A time zone is a region of the globe that observes a uniform standard time for legal, commercial, and social purposes. Time zones tend to follow the boundaries of countries and their subdivisions because it is convenient for areas in close commercial or other communication to keep the same time.
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 zones are offset by 30 or 45 minutes (e.g. Newfoundland Standard Time is UTC−03:30, Nepal Standard Time is UTC+05:45, and Indian Standard Time is UTC+05:30).
Some higher latitude and temperate zone countries use daylight saving time for part of the year, typically by adjusting local clock time 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 first invented, it was common practice to mark 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 location and dependent on longitude.
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) because of the elliptical 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 sums 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 while each city in England kept a different local time.
Local solar time became increasingly inconvenient as rail transport and telecommunications improved, because clocks differed between places by amounts corresponding to the differences in their geographical longitudes, which varied by four minutes of time for every degree of longitude. For example, Bristol is about 2.5 degrees west of Greenwich (East London), so when it is solar noon in Bristol, it is about 10 minutes past solar noon in London. The use of time zones accumulates these differences into longer units, usually hours, so that nearby places can share a common standard for timekeeping.
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 British clocks from this period have two minute hands—one for the local time, one for GMT.
Improvements in worldwide communication 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 that adopted 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 on the meridian 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 halfway 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.
The first known person to conceive of a worldwide system of time zones was the Italian mathematician Quirico Filopanti. He introduced the idea in his book Miranda! published in 1858. He proposed 24 hourly time zones, which he called "longitudinal days", the first centred on the meridian of Rome. He also proposed a universal time to be used in astronomy and telegraphy. But his book attracted no attention until long after his death.
Scottish-born Canadian Sir Sandford Fleming proposed a worldwide system of time zones in 1879. He advocated his system at several international conferences, and is credited with "the initial effort that led to the adoption of the present time meridians". In 1876, his first proposal was for a global 24-hour clock, conceptually located at the centre 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 1956.
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, 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 of New Zealand, use quarter-hour deviations. Some countries, such as China and India, use a single time zone even though the extent of their territory far exceeds 15° of longitude.
If a time is in Coordinated 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".
Offsets from UTC are written in the format ±[hh]:[mm], ±[hh] [mm], 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 in Berlin during 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 with daylight saving time, e.g. a time offset in Chicago, which is in the North American Central Time Zone, is "−06:00" for the winter (Central Standard Time) and "−05:00" for the summer (Central Daylight Time).
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 standard ISO 8601 and their use as sole designator for a time zone is discouraged. 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 that do not use DST||Example locations that in summer use DST|
|UTC−12:00||2019-01-18T05:11:09-12:00||Baker Island, Howland Island (both uninhabited)|
|UTC−11:00||2019-01-18T06:11:09-11:00||American Samoa, Niue|
|UTC−10:00||2019-01-18T07:11:09-10:00||French Polynesia (most), United States (Hawaii) (Cook Islands)||United States (Aleutian Islands)|
