Time zone

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A time zone is a region of the globe that observes a uniform standard time for legal, commercial, and social purposes. Time zones tend to follow the boundaries of countries and their subdivisions because it is convenient for areas in close commercial or other communication to keep the same time.

Standard time The synchronization of clocks within a geographical area or region

Standard time is the synchronization of clocks within a geographical area or region to a single time standard, rather than using solar time or a locally chosen meridian (longitude) to establish a local mean time standard. Historically, the concept was established during the 19th century to aid weather forecasting and train travel. Applied globally in the 20th century, the geographical areas became extended around evenly spaced meridians into time zones which (usually) centered on them. The standard time set in each time zone has come to be defined in terms of offsets from Universal Time. In regions where daylight saving time is used, that time is defined by another offset, from the standard time in its applicable time zones.

Commerce relates to "the exchange of goods and services, especially on a large scale". It includes legal, economic, political, social, cultural and technological systems that operate in a country or in international trade.

Social characteristic of living organisms

Living organisms including humans are social when they live collectively in interacting populations, whether they are aware of it, and whether the interaction is voluntary or involuntary.

Contents

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, and Indian Standard Time is UTC+05:30).

Coordinated Universal Time Primary time standard by which the world regulates clocks and time

Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is within about 1 second of mean solar time at 0° longitude, and is not adjusted for daylight saving time. In some countries where English is spoken, the term Greenwich Mean Time (GMT) is often used as a synonym for UTC and predates UTC by nearly 300 years.

UTC−12:00 identifier for a time offset from UTC of −12 hours

UTC−12:00 is a time offset that subtracts 12 hours from Coordinated Universal Time (UTC). It is the last to celebrate a New Year, and it is sometimes referred to as the International Date Line West time zone (IDLW).

UTC+14:00 Identifier for a time offset from UTC of +14

UTC+14:00 is an identifier for a +14-hour time offset from UTC. This is the earliest time zone on Earth, meaning that areas in this zone are the first to see a new day, and therefore the first to celebrate a New Year. It could also be called the latest time zone on earth, meaning that clocks in this zone show the 'latest' time of all time zones.

Some higher latitude and temperate zone countries use daylight saving time for part of the year, typically by adjusting local clock time by an hour. Many land time zones are skewed toward the west of the corresponding nautical time zones. This also creates a permanent daylight saving time effect.

Daylight saving time Practice of advancing clocks so that evenings have more daylight and mornings have less

Daylight saving time (DST), also daylight savings time or daylight time and summer time, is the practice of advancing clocks during summer months so that evening daylight lasts longer, while sacrificing normal sunrise times. Typically, regions that use daylight saving time adjust clocks forward one hour close to the start of spring and adjust them backward in the autumn to standard time. In effect, DST causes a lost hour of sleep in the spring and an extra hour of sleep in the fall.

History

Early timekeeping

Before clocks were invented, it was common practice to mark the time of day with apparent solar time (also called "true" solar time) – for example, the time on a sundial – which was typically different for every location and dependent on longitude.

Sundial device that tells the time of day by the apparent position of the Sun in the sky

A sundial is a device that tells the time of day when there is sunlight by the apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat plate and a gnomon, which casts a shadow onto the dial. As the Sun appears to move across the sky, the shadow aligns with different hour-lines, which are marked on the dial to indicate the time of day. The style is the time-telling edge of the gnomon, though a single point or nodus may be used. The gnomon casts a broad shadow; the shadow of the style shows the time. The gnomon may be a rod, wire, or elaborately decorated metal casting. The style must be parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude.

Longitude A geographic coordinate that specifies the east-west position of a point on the Earths surface

Longitude, is a geographic coordinate that specifies the east–west position of a point on the Earth's surface, or the surface of a celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek letter lambda (λ). Meridians connect points with the same longitude. By convention, one of these, the Prime Meridian, which passes through the Royal Observatory, Greenwich, England, was allocated the position of 0° longitude. The longitude of other places is measured as the angle east or west from the Prime Meridian, ranging from 0° at the Prime Meridian to +180° eastward and −180° westward. Specifically, it is the angle between a plane through the Prime Meridian and a plane through both poles and the location in question.

When well-regulated mechanical clocks became widespread in the early 19th century, [1] 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 difference between the two sums to zero after a year.

Equation of time apparent solar time minus mean solar time

The equation of time describes the discrepancy between two kinds of solar time. The word equation is used in the medieval sense of "reconcile a difference". The two times that differ are the apparent solar time, which directly tracks the diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun with uniform motion. Apparent solar time can be obtained by measurement of the current position of the Sun, as indicated by a sundial. Mean solar time, for the same place, would be the time indicated by a steady clock set so that over the year its differences from apparent solar time would resolve to zero.

Orbital eccentricity parameter that determines the amount by which an orbit deviates from a perfect circle

The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptic orbit, 1 is a parabolic escape orbit, and greater than 1 is a hyperbola. The term derives its name from the parameters of conic sections, as every Kepler orbit is a conic section. It is normally used for the isolated two-body problem, but extensions exist for objects following a Klemperer rosette orbit through the galaxy.

In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane. It differs from orbital inclination.

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.

Greenwich Mean Time time zone

Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, reckoned from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a consequence, it cannot be used to specify a precise time unless a context is given.

