Time zone

Last updated

This 2012 time zone map gives an example of the way time zones are mapped out. World Time Zones Map.png
This 2012 time zone map gives an example of the way time zones are mapped out.

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 an identifier for a time offset from UTC of −12:00. It is the offset of the time zone that is the last to celebrate a New Year, 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 time offset from UTC of +14:00. 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. Consequently, it is also referred to as the "latest time zone" on Earth, as clocks in it would always 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. 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.

Clock instrument that measures the passage of time

A clock is an instrument used to measure, keep, and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units: the day, the lunar month, and the year. Devices operating on several physical processes have been used over the millennia.

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 dimensionless 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

The Royal Observatory, Greenwich is an observatory situated on a hill in Greenwich Park, overlooking the River Thames. It played a major role in the history of astronomy and navigation, and is best known for the fact that the prime meridian passes through it, and thereby gave its name to Greenwich Mean Time. The ROG has the IAU observatory code of 000, the first in the list. ROG, the National Maritime Museum, the Queen's House and Cutty Sark are collectively designated Royal Museums Greenwich.

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 2019-08-30T20:51:34 -12:00 Flag of the United States.svg Baker Island

Flag of the United States.svg Howland Island

UTC−11:00 2019-08-30T21:51:34 -11:00 Flag of American Samoa.svg American Samoa
Flag of Niue.svg Niue
UTC−10:00 2019-08-30T22:51:34 -10:00 Flag of the Cook Islands.svg Cook Islands
Flag of French Polynesia.svg French Polynesia (Tahiti)
Flag of the United States.svg United States ( Flag of Hawaii.svg Hawaii)
Flag of the United States.svg United States (Aleutian Islands)
UTC−09:30 2019-08-30T23:21:34 -09:30 Flag of French Polynesia.svg French Polynesia (Marquesas Islands)
UTC−09:00 2019-08-30T23:51:35 -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 2019-08-31T00:51:35 -08:00 Flag of the Pitcairn Islands.svg Pitcairn Islands Flag of Canada (Pantone).svg Canada ( Flag of British Columbia.svg British Columbia (most), Flag of Yukon.svg Yukon)
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 2019-08-31T01:51:35 -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 the Northwest Territories.svg Northwest Territories, Flag of Nunavut.svg Nunavut (west of 102°W))
Flag of Mexico.svg Mexico ( Flag of Chihuahua.svg Chihuahua)
Flag of the United States.svg United States ( Flag of Colorado.svg Colorado, Flag of Idaho.svg Idaho (most), Flag of Montana.svg Montana, Flag of New Mexico.svg New Mexico, Flag of Utah.svg Utah, Flag of Wyoming.svg Wyoming)
UTC−06:00 2019-08-31T02:51:35 -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 Chile.svg Chile ( Flag of Rapa Nui, Chile.svg Easter Island
Flag of Mexico.svg Mexico (most)
Flag of the United States.svg United States ( Flag of Alabama.svg Alabama, Flag of Arkansas.svg Arkansas, Flag of Illinois.svg Illinois, Flag of Iowa.svg Iowa, Flag of Kansas.svg Kansas (most), Flag of Louisiana.svg Louisiana, Flag of Minnesota.svg Minnesota, Flag of Mississippi.svg Mississippi, Flag of Missouri.svg Missouri, Flag of Nebraska.svg Nebraska (most), Flag of North Dakota.svg North Dakota (most), Flag of Oklahoma.svg Oklahoma, Flag of South Dakota.svg South Dakota (most), Flag of Tennessee.svg Tennessee (most), Flag of Texas.svg Texas (most), Flag of Wisconsin.svg Wisconsin))