|UTC−09:30||2019-01-18T07:41:09-09:30||French Polynesia (Marquesas Islands)|
|UTC−09:00||2019-01-18T08:11:09-09:00||French Polynesia (Gambier Islands)||United States (most of Alaska)|
|UTC−08:00||2019-01-18T09:11:09-08:00||Pitcairn Islands||Canada (most of British Columbia), Mexico (Baja California), United States (California, most of Nevada, most of Oregon, Washington)|
|UTC−07:00||2019-01-18T10:11:09-07:00||Canada (northeastern British Columbia), Mexico (Sonora), United States (most of Arizona)||Canada (Alberta), Mexico (Chihuahua), United States (Colorado)|
|UTC−06:00||2019-01-18T11:11:09-06:00||Belize, Canada (most of Saskatchewan), Costa Rica, Ecuador (Galápagos Islands), El Salvador, Guatemala, Honduras, Nicaragua||Canada (Manitoba), United States (Illinois, most of Texas), Mexico (most), Chile (Easter Island)|
|UTC−05:00||2019-01-18T12:11:09-05:00||Brazil (Acre), Colombia, Ecuador (continental), Haiti, Jamaica, Mexico (most of Quintana Roo), Panama, Peru||Bahamas, Canada (most of Ontario, most of Quebec), Cuba, United States (most of Florida, Georgia, New York, Ohio, Pennsylvania)|
|UTC−04:00||2019-01-18T13:11:10-04:00||Barbados, Bolivia, Brazil (most of Amazonas, Rondônia, Roraima), Dominican Republic, Puerto Rico, Trinidad and Tobago, Venezuela||Brazil ( most of Mato Grosso, Mato Grosso do Sul), Canada ( Nova Scotia, New Brunswick and most of Labrador, Prince Edward Island), Chile (continental), Paraguay|
|UTC−03:30||2019-01-18T13:41:10-03:30||Canada ( island of Newfoundland, southeastern Labrador)|
|UTC−03:00||2019-01-18T14:11:10-03:00||Argentina, Brazil ( Bahia, Ceará, Maranhão, Pará, Pernambuco), Chile (Magallanes Region), Falkland Islands, Saint Pierre and Miquelon, Uruguay||Brazil (Federal District, Minas Gerais, Paraná, Rio Grande do Sul, Rio de Janeiro, São Paulo), Greenland (most)|
|UTC−02:00||2019-01-18T15:11:10-02:00||Brazil (Fernando de Noronha), South Georgia and the South Sandwich Islands|
|UTC−01:00||2019-01-18T16:11:10-01:00||Cape Verde||Portugal ( Azores)|
|UTC±00:00||2019-01-18T17:11:10+00:00||Ivory Coast, Ghana, Iceland, Saint Helena, Senegal, Mali||Faroe Islands, Ireland, Portugal (continental, Madeira), Spain ( Canary Islands), United Kingdom|
|UTC+01:00||2019-01-18T18:11:10+01:00||Algeria, Angola, Benin, Cameroon, Democratic Republic of the Congo (west), Gabon, Morocco, Niger, Nigeria, Tunisia, Western Sahara||Albania, Andorra, Austria, Belgium, Bosnia and Herzegovina, Croatia, Czech Republic, Denmark, France (metropolitan), Germany, Hungary, Italy, Liechtenstein, Luxembourg, Monaco, Malta, Macedonia, Netherlands (European), Norway, Poland, Serbia, Slovakia, Slovenia, Spain (continental), Sweden, Switzerland, Vatican City|
|UTC+02:00||2019-01-18T19:11:10+02:00||Burundi, Egypt, Malawi, Mozambique, Namibia, Russia ( Kaliningrad), Rwanda, South Africa, Sudan, Swaziland, Zambia, Zimbabwe||Bulgaria, Cyprus, Estonia, Finland, Greece, Israel, Jordan, Latvia, Lebanon, Lithuania, Moldova, Palestine, Romania, Syria, Ukraine|
|UTC+03:00||2019-01-18T20:11:10+03:00||Belarus, Djibouti, Eritrea, Ethiopia, Iraq, Kenya, Kuwait, Madagascar, Russia (most of European part), Saudi Arabia, Qatar, Somalia, South Sudan, Tanzania, Turkey, Uganda, Yemen,|
|UTC+04:00||2019-01-18T21:11:10+04:00||Armenia, Azerbaijan, Georgia, Mauritius, Oman, Russia ( Samara), Seychelles, United Arab Emirates|
|UTC+05:00||2019-01-18T22:11:10+05:00||Kazakhstan (west), Maldives, Pakistan, Russia ( Sverdlovsk, Chelyabinsk), Uzbekistan|
|UTC+05:30||2019-01-18T22:41:10+05:30||India, Sri Lanka|
|UTC+06:00||2019-01-18T23:11:10+06:00||Bangladesh, Bhutan, British Indian Ocean Territory, Kazakhstan (most), Russia ( Omsk)|
|UTC+06:30||2019-01-18T23:41:10+06:30||Cocos Islands, Myanmar|
|UTC+07:00||2019-01-19T00:11:10+07:00||Cambodia, Indonesia (west), Laos, Mongolia (west), Russia ( Krasnoyarsk), Thailand, Vietnam|
|UTC+08:00||2019-01-19T01:11:11+08:00||Australia ( Western Australia), Brunei, People's Republic of China, Hong Kong, Indonesia (central), Macau, Malaysia, Mongolia (most), Philippines, Russia ( Irkutsk), Singapore, Republic of China|
|UTC+08:45||2019-01-19T01:56:11+08:45||Australia (Eucla unofficial)|
