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 instead of strictly following longitude because it is convenient for areas in close commercial or other communication to keep the same time.
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 permanent daylight saving time effect.
ISO 8601 is an international standard that defines methods of representing dates and times in textual form, including specifications for representing time zones.
Most of the time zones on land are offset from Coordinated Universal Time (UTC) by a whole number of hours (UTC−12:00 to UTC+14:00), 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, Indian Standard Time is UTC+05:30 and Myanmar Standard Time is UTC+06:30).
| GMT | Vancouver | San Francisco | Seattle |
| Los Angeles | Denver | Mexico City | Houston |
| Minneapolis | New Orleans | Chicago | Montgomery |
| Indianapolis | Atlanta | Detroit | Miami |
| Washington DC | Philadelphia | New York | Montreal |
| Boston | Buenos Aires | Sao Paulo | Rio De Janeiro |
| Lisbon | Dublin | London | Madrid |
| Barcelona | Paris | Brussels | Amsterdam |
| Frankfurt | Rome | Berlin | Prague |
| Vienna | Stockholm | Athens | Helsinki |
| Minsk | Istanbul | Cairo | Jerusalem |
| Beirut | Moscow | Baghdad | Dubai |
| Bangkok | Jakarta | Hong Kong | Beijing |
| Shanghai | Seoul | Tokyo | Melbourne |
| Sydney | Brisbane | Vladivostok | Kamchatka |
When well-regulated mechanical clocks became widespread in the early 19th century, each city began to use 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 accumulated 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, England 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 in November 1840, 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 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 centered 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 inhabited places on Earth had adopted one or other standard time zone; but only some of these 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/UTC. By 1929, the majority of countries had adopted hourly time zones, though a number of countries from Iran to Australia had time zones with a 30-minute offset. 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, Myanmar, 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, which causes problems as some places in China, like Xinjiang (westernmost province of China), uses local time and when planning to meet with the Chinese living in Beijing, a very eastern city, they will have trouble understanding each other as they are two hours apart. Russia is traditionally divided into 11 time zones, but in 2010 the number was reduced to nine. Then-President Dmitry Medvedev said at the time that he would like to see even fewer in place. In 2014, the two removed time zones were reinstated, making the number of time zones 11 again.
France, including its overseas territories, has the most time zones of any country, with a total of 12.
| Time Zone | UTC Offset | UTC DST Offset |
|---|
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], ±[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, "CST" can mean China Standard Time (UTC+8), Cuba Standard Time (UTC−5), and (North American) Central Standard Time (UTC−6), and it is also a widely used variant of ACST (Australian Central Standard Time, UTC+9:30).
Such designations predate both ISO 8601 and the internet era; in an earlier era, they were sufficiently unambiguous for many practical uses within a national context (for example, in railway timetables and business correspondence), but their ambiguity explains their deprecation in the internet era, when communications more often cannot rely on implicit geographic context to supply part of the meaning.
| Abbreviation | Time Zone | Offset |
|---|---|---|
| ACDT | Australian Central Daylight Saving Time | UTC+10:30 |
| ACST | Australian Central Standard Time | UTC+09:30 |
| ACT | Acre Time | UTC−05 |
| ACT | ASEAN Common Time | UTC+06:30 – UTC+09 |
| ACWST | Australian Central Western Standard Time | UTC+08:45 |
| ADT | Atlantic Daylight Time | UTC−03 |
| AEDT | Australian Eastern Daylight Saving Time | UTC+11 |