Royal Observatory, Greenwich observatory in Greenwich, London, UK

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Railway time

Plaque commemorating the Railway General Time Convention of 1883 in North America Time zone chicago.jpg
Plaque commemorating the Railway General Time Convention of 1883 in North America

Local solar time became increasingly inconvenient as rail transport and telecommunications improved, because clocks differed between places by amounts corresponding to the differences in their geographical longitudes, which varied by four minutes of time for every degree of longitude. For example, Bristol is about 2.5 degrees west of Greenwich (East London), so when it is solar noon in Bristol, it is about 10 minutes past solar noon in London. [2] The use of time zones accumulates these differences into longer units, usually hours, so that nearby places can share a common standard for timekeeping.

The first adoption of a standard time was on December 1, 1847, in Great Britain by railway companies using GMT kept by portable chronometers. The first of these companies to adopt standard time was the Great Western Railway (GWR) in November 1840. This quickly became known as Railway Time. About August 23, 1852, time signals were first transmitted by telegraph from the Royal Observatory, Greenwich. Even though 98% of Great Britain's public clocks were using GMT by 1855, it was not made Britain's legal time until August 2, 1880. Some British clocks from this period have two minute hands—one for the local time, one for GMT. [3]

Improvements in worldwide communication further increased the need for interacting parties to communicate mutually comprehensible time references to one another. The problem of differing local times could be solved across larger areas by synchronizing clocks worldwide, but in many places that adopted time would then differ markedly from the solar time to which people were accustomed.

On November 2, 1868, the then British colony of New Zealand officially adopted a standard time to be observed throughout the colony, and was perhaps the first country to do so. It was based on the longitude 172°30′ East of Greenwich, that is 11 hours 30 minutes ahead of GMT. This standard was known as New Zealand Mean Time. [4]

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. [5]

1913 time zone map of the United States, showing boundaries very different from today Time zone map of the United States 1913 (colorized).png
1913 time zone map of the United States, showing boundaries very different from today

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. [6] 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", [7] 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. [8] 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.

Worldwide time zones

The first known person to conceive of a worldwide system of time zones was the Italian mathematician Quirico Filopanti. He introduced the idea in his book Miranda! published in 1858. He proposed 24 hourly time zones, which he called "longitudinal days", the first centred on the meridian of Rome. He also proposed a universal time to be used in astronomy and telegraphy. But his book attracted no attention until long after his death. [9] [10]

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". [11] 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". [12]

By about 1900, almost all time on Earth was in the form of standard time zones, only some of which used an hourly offset from GMT. Many applied the time at a local astronomical observatory to an entire country, without any reference to GMT. It took many decades before all time on Earth was in the form of time zones referred to some "standard offset" from GMT/UTC. By 1929, most major countries had adopted hourly time zones. Nepal was the last country to adopt a standard offset, shifting slightly to UTC+5:45 in 1956. [13]

Today, all nations use standard time zones for secular purposes, but they do not all apply the concept as originally conceived. Newfoundland, India, Iran, Afghanistan, Burma, Sri Lanka, the Marquesas, as well as parts of Australia use half-hour deviations from standard time, and some nations, such as Nepal, and some provinces, such as the Chatham Islands of New Zealand, use quarter-hour deviations. Some countries, such as China and India, use a single time zone even though the extent of their territory far exceeds 15° of longitude. [14] Russia is traditionally divided into 11 time zones, but in 2011 the number was reduced to nine. Then-President Dmitry Medvedev said at the time that he would like to see even fewer in place. [15] Still in 2014, the two removed time zones were reinstated, making them 11 again.

Notation of time

ISO 8601

ISO 8601 is an international standard that defines methods of representing dates and times in textual form, including specifications for representing time zones. [16]

UTC

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". [17]

UTC time is also known as "Zulu" time, since "Zulu" is a phonetic alphabet code word for the letter "Z". [18]

Offsets from UTC

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). [19]

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 and their use as sole designator for a time zone is discouraged. Such designations can be ambiguous; for example, "ECT" could be interpreted as "Eastern Caribbean Time" (UTC−4h), "Ecuador Time" (UTC−5h), or "European Central Time" (UTC+1h). [20]

