A time zone is an area which observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it is convenient for areas in frequent communication to keep the same time.
Each time zone is defined by a standard offset from Coordinated Universal Time (UTC). The offsets range from UTC−12:00 to UTC+14:00, and are usually a whole number of hours, but a few zones are offset by an additional 30 or 45 minutes, such as in India and Nepal. Some areas in a time zone may use a different offset for part of the year, typically one hour ahead during spring and summer, a practice known as daylight saving time (DST).
List of UTC offsets
In the table below, the locations that use daylight saving time (DST) are listed in their UTC offset when DST is not in effect. When DST is in effect, approximately during spring and summer, their UTC offset is increased by one hour (except for Lord Howe Island, where it is increased by 30 minutes). For example, during the DST period California observes UTC−07:00 and the United Kingdom observes UTC+01:00.
| UTC offset | Locations that do not use DST | Locations that use DST | ||
|---|---|---|---|---|
| UTC−12:00 | ||||
| UTC−11:00 |  American Samoa Jarvis Island Kingman Reef Midway Atoll Niue Palmyra Atoll | |||
| UTC−10:00 |  Cook Islands French Polynesia (most) Johnston Atoll United States: Hawaii | United States: Andreanof Islands, Islands of Four Mountains, Near Islands, Rat Islands (Aleutian Islands, Alaska) | ||
| UTC−09:30 | French Polynesia: Marquesas Islands | |||
| UTC−09:00 | French Polynesia: Gambier Islands | United States: Alaska (most) | ||
| UTC−08:00 |  Clipperton Island Pitcairn Islands |  Canada: British Columbia (most) Mexico: Baja California United States: California, Idaho (north), Nevada (most), Oregon (most), Washington | ||
| UTC−07:00 | Canada: British Columbia (northeast), Yukon Mexico: Baja California Sur, Nayarit (most), Sinaloa, Sonora United States: Arizona (most) | Canada: Alberta, British Columbia (southeast), Northwest Territories, Nunavut (west), Saskatchewan (Lloydminster area) Mexico: Chihuahua (northwest border) United States: Arizona (Navajo Nation), Colorado, Idaho (most), Kansas (west), Montana, Nebraska (west), New Mexico, Nevada (northeast border), North Dakota (southwest), Oregon (east), South Dakota (west), Texas (west), Utah, Wyoming | ||
| UTC−06:00 |  Belize Canada: Saskatchewan (most) Costa Rica Ecuador: Galápagos El Salvador Guatemala Honduras Mexico (most) Nicaragua | Canada: Manitoba, Nunavut (central), Ontario (west) Chile: Easter Island Mexico (northeast border) United States: Alabama, Arkansas, Florida (northwest), Illinois, Indiana (northwest, southwest), Iowa, Kansas (most), Kentucky (west), Louisiana, Michigan (northwest border), Minnesota, Mississippi, Missouri, Nebraska (most), North Dakota (most), Oklahoma, South Dakota (most), Tennessee (most), Texas (most), Wisconsin | ||
| UTC−05:00 |  Brazil: Acre, Amazonas (southwest) Canada: Atikokan, Mishkeegogamang, Southampton Island Cayman Islands Colombia Ecuador (most) Jamaica Mexico: Quintana Roo Navassa Island Panama Peru |  Bahamas Canada: Nunavut (east), Ontario (most), Quebec (most) Cuba Haiti Turks and Caicos Islands United States: Connecticut, Delaware, District of Columbia, Florida (most), Georgia, Indiana (most), Kentucky (east), Maine, Maryland, Massachusetts, Michigan (most), New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Tennessee (east), Vermont, Virginia, West Virginia | ||
| UTC−04:00 |  Anguilla Antigua and Barbuda Aruba Barbados Bolivia Brazil: Amazonas (most), Mato Grosso, Mato Grosso do Sul, Rondônia, Roraima British Virgin Islands Canada: Quebec (east) Caribbean Netherlands Curaçao Dominica Dominican Republic Grenada Guadeloupe Guyana Martinique Montserrat Puerto Rico Saint Barthélemy Saint Kitts and Nevis Saint Lucia Saint Martin Saint Vincent and the Grenadines Sint Maarten Trinidad and Tobago U.S. Virgin Islands Venezuela |  Bermuda Canada: Labrador (most), New Brunswick, Nova Scotia, Prince Edward Island Chile (most) Greenland: Pituffik Space Base | ||
| UTC−03:30 | Canada: Newfoundland, Labrador (southeast) | |||
| UTC−03:00 |  Argentina Brazil (most) Chile: Magallanes Region Falkland Islands French Guiana Paraguay Suriname Uruguay | Saint Pierre and Miquelon | ||
| UTC−02:00 | Brazil: Fernando de Noronha South Georgia and the South Sandwich Islands | Greenland (most) | ||
| UTC−01:00 |  Cape Verde |  Portugal: Azores | ||
| UTC+00:00 |  Burkina Faso Gambia Ghana Greenland: National Park (east coast) Guinea Guinea-Bissau Iceland Ivory Coast Liberia Mali Mauritania Saint Helena, Ascension and Tristan da Cunha Senegal Sierra Leone São Tomé and Príncipe Togo |  Faroe Islands Guernsey Ireland Isle of Man Jersey Portugal (most) Spain: Canary Islands United Kingdom | ||
| UTC+01:00 |  Albania Andorra Austria Belgium Bosnia and Herzegovina Croatia Czech Republic Denmark France (metropolitan) Germany Gibraltar Hungary Italy Kosovo Liechtenstein Luxembourg Malta Monaco Montenegro Netherlands (European) North Macedonia Norway Poland San Marino Serbia Slovakia Slovenia Spain (most) Sweden Switzerland Vatican City | |||
| UTC+02:00 | ||||
| UTC+03:00 |  Abkhazia Bahrain Belarus Comoros Djibouti Eritrea Ethiopia French Southern and Antarctic Lands: Scattered Islands Iraq Jordan Kenya Kuwait Madagascar Mayotte Qatar Russia (most of European part) Saudi Arabia Somalia Somaliland South Africa: Prince Edward Islands South Ossetia Syria Tanzania Turkey Uganda Ukraine: occupied territories Yemen | |||
| UTC+03:30 |  Iran | |||
| UTC+04:00 |  Armenia Azerbaijan French Southern and Antarctic Lands: Crozet Islands Georgia Mauritius Oman Russia: Astrakhan, Samara, Saratov, Udmurtia, Ulyanovsk Réunion Seychelles United Arab Emirates | |||
| UTC+04:30 |  Afghanistan | |||
| UTC+05:00 | French Southern and Antarctic Lands: Kerguelen Islands, Saint Paul Island, Amsterdam Island Heard Island and McDonald Islands Kazakhstan Maldives Pakistan Russia: Bashkortostan, Chelyabinsk, Khanty-Mansi, Kurgan, Orenburg, Perm, Sverdlovsk, Tyumen, Yamalia Tajikistan Turkmenistan Uzbekistan | |||
| UTC+05:30 |  India Sri Lanka | |||
| UTC+05:45 |  Nepal | |||
| UTC+06:00 | ||||
| UTC+06:30 |  Cocos (Keeling) Islands Myanmar | |||
| UTC+07:00 |  Cambodia Christmas Island Indonesia: Sumatra, Java, West Kalimantan, Central Kalimantan Laos Mongolia: Bayan-Ölgii, Khovd, Uvs Russia: Altai Krai, Altai Republic, Kemerovo, Khakassia, Krasnoyarsk, Novosibirsk, Tomsk, Tuva Thailand Vietnam | |||
| UTC+08:00 | Australia: Western Australia (most) Brunei China Hong Kong Indonesia: South Kalimantan, East Kalimantan, North Kalimantan, Sulawesi, Lesser Sunda Islands Macau Malaysia Mongolia (most) Philippines Russia: Buryatia, Irkutsk Singapore Taiwan | |||
| UTC+08:45 | Australia: Eucla | |||
| UTC+09:00 |  Timor-Leste Indonesia: Maluku Islands, Western New Guinea Japan North Korea Palau Russia: Amur, Sakha (most), Zabaykalsky South Korea | |||
| UTC+09:30 | Australia: Northern Territory | Australia: South Australia, Yancowinna County | ||
| UTC+10:00 | Australia: Queensland Guam Micronesia: Chuuk, Yap Northern Mariana Islands Papua New Guinea (most) Russia: Jewish, Khabarovsk, Primorsky, Sakha (central-east) | Australia: Australian Capital Territory, Jervis Bay Territory, New South Wales (most), Tasmania, Victoria | ||
