GMT is the standard time zone in Ireland and the United Kingdom, including England, Wales, Scotland, and Northern Ireland. All of these countries use DST during part of the year, but under different names. The United Kingdom and its Crown dependencies use British Summer Time during the DST period. GMT and Daylight Saving Time. Believe it or not, neither the GMT time zone nor the UTC standard are affected by Daylight Saving Time or DST. To account for this, some countries in the GMT time zone actually switch to another zone during Daylight Saving. For instance, during summer months the UK switches from GMT to British Summer Time or BST. Daylight Saving Time or summer time, -except Lord Howe Island- it is 1h. More with respect to the standard o local time zone. Winter Time or Standard local time UTC/ GMT, is the one used the rest of the year, when there is no DST summer time. Do not use daylight saving time throughout the year. (No Daylight Saving Time). Daylight saving time (DST), also known as daylight savings time or daylight time (the United States and Canada), and summer time (United Kingdom, European Union, and some others), is the practice of advancing clocks (typically by one hour) during warmer months so that darkness falls at a later clock time. The history of daylight saving time is surveyed in About Daylight Saving Time – History, rationale, laws & dates and summarized in A Brief History of Daylight Saving Time. Time Lords discusses how authoritarians manipulate civil time. Working with Time Zones contains guidelines and best practices for software applications that deal with civil.
Central Daylight Time is 5 hours behind of Greenwich Mean Time
9:00 pm21:00 in CDT is 2:00 am02:00 in GMT
CST to GMT call time
Best time for a conference call or a meeting is between 8am-12pm in CST which corresponds to 2pm-6pm in GMT
9:00 pm21:00 Central Daylight Time (CDT). Offset UTC -5:00 hours
2:00 am02:00 Greenwich Mean Time (GMT). Offset UTC 0:00 hours
9:00 pm21:00 CDT / 2:00 am02:00 GMT
Central Standard Time
Offset: CST is 6 hours behind Greenwich Mean Time (GMT) and is used in North America
Countries: It is used in following countries: Belize, Canada, Costa Rica, Guatemala, Honduras, Mexico, Nicaragua, El Salvador, United States
Principal Cities: The largest city in the CST timezone is Mexico City from Mexico with population about 12.294 million people. Other major cities in the area are Chicago, Houston, Iztapalapa, Ecatepec
Daylight Saving: This is a standard timezone, however during summer some places adjust time for one hour forward for daylight saving and observe Central Daylight Time (CDT).
End: CST ended and clocks were set one hour forward on Sunday, 14 March 2021, at 2:00 (2:00 am) local time.
Start: CST starts and clocks are set one hour back on Sunday, 07 November 2021, at 2:00 (2:00 am) local time.
French: HNC - Heure Normale du Centre
Spanish: CT - Tiempo Central Estándar, CT - Zona Centro
CST representations, usage and related time zonesW3C/ISO-8601: International standard covering representation and exchange of dates and time-related data
- -06 - basic short
- -0600 - basic
- -06:00 - extended
- -0600 - sign character (-) followed by a four digit time providing hours (06) and minutes (00) of the offset. Indicates six hour and zero minutes time differences to the west of the zero meridian.
- Sierra - Military abbreviation for CST
- S - short form of 'Sierra'
- Central Standard Time - UTC -6
- China Standard Time - UTC +8
- Cuba Standard Time - UTC -5
- CST - Central Standard Time
- CT - Central Time
- MDT - Mountain Daylight Time
- EAST - Easter Island Standard Time
- GALT - Galapagos Time
- S - Sierra Time Zone
- -06 -
Greenwich Mean Time
Offset: GMT is 0 hours ahead Greenwich Mean Time (GMT) and is used in Europe
Countries: It is used in following countries: Burkina Faso, Côte d'Ivoire, Britain (UK), Guernsey, Ghana, Greenland, Gambia, Guinea, Guinea-Bissau, Ireland, Isle of Man, Iceland, Jersey, Liberia, Mali, Mauritania, St Helena, Sierra Leone, Senegal, Sao Tome & Principe, Togo
Principal Cities: The largest city in the GMT timezone is London from United Kingdom with population about 7.557 million people. Other major cities in the area are Abidjan, Dakar, Grand Dakar, Accra
Greenwich Mean Time (GMT) originally referred to the mean solar time at the Royal Observatory in Greenwich, London. It is now often used to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone, and in casual use for the most parts it is the same. In military GMT is known as Zulu time.Greenwich Mean Time was the same as Universal Time (UT), until the introduction of UTC in 1972 a standard astronomical concept used in many technical fields.