UTC offsets worldwide

Standard World Time Zones.png
A great part of the world has a gap between the official time and the solar time Solar time vs standard time.png
A great part of the world has a gap between the official time and the solar time
UTC−12:00 ...
UTC−07:00
UTC−06:00 ...
UTC−01:00
UTC±00:00 ...
UTC+05:45
UTC+06:00 ...
UTC+11:30
UTC+12:00 ...
UTC+14:00
Oceania / North America / Antarctica North and South America / Antarctica Europe / Africa / Asia / Antarctica Asia / Antarctica Asia / Oceania / Antarctica
No DST in summerDST in summerNo DST in summerDST in summerNo DST in summerDST in summerNo DST in summerDST in summerNo DST in summerDST in summer
−12:00 −12:00
/−11:00
N: US-
−06:00 −06:00
/−05:00
N: US-, MX-
±00:00
IS
±00:00
/+01:00
N: GB, IE, PT
+06:00
RU-, KZ--
+06:00
/+07:00
+12:00
KI-, RU-
+12:00
/+13:00
S: NZ-
+06:30
MM
+12:45 +12:45
/+13:45
S: NZ
−11:00
US-
−11:00
/−10:00
−05:00
BO, CO, PA, PE
−05:00
/−04:00
N: CA-, CU, US-
+01:00
TN, CG, CD-, DZ, NE, NG
+01:00
/+02:00
N: AT, BA, BE, CH, CZ, DE, DK, ES-, FR, HR, HU, IT, LI, LU, MK, NL, NO, PL, SE, SI, SK
S: NA
+07:00
RU-, VN, LA, TH, KH, ID-
+07:00
/+08:00
N: MN-
+13:00
KI-
−10:00
US-
−10:00
/−09:00
US-
−04:00 −04:00
/−03:00
S: CL-
+02:00
Africa: BI, BW, CD-, EG, LY, MW, MZ, RW, ZA, ZM, ZW
+02:00
/+03:00
N: FI, EE, LV, LT, UA, BG, GR, MD, RO
+08:00
AU-, CN, HK, ID, MY, RU-, PH, SG, TW,
+08:00
/+09:00
N: MN-
+14:00
KI-
−03:30 −03:30
/−02:30
S: CA-
−09:00 −09:00
/−08:00
N: US-
−03:00
S: AR
CL-
−03:00
/−02:00
S: BR-
+03:00
Europe: BY, RU-, TR, Africa: KE, SD, SO, SS, ER, Asia: IQ, SA
+03:00
/+04:00
+09:00
RU-, JP, KR, ID-
+09:00
/+10:00
+03:30 +03:30
/+04:30
IR
+09:30 +09:30
/+10:30
AU-
−08:00 −08:00
/−07:00
N: CA-, US-, MX-
−02:00
BR-
−02:00
/−01:00
+04:00
RU-, GE
+04:00
/+05:00
+10:00
RU-
+10:00
/+11:00
+04:30
AF
−07:00
US-, MX-
−07:00
/−06:00
N: CA-, US-, MX-
S: CL-
−01:00 −01:00
/±00:00
+05:00
KZ-, PK
+05:00
/+06:00
+11:00
RU-
+11:00
/+12:00
+05:30
IN
+11:30
NF
+05:45
NP

XX = ISO 3166-1 alpha-2 country code, XX- = parts of the country, N = North, S = South, UTC = Universal Coordinated Time, DST = Daylight Saving Time

List of UTC offsets

These examples give the local time at various locations around the world when daylight saving time is not in effect:

Time offsetExample time
(ISO 8601 notation)
Example locations that do not use DSTExample locations that in summer use DST
UTC−12:00 2019-05-20T18:39:53 -12:00 Baker Island

Howland Island

UTC−11:00 2019-05-20T19:39:53 -11:00 Flag of American Samoa.svg American Samoa

Flag of Niue.svg Niue

UTC−10:00 2019-05-20T20:39:53 -10:00 Flag of French Polynesia.svg French Polynesia (most)

Flag of the United States.svg United States ( Flag of Hawaii.svg Hawaii)

Flag of the Cook Islands.svg Cook Islands

Flag of the United States.svg United States (Aleutian Islands)
UTC−09:30 2019-05-20T21:09:53 -09:30 Flag of French Polynesia.svg French Polynesia (Marquesas Islands)
UTC−09:00 2019-05-20T21:39:53 -09:00 Flag of French Polynesia.svg French Polynesia (Gambier Islands) Flag of the United States.svg United States ( Flag of Alaska.svg Alaska (most))
UTC−08:00 2019-05-20T22:39:53 -08:00 Pitcairn Islands Flag of Canada (Pantone).svg Canada ( Flag of British Columbia.svg British Columbia (most))

Flag of Mexico.svg Mexico ( Flag of Baja California.svg Baja California)

Flag of the United States.svg United States ( Flag of California.svg California, Flag of Nevada.svg Nevada (most), Flag of Oregon.svg Oregon (most), Flag of Washington.svg Washington)

UTC−07:00 2019-05-20T23:39:53 -07:00 Flag of Canada (Pantone).svg Canada (northeastern Flag of British Columbia.svg British Columbia)

Flag of Mexico.svg Mexico ( Flag of Sonora.svg Sonora)

Flag of the United States.svg United States ( Flag of Arizona.svg Arizona (most))

Flag of Canada (Pantone).svg Canada ( Flag of Alberta.svg Alberta)

Flag of Mexico.svg Mexico ( Flag of Chihuahua.svg Chihuahua)

Flag of the United States.svg United States ( Flag of Colorado.svg Colorado)

UTC−06:00 2019-05-21T00:39:53 -06:00 Flag of Belize.svg  Belize Flag of Canada (Pantone).svg Canada ( Flag of Saskatchewan.svg Saskatchewan (most))

Flag of Costa Rica.svg  Costa Rica

Flag of Ecuador.svg  Ecuador (Galápagos Islands)

Flag of El Salvador.svg  El Salvador

Flag of Guatemala.svg  Guatemala

Flag of Honduras.svg  Honduras

Flag of Nicaragua.svg  Nicaragua

Flag of Canada (Pantone).svg Canada ( Flag of Manitoba.svg Manitoba)

Flag of the United States.svg United States ( Flag of Illinois.svg Illinois, Flag of Texas.svg Texas (most))

Flag of Mexico.svg Mexico (most)

Flag of Chile.svg Chile ( Flag of Rapa Nui, Chile.svg Easter Island)

UTC−05:00 2019-05-21T01:39:54 -05:00 Flag of Brazil.svg Brazil (Acre)

Flag of Colombia.svg  Colombia

Flag of Ecuador.svg  Ecuador (continental)

Flag of Haiti.svg  Haiti

Flag of Jamaica.svg  Jamaica

Flag of Mexico.svg Mexico ( Flag of Quintana Roo.svg Quintana Roo (most))