| UTC+10:30 | Australia: Lord Howe Island (DST increase is 30 minutes) | |||
| UTC+11:00 | Micronesia: Kosrae, Pohnpei New Caledonia Papua New Guinea: Bougainville Russia: Magadan, Sakha (east), Sakhalin Solomon Islands Vanuatu |  Norfolk Island | ||
| UTC+12:00 |  Fiji Kiribati: Gilbert Islands Marshall Islands Nauru Russia: Chukotka, Kamchatka Tuvalu Wake Island Wallis and Futuna |  New Zealand (most) | ||
| UTC+12:45 | New Zealand: Chatham Islands | |||
| UTC+13:00 | Kiribati: Phoenix Islands Samoa Tokelau Tonga | |||
| UTC+14:00 | Kiribati: Line Islands | |||
History
The apparent position of the Sun in the sky, and thus solar time, varies by location due to the spherical shape of the Earth. This variation corresponds to four minutes of time for every degree of longitude, so for example when it is solar noon in London, it is about 10 minutes before solar noon in Bristol, which is about 2.5 degrees to the west.
The Royal Observatory, Greenwich, founded in 1675, established Greenwich Mean Time (GMT), the mean solar time at that location, as an aid to mariners to determine longitude at sea, providing a standard reference time while each location in England kept a different time.
Railway time
In the 19th century, as transportation and telecommunications improved, it became increasingly inconvenient for each location to observe its own solar time. In November 1840, the British Great Western Railway started using GMT kept by portable chronometers.[failed verification] This practice was soon followed by other railway companies in Great Britain and became known as railway time.
Around August 23, 1852, time signals were first transmitted by telegraph from the Royal Observatory. By 1855, 98% of Great Britain's public clocks were using GMT, but it was not made the island's legal time until August 2, 1880. Some British clocks from this period have two minute hands, one for the local time and one for GMT.
On November 2, 1868, the British Colony of New Zealand officially adopted a standard time to be observed throughout the colony. It was based on 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 North American railroads in the 19th century was complex. Each railroad used its own standard time, usually based on the local time of its headquarters or most important terminus, and the railroad's train schedules were published using its own time. Some junctions served by several railroads had a clock for each railroad, each showing a different time. Because of this a number of accidents occurred when trains from different companies using the same tracks mistimed their passings.
Around 1863, Charles F. Dowd proposed a system of hourly standard time zones for North American railroads, although he published nothing on the matter at that time and did not consult railroad officials until 1869. In 1870 he proposed four ideal time zones having north–south borders, the first centered on Washington, D.C., but by 1872 the first was centered on meridian 75° west of Greenwich, with natural borders such as sections of the Appalachian Mountains. Dowd's system was never accepted by North American railroads. Chief meteorologist at the United States Weather Bureau Cleveland Abbe divided the United States into four standard time zones for consistency among the weather stations. In 1879, he published a paper titled Report on Standard Time. In 1883, he convinced North American railroad companies to adopt his time-zone system. In 1884, Britain, which had already adopted its own standard time system for England, Scotland, and Wales, helped gather international consent for global time. In time, the American government, influenced in part by Abbe's 1879 paper, adopted the time-zone system. It was 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 North American zones were named Intercolonial, Eastern, Central, Mountain, and Pacific. Within a year 85% of all cities with populations over 10,000 (about 200 cities) were using standard time. A notable exception was Detroit (located about halfway between the meridians of Eastern and Central time), which kept local time until 1900, then tried Central Standard Time, local mean time, and Eastern Standard Time (EST) before a May 1915 ordinance settled on EST and was ratified by popular vote in August 1916. The confusion of times came to an end when standard time zones were formally adopted by the U.S. Congress in the Standard Time Act of March 19, 1918.
Worldwide time zones
Italian mathematician Quirico Filopanti introduced the idea of a worldwide system of time zones 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. However, his book attracted no attention until long after his death.
Scottish-born Canadian Sir Sandford Fleming proposed a worldwide system of time zones in 1876 - see Sandford Fleming § Inventor of worldwide standard time. The proposal divided the world into twenty-four time zones labeled A-Y (skipping J), each one covering 15 degrees of longitude. All clocks within each zone would be set to the same time as the others, but differed by one hour from those in the neighboring zones. He advocated his system at several international conferences, including the International Meridian Conference, where it received some consideration. The system has not been directly adopted, but some maps divide the world into 24 time zones and assign letters to them, similarly to Fleming's system.
By about 1900, almost all inhabited places on Earth had adopted a standard time zone, but only some of them used an hourly offset from GMT. Many applied the time at a local astronomical observatory to an entire country, without any reference to GMT. It took many decades before all time zones were based on some standard offset from GMT or Coordinated Universal Time (UTC). By 1929, the majority of countries had adopted hourly time zones, though some countries such as Iran, India, Myanmar and parts of Australia had time zones with a 30-minute offset. Nepal was the last country to adopt a standard offset, shifting slightly to UTC+05:45 in 1986.
All nations currently use standard time zones for secular purposes, but not all of them apply the concept as originally conceived. Several countries and subdivisions use half-hour or quarter-hour deviations from standard time. Some countries, such as China and India, use a single time zone even though the extent of their territory far exceeds the ideal 15° of longitude for one hour; other countries, such as Spain and Argentina, use standard hour-based offsets, but not necessarily those that would be determined by their geographical location. The consequences, in some areas, can affect the lives of local citizens, and in extreme cases contribute to larger political issues, such as in the western reaches of China. In Russia, which has 11 time zones, two time zones were removed in 2010 and reinstated in 2014.
Notation
ISO 8601
ISO 8601 is a standard established by the International Organization for Standardization defining methods of representing dates and times in textual form, including specifications for representing time zones.
