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Time zone
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

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This article is about time zones in general. For a list of time zones by country, see List of time zones by country. For more time zone lists, see Lists of time zones. For other uses, see Time zone (disambiguation). This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.
Find sources: "Time zone" – news · newspapers · books · scholar · JSTOR (September 2017) (Learn how and when to remove this template message) This article possibly contains original research. Please improve it by verifying the claims made and adding inline citations. Statements consisting only of original research should be removed. (August 2018) (Learn how and when to remove this template message)

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.

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).

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.

Contents
  • 1 History
    • 1.1 Early timekeeping
    • 1.2 Railway time
    • 1.3 Worldwide time zones
  • 2 Notation of time
    • 2.1 ISO 8601
    • 2.2 UTC
      • 2.2.1 Offsets from UTC
    • 2.3 Abbreviations
  • 3 UTC offsets worldwide
  • 4 List of UTC offsets
  • 5 Time zone conversions
  • 6 Nautical time zones
  • 7 Skewing of zones
  • 8 Daylight saving time
  • 9 Computer systems and the Internet
    • 9.1 Operating systems
      • 9.1.1 Unix
      • 9.1.2 Microsoft Windows
    • 9.2 Programming languages
      • 9.2.1 Java
      • 9.2.2 JavaScript
      • 9.2.3 Perl
      • 9.2.4 PHP
      • 9.2.5 Python
      • 9.2.6 Smalltalk
  • 10 Time zones in outer space
  • 11 See also
  • 12 Notes
  • 13 Further reading
  • 14 References
  • 15 External links
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.

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.

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.

Railway time 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

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 Main article: 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 ±:, ± , or ± (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 Main article: List of time zone 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 Main article: List of UTC time offsets 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 summer DST in summer No DST in summer DST in summer No DST in summer DST in summer No DST in summer DST in summer No DST in summer DST 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 It has been suggested that List of UTC time offsets be merged into this article. (Discuss) Proposed since May 2018.

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

Time offset Example time
(ISO 8601 notation) Example locations that do not use DST Example locations that in summer use DST UTC−12:00 2019-07-16T17:27:30-12:00 Baker Island

Howland Island

UTC−11:00 2019-07-16T18:27:30-11:00 American Samoa

Niue

UTC−10:00 2019-07-16T19:27:30-10:00 French Polynesia (most)

United States ( Hawaii)

Cook Islands

United States (Aleutian Islands) UTC−09:30 2019-07-16T19:57:30-09:30 French Polynesia (Marquesas Islands) UTC−09:00 2019-07-16T20:27:30-09:00 French Polynesia (Gambier Islands) United States ( Alaska (most)) UTC−08:00 2019-07-16T21:27:30-08:00 Pitcairn Islands Canada ( British Columbia (most))

Mexico ( Baja California)

United States ( California, Nevada (most), Oregon (most), Washington)

UTC−07:00 2019-07-16T22:27:30-07:00 Canada (northeastern British Columbia)

Mexico ( Sonora)

United States ( Arizona (most))

Canada ( Alberta)

Mexico ( Chihuahua)

United States ( Colorado)

UTC−06:00 2019-07-16T23:27:30-06:00  BelizeCanada ( Saskatchewan (most))

 Costa Rica

 Ecuador (Galápagos Islands)

 El Salvador

 Guatemala

 Honduras

 Nicaragua

Canada (Manitoba)

United States (Illinois, Texas (most))

Mexico (most)

Chile ( Easter Island)

UTC−05:00 2019-07-17T00:27:30-05:00 Brazil (Acre)

 Colombia

 Ecuador (continental)

 Haiti

 Jamaica

Mexico ( Quintana Roo (most))

 Panama

 Peru

Bahamas

Canada ( Ontario (most), Quebec (most))

 Cuba

United States ( Florida (most), Georgia, New York, Ohio, Pennsylvania)

UTC−04:00 2019-07-17T01:27:30-04:00  Barbados Bolivia

Brazil (Amazonas (most), Rondônia, Roraima)

 Dominican Republic

Puerto Rico

 Trinidad and Tobago

Venezuela

Brazil ( Mato Grosso (most), Mato Grosso do Sul)

Canada ( Nova Scotia, New Brunswick, Labrador (most), Prince Edward Island)

 Chile (continental)

 Paraguay

UTC−03:30 2019-07-17T01:57:30-03:30 Canada (southeastern Labrador, Newfoundland) UTC−03:00 2019-07-17T02:27:30-03:00 Argentina

Brazil ( Bahia, Ceará, Maranhão, Pará, Pernambuco)

Chile (Magallanes)

Falkland Islands

Saint Pierre and Miquelon

 Uruguay

Brazil ( Espírito Santo, Federal District, Minas Gerais, Paraná, Rio Grande do Sul, Rio de Janeiro, São Paulo)

Greenland (most)

UTC−02:00 2019-07-17T03:27:30-02:00 Brazil (Fernando de Noronha)

South Georgia and the South Sandwich Islands

UTC−01:00 2019-07-17T04:27:30-01:00  Cape Verde  Portugal ( Azores) UTC±00:00 2019-07-17T05:27:30+00:00  Ivory Coast Ghana

 Iceland

Saint Helena

 Senegal

 Mali

Faroe Islands

 Ireland

 Portugal (continental, Madeira)

 Spain ( Canary Islands)

 United Kingdom

UTC+01:00 2019-07-17T06:27:31+01:00  Algeria Angola

 Benin

 Cameroon

 Democratic Republic of the Congo (west)

 Gabon

 Morocco

 Niger

 Nigeria

 Tunisia

 Western Sahara

 Albania

 Andorra

 Austria

 Belgium

 Bosnia and Herzegovina

 Croatia

 Czech Republic

 Denmark

 France (metropolitan)