Flag of Panama.svg  Panama

Flag of Peru.svg  Peru

Flag of the Bahamas.svg Bahamas

Flag of Canada (Pantone).svg Canada ( Flag of Ontario.svg Ontario (most), Flag of Quebec.svg Quebec (most))

Flag of Cuba.svg  Cuba

Flag of the United States.svg United States ( Flag of Florida.svg Florida (most), Flag of Georgia (U.S. state).svg Georgia, Flag of New York.svg New York, Flag of Ohio.svg Ohio, Flag of Pennsylvania.svg Pennsylvania)

UTC−04:00 2019-05-21T02:39:53 -04:00 Flag of Barbados.svg  Barbados Flag of Bolivia.svg  Bolivia

Flag of Brazil.svg Brazil (Amazonas (most), Bandeira de Rondonia.svg Rondônia, Bandeira de Roraima.svg Roraima)

Flag of the Dominican Republic.svg  Dominican Republic

Flag of Puerto Rico.svg Puerto Rico

Flag of Trinidad and Tobago.svg  Trinidad and Tobago

Flag of Venezuela.svg Venezuela

Flag of Brazil.svg Brazil ( Bandeira de Mato Grosso.svg Mato Grosso (most), Bandeira de Mato Grosso do Sul.svg Mato Grosso do Sul)

Flag of Canada (Pantone).svg Canada ( Flag of Nova Scotia.svg Nova Scotia, Flag of New Brunswick.svg New Brunswick, Flag of Labrador.svg Labrador (most), Flag of Prince Edward Island.svg Prince Edward Island)

Flag of Chile.svg  Chile (continental)

Flag of Paraguay.svg  Paraguay

UTC−03:30 2019-05-21T03:09:54 -03:30 Flag of Canada (Pantone).svg Canada (southeastern Flag of Labrador.svg Labrador, Flag of Newfoundland and Labrador.svg Newfoundland)
UTC−03:00 2019-05-21T03:39:54 -03:00 Flag of Argentina.svg Argentina

Flag of Brazil.svg Brazil ( Bandeira da Bahia.svg Bahia, Bandeira Estado Ceara Brasil.svg Ceará, Bandeira do Maranhao.svg Maranhão, Bandeira do Para.svg Pará, Bandeira de Pernambuco.svg Pernambuco)

Flag of Chile.svg Chile (Magallanes)

Flag of the Falkland Islands.svg Falkland Islands

Flag of France.svg Saint Pierre and Miquelon

Flag of Uruguay.svg  Uruguay

Flag of Brazil.svg Brazil ( Bandeira do Espirito Santo.svg Espírito Santo, Federal District, Bandeira de Minas Gerais.svg Minas Gerais, Paraná, Bandeira do Rio Grande do Sul.svg Rio Grande do Sul, Rio de Janeiro, São Paulo)

Flag of Greenland.svg Greenland (most)

UTC−02:00 2019-05-21T04:39:54 -02:00 Flag of Brazil.svg Brazil (Fernando de Noronha)

Flag of South Georgia and the South Sandwich Islands.svg South Georgia and the South Sandwich Islands

UTC−01:00 2019-05-21T05:39:54 -01:00 Flag of Cape Verde.svg  Cape Verde Flag of Portugal.svg  Portugal ( Flag of the Azores.svg Azores)
UTC±00:00 2019-05-21T06:39:54 +00:00 Flag of Cote d'Ivoire.svg  Ivory Coast Flag of Ghana.svg  Ghana

Flag of Iceland.svg  Iceland

Flag of Saint Helena.svg Saint Helena

Flag of Senegal.svg  Senegal

Flag of Mali.svg  Mali

Flag of the Faroe Islands.svg Faroe Islands

Flag of Ireland.svg  Ireland

Flag of Portugal.svg  Portugal (continental, Flag of Madeira.svg Madeira)

Flag of Spain.svg  Spain ( Flag of the Canary Islands.svg Canary Islands)

Flag of the United Kingdom.svg  United Kingdom

UTC+01:00 2019-05-21T07:39:53 +01:00 Flag of Algeria.svg  Algeria Flag of Angola.svg  Angola

Flag of Benin.svg  Benin

Flag of Cameroon.svg  Cameroon

Flag of the Democratic Republic of the Congo.svg  Democratic Republic of the Congo (west)

Flag of Gabon.svg  Gabon

Flag of Morocco.svg  Morocco

Flag of Niger.svg  Niger

Flag of Nigeria.svg  Nigeria

Flag of Tunisia.svg  Tunisia

Flag of the Sahrawi Arab Democratic Republic.svg  Western Sahara

Flag of Albania.svg  Albania

Flag of Andorra.svg  Andorra

Flag of Austria.svg  Austria

Flag of Belgium (civil).svg  Belgium

Flag of Bosnia and Herzegovina.svg  Bosnia and Herzegovina

Flag of Croatia.svg  Croatia

Flag of the Czech Republic.svg  Czech Republic

Flag of Denmark.svg  Denmark

Flag of France.svg  France (metropolitan)

Flag of Germany.svg Germany

Flag of Hungary.svg  Hungary

Flag of Italy.svg  Italy

Flag of Liechtenstein.svg  Liechtenstein

Flag of Luxembourg.svg  Luxembourg

Flag of Monaco.svg  Monaco

Flag of Malta.svg  Malta

Flag of the Netherlands.svg  Netherlands (European)

