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This article is about time zones in general. For a list of time zones by country, seeList of time zones by country.For more time zone lists, seeLists of time zones.For other uses, seeTime zone (disambiguation).

Time zones of the world

Atime zoneis an area which observes a uniformstandard timeforlegal,commercialandsocialpurposes. Time zones tend to follow the boundaries betweencountriesand theirsubdivisionsinstead of strictly followinglongitude,because it is convenient for areas in frequent communication to keep the same time.

Each time zone is defined by a standard offset fromCoordinated Universal Time(UTC). The offsets range fromUTC−12:00toUTC+14:00,and are usually a whole number of hours, but a few zones are offset by an additional 30 or 45 minutes, such as inIndiaandNepal.Some areas in a time zone may use a different offset for part of the year, typically one hour ahead duringspringandsummer,a practice known asdaylight saving time(DST).

List of UTC offsets

Main article:List of UTC time offsets
Time zones of the world

In the table below, the locations that use daylight saving time (DST) are listed in their UTC offset when DST isnotin effect. When DST is in effect, approximately during spring and summer, their UTC offset is increased by one hour (except forLord Howe Island,where it is increased by 30 minutes). For example, during the DST periodCaliforniaobservesUTC−07:00and theUnited KingdomobservesUTC+01:00.

UTC offset Locations that do not use DST Locations that use DST
UTC−12:00
UTC−11:00
UTC−10:00
Cook Islands
French Polynesia(most)
Johnston Atoll
United States:Hawaii
 United States:Andreanof Islands,Islands of Four Mountains,Near Islands,Rat Islands(Aleutian Islands,Alaska)
UTC−09:30 French Polynesia:Marquesas Islands
UTC−09:00 French Polynesia:Gambier Islands United States:Alaska(most)
UTC−08:00 Clipperton Island
Pitcairn Islands
UTC−07:00 Canada:British Columbia(northeast),Yukon
Mexico:Baja California Sur,Nayarit(most),Sinaloa,Sonora
United States:Arizona(most) 		Canada:Alberta,British Columbia(southeast),Northwest Territories,Nunavut(west),Saskatchewan(Lloydminsterarea)
Mexico:Chihuahua(northwest border)
United States:Arizona(Navajo Nation),Colorado,Idaho(most),Kansas(west),Montana,Nebraska(west),New Mexico,Nevada(northeast border),North Dakota(southwest),Oregon(east),South Dakota(west),Texas(west),Utah,Wyoming
UTC−06:00
UTC−05:00
UTC−04:00 Bermuda
Canada:Labrador(most),New Brunswick,Nova Scotia,Prince Edward Island
Chile(most)
Greenland:Pituffik Space Base
UTC−03:30 Canada:Newfoundland,Labrador(southeast)
UTC−03:00 Saint Pierre and Miquelon
UTC−02:00 Brazil:Fernando de Noronha
South Georgia and the South Sandwich Islands 		Greenland(most)
UTC−01:00 Cape Verde Portugal:Azores
UTC+00:00 Faroe Islands
Guernsey
Ireland
Isle of Man
Jersey
Portugal(most)
Spain:Canary Islands
United Kingdom
UTC+01:00
UTC+02:00
UTC+03:00
UTC+03:30 Iran
UTC+04:00
UTC+04:30 Afghanistan
UTC+05:00
UTC+05:30
UTC+05:45 Nepal
UTC+06:00
UTC+06:30
UTC+07:00
UTC+08:00
UTC+08:45 Australia:Eucla
UTC+09:00
UTC+09:30 Australia:Northern Territory Australia:South Australia,Yancowinna County[5]
UTC+10:00 Australia:Australian Capital Territory,Jervis Bay Territory,New South Wales(most),Tasmania,Victoria
UTC+10:30 Australia:Lord Howe Island(DST increase is 30 minutes)
UTC+11:00 Norfolk Island
UTC+12:00 New Zealand(most)
UTC+12:45 New Zealand:Chatham Islands
UTC+13:00
UTC+14:00 Kiribati:Line Islands

History

World time clock, 2.5 m high. Made inDresden1690. Technical Instrument Museum, Dresden

The apparent position of the Sun in the sky, and thussolar time,varies by location due to thesphericalshape of the Earth. This variation corresponds to fourminutesof time for everydegreeoflongitude,so for example when it is solar noon inLondon,it is about 10 minutes before solar noon inBristol,which is about 2.5 degrees to the west.[6]

TheRoyal Observatory, Greenwich,founded in 1675, establishedGreenwich Mean Time(GMT), the mean solar time at that location, as an aid to mariners to determine longitude at sea, providing a standard reference time while each location inEnglandkept a different time.

