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A time zone is a region that has a uniform standard time for legal, commercial, and social purposes. It is convenient for areas in close commercial or other communication to keep the same time, so time zones tend to follow the boundaries of countries and their subdivisions.
Most of the time zones on land are offset from Coordinated Universal Time (UTC) by a whole number of hours (UTC−12 to UTC+14), but a few are offset by 30 or 45 minutes (for example Newfoundland Standard Time is UTC -03:30 and Nepal Standard Time is UTC +05:45). Some higher latitude countries use daylight saving time for part of the year, typically by changing clocks by an hour. Many land time zones are skewed toward the west of the corresponding nautical time zones. This also creates a permanent daylight saving time effect.
Before clocks were invented, people marked the time of day with apparent solar time (also called "true" solar time) – for example, the time on a sundial – which was typically different for every settlement.
When well-regulated mechanical clocks became widespread in the early 19th century, each city began to use some local mean solar time. Apparent and mean solar time can differ by up to around 15 minutes (as described by the equation of time) due to the non-circular shape of the Earth's orbit around the sun (Eccentricity) and the tilt of the Earth's axis (Obliquity). Mean solar time has days of equal length, and the difference between the two averages to zero after a year.
Greenwich Mean Time (GMT) was established in 1675 when the Royal Observatory was built as an aid to mariners to determine longitude at sea, providing a standard reference time when each city in England kept a different local time.
Local solar time became increasingly awkward as rail transport and telecommunications improved, because clocks differed between places by an amount corresponding to the difference in their geographical longitude, which varied by four minutes for every degree of longitude. The difference between New York and Boston is about two degrees or 8 minutes, the difference between Sydney and Melbourne, Australia, is about 7 degrees or 28 minutes. Bristol is 2°35′ W(est) of Greenwich (East London), so when it is noon in Bristol, it is about 10 minutes past noon in London. The use of time zones smooths out these differences.
The first adoption of a standard time was on December 1, 1847, in Great Britain by railway companies using GMT kept by portable chronometers. The first of these companies to adopt standard time was the Great Western Railway (GWR) in November 1840. 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 hands—one for the local time, one for GMT.
The increase in worldwide communication had further increased the need for interacting parties to communicate mutually comprehensible time references to one another. The problem of differing local times could be solved across larger areas by synchronizing clocks worldwide, but in many places the local time would then differ markedly from the solar time to which people were accustomed.
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 172°30′ 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 junctions served by several railroads had a clock for each railroad, each showing a different time.
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 north–south borders), the first centered on Washington, D.C., but by 1872 the first was centered 75°W 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. 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", 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 a year 85% of all cities with populations over 10,000, about 200 cities, were using standard time. 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 in the Standard Time Act of March 19, 1918.
Although the first person to propose a worldwide system of time zones was 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.
Scottish-born 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".
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. By 1929, most major countries had adopted hourly time zones. Nepal was the last country to adopt a standard offset, shifting slightly to UTC+5:45 in 1986.
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, Sri Lanka, 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.
Until 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. 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.
Time zones are based on Greenwich Mean Time (GMT), the mean solar time at longitude 0° (the Prime Meridian). As the rate of rotation of the Earth is not constant, the time derived from atomic clocks is adjusted to stay within a second of UT1. In January 1972 the length of the second in both Greenwich Mean Time and atomic time was equalized. The readings of atomic clocks are averaged to give a uniform time scale.
Because the length of the average day is currently 0.002 second more than 24 hours, 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 the Earth's rotation is gradually slowing, leap seconds will need to be added more frequently in the future. However, from one year to the next the rotation rate is slightly irregular, so leap seconds are not added unless observations of Earth's rotation show that one is needed. In this way, local times will continue to stay close to mean solar time and the effects of variations in Earth's rotation rate will be confined to simple step changes relative 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 nations had altered the definition of local time in this way.
In the United Kingdom, this involved redefining Greenwich Mean Time to make it the same as UTC. 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 01:00 GMT on the last Sunday in October and 01:00 GMT on the last Sunday in March. Similar circumstances apply in many other places.
Leap seconds are considered by many to be a nuisance[who?], 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 5,000 years.
If the time is in Coordinated Universal Time (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".
