"Timezone" and "TimeZone" redirect here. For other uses see Time zone (disambiguation). "Local time" redirects here. For the mathematical concept see Local time (mathematics). This article is about time zones in general. For a list of time zones by country see List of time zones by country. For more time zone lists see Lists of time zones. This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (November 2010)

A time zone is a region on Earth that has a uniform legally mandated standard time.

Almost all time zones on land have legally defined borders which coincide with the borders of the country mandating the time or some subdivision thereof. Of the 40 time zones on land most are offset from Coordinated Universal Time (UTC) by a whole number of hours (UTC12 to UTC+14) but a few are offset by 30 or 45 minutes from a nearby hourly zone. In addition to land time zones there are 25 nautical time zones all separated by lines of longitude. Most (UTC11 to UTC+11) are 15 of longitude wide which is one hour of Earth's rotation relative to the Sun but an hourly zone in the central Pacific Ocean is split into two 7.5 wide zones (UTC12) by the 180th meridian part of which coincides with the International Date Line. Many land time zones are skewed toward the west relative to the corresponding nautical time zones.

Before 1972 all time zones were specified as an offset from Greenwich Mean Time (GMT) which was the mean solar time at the meridian passing through the Royal Observatory in Greenwich London United Kingdom. Since 1972 all official time services have broadcast radio time signals synchronized to UTC a form of atomic time that includes leap seconds to keep it within 0.9 seconds of this former GMT now called UT1. Many countries now legally define their standard time relative to UTC although some still legally refer to GMT including the United Kingdom itself. UTC also called Zulu time is used everywhere on Earth by astronomers and others who need to state the time of an event unambiguously.

Originally all time on Earth was some local apparent solar time the time on a sundial so every city had its own time. When well-regulated mechanical clocks became widespread in the early 19th century each city began to use some local mean solar time. The first time zone was created in 1847 by railroads on the island of Great Britain using GMT. Sandford Fleming of Canada proposed worldwide hourly time zones in 1879. By about 1900 almost all time on Earth was in the form of standard time zones only some of which 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 on Earth was in the form of time zones referred to some "standard offset" from GMT/UTC. Nepal was the last country to adopt a standard offset shifting slightly to UTC+5:45 in 1986. Daylight saving time or Summer Time an advance of one hour is mandated by many countries between spring and autumn. Computer operating systems use either UTC or a local time zone to time stamp events. Contents 1 Introduction 2 Definition 3 Notation of time (ISO 8601) 3.1 UTC 3.2 Time zone as offsets from UTC 3.3 Abbreviations 4 Examples 5 Time zone conversions 6 History 7 Nautical time zones 8 Skewing of zones 9 Daylight saving time 10 Additional information 11 Internet and computer systems 11.1 Operating systems 11.1.1 Unix 11.1.2 Microsoft Windows 11.2 Programming languages 11.2.1 Java 11.2.2 JavaScript 11.2.3 PHP 11.2.4 Python 11.2.5 Smalltalk 11.3 Databases 11.3.1 Oracle 11.3.2 PostgreSQL 11.4 Applications 11.4.1 Microsoft Outlook 12 See also 13 References 14 External links Introduction

Standard time zones can be defined by geometrically subdividing the Earth's spheroid into 24 lunes (wedge-shaped sections) bordered by meridians each 15 of longitude apart. The local time in neighboring zones would differ by one hour. However political boundaries geographical practicalities and convenience of inhabitants can result in irregularly shaped zones. Moreover in a few regions half-hour or quarter-hour differences are in effect.

Before the adoption of time zones people used local solar time. Originally this was apparent or true solar time as shown by a sundial and later it became mean solar time as kept by most mechanical clocks. Mean solar time has days of equal length but the difference between mean and apparent solar time called the equation of time averages to zero over a year.

The use of local solar time became increasingly awkward as railways and telecommunications improved because clocks differed between places by an amount corresponding to the difference in their geographical longitude which was usually not a convenient number. This problem could be solved by synchronizing the clocks in all localities but in many places the local time would then differ markedly from the solar time to which people were accustomed. Time zones are a compromise relaxing the complex geographic dependence while still allowing local time to approximate the mean solar time. There has been a general trend to set the boundaries of time zones west of their designated meridians in order to create a permanent daylight saving time effect. The increase in worldwide communication has further increased the need for interacting parties to communicate mutually comprehensible time references to one another. Thus the advance of technology has both forced (rail transport) and enabled (modern timepieces) the development of arbitrary official "time."

Time zones may be adjusted seasonally into standard and daylight saving (or summer) variants. Daylight saving time zones (or summer time zones) include an offset (typically +1 hour) for daylight saving time. Definition Time zones present (from March 27 2011).