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". UTC time is also known as "Zulu" time, since "Zulu" is a phonetic alphabet code word for the letter "Z".
Offsets from UTC are written in the format ±hh:mm, ±hhmm, or ±hh (either hours ahead or behind UTC). For example, if the time being described is one hour ahead of UTC (such as the time in Germany 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).
Abbreviations
Time zones are often represented by alphabetic abbreviations such as "EST", "WST", and "CST", but these are not part of the international time and date standard ISO 8601. Such designations can be ambiguous; for example, "CST" can mean (North American) Central Standard Time (UTC−06:00), Cuba Standard Time (UTC−05:00) and China Standard Time (UTC+08:00), and it is also a widely used variant of ACST (Australian Central Standard Time, UTC+09:30).
Conversions
Conversion between time zones obeys the relationship
- "time in zone A" − "UTC offset for zone A" = "time in zone B" − "UTC offset for zone B",
in which each side of the equation is equivalent to UTC.
The conversion equation can be rearranged to
- "time in zone B" = "time in zone A" − "UTC offset for zone A" + "UTC offset for zone B".
For example, the New York Stock Exchange opens at 09:30 (EST, UTC offset= −05:00). In California (PST, UTC offset= −08:00) and India (IST, UTC offset= +05:30), the New York Stock Exchange opens at
- time in California = 09:30 − (−05:00) + (−08:00) = 06:30;
- time in India = 09:30 − (−05:00) + (+05:30) = 20:00.
These calculations become more complicated near the time switch to or from daylight saving time, as the UTC offset for the area becomes a function of UTC time.
The time differences may also result in different dates. For example, when it is 22:00 on Monday in Egypt (UTC+02:00), it is 01:00 on Tuesday in Pakistan (UTC+05:00).
The table "Time of day by zone" gives an overview on the time relations between different zones.
| Time of day by zone | ||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| UTC offset | Monday | |||||||||||||||||||||||
| UTC−12:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 |
| UTC−11:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 |
| UTC−10:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 |
| UTC−09:30 | 02:30 | 03:30 | 04:30 | 05:30 | 06:30 | 07:30 | 08:30 | 09:30 | 10:30 | 11:30 | 12:30 | 13:30 | 14:30 | 15:30 | 16:30 | 17:30 | 18:30 | 19:30 | 20:30 | 21:30 | 22:30 | 23:30 | 00:30 | 01:30 |
| UTC−09:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 |
| UTC−08:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 |
| UTC−07:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 |
| UTC−06:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 |
| UTC−05:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 |
| UTC−04:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 |
| UTC−03:30 | 08:30 | 09:30 | 10:30 | 11:30 | 12:30 | 13:30 | 14:30 | 15:30 | 16:30 | 17:30 | 18:30 | 19:30 | 20:30 | 21:30 | 22:30 | 23:30 | 00:30 | 01:30 | 02:30 | 03:30 | 04:30 | 05:30 | 06:30 | 07:30 |
| UTC−03:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 |
| UTC−02:30 | 09:30 | 10:30 | 11:30 | 12:30 | 13:30 | 14:30 | 15:30 | 16:30 | 17:30 | 18:30 | 19:30 | 20:30 | 21:30 | 22:30 | 23:30 | 00:30 | 01:30 | 02:30 | 03:30 | 04:30 | 05:30 | 06:30 | 07:30 | 08:30 |
| UTC−02:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 |
| UTC−01:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 |
| UTC+00:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 |
| UTC+01:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 |
| UTC+02:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 |
| UTC+03:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 |
| UTC+03:30 | 15:30 | 16:30 | 17:30 | 18:30 | 19:30 | 20:30 | 21:30 | 22:30 | 23:30 | 00:30 | 01:30 | 02:30 | 03:30 | 04:30 | 05:30 | 06:30 | 07:30 | 08:30 | 09:30 | 10:30 | 11:30 | 12:30 | 13:30 | 14:30 |
| UTC+04:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 |
| UTC+04:30 | 16:30 | 17:30 | 18:30 | 19:30 | 20:30 | 21:30 | 22:30 | 23:30 | 00:30 | 01:30 | 02:30 | 03:30 | 04:30 | 05:30 | 06:30 | 07:30 | 08:30 | 09:30 | 10:30 | 11:30 | 12:30 | 13:30 | 14:30 | 15:30 |
| UTC+05:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 |
| UTC+05:30 | 17:30 | 18:30 | 19:30 | 20:30 | 21:30 | 22:30 | 23:30 | 00:30 | 01:30 | 02:30 | 03:30 | 04:30 | 05:30 | 06:30 | 07:30 | 08:30 | 09:30 | 10:30 | 11:30 | 12:30 | 13:30 | 14:30 | 15:30 | 16:30 |
| UTC+05:45 | 17:45 | 18:45 | 19:45 | 20:45 | 21:45 | 22:45 | 23:45 | 00:45 | 01:45 | 02:45 | 03:45 | 04:45 | 05:45 | 06:45 | 07:45 | 08:45 | 09:45 | 10:45 | 11:45 | 12:45 | 13:45 | 14:45 | 15:45 | 16:45 |
| UTC+06:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 |
| UTC+06:30 | 18:30 | 19:30 | 20:30 | 21:30 | 22:30 | 23:30 | 00:30 | 01:30 | 02:30 | 03:30 | 04:30 | 05:30 | 06:30 | 07:30 | 08:30 | 09:30 | 10:30 | 11:30 | 12:30 | 13:30 | 14:30 | 15:30 | 16:30 | 17:30 |
| UTC+07:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 |
| UTC+08:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 |
| UTC+08:45 | 20:45 | 21:45 | 22:45 | 23:45 | 00:45 | 01:45 | 02:45 | 03:45 | 04:45 | 05:45 | 06:45 | 07:45 | 08:45 | 09:45 | 10:45 | 11:45 | 12:45 | 13:45 | 14:45 | 15:45 | 16:45 | 17:45 | 18:45 | 19:45 |
| UTC+09:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 |
| UTC+09:30 | 21:30 | 22:30 | 23:30 | 00:30 | 01:30 | 02:30 | 03:30 | 04:30 | 05:30 | 06:30 | 07:30 | 08:30 | 09:30 | 10:30 | 11:30 | 12:30 | 13:30 | 14:30 | 15:30 | 16:30 | 17:30 | 18:30 | 19:30 | 20:30 |
| UTC+10:00 | 22:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 |
| UTC+10:30 | 22:30 | 23:30 | 00:30 | 01:30 | 02:30 | 03:30 | 04:30 | 05:30 | 06:30 | 07:30 | 08:30 | 09:30 | 10:30 | 11:30 | 12:30 | 13:30 | 14:30 | 15:30 | 16:30 | 17:30 | 18:30 | 19:30 | 20:30 | 21:30 |
| UTC+11:00 | 23:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 |
| UTC+12:00 | 00:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 |
| UTC+12:45 | 00:45 | 01:45 | 02:45 | 03:45 | 04:45 | 05:45 | 06:45 | 07:45 | 08:45 | 09:45 | 10:45 | 11:45 | 12:45 | 13:45 | 14:45 | 15:45 | 16:45 | 17:45 | 18:45 | 19:45 | 20:45 | 21:45 | 22:45 | 23:45 |
| UTC+13:00 | 01:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 |
| UTC+13:45 | 01:45 | 02:45 | 03:45 | 04:45 | 05:45 | 06:45 | 07:45 | 08:45 | 09:45 | 10:45 | 11:45 | 12:45 | 13:45 | 14:45 | 15:45 | 16:45 | 17:45 | 18:45 | 19:45 | 20:45 | 21:45 | 22:45 | 23:45 | 00:45 |
| UTC+14:00 | 02:00 | 03:00 | 04:00 | 05:00 | 06:00 | 07:00 | 08:00 | 09:00 | 10:00 | 11:00 | 12:00 | 13:00 | 14:00 | 15:00 | 16:00 | 17:00 | 18:00 | 19:00 | 20:00 | 21:00 | 22:00 | 23:00 | 00:00 | 01:00 |
| UTC offset | Tuesday | Wednesday | ||||||||||||||||||||||
Nautical time zones
Since the 1920s, a nautical standard time system has been in operation for ships on the high seas. As an ideal form of the terrestrial time zone system, nautical time zones consist of gores of 15° offset from GMT by a whole number of hours. A nautical date line follows the 180th meridian, bisecting one 15° gore into two 7.5° gores that differ from GMT by ±12 hours.