Offsets from UTC are written in the format ±[hh]:[mm], ±[hh][mm], or ±[hh] (either hour(s) ahead or behind UTC). 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.
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, "ECT", could be interpreted as "Eastern Caribbean Time" (UTC−4h), "Ecuador Time" (UTC−5h) or "European Central Time" (UTC+1h).
These examples give the local time at various locations around the world when daylight saving time is not in effect:
|Time offset||Example time|
(ISO 8601 notation)
|Example locations||Example locations that in summer use DST|
|UTC−12:00||2015-01-10T11:58:23-12:00||Baker Island, Howland Island (both uninhabited)|
|UTC−11:00||2015-01-10T12:58:23-11:00||American Samoa, Niue|
|UTC−10:00||2015-01-10T13:58:23-10:00||United States (Hawaii)||Most of French Polynesia, United States (Aleutian Islands)|
|UTC−09:00||2015-01-10T14:58:23-09:00||Gambier Islands||United States (most of Alaska)|
|UTC−08:00||2015-01-10T15:58:23-08:00||Canada (most of British Columbia), Mexico (Baja California), United States (California, most of Nevada, most of Oregon, Washington (state))|
|UTC−07:00||2015-01-10T16:58:23-07:00||Canada (northeastern British Columbia), Mexico (Sonora), United States (Arizona)||Canada (Alberta), Mexico (Chihuahua), United States (Colorado)|
|UTC−06:00||2015-01-10T17:58:23-06:00||Canada (almost all of Saskatchewan), Costa Rica, El Salvador, Ecuador (Galápagos Islands), Guatemala, Honduras, Mexico (most), Nicaragua,||Canada (Manitoba), United States (Illinois, most of Texas)|
|UTC−05:00||2015-01-10T18:58:23-05:00||Brazil (Acre), Colombia, Cuba, Ecuador (continental), Jamaica, Panama, Peru||Canada (most of Ontario, most of Quebec), Haiti, United States (most of Florida, Georgia, Massachusetts, most of Michigan, most of Indiana, New York, North Carolina, Ohio, Washington, D.C.), Bahamas|
|UTC−04:00||2015-01-10T19:58:23-04:00||Bolivia, Brazil (Amazonas, Rondônia, Roraima), Chile (continental), Dominican Republic, Canada (Nova Scotia), Puerto Rico, Barbados, Trinidad and Tobago||Brazil (Mato Grosso, Mato Grosso do Sul), Falkland Islands|
|UTC−03:30||2015-01-10T20:28:23-03:30||Canada (island of Newfoundland and southern Labrador)|
|UTC−03:00||2015-01-10T20:58:23-03:00||Argentina, Brazil (Bahia, Ceará, Maranhão, Pará, Pernambuco), Paraguay||Brazil (Federal District, Minas Gerais, Paraná, Rio Grande do Sul, Rio de Janeiro, São Paulo), most of Greenland, Uruguay|
|UTC−02:00||2015-01-10T21:58:23-02:00||Brazil (Fernando de Noronha), South Georgia and the South Sandwich Islands|
|UTC−01:00||2015-01-10T22:58:23-01:00||Cape Verde||Portugal (Azores)|
|UTC±00:00||2015-01-10T23:58:23+00:00||Côte d'Ivoire, Ghana, Iceland, Senegal, Saint Helena, Mali||Faroe Islands, Ireland, Morocco, Portugal (continental and Madeira), Spain (Canary Islands), United Kingdom, Western Sahara|
|UTC+01:00||2015-01-11T00:58:23+01:00||Algeria, Angola, Benin, Cameroon, Gabon, Niger, Nigeria, Democratic Republic of the Congo (west), Tunisia||Albania, Austria, Belgium, Bosnia and Herzegovina, Croatia, Czech Republic, Denmark, Germany, Hungary, Italy, Malta, Macedonia, France (metropolitan), Namibia, the Netherlands, Norway, Poland, Serbia, Slovakia, Slovenia, Spain (continental), Sweden, Switzerland|
|UTC+02:00||2015-01-11T01:58:23+02:00||Burundi, Egypt, Malawi, Mozambique, Russia (Kaliningrad Oblast), Rwanda, South Africa, Swaziland, Zambia, Zimbabwe||Bulgaria, Cyprus, Estonia, Finland, Greece, Israel, Jordan, Latvia, Lebanon, Libya, Lithuania, Moldova, Northern Cyprus, Palestine, Romania, Syria, Turkey, Ukraine|
|UTC+03:00||2015-01-11T02:58:23+03:00||Belarus, Djibouti, Eritrea, Ethiopia, Iraq, Kenya, Madagascar, Russia (European), Saudi Arabia, Somalia, South Sudan, Sudan, Tanzania, Uganda, Yemen||