Time zones are based on Greenwich Mean Time (GMT) the mean solar time at longitude 0 (the Prime Meridian). The definition of GMT was recently changed it was previously the same as UT1 a mean solar time calculated directly from the rotation of the Earth. As the rate of rotation of the Earth is not constant the time derived from atomic clocks was adjusted to closely match UT1. In January 1972 however the length of the second in both Greenwich Mean Time and atomic time was equalised. The readings of participating atomic clocks are averaged out to give a uniform time scale. Because the length of the average day is a small fraction of a second more than 24 hours (slightly more than 86400 seconds) leap seconds are periodically inserted into Greenwich Mean Time to make it approximate to UT1. This new time system is also called Coordinated Universal Time (UTC). Leap seconds are inserted to keep UTC within 0.9 seconds of UT1. Because of the secular (long term) slowing down of the Earth's rotation leap seconds will gradually need to be added more and more often. But on short time scales (from one year to the next) the rotation rate is irregular so leap seconds are not added unless observations of Earth's rotation show that one is required. In this way local times continue to correspond approximately to mean solar time while the effects of variations in Earth's rotation rate are confined to simple step changes that can be more easily applied to the uniform time scale (International Atomic Time or TAI). All local times differ from TAI by an integral number of seconds. With the implementation of UTC nations began to use it in the definition of their time zones. As of 2005 most but not all nations had altered the definition of local time in this way. In England this involved redefining Greenwich Mean Time to make it the same as UTC.1 British Summer Time (BST) is still one hour in advance of Greenwich Mean Time and is therefore also one hour in advance of Coordinated Universal Time. Thus Greenwich Mean Time is the local time at the Royal Observatory Greenwich between 0100 hours GMT on the last Sunday in October and 0100 hours GMT on the last Sunday in March. Similar circumstances apply in many other places. Looking to the future leap seconds are considered by many to be a nuisance and ways to abolish them are being considered. This means letting the time difference accumulate. One suggestion is to insert a "leap-hour" in about 5000 years. For more on this discussion read leap second#Proposal to abolish leap seconds. Notation of time (ISO 8601) UTC If the time is in UTC add a "Z" directly after the time without a space. "Z" is the zone designator for the zero UTC offset. "09:30 UTC" is therefore represented as "09:30Z" or "0930Z". "14:45:15 UTC" would be "14:45:15Z" or "144515Z". UTC time is also known as "Zulu" time since "Zulu" is the ICAO spelling alphabet code word for "Z". Time zone as offsets from UTC Time zone are written as offset from UTC in the format hh:mm hhmm or hh. So if the time being described is one hour ahead of UTC (such as the time in Berlin during the winter) the zone designator would be "+01:00" "+0100" or simply "+01". This is appended to the time in the same way that 'Z' was above. The offset from UTC changes with daylight saving time e.g. a time offset in Chicago which is in the North American Central Time Zone would be "06:00" for the winter (Central Standard Time) and "05:00" for the summer (Central Daylight Time). Abbreviations Time zones are often represented by abbreviations such as "EST WST CST" but these are not part of the international time and date standard ISO 8601 and their use as sole designator for a time zone is not recommended. Such designations can be ambiguous. For example "BST" which is British Summer Time was renamed "British Standard Time" between 1968 and 1971 when Central European Time was in force because legislators objected to calling it Central European Time. The same legislation affirmed that the Standard Time within the United Kingdom was and would continue to be Greenwich Mean Time. Examples Main article: List of time zones by UTC offset These examples give the local time at various locations around the world when daylight saving time is not in effect: Time zone Example time (ISO 8601 notation) Example locations UTC12:00 2011-06-12T07:20:44-12:00 Baker Island Howland Island (both uninhabited) UTC11:00 2011-06-12T08:20:44-11:00 Samoa American Samoa UTC10:00 2011-06-12T09:20:44-10:00 Hawaii Papeete UTC09:30 2011-06-12T09:50:44-09:30 Marquesas Islands UTC09:00 2011-06-12T10:20:44-09:00 Anchorage Fairbanks Juneau UTC08:00 2011-06-12T11:20:44-08:00 Vancouver Washington (state) Portland Las Vegas California Baja California UTC07:00 2011-06-12T12:20:44-07:00 Alberta Colorado Arizona Chihuahua Sonora UTC06:00 2011-06-12T13:20:44-06:00 Chicago Costa Rica Dallas El Salvador Guatemala Honduras Houston Manitoba Mexico City Nicaragua Saskatchewan UTC05:00 2011-06-12T14:20:44-05:00 Ottawa Toronto Montreal Boston New York North Carolina Washington D.C. Georgia Miami Cuba Jamaica Haiti Panama Colombia Continental Ecuador Peru UTC-04:30 2011-06-12T14:50:44-04:30 