|UTC+09:00||2019-01-19T02:11:11+09:00||Timor-Leste, Indonesia (east), Japan, North Korea, Russia ( most of Sakha), South Korea|
|UTC+09:30||2019-01-19T02:41:11+09:30||Australia ( Northern Territory)||Australia ( South Australia)|
|UTC+10:00||2019-01-19T03:11:11+10:00||Australia ( Queensland), Papua New Guinea, Russia ( Primorsky)||Australia ( New South Wales, Tasmania, Victoria)|
|UTC+10:30||2019-01-19T03:41:11+10:30||Lord Howe Island|
|UTC+11:00||2019-01-19T04:11:11+11:00||New Caledonia, Russia ( Magadan), Solomon Islands, Vanuatu|
|UTC+12:00||2019-01-19T05:11:11+12:00||Kiribati (Gilbert Islands), Russia ( Kamchatka)||Fiji, New Zealand (most)|
|UTC+12:45||2019-01-19T05:56:11+12:45||New Zealand (Chatham Islands)|
|UTC+13:00||2019-01-19T06:11:11+13:00||Kiribati (Phoenix Islands), Tokelau, Tonga||Samoa|
|UTC+14:00||2019-01-19T07:11:11+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|
|San Francisco, Los Angeles||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 City||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, Miami, 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 de Janeiro, 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
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. Another reason is that it can allow the more efficient use of sunlight. 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,[note 1] 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 westernmost 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, Muxía 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 August 9 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 because once a year there is a one-hour period when local times are ambiguous.
Calendar systems nowadays 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. There is also an option in newer versions of Microsoft Outlook to enter the time zone in which an event will happen, but often not in other calendar systems. Calendaring software must also deal with daylight 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. In Microsoft Outlook, time stamps are therefore stored and communicated without DST offsets. Hence, an appointment in London at noon in the summer will be represented as 12:00 (UTC+00:00) even though the event will actually take place at 13:00 UTC. In Google Calendar, calendar events are stored in UTC (although shown in local time) and might be changed by a time-zone changes, although normal daylight saving start and end are compensated for (similar to much other calendar software).
Most Unix-like systems, including Linux and Mac OS X, keep system time in time_t format, representing the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC) on Thursday, January 1, 1970,. By default the external representation is as UTC (Coordinated Universal Time), though individual processes can specify time zones using the LC_TIME environment variable or less commonly 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.
As of Java 8 new DATE TIME API is there that can help converting timezones. Java 8 Date Time
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 included with Python 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.
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 Greenwich Mean Time (GMT).
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.
The Atlantic Time Zone is a geographical region that keeps standard time—called Atlantic Standard Time (AST)—by subtracting four hours from Coordinated Universal Time (UTC), resulting in UTC−04:00. During part of the year, some portions of the zone observe daylight saving time, referred to as Atlantic Daylight Time (ADT), by moving their clocks forward one hour to result in UTC−03:00. The clock time in this zone is based on the mean solar time of the 60th meridian west of the Greenwich Observatory.
In Canada, the provinces of New Brunswick, Nova Scotia, and Prince Edward Island are in this zone, though legally they calculate time specifically as an offset of four hours from Greenwich Mean Time (GMT–4) rather than from UTC. Small portions of Quebec (eastern Côte-Nord and the Magdalen Islands) also observe Atlantic Time. Officially, the entirety of Newfoundland and Labrador observes Newfoundland Standard Time, but in practice Atlantic Time is used in most of Labrador.
