A time zone is an area that 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.
All time zones are defined as offsets from Coordinated Universal Time (UTC), ranging from UTC−12:00 to UTC+14:00. The offsets 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 of higher latitude use daylight saving time for part of the year, typically by adding one hour to local time during spring and summer.
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||Baker Island||Howland Island|
|UTC−11:00|| American Samoa |
| Midway 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 |
| 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 |
United States: Arizona (most)
| Canada: Alberta, British Columbia (southeast), Northwest Territories, Nunavut (west)|
Mexico: Baja California Sur, Chihuahua, Nayarit (most), Sinaloa
United States: Colorado, Idaho (most), Montana, New Mexico, Utah, Wyoming
|UTC−06:00|| Belize |
Canada: Saskatchewan (most)
| El Salvador |
| Canada: Manitoba, Nunavut (central), Ontario (west)|
Chile: Easter Island
|United States: Alabama, Arkansas, Illinois, Iowa, Kansas (most), Louisiana, Minnesota, Mississippi, Missouri, Nebraska (most), North Dakota (most), Oklahoma, South Dakota (most), Tennessee (most), Texas (most), Wisconsin|
|UTC−05:00|| Brazil: Acre |
Canada: Atikokan, Mishkeegogamang, Southampton Island
| Jamaica |
Mexico: Quintana Roo
| Bahamas |
Canada: Nunavut (east), Ontario (most), Quebec (most)
Turks and Caicos Islands
|United States: Connecticut, Delaware, District of Columbia, Florida (most), Georgia, Indiana (most), Kentucky (most), Maine, Maryland, Massachusetts, Michigan (most), New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Vermont, Virginia, West Virginia|
|UTC−04:00|| Anguilla |
Antigua and Barbuda
Brazil: Amazonas (most), Mato Grosso, Mato Grosso do Sul, Rondônia, Roraima
British Virgin Islands
Canada: Quebec (east)
| Guadeloupe |
Saint Kitts and Nevis
Saint Vincent and the Grenadines
Trinidad and Tobago
U.S. Virgin Islands
| Bermuda |
Canada: Labrador (most), New Brunswick, Nova Scotia, Prince Edward Island
Greenland: Thule Air Base
|UTC−03:30||Canada: Newfoundland, Labrador (southeast)|
|UTC−03:00|| Argentina |
| French Guiana |
| Greenland (most)|
Saint Pierre and Miquelon
|UTC−02:00|| Brazil: Fernando de Noronha |
South Georgia and the South Sandwich Islands
|UTC−01:00||Cape Verde|| Greenland: Ittoqqortoormiit |
|UTC±00:00|| Burkina Faso |
| Liberia |
Saint Helena, Ascension and Tristan da Cunha
São Tomé and Príncipe
| Faroe Islands |
Isle of Man
Spain: Canary Islands
|UTC+01:00|| Algeria |
Central African Republic
Democratic Republic of the Congo: Équateur, Kinshasa, Kongo Central, Kwango, Kwilu, Mai-Ndombe, Mongala, Nord-Ubangi, Sud-Ubangi, Tshuapa
| Equatorial Guinea |
| Albania |
Bosnia and Herzegovina
| Hungary |
| Norway |
|UTC+02:00|| Botswana |
Democratic Republic of the Congo (most)
| Mozambique |
South Africa (most)
| Akrotiri and Dhekelia |
| Israel |
| Northern Cyprus |
|UTC+03:00|| Abkhazia |
French Southern and Antarctic Lands: Scattered Islands
| Qatar |
Russia (most of European part)
South Africa: Prince Edward Islands
Ukraine: Donetsk PR, Luhansk PR, Crimea
|UTC+04:00|| Armenia |
French Southern and Antarctic Lands: Crozet Islands
| Oman |
Russia: Astrakhan, Samara, Saratov, Udmurtia, Ulyanovsk
United Arab Emirates
|UTC+05:00|| French Southern and Antarctic Lands: Kerguelen Islands, Saint Paul Island, Amsterdam Island |
Heard Island and McDonald Islands
Kazakhstan: Aktobe, Atyrau, Baikonur, Kyzylorda, Mangystau, West Kazakhstan
| Pakistan |
Russia: Bashkortostan, Chelyabinsk, Khanty-Mansi, Kurgan, Orenburg, Perm, Sverdlovsk, Tyumen, Yamalia
|UTC+05:30|| India |
|UTC+06:00|| Bangladesh |
British Indian Ocean Territory
| Kazakhstan (most)|
|UTC+06:30|| Cocos Islands |
|UTC+07:00|| Cambodia |
Indonesia: Sumatra, Java, West Kalimantan, Central Kalimantan
Mongolia: Bayan-Ölgii, Khovd, Uvs
| Russia: Altai Krai, Altai Republic, Kemerovo, Khakassia, Krasnoyarsk, Novosibirsk, Tomsk, Tuva |
|UTC+08:00|| Australia: Western Australia (most)|
Indonesia: South Kalimantan, East Kalimantan, North Kalimantan, Sulawesi, Bali, West Nusa Tenggara, East Nusa Tenggara
| Macau |
Russia: Buryatia, Irkutsk
|UTC+09:00|| East Timor |