Venezuela UTC04:00 2011-06-12T15:20:44-04:00 Nova Scotia Dominican Republic Puerto Rico Trinidad and Tobago Amazonas Bolivia Continental Chile Paraguay San Luis Province UTC03:30 2011-06-12T15:50:44-03:30 Newfoundland UTC03:00 2011-06-12T16:20:44-03:00 Rio de Janeiro So Paulo Argentina (except San Luis Province) Uruguay Nuuk UTC02:00 2011-06-12T17:20:44-02:00 Fernando de Noronha South Georgia and the South Sandwich Islands UTC01:00 2011-06-12T18:20:44-01:00 Azores Cape Verde UTC 2011-06-12T19:20:44-00:00 Iceland Faroe Islands United Kingdom Ireland Continental Portugal Madeira Canary Islands Morocco Senegal Ghana Cte d'Ivoire UTC+01:00 2011-06-12T20:20:44+01:00 Albania Slovenia Macedonia Norway Sweden Denmark Germany the Netherlands Belgium Metropolitan France Switzerland Austria Poland Czech Republic Slovakia Hungary Continental Spain Italy Croatia Serbia Kosovo Bosnia and Herzegovina Tunisia Algeria Nigeria Cameroon Angola Kinshasa UTC+02:00 2011-06-12T21:20:44+02:00 Finland Lithuania Latvia Estonia Belarus Ukraine Romania Bulgaria Greece Turkey Cyprus Syria Lebanon Jordan Palestine Israel Egypt Libya Mozambique Malawi Zambia Zimbabwe South Africa UTC+03:00 2011-06-12T22:20:44+03:00 Samara Iraq Saudi Arabia Yemen Sudan Ethiopia Somalia Kenya Uganda Tanzania Madagascar UTC+03:30 2011-06-12T22:50:44+03:30 Iran UTC+04:00 2011-06-12T23:20:44+04:00 Georgia Armenia Azerbaijan United Arab Emirates Oman Seychelles Mauritius Moscow Saint Petersburg UTC+04:30 2011-06-12T23:50:44+04:30 Afghanistan UTC+05:00 2011-06-13T00:20:44+05:00 Sverdlovsk Uzbekistan Pakistan Maldives Kazakhstan UTC+05:30 2011-06-13T00:50:44+05:30 India Sri Lanka UTC+05:45 2011-06-13T01:05:44+05:45 Nepal UTC+06:00 2011-06-13T01:20:44+06:00 Novosibirsk Almaty Bangladesh UTC+06:30 2011-06-13T01:50:44+06:30 Myanmar Cocos Islands UTC+07:00 2011-06-13T02:20:44+07:00 Krasnoyarsk Thailand Vietnam Jakarta UTC+08:00 2011-06-13T03:20:44+08:00 Irkutsk Ulan Bator China Taiwan Hong Kong Philippines Malaysia Singapore Western Australia UTC+09:00 2011-06-13T04:20:44+09:00 Zabaykalsky Japan North Korea South Korea East Timor UTC+09:30 2011-06-13T04:50:44+09:30 Northern Territory South Australia UTC+10:00 2011-06-13T05:20:44+10:00 Victoria Tasmania Queensland New South Wales Primorsky UTC+10:30 2011-06-13T05:50:44+10:30 Lord Howe Island UTC+11:00 2011-06-13T06:20:44+11:00 Kamchatka Solomon Islands New Caledonia UTC+11:30 2011-06-13T06:50:44+11:30 Norfolk Island UTC+12:00 2011-06-13T07:20:44+12:00 Fiji New Zealand UTC+12:45 2011-06-13T08:05:44+12:45 Chatham Islands UTC+13:00 2011-06-13T08:20:44+13:00 Tonga UTC+14:00 2011-06-13T09:20:44+14:00 Line Islands Where the adjustment for time zones results in a time at the other side of midnight from UTC then the date at the location is one day later or earlier. Some examples when UTC is 23:00 on Monday when or where daylight saving time is not in effect: Cairo Egypt: UTC+02; 01:00 on Tuesday Wellington New Zealand: UTC+12; 11:00 on Tuesday Some examples when UTC is 02:00 on Tuesday when or where daylight saving time is not in effect: Honolulu Hawaii United States: UTC10; 16:00 on Monday Toronto Ontario Canada: UTC05; 21:00 on Monday The time-zone adjustment for a specific location may vary because of daylight saving time. For example New Zealand which is usually UTC+12 observes a one-hour daylight saving time adjustment during the Southern Hemisphere summer resulting in a local time of UTC+13. Time zone 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 more familiar term "UTC offset" is used here rather than the term "zone designator" used by the standard.) 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 what time is it in Los Angeles (UTC offset 08) when the New York Stock Exchange opens at 09:30 (05) time in Los Angeles 09:30 (05:00) + (08:00) 06:30 In Delhi (UTC offset +5:30) the New York Stock Exchange opens at time in Delhi 09:30 (05:00) + (+5:30) 20:00 These calculations become more complicated near a daylight saving boundary (because the UTC offset for zone X is a function of the UTC time). History Plaque commemorating the Railway General Time Convention of 1883 Greenwich Mean Time (GMT) was established in 1675 when the Royal Observatory was built as an aid to (English) mariners to determine longitude at sea. At the time each town's local clock in the area was calibrated to its local noon. Therefore each clock across England had a slightly different time. The first time zone in the world was established by British railway companies on December 1 1847with GMT kept by portable chronometers. This quickly became known as Railway Time. About August 23 1852 time signals were first transmitted by telegraph from the Royal Observatory Greenwich. Even though 98% of Great Britain's public clocks were using GMT by 1855 it was not made Britain's legal time until August 2 1880. Some old British clocks from this period have two minute handsone for the local time one for GMT.2 On November 2 1868 the then-British colony of New Zealand officially adopted a standard time to be observed throughout the colony and was perhaps the first country to do so. It was based on the longitude 17230 East of Greenwich that is 11 hours 30 minutes ahead of GMT. This standard was known as New Zealand Mean Time. Timekeeping on the American railroads in the mid-19th century was somewhat confused. 