Railway time

Main article:Railway time
Plaque commemorating the Railway General Time Convention of 1883 in North America
The control panel of the Time Zone Clock in front ofCoventry Transport Museum

In the 19th century, as transportation and telecommunications improved, it became increasingly inconvenient for each location to observe its own solar time. In November 1840, the BritishGreat Western Railwaystarted using GMT kept by portablechronometers.[7][failed verification]This practice was soon followed by otherrailway companies in Great Britainand became known asrailway time.

Around August 23, 1852, time signals were first transmitted bytelegraphfrom the Royal Observatory. By 1855, 98% of Great Britain's public clocks were using GMT, but it was not made the island's legal time until August 2, 1880. Some British clocks from this period have two minute hands, one for the local time and one for GMT.[8]

On November 2, 1868, the BritishColony of New Zealandofficially adopted a standard time to be observed throughout the colony.[9]It was based on longitude172°30′east ofGreenwich,that is 11 hours 30 minutes ahead of GMT. This standard was known asNew Zealand Mean Time.[10]

1913 time zone map of the United States, showing boundaries very different from today

Timekeeping onNorth Americanrailroadsin the 19th century was complex. Each railroad used its own standard time, usually based on the local time of its headquarters or most important terminus, and the railroad's train schedules were published using its own time. Some junctions served by several railroads had a clock for each railroad, each showing a different time.[11]Because of this a number of accidents occurred when trains from different companies using the same tracks mistimed their passings.[12]

Around 1863,Charles F. Dowdproposed a system of hourly standard time zones for North American railroads, although he published nothing on the matter at that time and did not consult railroad officials until 1869. In 1870 he proposed four ideal time zones having north–south borders, the first centered onWashington, D.C.,but by 1872 the first was centered on meridian75° west of Greenwich,with natural borders such as sections of theAppalachian Mountains.Dowd's system was never accepted by North American railroads. Chief meteorologist at theUnited States Weather BureauCleveland Abbedivided the United States into four standard time zones for consistency among the weather stations. In 1879, he published a paper titledReport on Standard Time.[13]In 1883, he convinced North American railroad companies to adopt his time-zone system. In 1884, Britain, which had already adopted its own standard time system for England, Scotland, and Wales, helped gather international consent for global time. In time, the American government, influenced in part by Abbe's 1879 paper, adopted the time-zone system.[14] It was a version proposed by William F. Allen, the editor of theTraveler's Official Railway Guide.[15]The borders of its time zones ran through railroad stations, often in major cities. For example, the border between its Eastern and Central time zones ran throughDetroit,Buffalo,Pittsburgh,Atlanta,andCharleston.It was inaugurated on Sunday, November 18, 1883, also called "The Day of Two Noons",[16]when each railroad station clock was reset as standard-time noon was reached within each time zone.

The North American zones were named Intercolonial, Eastern, Central, Mountain, and Pacific. Within a year 85% of all cities with populations over 10,000 (about 200 cities) were using standard time.[17]A notable exception was Detroit (located about halfway between the meridians of Eastern and Central time), which kept local time until 1900, then tried Central Standard Time,local mean time,andEastern Standard Time(EST) before a May 1915 ordinance settled on EST and was ratified by popular vote in August 1916. The confusion of times came to an end when standard time zones were formally adopted by theU.S. Congressin theStandard Time Actof March 19, 1918.

Worldwide time zones

"World time" redirects here. For the global time standard, seeUniversal Time.