However, in practice each ship may choose what time to observe at each location. Ships may decide to adjust their clocks at a convenient time, usually at night, not exactly when they cross a certain longitude. Some ships simply remain on the time of the departing port during the whole trip.
Skewing of time zones
| 1h ± 30 min behind | |
| 0h ± 30m | |
| 1h ± 30 m ahead | |
| 2h ± 30 m ahead | |
| 3h ± 30 m ahead |
Ideal time zones, such as nautical time zones, are based on the mean solar time of a particular meridian in the middle of that zone with boundaries located 7.5 degrees east and west of the meridian. In practice, however, many time zone boundaries are drawn much farther to the west, and some countries are located entirely outside their ideal time zones.
For example, even though the Prime Meridian (0°) passes 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 other European states, began observing daylight saving (summer) time.
One reason to draw time zone boundaries far to the west of their ideal meridians is to allow the more efficient use of afternoon sunlight. Some of these locations also use daylight saving time (DST), further increasing the difference to local solar time. As a result, in summer, solar noon in the Spanish city of Vigo occurs at 14:41 clock time. This westernmost area of continental Spain never experiences sunset before 18:00 clock time, even in winter, despite lying 42 degrees north of the equator. Near the summer solstice, Vigo has sunset times after 22:00, similar to those of Stockholm, which is in the same time zone and 17 degrees farther north. Stockholm has much earlier sunrises, though.
In the United States, the reasons were more historical and business-related. In Midwestern states, like Indiana and Michigan, those living in Indianapolis and Detroit wanted to be on the same time zone as New York to simplify communications and transactions.
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 two sunsets on the same day in early August, one shortly after midnight at the start of the day, and the other shortly before midnight at the end of the day.
China extends as far west as 73°E, but all parts of it use UTC+08:00 (120°E), so solar "noon" can occur as late as 15:00 in western portions of China such as Xinjiang. The Afghanistan-China border marks the greatest terrestrial time zone difference on Earth, with a 3.5 hour difference between Afghanistan's UTC+4:30 and China's UTC+08:00.
Daylight saving time
Many countries, and sometimes just certain regions of countries, adopt daylight saving time (DST), also known as summer time, during part of the year. This typically involves 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. Countries around the equator usually do not observe daylight saving time, since the seasonal difference in sunlight there is minimal.
Computer systems
Many 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. 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.
Operating systems
Unix
Unix-like systems, including Linux and macOS, keep system time in Unix time format, representing the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC) on Thursday, January 1, 1970, excluding leap seconds. Unix time is usually converted to local time when displayed to the user, and times specified by the user in local time are converted to Unix time. The conversion takes into account the time zone and daylight saving time rules; by default the time zone and daylight saving time rules are set up when the system is configured, though individual processes can specify time zones and daylight saving time rules using the TZ environment variable. This allows users in multiple time zones, or in the same time zone but with different daylight saving time rules, to use the same computer, with their respective local times displayed correctly to each user. Information about time zones and daylight saving time rules most commonly comes from the IANA time zone database. Many systems, including anything using the GNU C Library, a C library based on the BSD C library, or the System V Release 4 C library, can make use of the IANA time zone database.
Microsoft Windows
Windows-based computer systems prior to Windows 95 and Windows NT used local time, but Windows 95 and later, and Windows NT, base system time on UTC. They allow a program to fetch the system time as UTC, represented as a year, month, day, hour, minute, second, and millisecond; Windows 95 and later, and Windows NT 3.5 and later, also allow the system time to be fetched as a count of 100 ns units since 1601-01-01 00:00:00 UTC. 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.
Programming languages
Java
While most application software will use the underlying operating system for time zone and daylight saving time rule information, the Java Platform, from version 1.3.1, has maintained its own database of time zone and daylight saving time rule information. This database is updated whenever time zone or daylight saving time rules change. Oracle provides an updater tool for this purpose.
As an alternative to the information bundled with the Java Platform, programmers may choose to use the Joda-Time library. This library includes its own data based on the IANA time zone database.
As of Java 8 there is a new date and time API that can help with converting times.
JavaScript
Traditionally, there was very little in the way of time zone support for JavaScript. Essentially the programmer had to extract the UTC offset by instantiating a time object, getting a GMT time from it, and differencing the two. This does not provide a solution for more complex daylight saving variations, such as divergent DST directions between northern and southern hemispheres.
ECMA-402, the standard on Internationalization API for JavaScript, provides ways of formatting Time Zones. However, due to size constraint, some implementations or distributions do not include it.
Perl
The DateTime object in Perl supports all entries in the IANA time zone database and includes the ability to get, set and convert between time zones.
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 time zone, and DateTime is aware of its own time zone internally. PHP.net provides extensive documentation on this. As noted there, the most current time zone database can be implemented via the PECL timezonedb.
Python
The standard module datetime included with Python stores and operates on the time zone 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. From Python 3.9, the zoneinfo module introduces timezone management without need for third party module.
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 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.
Time in outer space
Orbiting spacecraft may experience many sunrises and sunsets, or none, in a 24-hour period. Therefore, it is not possible to calibrate the time with respect to the Sun and still respect a 24-hour sleep/wake cycle. A common practice for space exploration is to use the Earth-based time of the launch site or mission control, synchronizing the sleeping cycles of the crew and controllers. 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 40 minutes, known as a sol. Earth controllers for some Mars missions have synchronized their sleep/wake cycles with the Martian day, when specifically solar-powered rover activity occurs.
See also
- Jet lag
- Lists of time zones
- Metric time
- Time by country
- Time in Europe
- Abolition of time zones – Replacing time zones with UTC
- World clock – Clock that displays the times in various locations around the globe
- International Date Line – Imaginary line that demarcates the change of one calendar day to the next
Notes
- Observes UTC+00:00 around Ramadan.