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 major railroad junctions served by several different railroads had a separate clock for each railroad each showing a different time; the main station in Pittsburgh Pennsylvania for example kept six different times. Charles F. Dowd proposed a system of one-hour standard time zones for American railroads about 1863 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 northsouth borders) the first centered on Washington D.C. but by 1872 the first was centered 75W of Greenwich with geographic borders (for example sections of the Appalachian Mountains). Dowd's system was never accepted by American railroads. Instead U.S. and Canadian railroads implemented a version proposed by William F. Allen the editor of the Traveler's Official Railway Guide.3 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 through Detroit Buffalo Pittsburgh Atlanta and Charleston. It was inaugurated on Sunday November 18 1883 also called "The Day of Two Noons"4 when each railroad station clock was reset as standard-time noon was reached within each time zone. The zones were named Intercolonial Eastern Central Mountain and Pacific. Within one year 85% of all cities with populations over 10000 about 200 cities were using standard time.5 A notable exception was Detroit (which is about half-way between the meridians of eastern time and central time) which kept local time until 1900 then tried Central Standard Time local mean time and Eastern Standard Time 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 zone time was formally adopted by the U.S. Congress on March 19 1918 in the Standard Time Act. U.S. Commissioner of Railroads William H. Armstrong gave the following account of the new railroad time system in his Report to the Secretary of the Interior for 1883. The question of uniform time standards for railways of the United States has long attracted the attention of railway managers but Mr. W. F. Allen editor of the Traveler's Official Guide and secretary of the time conventions is entitled to the credit of having perfected the admirable system which was adopted by the general time convention of railway managers held at Chicago October 11 1883 and ratified by the southern railway time convention held at New York October 17 1883. As this is a subject of great interest to the entire country a brief synopsis of the general principles governing the proposed plan is deemed appropriate in this report. Under the present system each railway is operated independently on the local time of some principal point or points on said road but this plan was found to be highly objectionable owing to the fact that some fifty standards intersecting and interlacing each other were in use throughout the country. By the plan which has been adopted this number will be reduced to four the difference in time being one hour between each viz the 75th 90th 105th and 120th degrees of longitude west from Greenwich. The adoption of these standards will not cause a difference of more than thirty minutes from the local time at any point which is now used as a standard. The new arrangement goes into effect November 18 1883 and all changes of time are to occur at the termini of roads or at the ends of divisions. The seventy-fifth meridian being almost precisely the central meridian for the system of roads now using standards based upon the time of the Eastern cities and the ninetieth meridian being equally central for roads now running by the time of Western cities the time of these meridians has been adopted for the territory which includes 90 per cent. of the whole railway system of the country. Nearly all of the larger cities have abolished local time and adopted that of the nearest standard meridian in use by the railways.678 Although the first person to propose a worldwide system of time zones was the Italian mathematician Quirico Filopanti in his book Miranda! published in 1858 his idea was unknown outside the pages of his book until long after his death so it did not influence the adoption of time zones during the 19th century. He proposed 24 hourly time zones which he called "longitudinal days" the first centered on the meridian of Rome. He also proposed a universal time to be used in astronomy and telegraphy.910 Canadian Sir Sandford Fleming proposed a worldwide system of time zones in 1879. He advocated his system at several international conferences thus is widely credited with their invention. In 1876 his first proposal was for a global 24-hour clock conceptually located at the center of the Earth and not linked to any surface meridian. In 1879 he specified that his universal day would begin at the anti-meridian of Greenwich (180th meridian) while conceding that hourly time zones might have some limited local use. He also proposed his system at the International Meridian Conference in October 1884 but it did not adopt his time zones because they were not within its purview. The conference did adopt a universal day