ItalianmathematicianQuirico Filopantiintroduced the idea of a worldwide system of time zones in his bookMiranda!,published in 1858. He proposed 24 hourly time zones, which he called "longitudinal days", the first centred on the meridian ofRome.He also proposed a universal time to be used in astronomy and telegraphy. However, his book attracted no attention until long after his death.[18][19]

Scottish-bornCanadianSirSandford Flemingproposed a worldwide system of time zones in 1876 - seeSandford Fleming § Inventor of worldwide standard time.The proposal divided the world into twenty-four time zones labeled A-Y (skipping J), each one covering 15 degrees of longitude. All clocks within each zone would be set to the same time as the others, but differed by one hour from those in the neighboring zones.[20]He advocated his system at several international conferences, including theInternational Meridian Conference,where it received some consideration. The system has not been directly adopted, but some maps divide the world into 24 time zones and assign letters to them, similarly to Fleming's system.[21]

World map of time zones in 1928

By about 1900, almost all inhabited places on Earth had adopted a standard time zone, but only some of them used an hourly offset from GMT. Many applied the time at a local astronomical observatory to an entire country, without any reference to GMT. It took many decades before all time zones were based on some standard offset from GMT orCoordinated Universal Time(UTC). By 1929, the majority of countries had adopted hourly time zones, though some countries such asIran,India,Myanmarand parts ofAustraliahad time zones with a 30-minute offset.Nepalwas the last country to adopt a standard offset, shifting slightly to UTC+05:45 in 1986.[22]

All nations currently use standard time zones for secular purposes, but not all of them apply the concept as originally conceived. Several countries and subdivisions use half-hour or quarter-hour deviations from standard time. Some countries, such asChinaandIndia,use a single time zone even though the extent of their territory far exceeds the ideal 15° of longitude for one hour; other countries, such asSpainandArgentina,use standard hour-based offsets, but not necessarily those that would be determined by their geographical location. The consequences, in some areas, can affect the lives of local citizens, and in extreme cases contribute to larger political issues, such as in the western reaches of China.[23]InRussia, which has 11 time zones,two time zones were removed in 2010[24][25]and reinstated in 2014.[26]

Notation

ISO 8601

Main article:ISO 8601

ISO 8601 is a standard established by theInternational Organization for Standardizationdefining methods of representing dates and times in textual form, including specifications for representing time zones.

If a time is inCoordinated Universal Time(UTC), a "Z" is added directly after the time without a separating space. "Z" is the zone designator for the zero UTC offset. "09:30 UTC" is therefore represented as "09:30Z" or "0930Z". Likewise, "14:45:15 UTC" is written as "14:45:15Z" or "144515Z".[27]UTC time is also known as "Zulu" time, since "Zulu" is aphonetic alphabetcode word for the letter "Z".[27]

Offsets from UTCare written in the format ±hh:mm, ±hhmm, or ±hh (either hours ahead or behind UTC). For example, if the time being described is one hour ahead of UTC (such as the time inGermanyduring the winter), the zone designator would be "+01:00","+0100 ", or simply" +01 ". This numeric representation of time zones is appended to local times in the same way that alphabetic time zone abbreviations (or" Z ", as above) are appended. The offset from UTC changes withdaylight saving time,e.g. a time offset inChicago,which is in theNorth American Central Time Zone,is "−06:00"for the winter (Central Standard Time) and"−05:00"for the summer (Central Daylight Time).[28]

Abbreviations

Main article:List of time zone abbreviations

Time zones are often represented by alphabetic abbreviations such as "EST", "WST", and "CST", but these are not part of the international time and date standardISO 8601.Such designations can be ambiguous; for example, "CST" can mean (North American)Central Standard Time(UTC−06:00),Cuba Standard Time(UTC−05:00) andChina Standard Time(UTC+08:00), and it is also a widely used variant of ACST (Australian Central Standard Time,UTC+09:30).[29]

Conversions

Conversion between time zones obeys the relationship

"time in zone A" − "UTC offset for zone A" = "time in zone B" − "UTC offset for zone B",

in which each side of the equation is equivalent to UTC.