|UTC+04:00||2015-01-11T03:58:23+04:00||Armenia, Azerbaijan, Georgia, Mauritius, Oman, Seychelles, United Arab Emirates|
|UTC+05:00||2015-01-11T04:58:23+05:00||Kazakhstan (west), Maldives, Pakistan, Uzbekistan|
|UTC+05:30||2015-01-11T05:28:23+05:30||India, Sri Lanka|
|UTC+06:00||2015-01-11T05:58:23+06:00||Kazakhstan (most), Bangladesh, Bhutan, Russia (Ural: Sverdlovsk Oblast, Chelyabinsk Oblast)|
|UTC+06:30||2015-01-11T06:28:23+06:30||Cocos Islands, Myanmar|
|UTC+07:00||2015-01-11T06:58:23+07:00||Western Indonesia, Russia (Novosibirsk Oblast), Thailand, Vietnam, Cambodia,Laos|
|UTC+08:00||2015-01-11T07:58:23+08:00||Hong Kong, Central Indonesia, China, Russia (Krasnoyarsk Krai), Malaysia, Philippines, Singapore, Taiwan, most of Mongolia, Australia (Western Australia), (Eucla) official, Macau,Brunei|
|UTC+08:45||2015-01-11T08:43:23+08:45||Australia (Eucla) unofficial|
|UTC+09:00||2015-01-11T08:58:23+09:00||Eastern Indonesia, East Timor, Russia (Irkutsk Oblast), Japan, North Korea, South Korea|
|UTC+09:30||2015-01-11T09:28:23+09:30||Australia (Northern Territory)||Australia (South Australia)|
|UTC+10:00||2015-01-11T09:58:23+10:00||Russia (Zabaykalsky Krai), Papua New Guinea, Australia (Queensland)||Australia (New South Wales, Tasmania, Victoria)|
|UTC+10:30||2015-01-11T10:28:23+10:30||Lord Howe Island|
|UTC+11:00||2015-01-11T10:58:23+11:00||New Caledonia, Russia (Primorsky Krai), Solomon Islands, Vanuatu|
|UTC+12:00||2015-01-11T11:58:23+12:00||Kiribati (Gilbert Islands), Fiji, Russia (Kamchatka Krai)||New Zealand|
|UTC+13:00||2015-01-11T12:58:23+13:00||Kiribati (Phoenix Islands), Tonga, Tokelau||Samoa|
|UTC+14:00||2015-01-11T13:58:23+14:00||Kiribati (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:
Some examples when UTC is 02:00 on Tuesday when or where daylight saving time is not in effect:
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 of day by zone|
|Juneau, San Francisco||PST||120||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3|
|Winnipeg, Chicago, Mexico||CST||90||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5|
|Ottawa, New York, Quito, Lima||EST||75||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6|
|Caracas, La Paz, Santiago||CLT||60||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7|
|Greenland, Rio, Buenos Aires||ART||45||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8|
|London, Lisbon, Algiers, Monrovia||UTC||0||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11|
|Paris, Rome, Lagos, Kinshasa||CET||15||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12|
|Helsinki, Moscow, Cairo, Cape Town||EET||30||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13|
|Archangelsk, Ankara, Addis Abeba||AST||45||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14|
|Magnitogorsk, Mauritius, Réunion||GST||60||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15|
|Salekhard, Bishkek, Kerguelen||PKT||75||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16|
|Irkutsk, Bangkok, Jakarta||ICT||105||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18|
|Yakutsk, Beijing, Manila, Perth||CST||120||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19|
|Magadan, Sydney, Melbourne||AEST||150||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21||22||23||0||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20||21|
Conversion between time zones obeys the relationship
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
In Delhi (IST, UTC offset= +5:30), the New York Stock Exchange opens at
These calculations become more complicated near a daylight saving boundary (because the UTC offset for zone X is a function of the UTC time).