of 24 hours beginning at Greenwich midnight but specified that it "shall not interfere with the use of local or standard time where desirable". Nevertheless most major countries had adopted hourly time zones by 1929. Today all nations use standard time zones for secular purposes but they do not all apply the concept as originally conceived. Newfoundland India Iran Afghanistan Venezuela Burma the Marquesas as well as parts of Australia use half-hour deviations from standard time and some nations such as Nepal and some provinces such as the Chatham Islands use quarter-hour deviations. Some countries most notably China and India use a single time zone even though the extent of their territory far exceeds 15 of longitude. Before 1949 China used five time zones (see Time in China). Nautical time zones Main article: Nautical time Since the 1920s a nautical standard time system has been in operation for ships on the high seas. Nautical time zones are an ideal form of the terrestrial time zone system. Under the system a time change of one hour is required for each change of longitude by 15. The 15 gore that is offset from GMT or UT1 (not UTC) by twelve hours is bisected by the nautical date line into two 7.5 gores that differ from GMT by 12 hours. A nautical date line is implied but not explicitly drawn on time zone maps. It follows the 180th meridian except where it is interrupted by territorial waters adjacent to land forming gaps: it is a pole-to-pole dashed line.111213 A ship within the territorial waters of any nation would use that nation's standard time but would revert to nautical standard time upon leaving its territorial waters. The captain is permitted to change the ship's clocks at a time of the captains choice following the ship's entry into another time zone. The captain often chooses midnight. Ships going in shuttle traffic over a time zone border often keeps the same time zone all the time to avoid confusion about work meal and shop opening hours. Still the time table for port calls must follow the land time zone. Skewing of zones Difference between sun time and clock time during daylight saving time: 0h 30m 1h 30m ahead 2h 30m ahead 3h 30m ahead Ideal time zones such as nautical time zones are based on the mean solar time of a particular meridian located in the middle of that zone with boundaries located 7.5 degrees east and west of the meridian. In practice zone boundaries are often drawn much farther to the west with often irregular boundaries and some locations base their time on meridians located far to the east. For example even though the Prime Meridian (0) passes through Spain and France 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 the German occupation of the country during World War II and did not switch back after the war.citation needed There is a tendency to draw time zone boundaries far to the west of their meridians. Many of these locations also use daylight saving time. As a result in the summer solar noon in the Spanish town of Muxia occurs at 14:37 (2:37pm) by the clock. This area of Spain never experiences sunset before 18:00 (6pm) local time even in midwinter despite its lying more than 40 degrees north of the equator. Near the summer solstice Muxia has sunset times similar to those of Stockholm which is in the same time zone and 16 degrees further north. A more extreme example is Nome Alaska which is at 16524W longitudejust west of center of the idealized Samoa Time Zone (165W). Nevertheless Nome observes Alaska Time (135W) with DST so it is slightly more than two hours ahead of the sun in winter and over three in summer.14 Kotzebue Alaska also near the same meridian but north of the Arctic Circle has an annual event on 9 August to celebrate two sunsets in the same 24-hour day one shortly after midnight at the start of the day and the other shortly before midnight at the end of the day. Also China extends as far west as 7334E but all parts of it use UTC+8 (120E) so solar "noon" can occur as late as 15:00. Daylight saving time   DST used   DST no longer used   DST never used Main article: Daylight saving time Many countries and sometimes just certain regions of countries adopt daylight saving time (also known as "Summer Time") during part of the year. This typically involves advancing clocks by an hour near the start of spring and adjusting back in autumn ("spring" forward "fall" back). Some countries also use backward daylight saving over the winter period. Modern DST was first proposed in 1907 and was in widespread use in 1916 as a wartime measure aimed at conserving coal. Despite controversy many countries have used it since then; details vary by location and change occasionally. Most countries around the equator do not observe daylight saving time since the seasonal difference in sunlight is minimal. Additional information France has twelve time zones including those of Metropolitan France French Guiana and numerous islands inhabited and uninhabited. Russia has nine time zones (and used to have 11 time zones before March 2010) eight contiguous zones plus Kaliningrad exclave on the Baltic Sea. The United States has ten time zones (nine official