Flag of North Macedonia.svg  North Macedonia

Flag of Norway.svg  Norway

Flag of Poland.svg  Poland

Flag of Serbia.svg  Serbia

Flag of Slovakia.svg  Slovakia

Flag of Slovenia.svg  Slovenia

Flag of Spain.svg  Spain (continental)

Flag of Sweden.svg  Sweden

Flag of Switzerland.svg   Switzerland

Flag of the Vatican City.svg   Vatican City

UTC+02:00 2019-05-21T08:39:54 +02:00 Flag of Burundi.svg  Burundi Flag of Egypt.svg  Egypt

Flag of Malawi.svg  Malawi

Flag of Mozambique.svg  Mozambique

Flag of Namibia.svg  Namibia

Flag of Russia.svg Russia ( Flag of Kaliningrad Oblast.svg Kaliningrad)

Flag of Rwanda.svg  Rwanda

Flag of South Africa.svg  South Africa

Flag of Sudan.svg  Sudan

Flag of Eswatini.svg  Eswatini

Flag of Zambia.svg  Zambia

Flag of Zimbabwe.svg  Zimbabwe

Flag of Bulgaria.svg  Bulgaria Flag of Cyprus.svg  Cyprus

Flag of Estonia.svg  Estonia

Flag of Finland.svg  Finland

Flag of Greece.svg  Greece

Flag of Israel.svg  Israel

Flag of Jordan.svg  Jordan

Flag of Latvia.svg  Latvia

Flag of Lebanon.svg  Lebanon

Flag of Lithuania.svg  Lithuania

Flag of Moldova.svg  Moldova

Flag of Palestine.svg Palestine

Flag of Romania.svg  Romania

Flag of Syria.svg  Syria

Flag of Ukraine.svg  Ukraine

UTC+03:00 2019-05-21T09:39:55 +03:00 Flag of Belarus.svg  Belarus Flag of Djibouti.svg  Djibouti

Flag of Eritrea.svg  Eritrea

Flag of Ethiopia.svg  Ethiopia

Flag of Iraq.svg  Iraq

Flag of Kenya.svg  Kenya

Flag of Kuwait.svg  Kuwait

Flag of Madagascar.svg  Madagascar

Flag of Russia.svg Russia (most of European part)

Flag of Saudi Arabia.svg  Saudi Arabia

Flag of Qatar.svg  Qatar

Flag of Somalia.svg  Somalia

Flag of South Sudan.svg  South Sudan

Flag of Tanzania.svg  Tanzania

Flag of Turkey.svg  Turkey

Flag of Uganda.svg  Uganda

Flag of Yemen.svg  Yemen

UTC+03:30 2019-05-21T10:09:55 +03:30 Flag of Iran.svg  Iran
UTC+04:00 2019-05-21T10:39:55 +04:00 Flag of Armenia.svg  Armenia Flag of Azerbaijan.svg  Azerbaijan

Flag of Georgia.svg  Georgia

Flag of Mauritius.svg  Mauritius

Flag of Oman.svg  Oman

Flag of Russia.svg Russia ( Flag of Samara Oblast.svg Samara)

Flag of the Seychelles.svg  Seychelles

Flag of the United Arab Emirates.svg  United Arab Emirates

UTC+04:30 2019-05-21T11:09:55 +04:30 Flag of Afghanistan.svg  Afghanistan
UTC+05:00 2019-05-21T11:39:55 +05:00 Flag of Kazakhstan.svg  Kazakhstan (west)

Flag of Maldives.svg  Maldives

Flag of Pakistan.svg  Pakistan

Flag of Russia.svg Russia ( Flag of Sverdlovsk Oblast.svg Sverdlovsk, Flag of Chelyabinsk Oblast.svg Chelyabinsk)

Flag of Uzbekistan.svg  Uzbekistan

UTC+05:30 2019-05-21T12:09:55 +05:30 Flag of India.svg  India Flag of Sri Lanka.svg  Sri Lanka
UTC+05:45 2019-05-21T12:24:55 +05:45 Flag of Nepal.svg    Nepal
UTC+06:00 2019-05-21T12:39:55 +06:00 Flag of Bangladesh.svg  Bangladesh Flag of Bhutan.svg Bhutan

Flag of the British Indian Ocean Territory.svg British Indian Ocean Territory

Flag of Kazakhstan.svg  Kazakhstan (most)

Flag of Russia.svg Russia ( Flag of Omsk Oblast.svg Omsk)

UTC+06:30 2019-05-21T13:09:55 +06:30 Flag of Australia (converted).svg  Australia ( Flag of the Cocos (Keeling) Islands.svg Cocos Islands)

Flag of Myanmar.svg  Myanmar

UTC+07:00 2019-05-21T13:39:55 +07:00 Flag of Cambodia.svg  Cambodia Flag of Indonesia.svg  Indonesia (west)

Flag of Laos.svg  Laos

Flag of Mongolia.svg Mongolia (west)

Flag of Russia.svg Russia ( Flag of Krasnoyarsk Krai.svg Krasnoyarsk)

Flag of Thailand.svg  Thailand

Flag of Vietnam.svg  Vietnam

UTC+08:00 2019-05-21T14:39:55 +08:00 Flag of Australia (converted).svg Australia ( Flag of Western Australia.svg Western Australia (most))

Flag of Brunei.svg Brunei

Flag of the People's Republic of China.svg  People's Republic of China

Flag of Hong Kong.svg  Hong Kong

Flag of Indonesia.svg  Indonesia (central)

Flag of Macau.svg  Macau

Flag of Malaysia.svg  Malaysia

Flag of Mongolia.svg Mongolia (most)