UTC−05:00 2019-08-31T03:51:35 -05:00 Flag of Brazil.svg Brazil (Acre)
Flag of the Cayman Islands.svg Cayman Islands
Flag of Colombia.svg  Colombia
Flag of Ecuador.svg  Ecuador (main territory)
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 Haiti.svg Haiti
Flag of the United States.svg United States ( Flag of Connecticut.svg Connecticut, Flag of Delaware.svg Delaware, Flag of Florida.svg Florida (most), Flag of Georgia (U.S. state).svg Georgia, Flag of Indiana.svg Indiana (most), Flag of Kentucky.svg Kentucky (most), Flag of Maine.svg Maine, Flag of Maryland.svg Maryland, Flag of Massachusetts.svg Massachusetts, Flag of Michigan.svg Michigan (most), Flag of New Hampshire.svg New Hampshire, Flag of New Jersey.svg New Jersey, Flag of New York.svg New York, Flag of North Carolina.svg North Carolina, Flag of Ohio.svg Ohio, Flag of Pennsylvania.svg Pennsylvania, Flag of Rhode Island.svg Rhode Island, Flag of South Carolina.svg South Carolina, Flag of Vermont.svg Vermont, Flag of Virginia.svg Virginia, Flag of Washington, D.C..svg Washington, D.C., Flag of West Virginia.svg West Virginia)
UTC−04:00 2019-08-31T04:51:35 -04:00 Flag of Anguilla.svg Anguilla
Flag of Antigua and Barbuda.svg Antigua and Barbuda
Flag of Aruba.svg Anguilla
Flag of Barbados.svg  Barbados
Flag of Bolivia.svg  Bolivia
Flag of Brazil.svg Brazil (Amazonas (most), Bandeira de Mato Grosso do Sul.svg Mato Grosso do Sul, Bandeira de Mato Grosso.svg Mato Grosso, Bandeira de Rondonia.svg Rondônia, Bandeira de Roraima.svg Roraima)
Flag of the British Virgin Islands.svg British Virgin Islands
Flag of the Netherlands.svg Caribbean Netherlands
Flag of Curacao.svg Curacao
Flag of Dominica.svg Dominica
Flag of the Dominican Republic.svg  Dominican Republic
Flag of Grenada.svg Grenada
Flag of France.svg Guadeloupe
Flag of Guyana.svg Guyana
Flag of France.svg Martinique
Flag of Montserrat.svg Montserrat
Flag of Puerto Rico.svg Puerto Rico
Flag of France.svg Saint Barthelemy
Flag of Saint Kitts and Nevis.svg Saint Kitts and Nevis
Flag of Saint Lucia.svg Saint Lucia
Flag of France.svg Saint Martin
Flag of Saint Vincent and the Grenadines.svg Saint Vincent and the Grenadines
Flag of Trinidad and Tobago.svg  Trinidad and Tobago
Flag of the Turks and Caicos Islands.svg Turks and Caicos Islands
Flag of the United States Virgin Islands.svg US Virgin Islands
Flag of Venezuela.svg Venezuela
Flag of Canada (Pantone).svg Canada ( Flag of New Brunswick.svg New Brunswick, Flag of Nova Scotia.svg Nova Scotia, Flag of Newfoundland and Labrador.svg Newfoundland and Labrador (Labrador, except of southeastern coast), Flag of Prince Edward Island.svg Prince Edward Island)
Flag of Chile.svg  Chile (main territory)
Flag of Paraguay.svg  Paraguay
UTC−03:30 2019-08-31T05:21:35 -03:30 Flag of Canada (Pantone).svg Canada ( Flag of Newfoundland and Labrador.svg Newfoundland and Labrador (Newfoundland and southeastern coast of Labrador))
UTC−03:00 2019-08-31T05:51:35 -03:00 Flag of Argentina.svg Argentina
Flag of Brazil.svg Brazil (most)
Flag of Chile.svg Chile (Magallanes)
Flag of the Falkland Islands.svg Falkland Islands
Flag of France.svg French Guiana
Flag of Suriname.svg Suriname
Flag of Uruguay.svg  Uruguay
Flag of Greenland.svg Greenland (most)
Flag of France.svg Saint Pierre and Miquelon
UTC−02:00 2019-08-31T06:51:37 -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 2019-08-31T07:51:37 -01:00 Flag of Cape Verde.svg  Cape Verde Flag of Portugal.svg  Portugal ( Flag of the Azores.svg Azores)
Flag of Greenland.svg Greenland (Ittoqqortoormiit)