No portion of the continental United States currently uses Atlantic Time, although it is used by the territories of Puerto Rico and the U.S. Virgin Islands. A number of New England states are considering a regional change to Atlantic Standard Time year-round (with no observance of daylight saving time), even though only a small portion of Maine lies to the east of the 67.5°W theoretical extent of this zone. Florida is in the process of enacting a similar change; in both cases any changes will need to be approved by the United States Department of Transportation and the United States Congress.Central Time Zone
The North American Central Time Zone (CT) is a time zone in parts of Canada, the United States, Mexico, Central America, some Caribbean Islands, and part of the Eastern Pacific Ocean.
Central Standard Time (CST) is six hours behind Coordinated Universal Time (UTC). During summer most of the zone uses daylight saving time (DST), and changes to Central Daylight Time (CDT) which is five hours behind UTC.East Africa Time
East Africa Time, or EAT, is a time zone used in eastern Africa. The time zone is three hours ahead of UTC (UTC+03:00), which is the same as Arabia Standard Time, Further-eastern European Time, Moscow Time and Eastern European Summer Time.As this Time Zone is predominantly in the equatorial region, there is no significant change in day length throughout the year and so daylight saving time is not observed.East Africa Time is used by the following countries:
UgandaEastern Time Zone
The Eastern Time Zone (ET) is a time zone encompassing part or all of 22 states in the eastern part of the contiguous United States, parts of eastern Canada, the state of Quintana Roo in Mexico, Panama in Central America, and the Caribbean Islands.
Places that use Eastern Standard Time (EST) when observing standard time (autumn/winter) are 5 hours behind Coordinated Universal Time (UTC−05:00).
Eastern Daylight Time (EDT), when observing daylight saving time DST (spring/summer) is 4 hours behind Coordinated Universal Time (UTC−04:00).
In the northern parts of the time zone, on the second Sunday in March, at 2:00 a.m. EST, clocks are advanced to 3:00 a.m. EDT leaving a one-hour "gap". On the first Sunday in November, at 2:00 a.m. EDT, clocks are moved back to 1:00 a.m. EST, thus "duplicating" one hour. Southern parts of the zone (Panama and the Caribbean) do not observe daylight saving time.Greenwich Mean Time
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, reckoned from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a consequence, it cannot be used to specify a precise time unless a context is given.
English speakers often use GMT as a synonym for Coordinated Universal Time (UTC). For navigation, it is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude); but this meaning can differ from UTC by up to 0.9 s. The term GMT should not thus be used for technical purposes.Because of Earth's uneven speed in its elliptical orbit and its axial tilt, noon (12:00:00) GMT is rarely the exact moment the sun crosses the Greenwich meridian and reaches its highest point in the sky there. This event may occur up to 16 minutes before or after noon GMT, a discrepancy calculated by the equation of time. Noon GMT is the annual average (i.e. "mean") moment of this event, which accounts for the word "mean" in "Greenwich Mean Time".
Originally, astronomers considered a GMT day to start at noon, while for almost everyone else it started at midnight. To avoid confusion, the name Universal Time was introduced to denote GMT as counted from midnight. Astronomers preferred the old convention to simplify their observational data, so that each night was logged under a single calendar date. Today Universal Time usually refers to UTC or UT1.The term "GMT" is especially used by bodies connected with the United Kingdom, such as the BBC World Service, the Royal Navy, the Met Office and others particularly in Arab countries, such as the Middle East Broadcasting Centre and OSN. It is a term commonly used in the United Kingdom and countries of the Commonwealth, including Australia, New Zealand, South Africa, India, Pakistan, Bangladesh and Malaysia; and in many other countries of the Eastern Hemisphere.Indian Standard Time
Indian Standard Time (IST) is the time observed throughout India, with a time offset of UTC+05:30. India does not observe daylight saving time (DST) or other seasonal adjustments. In military and aviation time IST is designated E* ("Echo-Star").Indian Standard Time is calculated on the basis of 82.5' E longitude, in Mirzapur (Amravati Chauraha), Uttar Pradesh, which is nearly on the corresponding longitude reference line.Iran Standard Time
Iran Standard Time (IRST) or Iran Time (IT) is the time zone used in Iran. Iran uses a UTC offset UTC+03:30. IRST is defined by the 52.5 degrees east meridian, the same meridian which defines the Iranian calendar and is the official meridian of Iran.