GMT representations, usage and related time zonesW3C/ISO-8601: International standard covering representation and exchange of dates and time-related data
- Z - is the zone designator for the zero UTC/GMT offset, also known as 'Zulu' time
- +00 - basic short
- +0000 - basic
- +00:00 - extended
- +0000 - sign character (+) followed by a four digit time providing hours (00) and minutes (00) of the offset. Indicates zero hour and zero minutes time differences of the zero meridian.
- Zulu - Military abbreviation for GMT
- Z - short form of 'Zulu'
- Greenwich Mean Time - UTC +0
- GMT+04:00 - UTC +4
- GMT-05:00 - UTC -5
- GMT - UTC +0
- GMT+13:00 - UTC +13
- GMT-03:00 - UTC -3
- EGST - Eastern Greenland Summer Time
- GMT - Greenwich Mean Time
- WET - Western European Time
- AZOST - Azores Summer Time
- UTC - Universal Time Coordinated
- WT - Western Sahara Standard Time
- Z - Zulu Time Zone
- GMT - GMT
- +00 -
Time zone anddaylight-savingrules are controlled by individualgovernments. They are sometimes changed with little notice, and theirhistories and planned futures are often recorded only fitfully. Hereis a summary of attempts to organize and record relevant data in thisarea.
The public-domaintime zone database contains code and datathat represent the history of local timefor many representative locations around the globe.It is updated periodically to reflect changes made by political bodiesto time zone boundaries and daylight saving rules.This database (known as
zoneinfo)is used by several implementations,includingtheGNUC Library (used inGNU/Linux),Android,FreeBSD,NetBSD,OpenBSD,Chromium OS,Cygwin,MariaDB,MINIX,MySQL,webOS,AIX,BlackBerry 10,iOS,macOS,Microsoft Windows,OpenVMS,Oracle Database, andOracle Solaris.
Each main entry in the database represents a timezonefor a set of civil-time clocks that have all agreed since 1970.Timezones are typically identified by continent or ocean and then by thename of the largest city within the region containing the clocks.For example,
America/New_Yorkrepresents most of the US eastern time zone;
America/Phoenix represents most of Arizona, whichuses mountain time without daylight saving time (DST);
America/Detroit represents most of Michigan, which useseastern time but with different DST rules in 1975;and other entries represent smaller regions like Starke County,Indiana, which switched from central to eastern time in 1991and switched back in 2006.To use the database on an extended POSIXimplementation set the
TZenvironment variable to the location's full name,e.g.,
Associated with each timezone is a history of offsets fromUniversalTime (UT), which is Greenwich MeanTime (GMT) with days beginning at midnight;for timestamps after 1960 this is more precisely CoordinatedUniversal Time (UTC).The database also records when daylight saving time was in use,along with some time zone abbreviations such as ESTfor Eastern Standard Time in the US.
The following shell commands downloadthe latest release's twotarballsto a GNU/Linux or similar host.
Alternatively, the following shell commands download the samerelease in a single-tarball format containing extra datauseful for regression testing:
These commands use convenience links to the latest releaseof the
tz database hosted by theTime Zone Database websiteof the Internet Assigned NumbersAuthority (IANA).Older releases are in files named
V is the version.Since 1996, each version has been a four-digit year followed bylower-case letter (a through z,then za through zz, then zzathrough zzz, and so on).Since version 2016h, each release has contained a text file named'version' whose first (and currently only) line is the version.Older releases are archived,and are also available in anFTP directory via aless-secure protocol.
Alternatively, a development repository of code and data can beretrieved from GitHub via the shellcommand:
Since version 2012e, each release has been tagged in development repositories.Untagged commits are less well tested and probably containmore errors.
After obtaining the code and data files, see the
README file for what to do next.The code lets you compile the
tz source files intomachine-readable binary files, one for each location. The binary filesare in a special timezone information format (TZif)specified by InternetRFC 8536.The code also letsyou read a TZif file and interpret timestamps for thatlocation.
Changes to the
tz code and dataare by no means authoritative. If you find errors, pleasesend changes to
[email protected],the time zone mailing list. You can also subscribe to itand browse the archive of oldmessages.Metadata for mailing listdiscussions and corresponding data changes can begenerated automatically.