UTC±00:00 2019-08-31T08:51:37 +00:00 Flag of Burkina Faso.svg Burkina Faso
Flag of The Gambia.svg Gambia
Flag of Ghana.svg  Ghana
Flag of Guinea.svg Guinea
Flag of Guinea-Bissau.svg Guinea-Bissau
Flag of Iceland.svg  Iceland
Flag of Cote d'Ivoire.svg  Ivory Coast
Flag of Liberia.svg Liberia
Flag of Mali.svg  Mali
Flag of Mauritania.svg Mauritania
Flag of Saint Helena.svg Saint Helena
Flag of Senegal.svg  Senegal
Flag of Sierra Leone.svg Sierra Leone
Flag of Sao Tome and Principe.svg Sao Tome and Principe
Flag of Togo.svg Togo
Flag of the Faroe Islands.svg Faroe Islands
Flag of Guernsey.svg Guernsey
Flag of Ireland.svg  Ireland
Flag of the Isle of Man.svg Isle of Man
Flag of Jersey.svg Jersey
Flag of Portugal.svg  Portugal (main territory)
Flag of Spain.svg  Spain ( Flag of the Canary Islands.svg Canary Islands)
Flag of the United Kingdom.svg  United Kingdom
UTC+01:00 2019-08-31T09:51:37 +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 Central African Republic.svg Central African Republic
Flag of the Republic of the Congo.svg Republic of the Congo
Flag of the Democratic Republic of the Congo.svg  Democratic Republic of the Congo (western part)
Flag of Equatorial Guinea.svg Equatorial Guinea
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 Gibraltar.svg Gibraltar
Flag of Hungary.svg  Hungary
Flag of Italy.svg  Italy
Flag of Liechtenstein.svg  Liechtenstein
Flag of Luxembourg.svg  Luxembourg
Flag of Malta.svg  Malta
Flag of Monaco.svg  Monaco
Flag of Montenegro.svg Montenegro
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 San Marino.svg San Marino
Flag of Serbia.svg  Serbia
Flag of Slovakia.svg  Slovakia
Flag of Slovenia.svg  Slovenia
Flag of Spain.svg  Spain (main territory)
Flag of Sweden.svg  Sweden
Flag of Switzerland.svg   Switzerland
Flag of the Vatican City.svg   Vatican City
UTC+02:00 2019-08-31T10:51:37 +02:00 Flag of Botswana.svg Botswana
Flag of Burundi.svg  Burundi
Flag of the Democratic Republic of the Congo.svg Democratic Republic of the Congo (eastern part)
Flag of Egypt.svg  Egypt
Flag of Eswatini.svg  Eswatini
Flag of Lesotho.svg Lesotho
Flag of Libya.svg Libya
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 Time)
Flag of Rwanda.svg  Rwanda
Flag of South Africa.svg  South Africa
Flag of Sudan.svg  Sudan
Flag of Zambia.svg  Zambia
Flag of Zimbabwe.svg  Zimbabwe
Flag of Aland.svg  Åland Islands
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 2019-08-31T11:51:37 +03:00 Flag of Bahrain.svg Bahrain
Flag of Belarus.svg  Belarus
Flag of the Comoros.svg Comoros
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 France.svg Mayotte
Flag of Qatar.svg Qatar
Flag of Russia.svg Russia ( Flag of Moscow.svg Moscow Time)
Flag of Saudi Arabia.svg  Saudi Arabia
Flag of Somalia.svg  Somalia
Flag of South Sudan.svg  South Sudan
Flag of Syria.svg Syria
Flag of Tanzania.svg  Tanzania
Flag of Turkey.svg  Turkey
Flag of Uganda.svg  Uganda
Flag of Yemen.svg  Yemen
UTC+03:30 2019-08-31T12:21:37 +03:30 Flag of Iran.svg  Iran
UTC+04:00 2019-08-31T12:51:37 +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 Time)
Flag of France.svg Réunion
Flag of the Seychelles.svg  Seychelles
Flag of the United Arab Emirates.svg  United Arab Emirates
UTC+04:30 2019-08-31T13:21:37 +04:30 Flag of Afghanistan.svg  Afghanistan
UTC+05:00 2019-08-31T13:51:37 +05:00 Flag of Kazakhstan.svg  Kazakhstan (western part)
Flag of Maldives.svg  Maldives
Flag of Pakistan.svg  Pakistan
Flag of Russia.svg Russia ( Flag of Sverdlovsk Oblast.svg Yekaterinburg Time)
Flag of Tajikistan.svg Tajikistan
Flag of Turkmenistan.svg Turkmenistan
Flag of Uzbekistan.svg  Uzbekistan
UTC+05:30 2019-08-31T14:21:37 +05:30 Flag of India.svg  India
Flag of Sri Lanka.svg  Sri Lanka