Indonesia: Maluku, North Maluku, Papua, West Papua
| Palau |
Russia: Amur, Sakha (most), Zabaykalsky
|UTC+09:30||Australia: Northern Territory||Australia: South Australia|
|UTC+10:00|| Australia: Queensland |
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|
|UTC+11:00|| Micronesia: Kosrae, Pohnpei |
Papua New Guinea: Bougainville
| Russia: Magadan, Sakha (east), Sakhalin |
|UTC+12:00|| Fiji |
Kiribati: Gilbert Islands
| Russia: Chukotka, Kamchatka |
Wallis and Futuna
|New Zealand (most)|
|UTC+12:45||New Zealand: Chatham Islands|
|UTC+13:00|| Kiribati: Phoenix Islands |
| Tokelau |
|UTC+14:00||Kiribati: Line Islands|
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.
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 Great Western Railway started using GMT kept by portable chronometers.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 then British Colony of New Zealand officially adopted a standard time to be observed throughout the colony. 172°30′ east of Greenwich, that is 11 hours 30 minutes ahead of GMT. This standard was known as New Zealand Mean Time.It was based on longitude
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.
Charles F. Dowd proposed a system of hourly standard time zones for North American railroads around 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 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. 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 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.
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 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 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 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.
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).
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 China Standard Time (UTC+08:00), Cuba Standard Time (UTC−05:00) and (North American) Central Standard Time (UTC−06:00), and it is also a widely used variant of ACST (Australian Central Standard Time, UTC+09:30).
Conversion between time zones obeys the relationship
in which each side of the equation is equivalent to UTC.
The conversion equation can be rearranged to
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
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|
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.
|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 located 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 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.
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.
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.
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. There is also an option in newer versions of Microsoft Outlook to enter the time zone in which an event will happen, but often not in other calendar systems. Calendaring software must also deal with daylight saving time (DST). If, for political reasons, the begin and end dates of daylight saving time are changed, calendar entries should stay the same in local time, even though they may shift in UTC time. In Microsoft Outlook, time stamps are therefore stored and communicated without DST offsets.Hence, an appointment in London at noon in the summer will be represented as 12:00 (UTC+00:00) even though the event will actually take place at 13:00 UTC. In Google Calendar, calendar events are stored in UTC (although shown in local time) and might be changed by a time-zone changes, although normal daylight saving start and end are compensated for (similar to much other calendar software).
Most Unix-like systems, including Linux and Mac OS X, keep system time in time_t format, representing the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC) on Thursday, January 1, 1970.By default the external representation is as UTC (Coordinated Universal Time), though individual processes can specify time zones using the TZ environment variable. This allows users in multiple time zones to use the same computer, with their respective local times displayed correctly to each user. Time zone information most commonly comes from the IANA time zone database. In fact, many systems, including anything using the GNU C Library, can make use of this database.