If your government plans to change its time zone boundaries ordaylight saving rules, inform
[email protected] well inadvance, as this will coordinate updates to many cell phones,computers, and other devices around the world.The change should be officially announced at least a year before it affectshow clocks operate; otherwise, there is a good chance that someclocks will operate incorrectly after the change, dueto delays in propagating updates to software and data. The shorterthe notice, the more likely clock problems will arise; see 'Onthe Timing of Time Zone Changes' for examples.The
tz data can represent planned changesfar into the future, and a long-planned change can easily be revertedor otherwise altered with a year's notice before the change would haveaffected clocks.
Changes to the
tz code and data are oftenpropagated to clients via operating system updates, soclient
tz data can often be corrected byapplying these updates. With GNU/Linux and similar systems, if yourmaintenance provider has not yet adopted thelatest
tz data, you can often short-circuitthe process by tailoring the generic instructions inthe
tz README file and installing the latestdata yourself. System-specific instructions for installing thelatest
tz data have also been publishedfor AIX,Android,ICU,IBMJDK,Joda-Time, MySQL,Noda Time, and OpenJDK/Oracle JDK.
Sources for the
tz database areUTF-8text fileswith lines terminated by LF,which can be modified by common text editors suchas GNU Emacs,gedit, andvim.Specialized source-file editing can be done via theSublimezoneinfo package for Sublime Text and the VSCodezoneinfo extension for VisualStudio Code.
For further information about updates, please seeProcedures forMaintaining the Time Zone Database (Internet RFC 6557). More detail can befound in Theory and pragmatics of the
tz code and data.A0 TimeZone Migrationdisplays changes between recent
Commentary on the
- The articletz database isan encyclopedic summary.
- How to Read thetz Database Source Files explains the
- Aliterary appreciation of the Olson/Zoneinfo/tz database comments on thedatabase's style.
Web sites using recent versions of the
These are listed roughly in ascending order of complexity and fanciness.
- Time.is shows locations'time and zones.
- TimeJones.com,Time Zone Converter andThe World Clockare time zone converters.
- TimeZoneDB Databasepublishes
tzdb-derived data inCSV andin SQL form.
- Date and Time Gatewaylets you see the
- CurrentTime in 1000 Places uses descriptions of the values.
- Time Zone Converteruses a pulldown menu.
- Completetimezone information for all countries displays tables of DST rules.
- The World Clock –Worldwide lets you sort zone names and convert times.
- 24TimeZones has a worldtime map and a time converter.
- Time Differencecalculates the current time difference between locations.
- Weather Now andThe Time Now list the weather too.
Network protocols for
- The Internet Engineering Task Force'sTime Zone DataDistribution Service (tzdist) working group defined TZDIST(Internet RFC 7808), a time zone data distribution service,along with CalDAV(Internet RFC 7809), a calendar access protocol fortransferring time zone data by reference.TZDISTimplementations are available.The tzdist-bismailing list discusses possible extensions.
- The Internet Calendaring and Scheduling Core Object Specification(iCalendar) (Internet RFC 5445)covers time zonedata; see its VTIMEZONE calendar component.The iCalendar format requires specialized parsers and generators; avariant xCal(Internet RFC 6321) usesXML format, and a variantjCal(Internet RFC 7265)uses JSON format.
Although some of these do not fully support
tz data, in recent
tzdbdistributions you can generally work around compatibility problems byrunning the command
make rearguard_tarballs and compilingfrom the resulting tarballs instead.
- Vzic is a Cprogram that compiles
tzsource into iCalendar-compatible VTIMEZONE files.Vzic is freelyavailable under the GNUGeneral Public License (GPL).
- tziCal – tzdatabase conversion utility is like Vzic, except for the .NET frameworkand with a BSD-style license.
- DateTime::TimeZonecontains a script
tzsource into Perlmodules. It is part of the Perl DateTime Project,which is freelyavailable under both the GPL and the Perl ArtisticLicense. DateTime::TimeZone also contains a script
tests_from_zdumpthat generates test cases for each clocktransition in the
- The Time ZoneDatabase Parser is aC++ parser andruntime library with APIadopted byC++20,the current iteration of the C++ standard.It is freely available under theMIT license.
- International Components forUnicode (ICU) contains C/C++ and Javalibraries for internationalization thathas a compiler from
tzsourceand from CLDR data(mentioned below)into an ICU-specific format.ICU is freely available under aBSD-style license.