References
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A time zone is an area which observes a uniform standard time for legal commercial and social purposes Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude because it is convenient for areas in frequent communication to keep the same time Time zones of the world Each time zone is defined by a standard offset from Coordinated Universal Time UTC The offsets range from UTC 12 00 to UTC 14 00 and are usually a whole number of hours but a few zones are offset by an additional 30 or 45 minutes such as in India and Nepal Some areas in a time zone may use a different offset for part of the year typically one hour ahead during spring and summer a practice known as daylight saving time DST List of UTC offsetsTime zones of the world In the table below the locations that use daylight saving time DST are listed in their UTC offset when DST is not in effect When DST is in effect approximately during spring and summer their UTC offset is increased by one hour except for Lord Howe Island where it is increased by 30 minutes For example during the DST period California observes UTC 07 00 and the United Kingdom observes UTC 01 00 UTC offset Locations that do not use DST Locations that use DSTUTC 12 00 Baker Island Howland IslandUTC 11 00 American Samoa Jarvis Island Kingman Reef Midway Atoll Niue Palmyra AtollUTC 10 00 Cook Islands French Polynesia most Johnston Atoll United States Hawaii United States Andreanof Islands Islands of Four Mountains Near Islands Rat Islands Aleutian Islands Alaska UTC 09 30 French Polynesia Marquesas IslandsUTC 09 00 French Polynesia Gambier Islands United States Alaska most UTC 08 00 Clipperton Island Pitcairn Islands Canada British Columbia most Mexico Baja California United States California Idaho north Nevada most Oregon most WashingtonUTC 07 00 Canada British Columbia northeast Yukon Mexico Baja California Sur Nayarit most Sinaloa Sonora United States Arizona most Canada Alberta British Columbia southeast Northwest Territories Nunavut west Saskatchewan Lloydminster area Mexico Chihuahua northwest border United States Arizona Navajo Nation Colorado Idaho most Kansas west Montana Nebraska west New Mexico Nevada northeast border North Dakota southwest Oregon east South Dakota west Texas west Utah WyomingUTC 06 00 Belize Canada Saskatchewan most Costa Rica Ecuador Galapagos El Salvador Guatemala Honduras Mexico most Nicaragua Canada Manitoba Nunavut central Ontario west Chile Easter Island Mexico northeast border United States Alabama Arkansas Florida northwest Illinois Indiana northwest southwest Iowa Kansas most Kentucky west Louisiana Michigan northwest border Minnesota Mississippi Missouri Nebraska most North Dakota most Oklahoma South Dakota most Tennessee most Texas most WisconsinUTC 05 00 Brazil Acre Amazonas southwest Canada Atikokan Mishkeegogamang Southampton Island Cayman Islands Colombia Ecuador most Jamaica Mexico Quintana Roo Navassa Island Panama Peru Bahamas Canada Nunavut east Ontario most Quebec most Cuba Haiti Turks and Caicos Islands United States Connecticut Delaware District of Columbia Florida most Georgia Indiana most Kentucky east Maine Maryland Massachusetts Michigan most New Hampshire New Jersey New York North Carolina Ohio Pennsylvania Rhode Island South Carolina Tennessee east Vermont Virginia West VirginiaUTC 04 00 Anguilla Antigua and Barbuda Aruba Barbados Bolivia Brazil Amazonas most Mato Grosso Mato Grosso do Sul Rondonia Roraima British Virgin Islands Canada Quebec east Caribbean Netherlands Curacao Dominica Dominican Republic Grenada Guadeloupe Guyana Martinique Montserrat Puerto Rico Saint Barthelemy Saint Kitts and Nevis Saint Lucia Saint Martin Saint Vincent and the Grenadines Sint Maarten Trinidad and Tobago U S Virgin Islands Venezuela Bermuda Canada Labrador most New Brunswick Nova Scotia Prince Edward Island Chile most Greenland Pituffik Space BaseUTC 03 30 Canada Newfoundland Labrador southeast UTC 03 00 Argentina Brazil most Chile Magallanes Region Falkland Islands French Guiana Paraguay Suriname Uruguay Saint Pierre and MiquelonUTC 02 00 Brazil Fernando de Noronha South Georgia and the South Sandwich Islands Greenland most UTC 01 00 Cape Verde Portugal AzoresUTC 00 00 Burkina Faso Gambia Ghana Greenland National Park east coast Guinea Guinea Bissau Iceland Ivory Coast Liberia Mali Mauritania Saint Helena Ascension and Tristan da Cunha Senegal Sierra Leone Sao Tome and Principe Togo Faroe Islands Guernsey Ireland Isle of Man Jersey Portugal most Spain Canary Islands United KingdomUTC 01 00 Algeria Angola Benin Cameroon Central African Republic Chad Congo Democratic Republic of the Congo Equateur Kinshasa Kongo Central Kwango Kwilu Mai Ndombe Mongala Nord Ubangi Sud Ubangi Tshuapa Equatorial Guinea Gabon Morocco Niger Nigeria Tunisia Western Sahara Albania Andorra Austria Belgium Bosnia and Herzegovina Croatia Czech Republic Denmark France metropolitan Germany Gibraltar Hungary Italy Kosovo Liechtenstein Luxembourg Malta Monaco Montenegro Netherlands European North Macedonia Norway Poland San Marino Serbia Slovakia Slovenia Spain most Sweden Switzerland Vatican CityUTC 02 00 Botswana Burundi Democratic Republic of the Congo most Eswatini Lesotho Libya Malawi Mozambique Namibia Russia Kaliningrad Rwanda South Africa most South Sudan Sudan Zambia Zimbabwe Akrotiri and Dhekelia Bulgaria Cyprus Egypt Estonia Finland Greece Israel Latvia Lebanon Lithuania Moldova Northern Cyprus Palestine Romania Transnistria Ukraine most UTC 03 00 Abkhazia Bahrain Belarus Comoros Djibouti Eritrea Ethiopia French Southern and Antarctic Lands Scattered Islands Iraq Jordan Kenya Kuwait Madagascar Mayotte Qatar Russia most of European part Saudi Arabia Somalia Somaliland South Africa Prince Edward Islands South Ossetia Syria Tanzania Turkey Uganda Ukraine occupied territories YemenUTC 03 30 IranUTC 04 00 Armenia Azerbaijan French Southern and Antarctic Lands Crozet Islands Georgia Mauritius Oman Russia Astrakhan Samara Saratov Udmurtia Ulyanovsk Reunion Seychelles United Arab EmiratesUTC 04 30 AfghanistanUTC 05 00 French Southern and Antarctic Lands Kerguelen Islands Saint Paul Island Amsterdam Island Heard Island and McDonald Islands Kazakhstan Maldives Pakistan Russia Bashkortostan Chelyabinsk Khanty Mansi Kurgan Orenburg Perm Sverdlovsk Tyumen Yamalia Tajikistan Turkmenistan UzbekistanUTC 05 30 India Sri LankaUTC 05 45 NepalUTC 06 00 Bangladesh Bhutan British Indian Ocean Territory Kyrgyzstan Russia OmskUTC 06 30 Cocos Keeling Islands MyanmarUTC 07 00 Cambodia Christmas Island Indonesia Sumatra Java West Kalimantan Central Kalimantan Laos Mongolia Bayan Olgii Khovd Uvs Russia Altai Krai Altai Republic Kemerovo Khakassia Krasnoyarsk Novosibirsk Tomsk Tuva Thailand VietnamUTC 08 00 Australia Western Australia most Brunei China Hong Kong Indonesia South Kalimantan East Kalimantan North Kalimantan Sulawesi Lesser Sunda Islands Macau Malaysia Mongolia most Philippines Russia Buryatia Irkutsk Singapore TaiwanUTC 08 45 Australia EuclaUTC 09 00 Timor Leste Indonesia Maluku Islands Western New Guinea Japan North Korea Palau Russia Amur Sakha most Zabaykalsky South KoreaUTC 09 30 Australia Northern Territory Australia South Australia Yancowinna CountyUTC 10 00 Australia Queensland Guam Micronesia Chuuk Yap Northern Mariana Islands Papua New Guinea most Russia Jewish Khabarovsk Primorsky Sakha central east Australia Australian Capital Territory Jervis Bay Territory New South Wales most Tasmania VictoriaUTC 10 30 Australia Lord Howe Island DST increase is 30 minutes UTC 11 00 Micronesia Kosrae Pohnpei New Caledonia Papua New Guinea Bougainville Russia Magadan Sakha east Sakhalin Solomon Islands Vanuatu Norfolk IslandUTC 12 00 Fiji Kiribati Gilbert Islands Marshall Islands Nauru Russia Chukotka Kamchatka Tuvalu Wake Island Wallis and Futuna New Zealand most UTC 12 45 New Zealand Chatham IslandsUTC 13 00 Kiribati Phoenix Islands Samoa Tokelau TongaUTC 14 00 Kiribati Line IslandsHistoryWorld time clock 2 5 m