The conversion equation can be rearranged to

"time in zone B" = "time in zone A" − "UTC offset for zone A" + "UTC offset for zone B".

For example, theNew York Stock Exchangeopens at 09:30 (EST,UTC offset= −05:00). InCalifornia(PST,UTC offset= −08:00) andIndia(IST,UTC offset= +05:30), the New York Stock Exchange opens at

time in California = 09:30 − (−05:00) + (−08:00) = 06:30;
time in India = 09:30 − (−05:00) + (+05:30) = 20:00.

These calculations become more complicated near the time switch to or from daylight saving time, as the UTC offset for the area becomes a function of UTC time.

The time differences may also result in different dates. For example, when it is 22:00 on Monday inEgypt(UTC+02:00), it is 01:00 on Tuesday inPakistan(UTC+05:00).

The table "Time of day by zone" gives an overview on the time relations between different zones.

Nautical time zones

Main article:Nautical time

Since the 1920s, anautical standard timesystem has been in operation for ships on thehigh seas.As an ideal form of the terrestrial time zone system, nautical time zones consist ofgoresof 15° offset from GMT by a whole number of hours. A nautical date line follows the 180th meridian, bisecting one 15° gore into two 7.5° gores that differ from GMT by ±12 hours.[30][31][32]

However, in practice each ship may choose what time to observe at each location. Ships may decide to adjust their clocks at a convenient time, usually at night, not exactly when they cross a certain longitude.[33]Some ships simply remain on the time of the departing port during the whole trip.[34]

Skewing of time zones

Difference between sun time and clock time during daylight saving time:
1h ± 30 min behind
0h ± 30m
1h ± 30 m ahead
2h ± 30 m ahead
3h ± 30 m ahead
DST observed
DST formerly observed
DST never observed

Ideal time zones, such as nautical time zones, are based on the mean solar time of a particular meridian in the middle of that zone with boundaries located 7.5 degrees east and west of the meridian. In practice, however, many time zone boundaries are drawn much farther to the west, and some countries are located entirely outside their ideal time zones.

For example, even though the Prime Meridian (0°) passes throughSpainandFrance,they use the mean solar time of 15 degrees east (Central European Time) rather than 0 degrees (Greenwich Mean Time). France previously used GMT, but was switched to CET (Central European Time) during theGerman occupationof the country during World War II and did not switch back after the war.[35]Similarly, prior to World War II, the Netherlands observed "Amsterdam Time", which was twenty minutes ahead of Greenwich Mean Time. They were obliged to follow German time during the war, and kept it thereafter. In the mid-1970s the Netherlands, as other European states, began observing daylight saving (summer) time.

One reason to draw time zone boundaries far to the west of their ideal meridians is to allow the more efficient use of afternoon sunlight.[36]Some of these locations also usedaylight saving time(DST), further increasing the difference to local solar time. As a result, in summer, solar noon in the Spanish city ofVigooccurs at 14:41 clock time. This westernmost area of continental Spain never experiences sunset before 18:00 clock time, even in winter, despite lying 42 degrees north of the equator.[37]Near the summersolstice,Vigo has sunset times after 22:00, similar to those ofStockholm,which is in the same time zone and 17 degrees farther north. Stockholm has much earlier sunrises, though.[38]

In the United States, the reasons were more historical and business-related. In Midwestern states, likeIndianaandMichigan,those living inIndianapolisandDetroitwanted to be on the same time zone asNew Yorkto simplify communications and transactions.[39]

A more extreme example isNome, Alaska,which is at 165°24′W longitude – just west of center of the idealizedSamoa Time Zone(165°W). Nevertheless, Nome observesAlaska Time(135°W) with DST so it is slightly more than two hours ahead of the sun in winter and over three in summer.[40] Kotzebue, Alaska,also near the same meridian but north of the Arctic Circle, hastwosunsets on the same day in early August, one shortly after midnight at the start of the day, and the other shortly before midnight at the end of the day.[41]

China extends as far west as 73°E,but all parts of it useUTC+08:00(120°E), so solar "noon" can occur as late as 15:00 in western portions of China such asXinjiang.[42]TheAfghanistan-China bordermarks the greatest terrestrial time zone difference on Earth, with a 3.5 hour difference between Afghanistan's UTC+4:30 and China'sUTC+08:00.