- The Tzdata package forthe Elixir language downloadsand compiles
tzsource and exposes APIs for use. It isfreely available under the MIT license.
- Java-based compilers and libraries include:
- The TZUpdatertool compiles
tzsource into the format used byOpenJDK andOracle JDK.Although its source code is proprietary, its executable is available under theJava SETimezone Updater License Agreement.
- The JavaSE 8 Date and TimeAPI can be supplemented by ThreeTen-Extra,which is freely available under a BSD-style license.
- Joda-Time – Java dateand time API contains a class
tzsource into a binary format. It inspiredJava 8
java.time, which its users should migrate to oncethey can assume Java 8 or later. It is available under the Apache License.
- IANA Updater and ZIUpdaterare alternatives to TZUpdater. IANA Updater's license is unclear;ZIUpdater is licensed under the GPL.
- Time4A: Advanced date andtime library for Android andTime4J: Advanced date,time and interval library for Java compile
tzsource into a binary format.Time4A is available under the Apache License and Time4J isavailable under the GNU LesserGeneral Public License (LGPL).
- ICU (mentioned above) contains compilers andJava-based libraries.
- The TZUpdatertool compiles
- Noda Time – Date andtime API for .NETis like Joda-Time and Time4J, but for the .NET framework instead of Java.It is freely available under the Apache License.
tznatively via thetimeZone option of Intl.DateTimeFormat.On other runtimes, you can use one of the following.
- The Intl.DateTimeFormattimezone polyfillis freely available under a BSD-style license.
- Moment Timezone is aplugin for the Moment.js datemanipulation library. It is freely available under the MITlicense.
- JuliaTime contains acompiler from
tzsource intoJulia. It is freely availableunder the MIT license.
- TZDB –IANA Time Zone Database for Delphi/FPCcompiles from
tzsource intoObject Pascalas compiled by Delphiand FPC.It is freely available under a BSD-style license.
- pytz – World TimezoneDefinitions for Python compiles
tzsource intoPython.It is freely available under a BSD-style license.In code that can assume Python 3.9 or later it is superseded by
- TZInfo –Ruby Timezone Librarycompiles
tzsource intoRuby.It is freely available under the MIT license.
- The Chronos Date/TimeLibrary isa Smalltalk classlibrary that compiles
tzsource into a timezone repository whose formatis either proprietary or an XML-encodedrepresentation.
- Tclcontains a developer-oriented parser that compiles
tzsource into text files, along with a runtime that can read thosefiles. Tcl is freely available under a BSD-stylelicense.
Other TZif readers
Gmt Time Daylight Savings
- The GNU CLibraryhas an independent, thread-safe implementation ofa TZif file reader.This library is freely available under the LGPLand is widely used in GNU/Linux systems.
- GNOME'sGLib hasa TZif file reader written in C thatcreates a
GTimeZoneobject representing setsof UT offsets.It is freely available under the LGPL.
- TheBDE Standard Library's
baltzo::TimeZoneUtilcomponent contains a C++implementation of a TZif file reader. It is freely available underthe Apache License.
- CCTZ is a simple C++library that translates between UT and civil time andcan read TZif files. It is freely available under the ApacheLicense.
- ZoneInfo.javais a TZif file reader written in Java.It is freely available under the LGPL.
- Timelib is a Clibrary that reads TZif files and convertstimestamps from one time zone or format to another.It is used by PHP,HHVM,and MongoDB.It is freely available under the MIT license.
- Tcl, mentioned above, also contains aTZif file reader.
- DateTime::TimeZone::Tzfileis a TZif file reader written in Perl.It is freely available under the same terms as Perl(dual GPL and Artistic license).
- Python has a
zoneinfo.ZoneInfoclass that reads TZif data and creates objectsthat represent
tzdbtimezones.Python is freely available under thePython Software FoundationLicense.A companion PyPI module
tzdatasupplies TZif data if the underlying system data cannot be found;it is freely available under the Apache License.
- The timezone-olsonpackage contains Haskell code thatparses and uses TZif data. It is freelyavailable under a BSD-style license.
tz-based time zone software
- FoxClocksis an extension for GoogleChrome and for MozillaToolkit applications like Firefox and Thunderbird.It displays multiple clocks in the application window, and has a mappinginterface to Google Earth.It is freely available under the GPL.
- Go programming languageimplementations contain a copy of a 32-bit subset of a recent
tzdatabase in aGo-specific format.
