Time zones are areas of the Earth that have adopted the same standard time, usually referred to as the local time. Formerly, people used local solar time, originally apparent and then mean solar time. Their difference is the equation of time. Mean solar time is the average over the course of a year of apparent solar time (sundial time). With the expansion of the railways and as telecommunications improved this became increasingly awkward because clocks in a given town would differ from those in any other by an amount corresponding to their difference in geographical longitude, which was generally not a convenient number. Forcing all localities to run their clocks in synchrony would solve this problem, but under such a scheme, in many places the time shown by clocks at sunrise and sunset would differ too markedly from the solar time values to which people are accustomed. As a compromise, a scheme was devised where the surface of the planet was divided into twenty four "time zones", each separated by 15° of longitude and offset by one hour from its neighbor. Under this scheme, local time is always close to mean solar time, while comparing the time in different places is a simple matter of adding or subtracting whole hours. However, the one hour separation is not universal and, as the map below shows, the shapes of time zones can be quite irregular because they usually follow the boundaries of states, countries or other administrative areas.
Originally, time zones were aligned such that the Prime Meridian (longitude 0°) was the centre of its own time zone, with the mean solar time on that meridian (Greenwich Mean Time or GMT) defining its local time. As a result, all other time zones used GMT as the "base time", from which they were offset by a whole number of hours. However, as a mean solar time, GMT, or UT1 as it is also now known, is defined by the rotation of the Earth, which is not constant in rate. Initially, the rate of atomic clocks was annually changed or steered to closely match GMT, but effective 1 January 1972 the rate of atomic clocks became fixed, and time steps (leap seconds) replaced the rate changes. This new time system was called Coordinated Universal Time (UTC). Leap seconds are inserted to keep UTC within 0.9 seconds of UT1. In this way, local times continue to correspond approximately to mean solar time, while the effects of variations in Earth's rotation rate are confined to simple step changes that can be easily subtracted if a uniform time scale (TAI) is desired. With the implementation of UTC, nations began to use it in the definition of their time zones instead of GMT. As of 2005, most but not all nations have altered the definition of local time in this way (though many media outlets fail to make a distinction between GMT and UTC). Further change to the basis of time zones may occur if proposals to abandon leap seconds succeed.
UTC is, incidentally, local time at Greenwich itself only between 01:00 UTC on the last Sunday in October and 01:00 UTC on the last Sunday in March. For the remainder of the year local time is UTC + 1, known in the UK as British Summer Time (BST). Similar circumstances apply in many places: see Daylight saving time.
The definition for time zones can be written in short form as UTC±n (or GMT±n), where n is the offset in hours. Here are some examples:
- Los Angeles, California, USA: UTC − 8 (e.g. if it is 12:00 UTC, then it is 04:00 in Los Angeles)
- Toronto, Ontario, Canada: UTC − 5 (e.g. if it is 12:00 UTC, then it is 07:00 in Toronto)
- Stockholm, Sweden: UTC + 1 (e.g. if it is 12:00 UTC, then it is 13:00 in Stockholm)
- Istanbul, Turkey: UTC + 2 (e.g. if it is 12:00 UTC, then it is 14:00 in Istanbul)
- Mumbai, India: UTC + 5:30 (e.g. if it is 12:00 UTC, then it is 17:30 in Mumbai)
- Tokyo, Japan: UTC + 9 (e.g. if it is 12:00 UTC, then it is 21:00 in Tokyo)
Where the adjustment for time zones results in a time the other side of midnight from UTC, then the date at the location is one day later or earlier. Some examples:
- Cairo, Egypt: UTC + 2 (e.g. if it is 23:00 UTC on Monday 15 March, then the time in Cairo is 01:00, Tuesday 16 March)
- Wellington, New Zealand: UTC + 12 (e.g. if it is 21:00 UTC on Wednesday 30 June, then the time in Wellington is 09:00, Thursday 1 July)
- New York, New York, USA: UTC − 5 (e.g. if it is 02:00 UTC on Tuesday, then the time in NY is 21:00 on Monday)
- Honolulu, Hawaii, USA: UTC − 10 (e.g. if it is 06:00 UTC on Monday 1 May, then the time in Honolulu is 20:00, Sunday 30 April)
Note: The time zone adjustment for a specific location may vary due to the use of daylight saving time, e.g. 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.