A visualization of the mismatch between clock time and solar time in different locations. In blue areas, clock time lags behind solar time; in red areas, the reverse is true. The two are synchronized in the white areas.

Daylight saving time

Main article:Daylight saving time

Many countries, and sometimes just certain regions of countries, adopt daylight saving time (DST), also known as summer time, during part of the year. This typically involves advancingclocksby anhournear the start ofspringand adjusting back inautumn( "spring forward", "fall back" ). Modern DST was first proposed in 1907 and was in widespread use in 1916 as awartimemeasure aimed atconserving coal.Despitecontroversy,many countries have used it off and on since then;details vary by locationand change occasionally. Countries around the equator usually do not observe daylight saving time, since the seasonal difference in sunlight there is minimal.

Computer systems

Many computeroperating systemsinclude the necessary support for working with all (or almost all) possible local times based on the various time zones. Internally, operating systems typically useUTCas their basictime-keeping standard,while providing services for converting local times to and from UTC, and also the ability to automatically change local time conversions at the start and end of daylight saving time in the various time zones. (See the article ondaylight saving timefor more details on this aspect.)

Web servers presenting web pages primarily for an audience in a single time zone or a limited range of time zones typically show times as a local time, perhaps with UTC time in brackets. More internationally oriented websites may show times in UTC only or using an arbitrary time zone. For example, the international English-language version of CNN includes GMT and Hong Kong Time,[43]whereas the US version showsEastern Time.[44]US Eastern Time andPacific Timeare also used fairly commonly on many US-based English-language websites with global readership. The format is typically based in theW3CNote "datetime".

Emailsystems and other messaging systems (IRC chat,etc.)[45]time-stamp messages using UTC, or else include the sender's time zone as part of the message, allowing the receiving program to display the message's date and time of sending in the recipient's local time.

Databaserecords that include a time stamp typically use UTC, especially when the database is part of a system that spans multiple time zones. The use of local time for time-stamping records is not recommended for time zones that implement daylight saving time because once a year there is a one-hour period when local times are ambiguous.

Calendar systemsnowadays usually tie their time stamps to UTC, and show them differently on computers that are in different time zones. That works when having telephone or internet meetings. It works less well when travelling, because the calendar events are assumed to take place in the time zone the computer or smartphone was on when creating the event. The event can be shown at the wrong time. For example, if a New Yorker plans to meet someone in Los Angeles at 9 am, and makes a calendar entry at 9 am (which the computer assumes is New York time), the calendar entry will be at 6 am if taking the computer's time zone. Calendaring software must also deal withdaylight saving time(DST). If, for political reasons, the begin and end dates of daylight saving time are changed, calendar entries should stay the same in local time, even though they may shift in UTC time.

Operating systems

Unix

Main article:Unix time

Unix-likesystems, includingLinuxandmacOS,keep system time inUnix timeformat, representing the number of seconds (excludingleap seconds) that have elapsed since 00:00:00Coordinated Universal Time(UTC) on Thursday, January 1, 1970.[46]Unix time is usually converted to local time when displayed to the user, and times specified by the user in local time are converted to Unix time. The conversion takes into account the time zone and daylight saving time rules; by default the time zone and daylight saving time rules are set up when the system is configured, though individual processes can specify time zones and daylight saving time rules using theTZenvironment variable.[47]This allows users in multiple time zones, or in the same time zone but with different daylight saving time rules, to use the same computer, with their respective local times displayed correctly to each user. Time zone and daylight saving time rule information most commonly comes from theIANA time zone database.In fact, many systems, including anything using theGNU C Library,a C library based on theBSDC library, or theSystem V Release 4C library, can make use of this database.