Between 2005 and 2008, by decree of President Mahmoud Ahmadinejad, Iran did not observe daylight saving time (called Iran Daylight Time or IRDT). It was reintroduced from 21 March 2008.Japan Standard Time
Japan Standard Time or JST (日本標準時/-ja "Nippon Hyōjunji" and Japanese pronunciation: [nipːɔ̞n çɔ̞ː(d)ʒʉn(d)ʒi], or 中央標準時/Japanese pronunciation: [tʃʉːɔ̞ː çɔ̞ː(d)ʒʉn(d)ʒi] Chūō Hyōjunji) is the standard timezone in Japan, 9 hours ahead of UTC (i.e. it is UTC+09:00). There is no daylight saving time, though its introduction has been debated several times. During World War II, it was often called Tokyo Standard Time.
Japan Standard Time is the same as Korean Standard Time, Indonesian Eastern Standard Time, East-Timorese Standard Time and Yakutsk Time (Russia)Mountain Time Zone
The Mountain Time Zone of North America keeps time by subtracting seven hours from Coordinated Universal Time (UTC) when standard time is in effect, and by subtracting six hours during daylight saving time (UTC−06:00). The clock time in this zone is based on the mean solar time at the 105th meridian west of the Greenwich Observatory. In the United States, the exact specification for the location of time zones and the dividing lines between zones is set forth in the Code of Federal Regulations at 49 CFR 71.In the United States and Canada, this time zone is generically called Mountain Time (MT). Specifically, it is Mountain Standard Time (MST) when observing standard time, and Mountain Daylight Time (MDT) when observing daylight saving time. The term refers to how the Rocky Mountains, which range from northwestern Canada to the US state of New Mexico, are located almost entirely in the time zone. In Mexico, this time zone is known as the Zona Pacífico (Pacific Zone). In the US and Canada, the Mountain Time Zone is to the east of the Pacific Time Zone and to the west of the Central Time Zone.
In some areas, starting in 2007, the local time changes from MST to MDT at 2 am MST to 3 am MDT on the second Sunday in March and returns at 2 am MDT to 1 am MST on the first Sunday in November.
Sonora in Mexico and most of Arizona in the United States do not observe daylight saving time, and during the spring, summer, and autumn months they are on the same time as Pacific Daylight Time. The Navajo Nation, most of which lies within Arizona, does observe DST, although the Hopi Nation, as well as some Arizona state offices lying within the Navajo Nation, do not.
The largest city in the Mountain Time Zone is Phoenix, Arizona. The Phoenix metropolitan area is the largest metropolitan area in the zone; the next largest metropolitan area that observes Mountain Time is Denver, closely followed by the El Paso–Juárez area.
TV broadcasting in the Mountain Time Zone is typically tape-delayed one hour, so that shows match the broadcast times of the Central Time Zone (i.e. prime time begins at 7 pm MT following the same order of programming as the Central Time Zone).Nepal Standard Time
Nepal Standard Time (NPT) is the time zone for Nepal. With a time offset from Coordinated Universal Time (UTC) of UTC+05:45 all over Nepal, it is one of only three time zones with a 45-minute offset from UTC. (The others are Chatham Island Standard Time, with an offset of UTC+12:45, and the unofficial Australian Central Western Time, with an offset of UTC+08:45.)NPT is an approximation of Kathmandu mean time, which is 5:41:16 ahead of UTC. The standard meridian passes through the peak of Gaurishankar mountain about 100 km east of Kathmandu.Nepal used local solar time until 1920, in Kathmandu UTC+5:41:16. In 1920, Nepal adopted Indian Standard Time, UTC+05:30. In 1986 Nepal advanced their clocks by 15 minutes, giving them a time zone of UTC+05:45.Pacific Time Zone
The Pacific Time Zone (PT) is a time zone encompassing parts of western Canada, the western United States, and western Mexico. Places in this zone observe standard time by subtracting eight hours from Coordinated Universal Time (UTC−8). During daylight saving time, a time offset of UTC−7 is used.