| AEST | Australian Eastern Standard Time | UTC+10 |
| AET | Australian Eastern Time | UTC+10/UTC+11 |
| AFT | Afghanistan Time | UTC+04:30 |
| AKDT | Alaska Daylight Time | UTC−08 |
| AKST | Alaska Standard Time | UTC−09 |
| ALMT | Alma-Ata Time | UTC+06 |
| AMST | Amazon Summer Time (Brazil) | UTC−03 |
| AMT | Amazon Time (Brazil) | UTC−04 |
| AMT | Armenia Time | UTC+04 |
| ANAT | Anadyr Time | UTC+12 |
| AQTT | Aqtobe Time | UTC+05 |
| ART | Argentina Time | UTC−03 |
| AST | Arabia Standard Time | UTC+03 |
| AST | Atlantic Standard Time | UTC−04 |
| AWST | Australian Western Standard Time | UTC+08 |
| AZOST | Azores Summer Time | UTC±00 |
| AZOT | Azores Standard Time | UTC−01 |
| AZT | Azerbaijan Time | UTC+04 |
| BDT | Brunei Time | UTC+08 |
| BIOT | British Indian Ocean Time | UTC+06 |
| BIT | Baker Island Time | UTC−12 |
| BOT | Bolivia Time | UTC−04 |
| BRST | Brasília Summer Time | UTC−02 |
| BRT | Brasília Time | UTC−03 |
| BST | Bangladesh Standard Time | UTC+06 |
| BST | Bougainville Standard Time | UTC+11 |
| BST | British Summer Time (British Standard Time from Feb 1968 to Oct 1971) | UTC+01 |
| BTT | Bhutan Time | UTC+06 |
| CAT | Central Africa Time | UTC+02 |
| CCT | Cocos Islands Time | UTC+06:30 |
| CDT | Central Daylight Time (North America) | UTC−05 |
| CDT | Cuba Daylight Time | UTC−04 |
| CEST | Central European Summer Time (Cf. HAEC) | UTC+02 |
| CET | Central European Time | UTC+01 |
| CHADT | Chatham Daylight Time | UTC+13:45 |
| CHAST | Chatham Standard Time | UTC+12:45 |
| CHOT | Choibalsan Standard Time | UTC+08 |
| CHOST | Choibalsan Summer Time | UTC+09 |
| CHST | Chamorro Standard Time | UTC+10 |
| CHUT | Chuuk Time | UTC+10 |
| CIST | Clipperton Island Standard Time | UTC−08 |
| WITA | Central Indonesia Time | UTC+08 |
| CKT | Cook Island Time | UTC−10 |
| CLST | Chile Summer Time | UTC−03 |
| CLT | Chile Standard Time | UTC−04 |
| COST | Colombia Summer Time | UTC−04 |
| COT | Colombia Time | UTC−05 |
| CST | Central Standard Time (North America) | UTC−06 |
| CST | China Standard Time | UTC+08 |
| CST | Cuba Standard Time | UTC−05 |
| CT | Central Time | UTC−06/UTC−05 |
| CVT | Cape Verde Time | UTC−01 |
| CWST | Central Western Standard Time (Australia) unofficial | UTC+08:45 |
| CXT | Christmas Island Time | UTC+07 |
| DAVT | Davis Time | UTC+07 |
| DDUT | Dumont d'Urville Time | UTC+10 |
| DFT | AIX-specific equivalent of Central European Time[NB 1] | UTC+01 |
| EASST | Easter Island Summer Time | UTC−05 |
| EAST | Easter Island Standard Time | UTC−06 |
| EAT | East Africa Time | UTC+03 |
| ECT | Eastern Caribbean Time (does not recognise DST) | UTC−04 |
| ECT | Ecuador Time | UTC−05 |
| EDT | Eastern Daylight Time (North America) | UTC−04 |
| EEST | Eastern European Summer Time | UTC+03 |
| EET | Eastern European Time | UTC+02 |
| EGST | Eastern Greenland Summer Time | UTC±00 |
| EGT | Eastern Greenland Time | UTC−01 |
| WIT | Eastern Indonesian Time | UTC+09 |
| EST | Eastern Standard Time (North America) | UTC−05 |
| FET | Further-eastern European Time | UTC+03 |
| FJT | Fiji Time | UTC+12 |
| FKST | Falkland Islands Summer Time | UTC−03 |
| FKT | Falkland Islands Time | UTC−04 |
| FNT | Fernando de Noronha Time | UTC−02 |
| GALT | Galápagos Time | UTC−06 |
| GAMT | Gambier Islands Time | UTC−09 |
| GET | Georgia Standard Time | UTC+04 |
| GFT | French Guiana Time | UTC−03 |
| GILT | Gilbert Island Time | UTC+12 |
| GIT | Gambier Island Time | UTC−09 |
| GMT | Greenwich Mean Time | UTC±00 |
| GST | South Georgia and the South Sandwich Islands Time | UTC−02 |
| GST | Gulf Standard Time | UTC+04 |
| GYT | Guyana Time | UTC−04 |
| HDT | Hawaii–Aleutian Daylight Time | UTC−09 |
| HAEC | Heure Avancée d'Europe Centrale French-language name for CEST | UTC+02 |
| HST | Hawaii–Aleutian Standard Time | UTC−10 |
| HKT | Hong Kong Time | UTC+08 |
| HMT | Heard and McDonald Islands Time | UTC+05 |
| HOVST | Hovd Summer Time (not used from 2017-present) | UTC+08 |
| HOVT | Hovd Time | UTC+07 |
| ICT | Indochina Time | UTC+07 |
| IDLW | International Day Line West time zone | UTC−12 |
| IDT | Israel Daylight Time | UTC+03 |
| IOT | Indian Ocean Time | UTC+03 |
| IRDT | Iran Daylight Time | UTC+04:30 |