The need to keep accurate time was a problem faced by seafarers wishing to calculate their position, based on the difference between local time (observed by the position of the Sun relative to the observer, with noon being its highest point in the sky) and a standard time in a known location. Clocks accurate enough not to lose time over a journey of months were not created until the 18th century, spurred on by the board of longditute in 1714. This time, known as Greenwich Mean Time (GMT) was being hand carried on chronometers. About August 23, 1852, time signals were first transmitted by telegraph from the Royal Greenwich Observatory. British railways adopted GMT as a standard way of publishing railway timetables on December 1, 1847 as before this, local time was used in indivudial towns and cities. 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 old clocks from this period have two minute hands — one for the local time, one for GMT . This only applied to the island of Great Britain, and not to the island of Ireland.
On November 2, 1868, New Zealand officially adopted a standard time to be observed nationally, 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 Greenwich Mean Time. This standard was known as New Zealand Mean Time.
Timekeeping on the American railroads in the mid 19th century was somewhat confused. Each railroad used its own standard time, usually based on the local time of its headquarters or most important terminus, and the railroad's train schedules were published using its own time. Some major railroad junctions served by several different railroads had a separate clock for each railroad, each showing a different time. The Pittsburgh main station used six different times! The confusion for travellers making a long journey involving several changes of train can be imagined.
A system of one-hour standard time zones for American railroads was first proposed by Charles F. Dowd about 1863. He made this proposal while teaching teenage girls, but without publishing anything. He did not even consult railroad officials until 1869. In 1870, he proposed four ideal time zones (having north-south borders), the first centered on Washington, DC, but by 1872 the first was centered 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 their own version on Sunday, November 18, 1883, when each railroad station clock was either advanced or delayed as noon, standard time, was reached within each time zone, east to west. The zones were named Intercolonial, Eastern, Central, Mountain, and Pacific. Within one year, 85% of all cities having populations over 10,000, about 200 cities, were using standard time. A notable exception was Detroit, Michigan, which kept local time until 1900, then vacillated between Central Standard Time, local mean time, and Eastern Standard Time until it settled on EST by ordinance (May 1915), and ratified by popular vote (August 1916). This hodgepodge was made uniform when Standard zone time was made legal by the U.S. Congress in 1918.
Time zones were first proposed for the entire world by Canada's Sir Sandford Fleming in 1876 as an appendage to the single 24-hour clock he proposed for the entire world (located at the center 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 (now called 180°), while conceding that hourly time zones might have some limited local use. He continued to advocate his system at subsequent international conferences. In October 1884 the International Meridian Conference 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." Nevertheless, most major countries had adopted hourly time zones by 1929. Even today, they have not been fully realized, with several time zones keeping a standard time that is not offset by a number of whole hours from Greenwich Mean Time.
Before 1920, all ships kept local apparent time on the high seas by setting their clocks at night or at the morning sight so that, given the ship's speed and direction, it would be 12 o'clock when the Sun crossed the ship's meridian (12 o'clock = local apparent noon). During 1917, at the Anglo-French Conference on Time-keeping at Sea, it was recommended that all ships, both military and civilian, should adopt hourly standard time zones on the high seas. Whenever a ship was within the territorial waters of any nation it would use that nation's standard time. The captain was permitted to change his ship's clocks at a time of his choice following his ship's entry into another time zone—he often chose midnight. These zones were adopted by all major fleets between 1920 and 1925 but not by many independent merchant ships until World War II. On February 9, 1942, the United States switched from Eastern Standard Time to Eastern War Time (Eastern Daylight Time maintained year round) and switched back to Eastern Standard Time on September 30, 1945.
Time on a ship's clocks and in a ship's log had to be stated along with a "zone description", which was the number of hours that was to be added to zone time to obtain GMT, hence zero in the Greenwich time zone, and negative numbers from −1 to −12 for time zones to the east and positive numbers from +1 to +12 to the west (hours, minutes, and seconds for nations without an hourly offset). These signs are opposite to those given below because ships must obtain GMT from zone time, not zone time from GMT. All zones were pole-to-pole staves 15° wide except for −12 and +12, which were each 7.5° wide separated by a longitude of 180°. Unlike the zig-zagging land-based International Date Line, the nautical International Date Line follows 180° except where it is interrupted by territorial waters and the lands they border, including islands. About 1950, a letter suffix was added to the zone description, assigning Z to the zero zone, and A–M (except J) to the east and N–Y to the west (J may be assigned to local time in non-nautical applications). These were to be vocalized using a phonetic alphabet which included Zulu for GMT, leading sometimes to the use of the term "Zulu Time".