In the United States and Canada, this time zone is generically called the "Pacific Time Zone". Specifically, time in this zone is referred to as "Pacific Standard Time" (PST) when standard time is being observed (early November to mid-March), and "Pacific Daylight Time" (PDT) when daylight saving time (mid-March to early November) is being observed. In Mexico, the corresponding time zone is known as the Zona Noroeste (Northwest Zone) and observes the same daylight saving schedule as the U.S. and Canada. The largest city in the Pacific Time Zone is Los Angeles; the city's metropolitan area is the largest in the zone.
The zone is two hours ahead of the Hawaii–Aleutian Time Zone, one hour ahead of the Alaska Time Zone, one hour behind the Mountain Time Zone, two hours behind the Central Time Zone, three hours behind the Eastern Time Zone, and four hours behind the Atlantic Time Zone.Time in China
The time in China follows a single standard time offset of UTC+08:00 (eight hours ahead of Coordinated Universal Time), despite China spanning five geographical time zones. The official national standard time is called Beijing Time (Chinese: 北京时间) domestically and China Standard Time (CST) internationally. Daylight saving time has not been observed since 1991.The special administrative regions (SARs) maintain their own time authorities, with standards called Hong Kong Time (香港時間) and Macau Standard Time (澳門標準時間). These have been equivalent to Beijing time since 1992.
In addition, it has been proposed during 2005's NPC & CPPCC of China that provinces in the west (such as Shaanxi, Sichuan, and Chongqing) should use the time offset of UTC+07:00. However, this proposal has not been voted upon yet.Time in the United States
Time in the United States, by law, is divided into nine standard time zones covering the states and its possessions, with most of the United States observing daylight saving time (DST) for approximately the spring, summer, and fall months. The time zone boundaries and DST observance are regulated by the Department of Transportation. Official and highly precise timekeeping services (clocks) are provided by two federal agencies: the National Institute of Standards and Technology (NIST) (an agency of the Department of Commerce); and its military counterpart, the United States Naval Observatory (USNO). The clocks run by these services are kept synchronized with each other as well as with those of other international timekeeping organizations.
It is the combination of the time zone and daylight saving rules, along with the timekeeping services, which determines the legal civil time for any U.S. location at any moment.Western European Time
Western European Time (WET, UTC±00:00) is a time zone covering parts of western and northwestern Europe. The following countries and regions use WET in winter months:
Canary Islands, since 1946 (rest of Spain is CET, UTC+1)
Faroe Islands, since 1908
North Eastern Greenland (Danmarkshavn and surrounding area)
Iceland, since 1968, without summer time changes
Portugal, since 1912 with pauses (except Azores, UTC−1)
Madeira islands, since 1912 with pauses
Ireland (legally known as Greenwich Mean Time), since 1916, except between 1968 and 1971
United Kingdom and Crown dependencies (legally known as Greenwich Mean Time), since 1847 in England, Scotland, Wales, the Channel Islands, and the Isle of Man, and since 1916 in Northern Ireland, with pausesAll the above countries except Iceland implement daylight saving time in summer (from the last Sunday in March to the last Sunday in October each year), switching to Western European Summer Time (WEST, UTC+1), which is one hour ahead of WET. WEST is called British Summer Time in the UK and is officially known as Irish Standard Time in Ireland.
The nominal span of the time zone is 7.5°E to 7.5°W (0° ± 7.5°), but the WET zone does not include the Netherlands, Belgium, Luxembourg, France, Gibraltar or Spain which use Central European Time (CET), although these are mostly (France) or completely (the rest) west of 7.5°E. Conversely, Iceland and eastern Greenland are included although both are west of 7.5°W. In September 2013, a Spanish parliamentary committee recommended switching to WET.