Germany

 Hungary

 Italy

 Liechtenstein

 Luxembourg

 Monaco

 Malta

 Netherlands (European)

 North Macedonia

 Norway

 Poland

 Serbia

 Slovakia

 Slovenia

 Spain (continental)

 Sweden

  Switzerland

  Vatican City

UTC+02:00 2019-07-17T07:27:31+02:00  Burundi Egypt

 Malawi

 Mozambique

 Namibia

Russia ( Kaliningrad)

 Rwanda

 South Africa

 Sudan

 Eswatini

 Zambia

 Zimbabwe

 Bulgaria Cyprus

 Estonia

 Finland

 Greece

 Israel

 Jordan

 Latvia

 Lebanon

 Lithuania

 Moldova

Palestine

 Romania

 Syria

 Ukraine

UTC+03:00 2019-07-17T08:27:31+03:00  Belarus Djibouti

 Eritrea

 Ethiopia

 Iraq

 Kenya

 Kuwait

 Madagascar

Russia (most of European part)

 Saudi Arabia

 Qatar

 Somalia

 South Sudan

 Tanzania

 Turkey

 Uganda

 Yemen

UTC+03:30 2019-07-17T08:57:31+03:30  Iran UTC+04:00 2019-07-17T09:27:31+04:00  Armenia Azerbaijan

 Georgia

 Mauritius

 Oman

Russia ( Samara)

 Seychelles

 United Arab Emirates

UTC+04:30 2019-07-17T09:57:31+04:30  Afghanistan UTC+05:00 2019-07-17T10:27:31+05:00  Kazakhstan (west)

 Maldives

 Pakistan

Russia ( Sverdlovsk, Chelyabinsk)

 Uzbekistan

UTC+05:30 2019-07-17T10:57:31+05:30  India Sri Lanka UTC+05:45 2019-07-17T11:12:31+05:45    Nepal UTC+06:00 2019-07-17T11:27:31+06:00  Bangladesh Bhutan

British Indian Ocean Territory

 Kazakhstan (most)

Russia ( Omsk)

UTC+06:30 2019-07-17T11:57:31+06:30  Australia ( Cocos Islands)

 Myanmar

UTC+07:00 2019-07-17T12:27:31+07:00  Cambodia Indonesia (west)

 Laos

Mongolia (west)

Russia ( Krasnoyarsk)

 Thailand

 Vietnam

UTC+08:00 2019-07-17T13:27:31+08:00 Australia ( Western Australia (most))

Brunei

 People's Republic of China

 Hong Kong

 Indonesia (central)

 Macau

 Malaysia

Mongolia (most)

 Philippines

Russia ( Irkutsk)

 Singapore

Taiwan

UTC+08:45 2019-07-17T14:12:31+08:45  Australia ( Western Australia (Eucla)) UTC+09:00 2019-07-17T14:27:31+09:00  Timor-Leste Indonesia (east)

 Japan

 North Korea

Russia ( Sakha (most))

 South Korea

UTC+09:30 2019-07-17T14:57:31+09:30 Australia ( Northern Territory) Australia ( South Australia) UTC+10:00 2019-07-17T15:27:31+10:00 Australia ( Queensland)

 Papua New Guinea

Russia ( Primorsky)

Australia ( New South Wales, Tasmania, Victoria) UTC+10:30 2019-07-17T15:57:31+10:30 Lord Howe Island UTC+11:00 2019-07-17T16:27:31+11:00 New Caledonia

Russia ( Magadan)

 Solomon Islands

 Vanuatu

UTC+12:00 2019-07-17T17:27:31+12:00  Kiribati (Gilbert Islands)

Russia ( Kamchatka)

 Fiji New Zealand (most) UTC+12:45 2019-07-17T18:12:31+12:45 New Zealand (Chatham Islands) UTC+13:00 2019-07-17T18:27:31+13:00  Kiribati (Phoenix Islands)

Tokelau

 Tonga

 Samoa UTC+14:00 2019-07-17T19:27:31+14:00  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:

  • Cairo, Egypt: UTC+02; 01:00 on Tuesday
  • Wellington, New Zealand: UTC+12; 11:00 on Tuesday

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

  • Honolulu, Hawaii, United States: UTC−10; 16:00 on Monday
  • Toronto, Ontario, Canada: UTC−05; 21:00 on Monday

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 Time of day by zone City, Region Zone °W Tue Wed Baker Island, Howland Island AoE 180 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 American Samoa SST 165 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 Hawaii HAST 150 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 Juneau, Alaska AKST 135 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 San Francisco, Los Angeles PST 120 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 Denver MST 105 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 Winnipeg, Chicago, Mexico City CST 90 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 Ottawa, New York, Miami, Quito, Lima EST 75 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 Caracas, La Paz, Santiago CLT 60 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 Greenland, Rio de Janeiro, Buenos Aires ART 45 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 GST 30 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 CVT 15 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 London, Lisbon, Algiers, Monrovia UTC 0 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 Paris, Rome, Lagos, Kinshasa CET 15 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Helsinki, Moscow, Cairo, Cape Town EET 30 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Archangelsk, Ankara, Addis Abeba AST 45 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Magnitogorsk, Mauritius, Réunion GST 60 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Salekhard, Bishkek, Kerguelen PKT 75 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Norilsk, Novosibirsk BST 90 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Irkutsk, Bangkok, Jakarta ICT 105 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Yakutsk, Beijing, Manila, Perth CST 120 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Vladivostok, Tokyo JST 135 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Magadan, Sydney, Melbourne AEST 150 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 NCT 165 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Fiji, Wellington NZST 180 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 Phoenix Islands TKT 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 Kiritimati LINT 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 °E Wed Thu Fri

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 Main article: Nautical time

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 t