plus that for Wake Island and its Antarctic stations; no official time zone is specified for uninhabited Howland Island and Baker Island). Australia has nine time zones (one unofficial and three official on the mainland plus four for its territories and one more for an Antarctic station not included in other time zones). The United Kingdom has eight time zones for itself and its overseas territories. Canada has six official time zones. The Danish Realm has five time zones. In terms of area China is the largest country with only one time zone (UTC+08). China also has the widest spanning time zone. Before 1949 China was separated into five time zones. Stations in Antarctica generally keep the time of their supply bases thus both the Amundsen-Scott South Pole Station (U.S.) and McMurdo Station (U.S.) use New Zealand time (UTC+12 southern winter UTC+13 southern summer). The 27N latitude passes back and forth across time zones in South Asia. Pakistan: +05:00 India +05:30 Nepal +05:45 +05:30 Myanmar +06:30. This switching was more odd in 2002 when Pakistan enabled daylight saving time. Thus from west to east time zones were: +06:00 +05:30 +05:45 +05:30 +08:00 +06:00 +05:30 and +06:30. Because the earliest and latest time zones are 26 hours apart any given calendar date exists at some point on the globe for 50 hours. For example April 11 begins in time zone UTC+14 at 10:00 UTC April 10 and ends in time zone UTC12 at 12:00 UTC April 12. There are 22 places where three or more time zones meet for instance at the tri-country border of Finland Norway and Russia. 28 countries present such triple points with China being the most present (in 13 of the 22 triple points). Then come India (7) Russia India and Afghanistan (4). There are 40 time zones. This is due to fractional hour offsets and zones with offsets larger than 12 hours near the International Date Line as well as one unofficial zone in Australia. See the list of time zones. The largest time gap along a political border is the 3.5 hour gap along the border of China (UTC+08) and Afghanistan (UTC+04:30). One of the most unusual time zones is the Australian Central Western Time zone (CWST) which is a small strip of Western Australia from the border of South Australia west to 125.5E just before Caiguna. It is 8 hours ahead of UTC (UTC+08:45) and covers an area of about 35000 km2 larger than Belgium but has a population of about 200. Although unofficial it is universally respected in the areawithout it the time gap in standard time at 129E (the WA/SA border) would be 1.5 hours. See Time in Australia. Internet and computer systems UTC is often used on the Internet for meetings (e.g. IRC chats news shows and so on).15 For e-mail the sender time zone is used to calculate the send time but this time is recalculated by the receiver mail client and shown according to the receiver time zone. On websites with mainly domestic audiences local time is often used sometimes with UTC in brackets: e.g. the international English-language version of CNN includes GMT and Hong Kong Time16 whilst the US version shows Eastern Time.17 US Eastern Time and Pacific Time are also used fairly commonly on many US-based English-language websites with global readership. The format is based in the W3C Note "datetime". On the other hand most modern computer operating systems include information about time zones including the capability to automatically change the local time when daylight saving starts and finishes (see the article on daylight saving time for more details on this aspect). Operating systems Unix Main article: Unix time Most Unix-like systems including Linux and Mac OS X keep system time as UTC (Coordinated Universal Time). Rather than having a single time zone set for the whole computer timezone offsets can vary for different processes. Standard library routines are used to calculate the local time based on the current timezone normally supplied to processes through the TZ environment variable. This allows users in multiple timezones to use the same computer with their respective local times displayed correctly to each user. Timezone information is most commonly stored in a timezone database known as tz database (or sometimes zoneinfo or Olson format). In fact many systems including anything using the GNU C Library can make use of this database. Microsoft Windows Windows-based computer systems normally keep system time as local time in a particular time zone. A system database of timezone information includes the offset from UTC and rules that indicate the start and end dates for daylight saving in each zone. Application software is able to calculate the time in various zones. Terminal Servers allow 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 timezone information the Java Platform from version 1.3.1 has maintained its own timezone database. This database will need to be updated whenever timezone rules change. Sun provides an updater tool for this purpose.18 As an alternative to the timezone information bundled with the Java Platform programmers may choose to use the Joda-Time library.19 This library includes its own timezone data based on the frequently