Windows-based computer systems prior to Windows 2000 used local time, but Windows 2000 and later can use UTC as the basic system time.The system registry contains time zone information that includes the offset from UTC and rules that indicate the start and end dates for daylight saving in each zone. Interaction with the user normally uses local time, and application software is able to calculate the time in various zones. Terminal Servers allow remote computers to redirect their time zone settings to the Terminal Server so that users see the correct time for their time zone in their desktop/application sessions. Terminal Services uses the server base time on the Terminal Server and the client time zone information to calculate the time in the session.
While most application software will use the underlying operating system for time zone information, the Java Platform, from version 1.3.1, has maintained its own time zone database. This database is updated whenever time zone rules change. Oracle provides an updater tool for this purpose.
As an alternative to the time zone information bundled with the Java Platform, programmers may choose to use the Joda-Time library.This library includes its own time zone 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 time zones. Java 8 Date Time
The DateTime object in Perl supports all time zones in the Olson DB and includes the ability to get, set and convert between time zones.
The DateTime objects and related functions have been compiled into the PHP core since 5.2. This includes the ability to get and set the default script 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.
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.
Each Smalltalk dialect comes with its own built-in classes for dates, times and timestamps, only a few of which implement the DateAndTime and Duration classes as specified by the ANSI Smalltalk Standard. VisualWorks provides a TimeZone class that supports up to two annually recurring offset transitions, which are assumed to apply to all years (same behavior as Windows time zones). Squeak provides a Timezone class that does not support any offset transitions. Dolphin Smalltalk does not support time zones at all.
For full support of the tz database (zoneinfo) in a Smalltalk application (including support for any number of annually recurring offset transitions, and support for different intra-year offset transition rules in different years) the third-party, open-source, ANSI-Smalltalk-compliant Chronos Date/Time Library is available for use with any of the following Smalltalk dialects: VisualWorks, Squeak, Gemstone, or Dolphin.
Orbiting spacecraft 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, because solar-powered rover activity on the surface was tied to periods of light and dark.
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a consequence, it cannot be used to specify a particular time unless a context is given. The term 'GMT' is also used as one of the names for the time zone UTC+00:00.
Daylight saving time (DST), also known as daylight savings time or daylight time, and summer time, is the practice of advancing clocks during warmer months so that darkness falls at a later clock time. The typical implementation of DST is to set clocks forward by one hour in the spring and set clocks back by one hour in autumn to return to standard time. As a result, there is one 23-hour day in late winter or early spring and one 25-hour day in the autumn.
Japan Standard Time, or Japan Central Standard Time, is the standard time zone in Japan, 9 hours ahead of UTC. Japan does not observe daylight saving time, though its introduction has been debated on several occasions. During World War II, the time zone was often referred to as Tokyo Standard Time.
The Hawaii–Aleutian Time Zone observes Hawaii–Aleutian Standard Time (HST) by subtracting ten hours from Coordinated Universal Time (UTC−10:00). The clock time in this zone is based on the mean solar time of the 150th meridian west of the Greenwich Observatory.
Central European Time (CET) is a standard time which is 1 hour ahead of Coordinated Universal Time (UTC). The time offset from UTC can be written as UTC+01:00. It is used in most parts of Europe and in a few North African countries. CET is also known as Middle European Time and by colloquial names such as Amsterdam Time, Berlin Time, Brussels Time, Madrid Time, Paris Time, Rome Time, and Warsaw Time.
Time in the United States, by law, is divided into nine standard time zones covering the states, territories and other US possessions, with most of the United States observing daylight saving time (DST) for approximately the spring, summer, and fall months. The time zone boundaries and DST observance are regulated by the Department of Transportation. Official and highly precise timekeeping services (clocks) are provided by two federal agencies: the National Institute of Standards and Technology (NIST) ; and the United States Naval Observatory (USNO). The clocks run by these services are kept synchronized with each other as well as with those of other international timekeeping organizations.