- Microsoft Windows 8.1and later has
tzdata and CLDRdata (mentioned below) used by theWindows Runtime /Universal Windows Platform classes
Calendar.ExploringWindows Time Zones with
System.TimeZoneInfodescribesthe older, proprietary method of Microsoft Windows 2000 and later,which stores time zone data in theWindows Registry. TheZone →Tzid table or XMLfile of the CLDR data maps proprietary zone IDsto
tznames.These mappings can be performed programmatically via the TimeZoneConverter .NET library,or the ICU Java and C++ libraries mentioned above.
- OracleJava contains a copy of a subset of a recent
tzdatabase in aJava-specific format.
- Time ZoneMaster is a Microsoft Windows clock program that can automaticallydownload, compile and use
tzreleases.The Basic version is free.
- VelaTerra isa macOS program. Its developersoffer freelicenses to
Other time zone databases
- Time-zone Atlasis Astrodienst's Web version of Shanks and Pottenger's out-of-printtime zone history atlasesfor the US andfor the world.Although these extensive atlasesweresources for much of the older
tzdata,they are unreliable as Shanks appears to haveguessed many UT offsets and transitions. The atlases cite nosources and do not indicate which entries are guesswork.
- HP-UX has a database inits own
- Microsoft Windows has proprietary data mentionedabove.
- World Time Serveris another time zone database.
- The StandardSchedules Information Manual of theInternational Air Transport Associationgives current time zone rules for airports served by commercial aviation.
Does Gmt Change
- The Worldand Regional Maps section of The World Factbook, published by theUS Central IntelligenceAgency (CIA), contains a timezone map; thePerry–CastañedaLibrary Map Collectionof the University of Texas at Austin has copies ofrecent editions.The pictorial quality is good,but the maps do not indicate daylight saving time,and parts of the data are a few years out of date.
- World Time Zone Mapwith current timehas several fancy time zone maps; it covers Russia particularly well.The maps' pictorial quality is not quite as good as theCIA'sbut the maps are more up to date.
- Howmuch is time wrong around the world? maps the difference betweenmean solar and standard time, highlighting areas such as western Chinawhere the two differ greatly. It's a bit out of date, unfortunately.
Time zone boundaries
Geographical boundaries between timezones are availablefrom several Internetgeolocationservices and other sources.
- TimezoneBoundary Builder extractsOpen Street Map data to buildboundaries of
tzdbtimezones.Its code is freely available under the MIT license, andits data entries are freely available under theOpen Data CommonsOpen Database License. The maps' borders appear to be quite accurate.
- Programmatic interfaces that map geographical coordinates via tz_world to
- GeoTimeZone iswritten in C#and is freely available under the MIT license.
- The latlong packageis written in Go and is freely available under the Apache License.
- LatLongToTimezone,in both Java andSwiftform, is freely available under the MIT license.
- For Node.js,the geo-tz moduleis freely available under the MIT license, andthe tz-lookup moduleis in the public domain.
- The timezonefinderlibrary for Python is freely available under the MIT license.
- The timezone_finderlibrary for Ruby is freely available under the MIT license.
- Free access via a network API, if you register a key, is provided bythe GeoNamesTimezone web service, the GoogleMaps Time Zone API, andthe TimeZoneDB API.Commercial network API access is providedby AskGeoand GeoGarage.
- 'Howto get a time zone from a location using latitude and longitudecoordinates?' discusses other geolocation possibilities.
- AdministrativeDivisions of Countries ('Statoids') listspolitical subdivision data related to time zones.
- Timezone boundaries for multizone countries summarizes legalboundaries between time zones within countries.
- Manifold Software– GIS and Database Tools includes a Manifold-format map ofworld time zone boundaries distributed under theGPL.
- A ship within the territorialwaters of any nation uses that nation's time. In internationalwaters, time zone boundaries are meridians 15° apart, except thatUT−12 and UT+12 are each 7.5°wide and are separated bythe 180° meridian (not by the International Date Line, which isfor land and territorial waters only). A captain can change ship'sclocks any time after entering a new time zone; midnight changes arecommon.
Civil time concepts and history
- AWalk through Timesurveys the evolution of timekeeping.
- The history of daylight saving time is surveyed in About DaylightSaving Time – History, rationale, laws & dates and summarized inA BriefHistory of Daylight Saving Time.
- TimeLords discusses how authoritarians manipulate civil time.