high Made in Dresden 1690 Technical Instrument Museum Dresden The apparent position of the Sun in the sky and thus solar time varies by location due to the spherical shape of the Earth This variation corresponds to four minutes of time for every degree of longitude so for example when it is solar noon in London it is about 10 minutes before solar noon in Bristol which is about 2 5 degrees to the west The Royal Observatory Greenwich founded in 1675 established Greenwich Mean Time GMT the mean solar time at that location as an aid to mariners to determine longitude at sea providing a standard reference time while each location in England kept a different time Railway time Plaque commemorating the Railway General Time Convention of 1883 in North AmericaThe control panel of the Time Zone Clock in front of Coventry Transport Museum In the 19th century as transportation and telecommunications improved it became increasingly inconvenient for each location to observe its own solar time In November 1840 the British Great Western Railway started using GMT kept by portable chronometers failed verification This practice was soon followed by other railway companies in Great Britain and became known as railway time Around August 23 1852 time signals were first transmitted by telegraph from the Royal Observatory By 1855 98 of Great Britain s public clocks were using GMT but it was not made the island s legal time until August 2 1880 Some British clocks from this period have two minute hands one for the local time and one for GMT On November 2 1868 the British Colony of New Zealand officially adopted a standard time to be observed throughout the colony It was based on longitude 172 30 east of Greenwich that is 11 hours 30 minutes ahead of GMT This standard was known as New Zealand Mean Time 1913 time zone map of the United States showing boundaries very different from today Timekeeping on North American railroads in the 19th century was complex Each railroad used its own standard time usually based on the local time of its headquarters or most important terminus and the railroad s train schedules were published using its own time Some junctions served by several railroads had a clock for each railroad each showing a different time Because of this a number of accidents occurred when trains from different companies using the same tracks mistimed their passings Around 1863 Charles F Dowd proposed a system of hourly standard time zones for North American railroads although he published nothing on the matter at that time and did not consult railroad officials until 1869 In 1870 he proposed four ideal time zones having north south borders the first centered on Washington D C but by 1872 the first was centered on meridian 75 west of Greenwich with natural borders such as sections of the Appalachian Mountains Dowd s system was never accepted by North American railroads Chief meteorologist at the United States Weather Bureau Cleveland Abbe divided the United States into four standard time zones for consistency among the weather stations In 1879 he published a paper titled Report on Standard Time In 1883 he convinced North American railroad companies to adopt his time zone system In 1884 Britain which had already adopted its own standard time system for England Scotland and Wales helped gather international consent for global time In time the American government influenced in part by Abbe s 1879 paper adopted the time zone system It was 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 North American zones were named Intercolonial Eastern Central Mountain and Pacific Within a year 85 of all cities with populations over 10 000 about 200 cities were using standard time A notable exception was Detroit located about halfway between the meridians of Eastern and Central time which kept local time until 1900 then tried Central Standard Time local mean time and Eastern Standard Time EST before a May 1915 ordinance settled on EST and was ratified by popular vote in August 1916 The confusion of times came to an end when standard time zones were formally adopted by the U S Congress in the Standard Time Act of March 19 1918 Worldwide time zones Italian mathematician Quirico Filopanti introduced the idea of a worldwide system of time zones 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 However his book attracted no attention until long after his death Scottish born Canadian Sir Sandford Fleming proposed a worldwide system of time zones in 1876 see Sandford Fleming Inventor of worldwide standard time The proposal divided the world into twenty four time zones labeled A Y skipping J each one covering 15 degrees of longitude All clocks within each zone would be set to the same time as the others but differed by one hour from those in the neighboring zones He advocated his system at several international conferences including the International Meridian Conference where it received some consideration The system has not been directly adopted but some maps divide the world into 24 time zones and assign letters to them similarly to Fleming s system World map of time zones in 1928 By about 1900 almost all inhabited places on Earth had adopted a standard time zone but only some of them used an hourly offset from GMT Many applied the time at a local astronomical observatory to an entire country without any reference to GMT It took many decades before all time zones were based on some standard offset from GMT or Coordinated Universal Time UTC By 1929 the majority of countries had adopted hourly time zones though some countries such as Iran India Myanmar and parts of Australia had time zones with a 30 minute offset Nepal was the last country to adopt a standard offset shifting slightly to UTC 05 45 in 1986 All nations currently use standard time zones for secular purposes but not all of them apply the concept as originally conceived Several countries and subdivisions use half hour or quarter hour deviations from standard time Some countries such as China and India use a single time zone even though the extent of their territory far exceeds the ideal 15 of longitude for one hour other countries such as Spain and Argentina use standard hour based offsets but not necessarily those that would be determined by their geographical location The consequences in some areas can affect the lives of local citizens and in extreme cases contribute to larger political issues such as in the western reaches of China In Russia which has 11 time zones two time zones were removed in 2010 and reinstated in 2014 NotationISO 8601 ISO 8601 is a standard established by the International Organization for Standardization defining methods of representing dates and times in textual form including specifications for representing time zones 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 UTC time is also known as Zulu time since Zulu is a phonetic alphabet code word for the letter Z Offsets from UTC are written in the format hh mm hhmm or hh either hours ahead or behind UTC For example if the time being described is one hour ahead of UTC such as the time in Germany 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 Abbreviations Time zones are often represented by alphabetic abbreviations such as EST WST and CST but these are not part of the international time and date standard ISO 8601 Such designations can be ambiguous for example CST can mean North American Central Standard Time UTC 06 00 Cuba Standard Time UTC 05 00 and China Standard Time UTC 08 00 and it is also a widely used variant of ACST Australian Central Standard Time UTC 09 30 ConversionsConversion between time zones obeys the relationship time in zone A UTC offset for zone A time in zone B UTC offset for zone B in which each side of the equation is equivalent to UTC The conversion equation can be rearranged to time in zone B time in zone A UTC offset for zone A UTC offset for zone B For example the New York Stock Exchange opens at 09 30 EST UTC offset 05 00 In California PST UTC offset 08 00 and