li><a href="http://hpiers.obspm.fr/eop-pc/products/bulletins/bulletins.html">Bulletins maintained by the <abbr title="International Earth Rotation Service">IERS</abbr> <abbr title="Earth Orientation Parameters">EOP</abbr> (<abbr title="Product Center">PC</abbr>)</a> contains official publications of the Earth Orientation Parameters Product Center of the International Earth Rotation Service, the committee that decides when leap seconds occur.</li> <li>The <a href="http://six.pairlist.net/mailman/listinfo/leapsecs">Leap Second Discussion List</a> covers <a href="http://gauss.gge.unb.ca/papers.pdf/gpsworld.november99.pdf">McCarthy and Klepczynski's proposal to discontinue leap seconds</a>, discussed further in <a href="http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf">The leap second: its history and possible future</a>. The (now disbanded) <a href="http://members.aas.org/comms/leap.cfm"><abbr title="American Astronomical Society">AAS</abbr> Leap Second Committee</a> has solicited input on this proposal. <a href="http://www.ucolick.org/~sla/leapsecs/">The Future of Leap Seconds</a> covers this contentious issue.</li> </ul> <h2>Time notation</h2> <ul> <li> <a href="http://www.cl.cam.ac.uk/~mgk25/iso-time.html">A Summary of the International Standard Date and Time Notation</a> is a good summary of <a href="http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=40874"><abbr title="International Organization for Standardization">ISO</abbr> 8601:2004 -- Data elements and interchange formats -- Information interchange -- Representation of dates and times</a>.</li> <li> <a href="http://www.w3.org/TR/xmlschema-2/#dateTime"><abbr>XML</abbr> Schema: Datatypes - dateTime</a> specifies a format inspired by <abbr>ISO</abbr> 8601 that is in common use in XML data.</li> <li> <a href="ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt">Internet Message Format</a> (Internet <abbr>RFC</abbr> 2822) §3.3 specifies the time notation used in email and <a href="ftp://ftp.rfc-editor.org/in-notes/rfc2616.txt"><abbr>HTTP</abbr></a> headers.</li> <li> <a href="ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt">Date and Time on the Internet: Timestamps</a> (Internet <abbr>RFC</abbr> 3339) specifies an <abbr>ISO</abbr> 8601 profile for use in new Internet protocols.</li> <li> <a href="http://www.hackcraft.net/web/datetime/">Date & Time Formats on the Web</a> surveys web- and Internet-oriented date and time formats.</li> <li> <a href="http://exit109.com/~ghealton/y2k/yrexamples.html">The Best of Dates, the Worst of Dates</a> covers many problems encountered by software developers when handling dates and time stamps.</li> <li>The <a href="http://unicode.org/cldr/">Unicode Common Locale Data Repository (<abbr>CLDR</abbr>) Project</a> has localizations for time zone names, abbreviations, identifiers, and formats. For example, it contains French translations for "Eastern European Summer Time", "<abbr title="Eastern European Summer Time">EEST</abbr>", and "Bucharest". <a href="http://unicode.org/cldr/data/charts/by_type/names.metazone.html">By-Type Chart: names.metazone</a> shows these values for many locales. <abbr>ICU</abbr> contains a mechanism for using this data.</li> <li>Alphabetic time zone abbreviations should not be used as unique identifiers for <abbr>UTC</abbr> offsets as they are ambiguous in practice. For example, "<abbr>EST</abbr>" denotes 5 hours behind <abbr>UTC</abbr> in English-speaking North America, but it denotes 10 or 11 hours ahead of <abbr>UTC</abbr> in Australia; and French-speaking North Americans prefer "<abbr title="Heure Normale de l'Est">HNE</abbr>" to "<abbr>EST</abbr>". For <abbr>POSIX</abbr> the <code>tz</code> database contains English abbreviations for all time stamps but in many cases these are merely inventions of the database maintainers.</li> <li>Numeric time zone abbreviations typically count hours east of <abbr>UTC</abbr>, e.g., <code>+09</code> for Japan and <code>-10</code> for Hawaii. However, the <abbr>POSIX</abbr> <code>TZ</code> environment variable uses the opposite convention. For example, one might use <code>TZ="JST-9"</code> and <code>TZ="HST10"</code> for Japan and Hawaii, respectively. If the <code>tz</code> database is available, it is usually better to use settings like <code>TZ="Asia/Tokyo"</code> and <code>TZ="Pacific/Honolulu"</code> instead, as this should avoid confusion, handle old time stamps better, and insulate you better from any future changes to the rules. One should never set <abbr>POSIX</abbr> <code>TZ</code> to a value like <code>"GMT-9"</code>, though, since this would falsely claim that local time is nine hours ahead of <abbr>UTC</abbr> and the time zone is called "<abbr>GMT</abbr>".</li> </ul> <h2>Related indexes</h2> <ul> <li><a href="http://www.dmoz.org/Reference/Time/">Open Directory - Reference: Time</a></li> <li><a href="http://directory.google.com/Top/Reference/Time/">Google Directory - Reference > Time</a></li> <li><a href="http://dir.yahoo.com/Science/Measurements_and_Units/Time">Yahoo! Directory > Science > Measurements and Units > Time</a></li> </ul> </body> </html>