- Working with Time Zonescontains guidelines and best practices for software applications thatdeal with civil time.
- A History ofthe International Date Line tells the story of the most importanttime zone boundary.
- Basic TimeZone Concepts discusses terminological issues behind time zones.
National histories of legal time
- The Parliamentary Library commissioned a researchpaper on daylight saving time in Australia.The Bureau of Meteorology publishes a list of ImplementationDates of Daylight Savings Time within Australia.
- The Royal Observatory of Belgium maintains a table of time inBelgium (inDutch and French).
- The Time Service Department of the National Observatoryrecords Brazil's daylight saving time decrees (inPortuguese).
- National Research Council Canada publishes currentand some older information about timezones and daylight saving time.
- The Hydrographic and Oceanographic Service of the Chilean Navy publishes ahistory ofChile's official time (in Spanish).
- The Hong Kong Observatory maintains ahistory of summer time in Hong Kong,and Macau's Meteorological and Geophysical Bureau maintains a similarhistory for Macau.Unfortunately the latter is incomplete and has errors.
- Czech Republic
- When daylight saving time starts and ends (in Czech)summarizes and cites historical DST regulations.
- The National Institute for Science and Technology maintains the Realisationof Legal Time in Germany.
- The Interior Ministry periodically issues announcements (in Hebrew).
- See Singapore below.
- The Investigation and Analysis Service of the Mexican Library ofCongress has published a history of Mexican local time (in Spanish).
- Legal time in the Netherlands (in Dutch)covers the history of local time in the Netherlands from ancient times.
- New Zealand
- The Department of Internal Affairs maintains a brief History ofDaylight Saving.
- Whyis Singapore in the 'Wrong' Time Zone? details thehistory of legal time in Singapore and Malaysia.
- United Kingdom
- History oflegal time in Britain discusses in detail the countrywith perhaps the best-documented history of clock adjustments.
- United States
- The Department of Transportation's RecentTime Zone Proceedings lists changes to time zone boundaries.
- The Oceanography, Hydrography, and Meteorology Service of the UruguayanNavy (SOHMA) publishes an annual almanac(in Spanish).
Costs and benefits of time shifts
Various sources argue for and against daylight saving time and timezone shifts, and many scientific studies have been conducted. Thissection summarizes reviews and position statements based onscientific literature in the area.
- Carey RN, Sarma KM.Impact ofdaylight saving time on road traffic collision risk: a systematicreview.BMJ Open. 2017;7(6):e014319. doi:10.1136/bmjopen-2016-014319.This reviews research literature and concludes that the evidenceneither supports nor refutes road safety benefits fromshifts in time zones.
- Havranek T, Herman D, Irsova D.Does daylightsaving save electricity? A meta-analysis. Energy J.2018;39(2).doi:10.5547/01956574.39.2.thav.This analyzes research literature and concludes, 'Electricity savingsare larger for countries farther away from the equator, whilesubtropical regions consume more electricity because of DST.'
- Rishi MA, Ahmed O, Barrantes Perez JH et al.Daylight saving time:an American Academy of Sleep Medicine position statement.J Clin Sleep Med.2020;10.5664/jcsm.8780.This argues for permanent standard time due to health risks of bothDST transitions and permanent DST.
- Roenneberg T, Winnebeck EC, Klerman EB.Daylightsaving time and artificial time zones – a battle betweenbiological and social times. Front Physiol. 2019;10:944.doi:10.3389/fphys.2019.00944.This reviews evidence about the health effects of DST and concludes,'In summary, the scientific literature strongly argues against theswitching between DST and Standard Time and even more so againstadopting DST permanently.'
Daylight Savings Time In Gmt
- TheScience of Timekeeping is a thorough introductionto the theory and practice of precision timekeeping.
- The Science ofTime 2016 contains several freely-readable papers.
- NTP: The NetworkTime Protocol (Internet RFC 5905)discusses how to synchronize clocks ofInternet hosts.
- The Huygensfamily of software algorithms can achieve accuracy to a few tens ofnanoseconds in scalable server farms without special hardware.
- The PrecisionTime Protocol (IEEE 1588)can achieve submicrosecond clock accuracy on a local area networkwith special-purpose hardware.
- TimezoneOptions for DHCP(Internet RFC 4833)specifies a DHCPoption for a server to configurea client's time zone and daylight saving settings automatically.