UTC+05:45 2019-08-31T14:36:37 +05:45 Flag of Nepal.svg    Nepal
UTC+06:00 2019-08-31T14:51:37 +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 (eastern part)
Flag of Russia.svg Russia ( Flag of Omsk Oblast.svg Omsk Time)
UTC+06:30 2019-08-31T15:21:37 +06:30 Flag of the Cocos (Keeling) Islands.svg Cocos Islands
Flag of Myanmar.svg  Myanmar
UTC+07:00 2019-08-31T15:51:37 +07:00 Flag of Cambodia.svg  Cambodia
Flag of Christmas Island.svg Christmas Island
Flag of Indonesia.svg  Indonesia (Sumatra Island, Java Island, Flag of West Kalimantan.svg West Kalimantan, Flag of Central Kalimantan.png Central Kalimantan)
Flag of Laos.svg  Laos
Flag of Mongolia.svg Mongolia (west)
Flag of Russia.svg Russia ( Flag of Krasnoyarsk Krai.svg Krasnoyarsk Time)
Flag of Thailand.svg  Thailand
Flag of Vietnam.svg  Vietnam
UTC+08:00 2019-08-31T16:51:37 +08:00 Flag of Australia (converted).svg Australia ( Flag of Western Australia.svg Western Australia)
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 (Sulawesi Island, Lesser Sunda Islands, Flag of North Kalimantan.png North Kalimantan, Flag of East Kalimantan.svg East Kalimantan, Flag of South Kalimantan.png South Kalimantan)
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 Time)
Flag of Singapore.svg  Singapore
Flag of the Republic of China.svg Taiwan
UTC+08:45 2019-08-31T17:36:37 +08:45 Flag of Australia (converted).svg  Australia ( Flag of Western Australia.svg Western Australia (Eucla))
UTC+09:00 2019-08-31T17:51:37 +09:00 Flag of East Timor.svg  Timor-Leste
Flag of Indonesia.svg  Indonesia (Maluku Islands, Flag of Papua.svg Papua, Flag of West Papua (vectorised).svg West Papua)
Flag of Japan.svg  Japan
Flag of North Korea.svg  North Korea
Flag of Palau.svg Palau
Flag of Russia.svg Russia ( Flag of Sakha.svg Yakutsk Time)
Flag of South Korea.svg  South Korea
UTC+09:30 2019-08-31T18:21:38 +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 2019-08-31T18:51:38 +10:00 Flag of Australia (converted).svg Australia ( Flag of Queensland.svg Queensland)
Flag of Federated States of Micronesia.svg Micronesia ( Flag of Kosrae.svg Kosrae, Flag of Pohnpei.svg Pohnpei)
Flag of Papua New Guinea.svg  Papua New Guinea
Flag of Russia.svg Russia ( Flag of Primorsky Krai.svg Vladivostok Time)
Flag of Australia (converted).svg Australia ( Flag of the Australian Capital Territory.svg Australian Capital Territory, Flag of New South Wales.svg New South Wales, Flag of Tasmania.svg Tasmania, Flag of Victoria (Australia).svg Victoria)
UTC+10:30 2019-08-31T19:21:39 +10:30 Flag of Australia (converted).svg Australia ( Flag of Lord Howe Island.svg Lord Howe Island)
UTC+11:00 2019-08-31T19:51:39 +11:00 Flag of Federated States of Micronesia.svg Micronesia ( Flag of Chuuk.svg Chuuk, Flag of Yap.svg Yap)
Flag of FLNKS.svg New Caledonia
Flag of Russia.svg Russia ( Flag of Magadan Oblast.svg Magadan Time)
Flag of the Solomon Islands.svg  Solomon Islands
Flag of Vanuatu.svg  Vanuatu
UTC+12:00 2019-08-31T20:51:39 +12:00 Flag of Kiribati.svg  Kiribati (Gilbert Islands)
Flag of the Marshall Islands.svg  Marshall Islands
Flag of Nauru.svg  Nauru
Flag of Russia.svg Russia ( Flag of Kamchatka Krai.svg Kamchatka Time)
Flag of Tuvalu.svg  Tuvalu
Flag of France.svg  Wallis and Futuna
Flag of Fiji.svg  Fiji
Flag of New Zealand.svg New Zealand (most)
UTC+12:45 2019-08-31T21:36:39 +12:45 Flag of New Zealand.svg New Zealand (Chatham Islands)
UTC+13:00 2019-08-31T21:51:39 +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 2019-08-31T22:51:39 +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 +- 30 m ahead
2h +- 30 m ahead
3h +- 30 m ahead Tzdiff-Europe-summer.png
Difference between sun time and clock time during daylight saving time:
1h ± 30 min behind
0h ± 30m
1h ± 30 m ahead