The table "Time of day by zone" gives an overview on the time relations between different zones.
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.
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 captain's 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 keep 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.
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. 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 with other European states, began observing daylight saving (summer) time.
There is a tendency to draw time zone boundaries far to the west of their meridians. The main reason for this is that similar working day schedules around the world have led to people rising on average at 07:00 clock time and going to bed at 23:00 clock time. This means that the middle of the period that people are awake ("awake time noon") occurs at 15:00 (= [7 + 23]/2) clock time, whereas - if using as clock time the time of the nautical time zone to which the location concerned geographically belongs - solar noon occurs at 12:00 (+/- 30 min) clock time. To make solar noon coincide more with awake time noon (i.e. make the sun reach its highest point closer to 15:00 clock time rather than 12:00 clock time), the time of one or even two nautical time zones to the east is chosen. Many of these locations also use DST, adding yet another nautical time zone to the east. As a result, in summer, solar noon in the Spanish town of Muxía occurs at 14:37 clock time, indeed very close to awake time noon (15:00). This western most area of continental Spain never experiences sunset before 18:00 clock time, even in midwinter, despite its lying more than 40 degrees north of the equator. Near the summer solstice, Muxia has sunset times (after 22:00) similar to those of Stockholm, which is in the same time zone and 16 degrees further north. Stockholm has much earlier sunrises, though.
A more extreme example is Nome, Alaska, which is at 165°24′W longitude—just west of center of the idealized Samoa Time Zone (165°W). Nevertheless, Nome observes Alaska Time (135°W) with DST so it is slightly more than two hours ahead of the sun in winter and over three in summer. 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.
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). 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 off and on 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.
Computer operating systems include the necessary support for working with all (or almost all) possible local times based on the various time zones. Internally, operating systems typically use UTC as their basic time-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 on daylight saving time for 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, whereas the US version shows Eastern Time. US Eastern Time and Pacific Time are also used fairly commonly on many US-based English-language websites with global readership. The format is typically based in the W3C Note "datetime".
Email systems and other messaging systems (IRC chat, etc.) 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.
Database records 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 due to the fact that once a year there is a one hour period when local times are ambiguous.
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. Time zone information most commonly comes from the IANA time zone database. In fact, many systems, including anything using the GNU C Library, can make use of this database.
Windows-based computer systems prior to Windows 2000 used local time, but Windows 2000 and later can use UTC as the basic system time. The system registry contains 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 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.
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 is updated whenever timezone rules change. Oracle provides an updater tool for this purpose.
As an alternative to the timezone information bundled with the Java Platform, programmers may choose to use the Joda-Time library. This library includes its own timezone data based on the IANA time zone database.
The DateTime object supports all time zones in the Olson DB and includes the ability to get, set and convert between time zones.
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. As noted there, the most current timezone database can be implemented via the PECL timezonedb.
The standard module datetime stores and operates on the timezone information class tzinfo. The third party pytz module provides access to the full IANA time zone database. Negated time zone offset in seconds is stored time.timezone and time.altzone attributes.
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 two 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, Gemstone, or Dolphin.
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 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:
Orbiting spacecraft typically experience many sunrises and sunsets in a 24-hour period, or in the case of Apollo program astronauts travelling to the moon, none. Thus it is not possible to calibrate time zones with respect to the sun, and still respect a 24-hour sleep/wake cycle. A common practice for space exploration is to use the Earth-based time zone of the launch site or mission control. This keeps the sleeping cycles of the crew and controllers in sync. The International Space Station normally uses Coordinated Universal Time (UTC).
Timekeeping on Mars can be more complex, since the planet has a solar day of approximately 24 hours and 39 minutes, known as a sol. Earth controllers for some Mars missions have synchronized their sleep/wake cycles with the Martian day, because solar-powered rover activity on the surface was tied to periods of light and dark. The difference in day length caused the sleep/wake cycles to slowly drift with respect to the day/night cycles on Earth, repeating approximately once every 36 days.
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