Standard time is the synchronization of clocks within a geographical region to a single time standard, rather than a local mean time standard. Generally, standard time agrees with the local mean time at some meridian that passes through the region, often near the center of the region. Historically, standard time was established during the 19th century to aid weather forecasting and train travel. Applied globally in the 20th century, the geographical regions became time zones. The standard time in each time zone has come to be defined as an offset from Universal Time. A further offset is applied for part of the year in regions with daylight saving time.
The Atlantic Time Zone is a geographical region that keeps standard time—called Atlantic Standard Time (AST)—by subtracting four hours from Coordinated Universal Time (UTC), resulting in UTC−04:00. During part of the year, some portions of the zone observe daylight saving time, referred to as Atlantic Daylight Time (ADT), by moving their clocks forward one hour to result in UTC−03:00. The clock time in this zone is based on the mean solar time of the 60th meridian west of the Greenwich Observatory.
Singapore Standard Time (SST), also known as Singapore Time (SGT), is used in Singapore and is 8 hours ahead of UTC (UTC+08:00). Singapore does not currently observe daylight saving time.
Australia uses three main time zones: Australian Western Standard Time, Australian Central Standard Time, and Australian Eastern Standard Time. Time is regulated by the individual state governments, some of which observe daylight saving time (DST). Australia's external territories observe different time zones.
The time in China follows a single standard time offset of UTC+08:00, even though China spans almost five geographical time zones. The official national standard time is called Beijing Time domestically and China Standard Time (CST) internationally. Daylight saving time has not been observed since 1991.
There are eleven time zones in Russia, which currently observe times ranging from UTC+02:00 to UTC+12:00. Daylight saving time (DST) has not been used in Russia since 26 October 2014. From 27 March 2011 to 26 October 2014, permanent DST was used.
Canada is divided into six time zones, based on proposals by Scottish Canadian railway engineer Sandford Fleming, who pioneered the use of the 24-hour clock, the world's time zone system, and a standard prime meridian. Most of Canada operates on standard time from the first Sunday in November to the second Sunday in March and daylight saving time the rest of the year.
Thailand follows UTC+07:00, which is 7 hours ahead of UTC. The local mean time in Bangkok was originally UTC+06:42:04. Thailand used this local mean time until 1920, when it changed to Indochina Time, UTC+07:00; ICT is used all year round as Thailand does not observe daylight saving time. Thailand shares the same time zone with Vietnam, Cambodia, Laos, Christmas Island, and Western Indonesia.
Time in Chile is divided into three time zones. Most of Continental Chile uses the time offset UTC−04:00 in winter time and UTC−03:00 in summer time, while the Magallanes and Chilean Antarctica region uses the time offset UTC-03:00 the whole year. Additionally, Easter Island uses the time offset UTC−06:00 in winter time and UTC−05:00 in summer time
Iran Standard Time (IRST) or Iran Time (IT) is the time zone used in Iran. Iran uses a UTC offset UTC+03:30. IRST is defined by the 52.5 degrees east meridian, the same meridian which defines the Iranian calendar and is the official meridian of Iran.
UTC±00:00 is an identifier for a time offset from UTC of +00:00. In ISO 8601, an example of the associated time would be written as 2019-02-07T23:28:34+00:00. It is also known by the following mnemonic or historical names:
Europe spans seven primary time zones, excluding summer time offsets. Most European countries use summer time and harmonise their summer time adjustments; see Summer time in Europe for details.
As of 2018, daylight saving time (DST) is permanently observed in Morocco. Previously, time was advanced to UTC+01:00 at 02:00 on the last Sunday of March, and reverted to UTC±00:00, defined as Greenwich Mean Time locally, at 03:00 on the last Sunday of October. This practise was continued through October 2018, after which clocks were permanently advanced. An exception was made during the month of Ramadan during which clocks reverted to UTC+00:00.
Denmark, including the dependencies Faroe Islands and Greenland, uses six time zones.
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