- AstronomicalTimes explains more abstruse astronomical time scales likeTDT,TCG, andTDB.TimeScales goes into more detail, particularly for historical variants.
- The IAU's SOFAcollection contains C and Fortrancode for converting among time scales likeTAI,TDB, TDT andUTC.
- Mars24 Sunclock– Time on Mars describes Airy Mean Time (AMT) and thediverse local timescales used by each landed mission on Mars.
- LeapSecond.com isdedicated not only to leap seconds but to precise time and frequencyin general. It covers the state of the art in amateur timekeeping, andhow the art has progressed over the past few decades.
- The rules for leap seconds are specified in Annex 1 (Time scales) of Standard-frequencyand time-signal emissions, International Telecommunication Union –Radiocommunication Sector (ITU-R) Recommendation TF.460-6 (02/2002).
- IERSBulletins contains official publications of the InternationalEarth Rotation and Reference Systems Service, which decides when leapseconds occur. The
tzcode and data support leap secondsvia an optional '
right' configuration where a computer's internal
time_tinteger clock counts every TAI second,as opposed to the default '
posix' configurationwhere the internal clock ignores leap seconds.The two configurations agree for timestamps starting with 1972-01-01 00:00:00UTC (
time_t63 072 000) and diverge fortimestamps starting with
time_t78 796 800,which corresponds to the first leap second1972-06-30 23:59:60 UTC in the '
right' configuration,and to1972-07-01 00:00:00 UTC in the '
posix' configuration.In practice the two configurations also agree for timestamps before1972 even though the historical situation is messy, partly becauseneither UTC nor TAIis well-defined for sufficiently-old timestamps.
- Leap Smeardiscusses how to gradually adjust POSIX clocks near aleap second so that they disagree with UTC by at most ahalf second, even though every POSIX minute has exactlysixty seconds. This approach works with the default
posix' configuration, is supported bythe NTP reference implementation, and is used by majorcloud service providers. However, according to§3.7.1 ofNetwork Time Protocol Best Current Practices(Internet RFC 8633), leap smearing is not suitable forapplications requiring accurate UTC or civil time,and is intended for use only in single, well-controlled environments.
- The LeapSecond Discussion List covers McCarthyand Klepczynski's 1999 proposal to discontinue leap seconds,discussed further inTheleap second: its history and possible future.UTCmight be redefinedwithout Leap Seconds gives pointers on thiscontentious issue, which was active until 2015 and could become activeagain.
Gmt Time Zone Daylight Savings
- The Unicode Common Locale DataRepository (CLDR) Project has localizations for timezone names, abbreviations, identifiers, and formats. For example, itcontains French translations for 'Eastern European Summer Time','EEST', and'Bucharest'. Itsby-typecharts show these values for many locales. Data values are available inboth LDML(an XML format) and JSON.
- A summary ofthe international standard date and time notation coversISO8601-1:2019 – Date and time – Representations for informationinterchange – Part 1: Basic rules.
- XMLSchema: Datatypes – dateTime specifies a format inspired byISO 8601 that is in common use in XML data.
- §3.3 ofInternet Message Format (Internet RFC 5322)specifies the time notation used in email and HTTPheaders.
- Date and Timeon the Internet: Timestamps (Internet RFC 3339)specifies an ISO 8601profile for use in new Internetprotocols.
- Date & TimeFormats on the Web surveys web- and Internet-oriented date and timeformats.
- Alphabetic time zone abbreviations should not be used as uniqueidentifiers for UT offsets as they are ambiguous inpractice. For example, in English-speaking North America'CST' denotes 6 hours behind UT,but in China it denotes 8 hours ahead of UT,and French-speaking North Americans prefer'HNC' to'CST'. The
tzdatabase contains English abbreviations for many timestamps;unfortunately some of these abbreviations were merely the database maintainers'inventions, and these have been removed when possible.
- Numeric time zone abbreviations typically count hours east ofUT, e.g., +09 for Japan and−10 for Hawaii. However, the POSIX
TZenvironment variable uses the opposite convention.For example, one might use
TZ='HST10'for Japan and Hawaii, respectively. If the
tzdatabase is available, it is usually better to usesettings like
TZ='Pacific/Honolulu'instead, as this should avoidconfusion, handle old timestamps better, and insulate you better fromany future changes to the rules. One should never setPOSIX
TZto a value like
'GMT-9', though, since this would incorrectly imply thatlocal time is nine hours ahead of UT and the time zoneis called 'GMT'.