updated tz database.20 JavaScript There is very little in the way of timezone support for JavaScript. Essentially the programmer has 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 daylight savings variations. PHP The DateTime objects and related functions have been compiled into the PHP core since 5.2. This includes the ability to get and set the default script timezone and DateTime is aware of its own timezone internally. PHP.net provides extensive documentation on this.21 As noted there the most current timezone database can be implemented via the PECL timezonedb. Python The standard module datetime stores and operates on the timezone information class tzinfo. The third party pytz module provides access to the full tz database.22 Negated time zone offset in seconds is stored time.timezone and time.altzone attributes. Smalltalk Each Smalltalk dialect 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. VisualWorks provides a TimeZone class that supports up to 2 annually recurring offset transitions which are assumed to apply to all years (same behavior as Windows time zones). Squeak provides a Timezone class that does not support any offset transitions. Dolphin Smalltalk does 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 or Dolphin. Databases Some databases allow storage of a datetime type having time zone information. The SQL standard defines two standard time data types: TIMESTAMP WITH TIME ZONE TIMESTAMP WITHOUT TIME ZONE However the standard has a somewhat naive understanding of time zones. It generally assumes a time zone can be specified by a simple offset from GMT. This causes problems when trying to do arithmetic on dates which span daylight saving time transitions or which span political changes in time zone rules. Oracle Oracle Database is configured with a database time zone and connecting clients are configured with session time zones. Oracle Database uses two data types to store time zone information: TIMESTAMP WITH TIME ZONE  Stores date and time information with the offset from UTC TIMESTAMP WITH LOCAL TIME ZONE  Stores date and time information with respect to the dbtimezone (which cannot be changed so long as there is a column in the db of this type) automatically adjusting the date and time from the stored time zone to the client's session time zone. PostgreSQL PostgreSQL uses the standard SQL data types but tries to impose an interpretation which avoids the problems described above. TIMESTAMP WITH TIME ZONE  Stores date and time in UTC and converts to the client's local time zone (which could be different for each client) for display purposes. TIMESTAMP WITHOUT TIME ZONE  Stores date and time without any conversion on input or output. Applications Microsoft Outlook Microsoft Outlook has a much criticized behavior regarding time zone handling. Appointments stored in Outlook move when the computer changes time zone since they are assumed to be fixed against UTC not against the hour number. As a consequence someone who inserts an appointment requiring a travel into another timezone (based on a telephone call or a free-text email) will not get a correct time for the appointment when travelling to the other time zone. For example a New Yorker is asked to meet someone in Los Angeles at 9:00 AM (Los Angeles time; meaning 12:00 New York time). He inserts an appointment at 9:00 AM in Outlook (his computer is on New York time). He travels to Los Angeles and adjust his computer time zone which causes the meeting to show up at 6:00 AM (9:00 New York time) in Outlook. One workaround is to adjust the clock but not the timezone of the computer when travelling. This will give sent e-mail wrong time stamp. Microsoft recommends23 to not change the clock at all and show a second time scale in the calendar. This will give reminder popups at the wrong time. The Outlook "feature" will give correct time if the organizer invites the guest to a meeting using the "invite attendees" feature (the Los Angeles meeting will show up as 12:00 noon in the New Yorkers calender before he adjusted the time zone) but only if the time zone is adjusted when travelling which is against the Microsoft recommendation. The Outlook feature will also give correct time for telephone meetings. See also Daylight saving time ISO 8601 Jet lag List of time zone abbreviations List of time zones by country Lists of time zones Metric time Sunpendulum Time by country World clock References The Astronomical Almanac 1983 U S Government Printing Office (Washington) and Her Majesty's Stationery Office (London) page B4. Bristol Time Economics of Time ZonesPDF (1.89 MB) Historymatters.gmu.edu Resolution concerning new standard time by Chicago Cooper Bruce Clement (Consultant Editor) The Classic Western American Railroad Routes. New York: Chartwell Books/Worth Press 2010. ISBN 978-0-7858-2573-9; ISBN 0-7858-2573-8; BINC: 3099794. pp 31 Cooper Bruce C. Railway & Travel Guides and the Pacific Railroad Central Pacific Railroad Photographic History Museum 2005 Annual report of the Commissioner of Railroads made to the Secretary of the Interior for the year ending June 30 1883 pp. 1920. Washington DC: Government Printing Office 1883. Quirico Filopanti from scienzagiovane Bologna University Italy. Gianluigi Parmeggiani (Osservatorio Astronomico di Bologna) The origin of time zones Bowditch Nathaniel. American Practical Navigator. Washington: Government Printing Office 1925 1939 1975. Hill John C. Thomas F. Utegaard Gerard Riordan. Dutton's Navigation and Piloting. Annapolis: United States Naval Institute 1958. Howse Derek. Greenwich Time and the Discovery of the Longitude. Oxford: Oxford University Press 1980. ISBN 0-19-215948-8. Doug O'Hara (2007-03-11). "Alaska: daylight stealing time". Far North Science. http://www.farnorthscience.com/2007/03/11/news-from-alaska/alaska-daylight-stealing-time/. Retrieved 2007-05-11.  "Guidelines for Ubuntu IRC Meetings". Canonical Ltd.. 2008-08-06. https://wiki.ubuntu.com/BuildingCommunity/BookingUbuntuMeetingChannel.  International CNN United States CNN Timezone Updater Tool Joda-Time tz database DateTime pytz module Work with time zones in Outlook Palin Michael. Pole to Pole. Bay Books 1995. ISBN 0-912333-41-3. External links ITU LEGAL TIME 2011 W3C Note Datetime US Official Time Clock Java-enabled clock to graphically display night and day around the globe. PlanetaryTime Local times world wide & time zone details. v d eTime Major concepts Time  Eternity  Arguments for eternity  Immortality Deep time  History  Past  Present  Future  Futurology Time Portal Measurement and standards Chronometry  UTC  UT  TAI  Second  Minute  Hour  Sidereal time  Solar time  Time zone Clock  Astrarium  History of timekeeping devices  Horology  Marine chronometer  Sundial  Water clock Calendar  Day  Week  Month  Year  Tropical year  Gregorian  Islamic  Julian Intercalation  Leap second  Leap year Chronology Astronomical chronology  Calendar era  Chronicle  Dating methodologies  Geochronology Geologic Time  Geological history  Periodization  Regnal year  Timeline Religion and mythology Dreamtime  Kla  Kalachakra  Prophecy  Time and fate deities  Wheel of time Philosophy A-series and B-series  B-Theory of time  Causality  Endurantism  Eternal return  Eternalism  Event Perdurantism  Presentism  Temporal finitism  Temporal parts  The Unreality of Time Physical sciences Time in physics  Absolute time and space  Arrow of time  Chronon  Coordinate time Planck epoch  Planck time  Proper time  Spacetime  Theory of relativity Time dilation  Gravitational time dilation  Time domain  T-symmetry Biology Chronobiology  Circadian rhythms Psychology Mental chronometry  Sense of time  Specious present Sociology and anthropology Long Now Foundation  Time discipline  Time use research Economics Time value of money  Time-based currency  Time Banking Related topics Carpe diem  Duration  Hexadecimal time  Metric time  Space  System time  Tempus fugit Time capsule  Time signature  Time travel v d eTime measurement and standards Major subjects Time  Chronometry  Orders of magnitude  Metrology International standards UTC  UTC offset  UT  DUT1  IERS  ISO 31-1  ISO 8601  TAI  12-hour clock  24-hour clock  Barycentric Coordinate Time  Civil time  Daylight saving time  Geocentric Coordinate Time  International Date Line  Leap second  Solar time  Terrestrial Time  Time zone Obsolete standards Barycentric Dynamical Time  Ephemeris time  Greenwich Mean Time  Prime Meridian Time in physics Absolute time and space  Spacetime  Chronon  Continuous time  Coordinate time  Cosmological decade  Discrete time  Planck epoch  Planck time  Proper time  Theory of relativity  Time dilation  Gravitational time dilation  Time domain  T-symmetry Horology Clock  Astrarium  Atomic clock  Complication  Equation of time  History of timekeeping devices  Hourglass  Marine chronometer  Marine sandglass  Radio clock  Sundial  Watch  Water clock Calendar Astronomical  Calculating the day of the week  Dominical letter  Epact  Equinox  Gregorian  Intercalation  Islamic  Julian  Leap year  Lunar  Lunisolar  Seven-day week  Solar  Solstice  Tropical year  Week-day names Archaeology & geology Dating methodologies  Geologic time scale  International Commission on Stratigraphy Astronomical chronology Galactic year  Nuclear time scale  Precession  Sidereal time Units of time Century  Day  Decade  Fortnight  Hour  Jiffy  Lustrum  Millennium  Minute  Month  Saeculum  Second  Shake  Tide  Week  Year Related topics Chronology  Duration  Mental chronometry  Metric time  System time  Time value of money  Timekeeper v d eCoordinated Universal Time (UTC) UTC offset for standard time and Daylight saving time (DST) Italics: historical only 12:00  11:00  10:30  10:00  09:30  09:00  08:30  08:00  07:00  06:00  05:00  04:30  04:00  03:30  03:00  02:30  02:00  01:00  00:44  00:25  00:00  +00:20  +00:30  +01:00  +01:24  +02:00  +02:30  +03:00  +03:30  +04:00  +04:30  +04:51  +05:00  +05:30  +05:40  +05:45  +06:00  +06:30  +07:00  +07:20  +07:30  +08:00  +08:30  +08:45  +09:00  +09:30  +09:45  +10:00  +10:30  +11:00  +11:30  +12:00  +12:45  +13:00  +13:45  +14:00 Time zone data sources tz database Lists of time zones time zones by country  time zones by UTC offset  tz database time zones  military time zones  time zone abbreviations  daylight saving time around the world