Flag of the Philippines.svg  Philippines

Flag of Russia.svg Russia ( Flag of Irkutsk Oblast.svg Irkutsk)

Flag of Singapore.svg  Singapore

Flag of the Republic of China.svg Taiwan

UTC+08:45 2019-05-21T15:24:55 +08:45 Flag of Australia (converted).svg  Australia ( Flag of Western Australia.svg Western Australia (Eucla))
UTC+09:00 2019-05-21T15:39:55 +09:00 Flag of East Timor.svg  Timor-Leste Flag of Indonesia.svg  Indonesia (east)

Flag of Japan.svg  Japan

Flag of North Korea.svg  North Korea

Flag of Russia.svg Russia ( Flag of Sakha.svg Sakha (most))

Flag of South Korea.svg  South Korea

UTC+09:30 2019-05-21T16:09:55 +09:30 Flag of Australia (converted).svg Australia ( Flag of the Northern Territory.svg Northern Territory) Flag of Australia (converted).svg Australia ( Flag of South Australia.svg South Australia)
UTC+10:00 2019-05-21T16:39:55 +10:00 Flag of Australia (converted).svg Australia ( Flag of Queensland.svg Queensland)

Flag of Papua New Guinea.svg  Papua New Guinea

Flag of Russia.svg Russia ( Flag of Primorsky Krai.svg Primorsky)

Flag of Australia (converted).svg Australia ( Flag of New South Wales.svg New South Wales, Flag of Tasmania.svg Tasmania, Flag of Victoria (Australia).svg Victoria)
UTC+10:30 2019-05-21T17:09:55 +10:30 Flag of Lord Howe Island.svg Lord Howe Island
UTC+11:00 2019-05-21T17:39:55 +11:00 Flag of FLNKS.svg New Caledonia

Flag of Russia.svg Russia ( Flag of Magadan Oblast.svg Magadan)

Flag of the Solomon Islands.svg  Solomon Islands

Flag of Vanuatu.svg  Vanuatu

UTC+12:00 2019-05-21T18:39:55 +12:00 Flag of Kiribati.svg  Kiribati (Gilbert Islands)

Flag of Russia.svg Russia ( Flag of Kamchatka Krai.svg Kamchatka)

Flag of Fiji.svg  Fiji Flag of New Zealand.svg New Zealand (most)
UTC+12:45 2019-05-21T19:24:55 +12:45 Flag of New Zealand.svg New Zealand (Chatham Islands)
UTC+13:00 2019-05-21T19:39:55 +13:00 Flag of Kiribati.svg  Kiribati (Phoenix Islands)

Flag of Tokelau.svg Tokelau

Flag of Tonga.svg  Tonga

Flag of Samoa.svg  Samoa
UTC+14:00 2019-05-21T20:39:55 +14:00 Flag of Kiribati.svg  Kiribati (Line Islands)

Where the adjustment for time zones results in a time at the other side of midnight from UTC, then the date at the location is one day later or earlier.

Some examples when UTC is 23:00 on Monday when or where daylight saving time is not in effect:

Some examples when UTC is 02:00 on Tuesday when or where daylight saving time is not in effect:

The time-zone adjustment for a specific location may vary because of daylight saving time. For example, New Zealand, which is usually UTC+12, observes a one-hour daylight saving time adjustment during the Southern Hemisphere summer, resulting in a local time of UTC+13.

Time zone 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 more familiar term "UTC offset" is used here rather than the term "zone designator" used by the standard.)

The conversion equation can be rearranged to

"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 Los Angeles (PST, UTC offset= −08:00) and Delhi (IST, UTC offset= +05:30), the New York Stock Exchange opens at

time in Los Angeles = 09:30 − (−05:00) + (−08:00) = 06:30.
time in Delhi = 09:30 − (−05:00) + (+05:30) = 20:00.

These calculations become more complicated near a daylight saving boundary (because the UTC offset for zone X is a function of the UTC time).

The table "Time of day by zone" gives an overview on the time relations between different zones.

Nautical time zones

Since the 1920s a nautical standard time system has been in operation for ships on the high seas. Nautical time zones are an ideal form of the terrestrial time zone system. Under the system, a time change of one hour is required for each change of longitude by 15°. The 15° gore that is offset from GMT or UT1 (not UTC) by twelve hours is bisected by the nautical date line into two 7.5° gores that differ from GMT by ±12 hours. A nautical date line is implied but not explicitly drawn on time zone maps. It follows the 180th meridian except where it is interrupted by territorial waters adjacent to land, forming gaps: it is a pole-to-pole dashed line. [21] [22] [23]

A ship within the territorial waters of any nation would use that nation's standard time, but would revert to nautical standard time upon leaving its territorial waters. The captain is permitted to change the ship's clocks at a time of the captain's choice following the ship's entry into another time zone. The captain often chooses midnight. Ships going in shuttle traffic over a time zone border often keep the same time zone all the time, to avoid confusion about work, meal, and shop opening hours. Still the time table for port calls must follow the land time zone.

Skewing of zones

Difference between sun time and clock time during daylight saving time:
1h +- 30 min behind
0h +- 30m
1h +- 30m ahead
2h +- 30m ahead
3h +- 30m ahead Tzdiff-Europe-summer.png
Difference between sun time and clock time during daylight saving time:
1h ± 30 min behind
0h ± 30m
1h ± 30m ahead
2h ± 30m ahead
3h ± 30m ahead
DST observed
DST formerly observed
DST never observed DaylightSaving-World-Subdivisions.png
  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 located in the middle of that zone with boundaries located 7.5 degrees east and west of the meridian. In practice, zone boundaries are often drawn much farther to the west with often irregular boundaries, and some locations base their time on meridians located far to the east.