| IRKT | Irkutsk Time | UTC+08 |
| IRST | Iran Standard Time | UTC+03:30 |
| IST | Indian Standard Time | UTC+05:30 |
| IST | Irish Standard Time | UTC+01 |
| IST | Israel Standard Time | UTC+02 |
| JST | Japan Standard Time | UTC+09 |
| KALT | Kaliningrad Time | UTC+02 |
| KGT | Kyrgyzstan Time | UTC+06 |
| KOST | Kosrae Time | UTC+11 |
| KRAT | Krasnoyarsk Time | UTC+07 |
| KST | Korea Standard Time | UTC+09 |
| LHST | Lord Howe Standard Time | UTC+10:30 |
| LHST | Lord Howe Summer Time | UTC+11 |
| LINT | Line Islands Time | UTC+14 |
| MAGT | Magadan Time | UTC+12 |
| MART | Marquesas Islands Time | UTC−09:30 |
| MAWT | Mawson Station Time | UTC+05 |
| MDT | Mountain Daylight Time (North America) | UTC−06 |
| MET | Middle European Time Same zone as CET | UTC+01 |
| MEST | Middle European Summer Time Same zone as CEST | UTC+02 |
| MHT | Marshall Islands Time | UTC+12 |
| MIST | Macquarie Island Station Time | UTC+11 |
| MIT | Marquesas Islands Time | UTC−09:30 |
| MMT | Myanmar Standard Time | UTC+06:30 |
| MSK | Moscow Time | UTC+03 |
| MST | Malaysia Standard Time | UTC+08 |
| MST | Mountain Standard Time (North America) | UTC−07 |
| MUT | Mauritius Time | UTC+04 |
| MVT | Maldives Time | UTC+05 |
| MYT | Malaysia Time | UTC+08 |
| NCT | New Caledonia Time | UTC+11 |
| NDT | Newfoundland Daylight Time | UTC−02:30 |
| NFT | Norfolk Island Time | UTC+11 |
| NOVT | Novosibirsk Time | UTC+07 |
| NPT | Nepal Time | UTC+05:45 |
| NST | Newfoundland Standard Time | UTC−03:30 |
| NT | Newfoundland Time | UTC−03:30 |
| NUT | Niue Time | UTC−11 |
| NZDT | New Zealand Daylight Time | UTC+13 |
| NZST | New Zealand Standard Time | UTC+12 |
| OMST | Omsk Time | UTC+06 |
| ORAT | Oral Time | UTC+05 |
| PDT | Pacific Daylight Time (North America) | UTC−07 |
| PET | Peru Time | UTC−05 |
| PETT | Kamchatka Time | UTC+12 |
| PGT | Papua New Guinea Time | UTC+10 |
| PHOT | Phoenix Island Time | UTC+13 |
| PHT | Philippine Time | UTC+08 |
| PKT | Pakistan Standard Time | UTC+05 |
| PMDT | Saint Pierre and Miquelon Daylight Time | UTC−02 |
| PMST | Saint Pierre and Miquelon Standard Time | UTC−03 |
| PONT | Pohnpei Standard Time | UTC+11 |
| PST | Pacific Standard Time (North America) | UTC−08 |
| PST | Philippine Standard Time | UTC+08 |
| PYST | Paraguay Summer Time | UTC−03 |
| PYT | Paraguay Time | UTC−04 |
| RET | Réunion Time | UTC+04 |
| ROTT | Rothera Research Station Time | UTC−03 |
| SAKT | Sakhalin Island Time | UTC+11 |
| SAMT | Samara Time | UTC+04 |
| SAST | South African Standard Time | UTC+02 |
| SBT | Solomon Islands Time | UTC+11 |
| SCT | Seychelles Time | UTC+04 |
| SDT | Samoa Daylight Time | UTC−10 |
| SGT | Singapore Time | UTC+08 |
| SLST | Sri Lanka Standard Time | UTC+05:30 |
| SRET | Srednekolymsk Time | UTC+11 |
| SRT | Suriname Time | UTC−03 |
| SST | Samoa Standard Time | UTC−11 |
| SST | Singapore Standard Time | UTC+08 |
| SYOT | Showa Station Time | UTC+03 |
| TAHT | Tahiti Time | UTC−10 |
| THA | Thailand Standard Time | UTC+07 |
| TFT | French Southern and Antarctic Time | UTC+05 |
| TJT | Tajikistan Time | UTC+05 |
| TKT | Tokelau Time | UTC+13 |
| TLT | Timor Leste Time | UTC+09 |
| TMT | Turkmenistan Time | UTC+05 |
| TRT | Turkey Time | UTC+03 |
| TOT | Tonga Time | UTC+13 |
| TVT | Tuvalu Time | UTC+12 |
| ULAST | Ulaanbaatar Summer Time | UTC+09 |
| ULAT | Ulaanbaatar Standard Time | UTC+08 |
| UTC | Coordinated Universal Time | UTC±00 |
| UYST | Uruguay Summer Time | UTC−02 |
| UYT | Uruguay Standard Time | UTC−03 |
| UZT | Uzbekistan Time | UTC+05 |
| VET | Venezuelan Standard Time | UTC−04 |
| VLAT | Vladivostok Time | UTC+10 |
| VOLT | Volgograd Time | UTC+04 |
| VOST | Vostok Station Time | UTC+06 |
| VUT | Vanuatu Time | UTC+11 |
| WAKT | Wake Island Time | UTC+12 |
| WAST | West Africa Summer Time | UTC+02 |
| WAT | West Africa Time | UTC+01 |
| WEST | Western European Summer Time | UTC+01 |
| WET | Western European Time | UTC±00 |
| WIB | Western Indonesian Time | UTC+07 |
| WGST | West Greenland Summer Time | UTC−02 |
| WGT | West Greenland Time | UTC−03 |
| WST | Western Standard Time | UTC+08 |
| YAKT | Yakutsk Time | UTC+09 |
| YEKT | Yekaterinburg Time | UTC+05 |