India IST UTC offset 05 30 the New York Stock Exchange opens at time in California 09 30 05 00 08 00 06 30 time in India 09 30 05 00 05 30 20 00 These calculations become more complicated near the time switch to or from daylight saving time as the UTC offset for the area becomes a function of UTC time The time differences may also result in different dates For example when it is 22 00 on Monday in Egypt UTC 02 00 it is 01 00 on Tuesday in Pakistan UTC 05 00 The table Time of day by zone gives an overview on the time relations between different zones Time of day by zoneUTC offset MondayUTC 12 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00UTC 11 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00UTC 10 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00UTC 09 30 02 30 03 30 04 30 05 30 06 30 07 30 08 30 09 30 10 30 11 30 12 30 13 30 14 30 15 30 16 30 17 30 18 30 19 30 20 30 21 30 22 30 23 30 00 30 01 30UTC 09 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00UTC 08 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00UTC 07 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00UTC 06 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00UTC 05 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00UTC 04 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00UTC 03 30 08 30 09 30 10 30 11 30 12 30 13 30 14 30 15 30 16 30 17 30 18 30 19 30 20 30 21 30 22 30 23 30 00 30 01 30 02 30 03 30 04 30 05 30 06 30 07 30UTC 03 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00UTC 02 30 09 30 10 30 11 30 12 30 13 30 14 30 15 30 16 30 17 30 18 30 19 30 20 30 21 30 22 30 23 30 00 30 01 30 02 30 03 30 04 30 05 30 06 30 07 30 08 30UTC 02 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00UTC 01 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00UTC 00 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00UTC 01 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00UTC 02 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00UTC 03 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00UTC 03 30 15 30 16 30 17 30 18 30 19 30 20 30 21 30 22 30 23 30 00 30 01 30 02 30 03 30 04 30 05 30 06 30 07 30 08 30 09 30 10 30 11 30 12 30 13 30 14 30UTC 04 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00UTC 04 30 16 30 17 30 18 30 19 30 20 30 21 30 22 30 23 30 00 30 01 30 02 30 03 30 04 30 05 30 06 30 07 30 08 30 09 30 10 30 11 30 12 30 13 30 14 30 15 30UTC 05 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00UTC 05 30 17 30 18 30 19 30 20 30 21 30 22 30 23 30 00 30 01 30 02 30 03 30 04 30 05 30 06 30 07 30 08 30 09 30 10 30 11 30 12 30 13 30 14 30 15 30 16 30UTC 05 45 17 45 18 45 19 45 20 45 21 45 22 45 23 45 00 45 01 45 02 45 03 45 04 45 05 45 06 45 07 45 08 45 09 45 10 45 11 45 12 45 13 45 14 45 15 45 16 45UTC 06 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00UTC 06 30 18 30 19 30 20 30 21 30 22 30 23 30 00 30 01 30 02 30 03 30 04 30 05 30 06 30 07 30 08 30 09 30 10 30 11 30 12 30 13 30 14 30 15 30 16 30 17 30UTC 07 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00UTC 08 00 20 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00UTC 08 45 20 45 21 45 22 45 23 45 00 45 01 45 02 45 03 45 04 45 05 45 06 45 07 45 08 45 09 45 10 45 11 45 12 45 13 45 14 45 15 45 16 45 17 45 18 45 19 45UTC 09 00 21 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00UTC 09 30 21 30 22 30 23 30 00 30 01 30 02 30 03 30 04 30 05 30 06 30 07 30 08 30 09 30 10 30 11 30 12 30 13 30 14 30 15 30 16 30 17 30 18 30 19 30 20 30UTC 10 00 22 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00UTC 10 30 22 30 23 30 00 30 01 30 02 30 03 30 04 30 05 30 06 30 07 30 08 30 09 30 10 30 11 30 12 30 13 30 14 30 15 30 16 30 17 30 18 30 19 30 20 30 21 30UTC 11 00 23 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00UTC 12 00 00 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00UTC 12 45 00 45 01 45 02 45 03 45 04 45 05 45 06 45 07 45 08 45 09 45 10 45 11 45 12 45 13 45 14 45 15 45 16 45 17 45 18 45 19 45 20 45 21 45 22 45 23 45UTC 13 00 01 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00UTC 13 45 01 45 02 45 03 45 04 45 05 45 06 45 07 45 08 45 09 45 10 45 11 45 12 45 13 45 14 45 15 45 16 45 17 45 18 45 19 45 20 45 21 45 22 45 23 45 00 45UTC 14 00 02 00 03 00 04 00 05 00 06 00 07 00 08 00 09 00 10 00 11 00 12 00 13 00 14 00 15 00 16 00 17 00 18 00 19 00 20 00 21 00 22 00 23 00 00 00 01 00UTC offset Tuesday WednesdayNautical time zonesSince the 1920s a nautical standard time system has been in operation for ships on the high seas As an ideal form of the terrestrial time zone system nautical time zones consist of gores of 15 offset from GMT by a whole number of hours A nautical date line follows the 180th meridian bisecting one 15 gore into two 7 5 gores that differ from GMT by 12 hours However in practice each ship may choose what time to observe at each location Ships may decide to adjust their clocks at a convenient time usually at night not exactly when they cross a certain longitude Some ships simply remain on the time of the departing port during the whole trip Skewing of time zonesDifference between sun time and clock time during daylight saving time 1h 30 min behind0h 30m1h 30 m ahead2h 30 m ahead3h 30 m ahead DST observed DST formerly observed DST never observed Ideal time zones such as nautical time zones are based on the mean solar time of a particular meridian in the middle of that zone with boundaries located 7 5 degrees east and west of the meridian In practice however many time zone boundaries are drawn much farther to the west and some countries are located entirely outside their ideal time zones For example even though the Prime Meridian 0 passes 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 other European states began observing daylight saving summer time One reason to draw time zone boundaries far to the west of their ideal meridians is to allow the more efficient use of afternoon sunlight Some of these locations also use daylight saving time DST further increasing the difference to local solar time As a result in summer solar noon in the Spanish city of Vigo occurs at 14 41 clock time This westernmost area of continental Spain never experiences sunset before 18 00 clock time even in winter despite lying 42 degrees north of the equator Near the summer solstice Vigo has sunset times after 22 00 similar to those of Stockholm which is in the same time zone and 17 degrees farther north Stockholm has much earlier sunrises though In the United States the reasons were more historical and business related In Midwestern states like Indiana and Michigan those living in Indianapolis and Detroit wanted to be on the same time zone as New York to simplify communications and transactions 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 two sunsets on the same day in early August one shortly after midnight at the start of the day and the other shortly before midnight at the end of the day China extends as far west as 73 E but all parts of it use UTC 08 00 120 E so solar noon can occur as late as 15 00 in western portions of China such as Xinjiang The Afghanistan China border marks the greatest terrestrial time zone difference on Earth with a 3 5 hour difference between Afghanistan s UTC 4 30 and China s UTC 08 00 A visualization of the mismatch between clock time and solar time in different locations In blue areas clock time lags behind solar time in red areas the reverse is true The two are synchronized in the white areas Daylight saving timeMany countries and sometimes just certain regions of countries adopt daylight saving time DST also known as summer time during part of the year This typically involves 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 Countries around the equator usually do not observe daylight saving time since the seasonal difference in sunlight there is minimal Computer systemsMany 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 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 Operating systems Unix Unix like systems including