For example, even though the Prime Meridian (0°) passes through Spain and France, they use the mean solar time of 15 degrees east (Central European Time) rather than 0 degrees (Greenwich Mean Time). France previously used GMT, but was switched to CET (Central European Time) during the German occupation of the country during World War II and did not switch back after the war. [24] Similarly, prior to World War II, the Netherlands observed "Amsterdam Time", which was twenty minutes ahead of Greenwich Mean Time. They were obliged to follow German time during the war, and kept it thereafter. In the mid 1970s the Netherlands, as with other European states, began observing daylight saving (summer) time.

There is a tendency to draw time zone boundaries far to the west of their meridians. A reason is that it can allow the more efficient use of sunlight. [25] Another reason for this is that similar working day schedules around the world have led to people rising on average at 07:00 clock time and going to bed at 23:00 clock time. This means that the middle of the period that people are awake ("awake time noon") occurs at 15:00 (= [7 + 23]/2) clock time, whereas - if using as clock time the time of the nautical time zone to which the location concerned geographically belongs - solar noon occurs at 12:00 (+/- 30 min) clock time. To make solar noon coincide more with awake time noon (i.e. make the sun reach its highest point closer to 15:00 clock time rather than 12:00 clock time), the time of one or even two nautical time zones to the east is chosen.[ citation needed ] Many of these locations also use DST, adding yet another nautical time zone to the east. As a result, [note 1] in summer, solar noon in the Spanish town of Muxía occurs at 14:37 clock time, indeed very close to awake time noon (15:00). This westernmost area of continental Spain never experiences sunset before 18:00 clock time, even in midwinter, despite its lying more than 40 degrees north of the equator. Near the summer solstice, Muxía has sunset times (after 22:00) similar to those of Stockholm, which is in the same time zone and 16 degrees 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. [26] Kotzebue, Alaska, also near the same meridian but north of the Arctic Circle, has an annual event on August 9 to celebrate two sunsets in the same 24-hour day, one shortly after midnight at the start of the day, and the other shortly before midnight at the end of the day.

Also, China extends as far west as 73°34′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 and Tibet.

Daylight saving time

Many countries, and sometimes just certain regions of countries, adopt daylight saving time (also known as "Summer Time") during part of the year. This typically involves advancing clocks by an hour near the start of spring and adjusting back in autumn ("spring forward", "fall back"). Modern DST was first proposed in 1907 and was in widespread use in 1916 as a wartime measure aimed at conserving coal. Despite controversy, many countries have used it off and on since then; details vary by location and change occasionally. Most countries around the equator do not observe daylight saving time, since the seasonal difference in sunlight is minimal.

Computer systems and the Internet

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, [27] whereas the US version shows Eastern Time. [28] 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.) [29] 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. [30] 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, [31] although normal daylight saving start and end are compensated for (similar to much other calendar software).

Operating systems

Unix

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, [32] . By default the external representation is as UTC (Coordinated Universal Time), though individual processes can specify time zones using the LC_TIME environment variable or less commonly the TZ environment variable.[ citation needed ] This allows users in multiple timezones to use the same computer, with their respective local times displayed correctly to each user. Time zone information most commonly comes from the IANA time zone database. In fact, many systems, including anything using the GNU C Library, can make use of this database.

Microsoft Windows

Windows-based computer systems prior to Windows 2000 used local time, but Windows 2000 and later can use UTC as the basic system time. [33] 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 timezone information, the Java Platform, from version 1.3.1, has maintained its own timezone database. This database is updated whenever timezone rules change. Oracle provides an updater tool for this purpose. [34]

As an alternative to the timezone information bundled with the Java Platform, programmers may choose to use the Joda-Time library. [35] This library includes its own timezone data based on the IANA time zone database. [36]

As of Java 8 there is a new date and time API that can help with converting timezones. Java 8 Date Time

JavaScript

Traditionally, there was very little in the way of timezone 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 do not support it.

Perl

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. [37]

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 timezone, and DateTime is aware of its own timezone internally. PHP.net provides extensive documentation on this. [38] As noted there, the most current timezone database can be implemented via the PECL timezonedb.

Python

The standard module datetime included with Python stores and operates on the timezone information class tzinfo. The third party pytz module provides access to the full IANA time zone database. [39] Negated time zone offset in seconds is stored time.timezone and time.altzone attributes.

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. [40]

Time zones in outer space

Orbiting spacecraft typically experience many sunrises and sunsets in a 24-hour period, or in the case of Apollo program astronauts travelling to the moon, none. Thus it is not possible to calibrate time zones with respect to the sun, and still respect a 24-hour sleep/wake cycle. A common practice for space exploration is to use the Earth-based time zone of the launch site or mission control. This keeps the sleeping cycles of the crew and controllers in sync. The International Space Station normally uses Greenwich Mean Time (GMT). [41] [42]

Timekeeping on Mars can be more complex, since the planet has a solar day of approximately 24 hours and 39 minutes, known as a sol. Earth controllers for some Mars missions have synchronized their sleep/wake cycles with the Martian day, [43] because solar-powered rover activity on the surface was tied to periods of light and dark. The difference in day length caused the sleep/wake cycles to slowly drift with respect to the day/night cycles on Earth, repeating approximately once every 36 days.