Microsoft Windows

Windows-based computer systems prior toWindows 95andWindows NTused local time, but Windows 95 and later, and Windows NT, base system time on UTC.[48][49]They allow a program to fetch the system time as UTC, represented as a year, month, day, hour, minute, second, and millisecond;[50][51]Windows 95 and later, and Windows NT 3.5 and later, also allow the system time to be fetched as a count of 100 ns units since 1601-01-01 00:00:00 UTC.[52][53]Thesystem registrycontains time zone information that includes the offset from UTC and rules that indicate the start and end dates for daylight saving in each zone. Interaction with the user normally uses local time, and application software is able to calculate the time in various zones.Terminal Serversallow remote computers to redirect their time zone settings to the Terminal Server so that users see the correct time for their time zone in their desktop/application sessions. Terminal Services uses the server base time on the Terminal Server and the client time zone information to calculate the time in the session.

Programming languages

Java

While most application software will use the underlying operating system for time zone and daylight saving time rule information,the Java Platform,from version 1.3.1, has maintained its own database of time zone and daylight saving time rule information. This database is updated whenever time zone or daylight saving time rules change.Oracleprovides an updater tool for this purpose.[54]

As an alternative to the information bundled with the Java Platform, programmers may choose to use the Joda-Time library.[55]This library includes its own data based on the IANA time zone database.[56]

As of Java 8 there is a new date and time API that can help with converting times.[57]

JavaScript

Traditionally, there was very little in the way of time zone support forJavaScript.Essentially the programmer had to extract the UTC offset by instantiating a time object, getting a GMT time from it, and differencing the two. This does not provide a solution for more complex daylight saving variations, such as divergent DST directions between northern and southern hemispheres.

ECMA-402, the standard on Internationalization API for JavaScript, provides ways of formatting Time Zones.[58]However, due to size constraint, some implementations or distributions do not include it.[59]

Perl

The DateTime object inPerlsupports all entries in theIANA time zone databaseand includes the ability to get, set and convert between time zones.[60]

PHP

The DateTime objects and related functions have been compiled into thePHPcore since 5.2. This includes the ability to get and set the default script time zone, and DateTime is aware of its own time zone internally. PHP.net provides extensive documentation on this.[61]As noted there, the most current time zone database can be implemented via thePECLtimezonedb.

Python

The standard module datetime included withPythonstores and operates on the time zone information class tzinfo. The third party pytz module provides access to the full IANA time zone database.[62]Negated time zone offset in seconds is stored time.timezone and time.altzone attributes. From Python 3.9, the zoneinfo module introduces timezone management without need for third party module.[63]

Smalltalk

EachSmalltalkdialect comes with its own built-in classes for dates, times and timestamps, only a few of which implement the DateAndTime and Duration classes as specified by the ANSI Smalltalk Standard.VisualWorksprovides a TimeZone class that supports up to two annually recurring offset transitions, which are assumed to apply to all years (same behavior as Windows time zones).Squeakprovides a Timezone class that does not support any offset transitions.Dolphin Smalltalkdoes not support time zones at all.

For full support of the tz database (zoneinfo) in a Smalltalk application (including support for any number of annually recurring offset transitions, and support for different intra-year offset transition rules in different years) the third-party, open-source, ANSI-Smalltalk-compliant Chronos Date/Time Library is available for use with any of the following Smalltalk dialects: VisualWorks, Squeak, Gemstone, or Dolphin.[64]

Time in outer space

Orbiting spacecraft may experience many sunrises and sunsets, or none, in a 24-hour period. Therefore, it is not possible to calibrate the time with respect to the Sun and still respect a 24-hour sleep/wake cycle. A common practice forspace explorationis to use the Earth-based time of the launch site or mission control, synchronizing the sleeping cycles of the crew and controllers. TheInternational Space Stationnormally uses Greenwich Mean Time (GMT).[65][66]

Timekeeping on Marscan be more complex, since the planet has a solar day of approximately 24 hours and 40 minutes, known as asol.Earth controllers for some Mars missions have synchronized their sleep/wake cycles with the Martian day, when specifically solar-powered rover activity occurs.[67]

See also

Notes

  1. ^abObserves UTC+00:00 aroundRamadan.[1][2][3]

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Further reading

Wikivoyage has a travel guide forTime zones.
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