Linux and macOS keep system time in Unix time format representing the number of seconds that have elapsed since 00 00 00 Coordinated Universal Time UTC on Thursday January 1 1970 excluding leap seconds Unix time is usually converted to local time when displayed to the user and times specified by the user in local time are converted to Unix time The conversion takes into account the time zone and daylight saving time rules by default the time zone and daylight saving time rules are set up when the system is configured though individual processes can specify time zones and daylight saving time rules using the TZ environment variable This allows users in multiple time zones or in the same time zone but with different daylight saving time rules to use the same computer with their respective local times displayed correctly to each user Information about time zones and daylight saving time rules most commonly comes from the IANA time zone database Many systems including anything using the GNU C Library a C library based on the BSD C library or the System V Release 4 C library can make use of the IANA time zone database Microsoft Windows Windows based computer systems prior to Windows 95 and Windows NT used local time but Windows 95 and later and Windows NT base system time on UTC They allow a program to fetch the system time as UTC represented as a year month day hour minute second and millisecond Windows 95 and later and Windows NT 3 5 and later also allow the system time to be fetched as a count of 100 ns units since 1601 01 01 00 00 00 UTC 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 Programming languages Java While most application software will use the underlying operating system for time zone and daylight saving time rule information the Java Platform from version 1 3 1 has maintained its own database of time zone and daylight saving time rule information This database is updated whenever time zone or daylight saving time rules change Oracle provides an updater tool for this purpose As an alternative to the information bundled with the Java Platform programmers may choose to use the Joda Time library This library includes its own data based on the IANA time zone database As of Java 8 there is a new date and time API that can help with converting times JavaScript Traditionally there was very little in the way of time zone support for JavaScript Essentially the programmer had to extract the UTC offset by instantiating a time object getting a GMT time from it and differencing the two This does not provide a solution for more complex daylight saving variations such as divergent DST directions between northern and southern hemispheres ECMA 402 the standard on Internationalization API for JavaScript provides ways of formatting Time Zones However due to size constraint some implementations or distributions do not include it Perl The DateTime object in Perl supports all entries in the IANA time zone database and includes the ability to get set and convert between time zones 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 time zone and DateTime is aware of its own time zone internally PHP net provides extensive documentation on this As noted there the most current time zone database can be implemented via the PECL timezonedb Python The standard module datetime included with Python stores and operates on the time zone 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 From Python 3 9 the zoneinfo module introduces timezone management without need for third party module 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 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 Time in outer spaceOrbiting spacecraft may experience many sunrises and sunsets or none in a 24 hour period Therefore it is not possible to calibrate the time with respect to the Sun and still respect a 24 hour sleep wake cycle A common practice for space exploration is to use the Earth based time of the launch site or mission control synchronizing the sleeping cycles of the crew and controllers 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 40 minutes known as a sol Earth controllers for some Mars missions have synchronized their sleep wake cycles with the Martian day when specifically solar powered rover activity occurs See alsoGeography portalWorld portalJet lag Lists of time zones Metric time Time by country Time in Europe Abolition of time zones Replacing time zones with UTC World clock Clock that displays the times in various locations around the globe International Date Line Imaginary line that demarcates the change of one calendar day to the nextNotesObserves UTC 00 00 around Ramadan References Morocco Re Introduces Clock Changes for Ramadan 2019 Timeanddate com April 19 2019 Archived from the original on December 28 2020 Time Zone in Casablanca Morocco Timeanddate com Archived from the original on March 30 2021 Time Zone in El Aaiun Western Sahara Timeanddate 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Papers Past Evening Post p 10 Archived from the original on October 8 2013 Retrieved October 2 2013 Alfred Randy November 18 2010 Nov 18 1883 Railroad Time Goes Coast to Coast Wired Archived from the original on August 19 2018 Retrieved July 30 2018 America s first time zone November 10 2011 Debus 1968 p 2 Asimov 1964 p 344 Economics of Time Zones PDF Archived from the original PDF on May 14 2012 1 89 MB The Times Reports on the Day of Two Noons History Matters Archived from the original on April 4 2012 Retrieved December 5 2011 Resolution concerning new standard time by Chicago Sos state il us Archived from the original on October 5 2011 Retrieved December 5 2011 Quirico Filopanti from scienzagiovane Bologna University Italy Archived from the original on January 17 2013 Suggestions from Gianluigi Parmeggiani Osservatorio Astronomico di Bologna The origin of time zones Archived from the original on August 24 2007 Fleming Sandford 1886 Time reckoning for the twentieth century Annual Report of the Board of Regents of the Smithsonian Institution 1 345 366 Archived from the original on October 5 2022 Retrieved March 24 2022 Reprinted in 1889 Time reckoning for the twentieth century at the Internet Archive Stromberg Joseph November 18 2011 Sandford Fleming Sets the World s Clock Smithsonian Magazine Archived from the original on March 24 2022 Retrieved March 24 2022 Time Zone amp Clock Changes in Kathmandu Nepal timeanddate com Archived from the original on January 22 2021 Retrieved December 1 2020 Schiavenza Matt November 5 2013 China Only Has One Time Zone and That s a Problem The Atlantic Archived from the original on August 22 2018 Retrieved August 22 2018 Russia Reduces Number of Time Zones TimeAndDate com March 23 2010 Archived from the original on August 9 2020 Retrieved May 31 2020 About Time Huge country nine time zones Video BBC March 22 2011 Archived from the original on February 13 2019 Retrieved February 12 2019 Russian clocks to retreat again in winter 11 time zones return Reuters July 2014 Archived from the original on October 28 2020 Retrieved October 25 2020 Z Zulu Time Zone Time Zone Abbreviation TimeAndDate com Archived from the original on August 22 2018 Retrieved August 22 2018 What is UTC or GMT Time National Hurricane Center Archived from the original on August 22 2018 Retrieved August 22 2018 Time Zone Abbreviations Worldwide List Timeanddate com Archived August 21 2018 at the Wayback Machine Bowditch Nathaniel 1975 1925 American Practical Navigator Washington Government Printing Office Hill John C Thomas F Utegaard Gerard Riordan 1958 Dutton s Navigation and Piloting Annapolis United States Naval Institute Howse Derek 1980 Greenwich Time and the Discovery of the Longitude Oxford Oxford University Press ISBN 0 19 215948 8 What Is Cruise Ship Time Cruise Critic January 7 2020 Frequently Asked Questions Caribbean Adventures Roatan Archived from the original on February 14 2021 Poulle Yvonne 1999 La France a l heure allemande 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