See also

The control panel of the Time Zone Clock in front of Coventry Transport Museum. Coventry Time Zone Clock.jpg
The control panel of the Time Zone Clock in front of Coventry Transport Museum.

Notes

  1. Spain may have chosen its time zone for other reasons, such as synchronising with trading partners, and adopting CET as a major member of the EU

Further reading

Related Research Articles

Japan Standard Time time zone

Japan Standard Time is the standard timezone in Japan, 9 hours ahead of UTC. There is no daylight saving time, though its introduction has been debated several times. During World War II, it was often called Tokyo Standard Time.

Indian Standard Time time zone, observed in India and Sri Lanka; UTC+05:30

Indian Standard Time (IST) is the time observed throughout India, with a time offset of UTC+05:30. India does not observe daylight saving time (DST) or other seasonal adjustments. In military and aviation time IST is designated E* ("Echo-Star").

Alaska Time Zone time zone observing UTC -9 during standard time and UTC -8 during daylight saving time

The Alaska Time Zone observes standard time by subtracting nine hours from Coordinated Universal Time (UTC−09:00). During daylight saving time its time offset is eight hours (UTC−08:00). The clock time in this zone is based on mean solar time at the 135th meridian west of the Greenwich Observatory.

Hawaii–Aleutian Time Zone UTC−10:00 during standard time; UTC−09:00 during daylight saving

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.

Time in the United States Timekeeping in the USA

Time in the United States, by law, is divided into nine standard time zones covering the states and its possessions, with most of the United States observing daylight saving time (DST) for approximately the spring, summer, and fall months. The time zone boundaries and DST observance are regulated by the Department of Transportation. Official and highly precise timekeeping services (clocks) are provided by two federal agencies: the National Institute of Standards and Technology (NIST) ; and its military counterpart, the United States Naval Observatory (USNO). The clocks run by these services are kept synchronized with each other as well as with those of other international timekeeping organizations.

Time in New Zealand

Time in New Zealand is divided by law into two standard time zones. The main islands use New Zealand Standard Time (NZST), 12 hours in advance of Coordinated Universal Time (UTC) / military M (Mike), while the outlying Chatham Islands use Chatham Standard Time (CHAST), 12 hours 45 minutes in advance of UTC / military M^ (Mike-Three).

The Atlantic Time Zone is a geographical region that keeps standard time—called Atlantic Standard Time (AST)—by subtracting four hours from Coordinated Universal Time (UTC), resulting in UTC−04:00. During part of the year, some portions of the zone observe daylight saving time, referred to as Atlantic Daylight Time (ADT), by moving their clocks forward one hour to result in UTC−03:00. The clock time in this zone is based on the mean solar time of the 60th meridian west of the Greenwich Observatory.

In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. Modern civil time is generally standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC) or from Greenwich Mean Time (GMT), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks, and was adopted in 1972. Older systems use telescope observations.

South African Standard Time time zone

South African Standard Time (SAST) is the time zone used by all of South Africa, Botswana as well as Eswatini and Lesotho. The zone is two hours ahead of UTC (UTC+02:00) and is the same as Central Africa Time. Daylight saving time is not observed in either time zone. Solar noon in this time zone occurs at 30° E in SAST, effectively making Pietermaritzburg at the correct solar noon point, with Johannesburg and Pretoria slightly west at 28° E and Durban slightly east at 31° E. Thus, most of South Africa's population experience true solar noon at approximately 12:00 daily.

The time in China follows a single standard time offset of UTC+08:00, despite China spanning 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.

Time in Russia About the 11 zones of Russia

There are eleven time zones in Russia, which currently observe times ranging from UTC+02:00 to UTC+12:00. Daylight saving time is not used in Russia.

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, GMT+07:00, ICT is used all year round as Thailand does not observe daylight saving time.

Time in Chile is divided into three time zones. Most of Continental Chile uses the time offset UTC−04:00 in winter time, 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

tz database collaborative compilation of information about the worlds time zones

The tz database is a collaborative compilation of information about the world's time zones, primarily intended for use with computer programs and operating systems. Paul Eggert is its current editor and maintainer, with the organizational backing of ICANN. The tz database is also known as tzdata, the zoneinfo database or IANA time zone database, and occasionally as the Olson database, referring to the founding contributor, Arthur David Olson.

Iran Standard Time time zone used in Iran

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.

Nautical time refers to the systems used by ships on high seas to express their local time. Nautical time keeping dates back to the early 20th century as a standard way to keep time at sea, although it largely only applied to military fleets pre-World War 2. This time-keeping method is only used for radio communications and to account for slight inaccuracies that using Greenwich Standard Time (GST) may lead to during navigation of the high seas. Nautical time zones are split into one hour intervals for every 15 degree change in a ship's longitudinal coordinate. This is typically only used for trans-oceanic travel, as captains will often not change the timekeeping for short distances such as channels or inland seas.

Alaska is covered by two time zones, as described below:

Time in Spain

Spain has two time zones and observes daylight saving time. Spain mainly uses Central European Time (GMT+01:00) and Central European Summer Time (GMT+02:00) in Peninsular Spain, the Balearic Islands, Ceuta, Melilla and plazas de soberanía. In the Canary Islands, the time zone is Western European Time (GMT±00:00) and Western European Summer Time (GMT+01:00). Daylight saving time is observed from the last Sunday in March to the last Sunday in October throughout Spain.

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