These nautical letters have been added to some time zone maps, like the map of Standard Time Zones by Her Majesty's Nautical Almanac Office (NAO), which extended the letters by adding an asterisk (*) or dagger (†) for areas that do not use a nautical time zone, and a double dagger (‡) for areas that do not have a legal standard time (Greenland's ice sheet and all of Antarctica—The United Kingdom specifies UTC − 3 for the Antarctic Peninsula, but no other country recognizes that). They conveniently ignore any zone that does not have an hour or half-hour offset, so a double dagger (‡) has been co-opted for these zones below.
In maritime usage, GMT retains its historical meaning of UT1, the mean solar time at Greenwich. UTC, atomic time at Greenwich, is too inaccurate, differing by as much as 0.9 s from UT1, creating an error of 0.4 km in longitude at the equator. However, DUT can be added to UTC to correct it to within 50 ms of UT1, reducing the error to only 20 m.
- In terms of number of time zones, Russia is first, with eleven time zones, including Kaliningrad on the Baltic Sea. The United States is second with nine time zones, six for states and three more for possessions. Canada is third with six time zones. Possessions of the United Kingdom and France may increase their number of time zones.
- In terms of area, China is the largest country with only one time zone (UTC+ 8). The next largest country with only one time zone is India (UTC + 5:30). Coincidentally, these are also the two most populous countries.
- Stations in Antarctica generally keep the time of their supply bases, thus both the Amundsen-Scott South Pole Station (U.S.) and McMurdo Station (U.S.) use New Zealand time (UTC + 12 southern winter, UTC + 13 southern summer).
- The 27° latitude passes back and forth across time zones in South Asia. Pakistan: +5, India +5:30, Nepal +5:45, India (Sikkim) +5:30, China +8:00, Bhutan +6:00, India (Arunachal Pradesh) +5:30, Myanmar +6:30. This switching was more odd in 2002, when Pakistan enabled Daylight Saving Time. Thus from west to east, time zones were: +6:00, +5:30, +5:45, +5:30, +8:00, +6:00, +5:30 and +6:30.
- Because the earliest and latest time zones are 26 hours apart, any given calendar date exists at some point on the globe for 50 hours. For example, April 11 begins in time zone UTC+14 at 10:00 UTC April 10, and April 11 ends in time zone UTC-12 fifty hours later, at 12:00 UTC April 12.
- There are numerous places where several time zones meet, for instance at the tri-country border of Finland, Norway and Russia.
List of time zones and contained areas
Regions marked with asterisks (* or **) observe Daylight Saving Time: add one hour in summer (* for Northern Hemisphere summer; ** for Southern Hemisphere). Note, some locations use GMT instead of UTC in the definition of local time. For the purposes of this summary, the distinction is ignored.
Some zones north-south of each other in the mid Pacific differ by 24 hours in time: they have the same time of the day but differ by a full day. The two extreme time zones on Earth (both in the mid Pacific) differ by 26 hours. A particular day starts earlier in countries with a more positive UTC offset. Thus the first occurrence of a date will be in UTC + 14 and the last of the same date in UTC − 12 (at sea). This gives the interesting feature that during one hour each day there are three different dates in use on land around the world, at 10:30 UTC Monday it is already 00:30 Tuesday in the Line Islands (UTC + 14) while the time is 23:30 Sunday in Samoa (UTC − 11) .
Time zone abbreviations are almost always customary, not legal—those listed here only exist in English and are somewhat arbitrary. English time zone names below generally only apply to English-speaking areas. The CIA and NAO disagree on the time kept by some Russian oblasts, so both are given below—this may be due to a recent time zone change.
UTC − 12, Y
Only ships at sea within 7.5° east of 180°.
UTC − 11, X
UTC − 10, W
- Cook Islands
- French Polynesia
- Johnston Atoll
- United States (HST—Hawaii-Aleutian Standard Time)
UTC − 9:30, V*
UTC − 9, V
UTC − 8, U
- Canada (PST—Pacific Standard Time; the corresponding daylight saving time is called Pacific Daylight Time - PDT, UTC − 7)
- Pitcairn Islands
- United States (PST—Pacific Standard Time)
- Nevada (most of state, except Idaho border towns and West Wendover)*
- Oregon (most of state, including southern 1/5 of Malheur County)*
UTC − 7, T
- Canada (MST—Mountain Standard Time)
- British Columbia
- northeastern (no DST)
- Northwest Territories*, except for Tungsten, Northwest Territories and the associated Cantung Mine (no DST)
- west of 102° West, and
- entire Kitikmeot Region,
- Lloydminster and surrounding area* (specifically exempted from the legal prohibition of DST in Saskatchewan)
- United States (MST—Mountain Standard Time)
- Arizona (the Navajo Nation* does observe DST, but the Hopi Reservation, an enclave within it, does not observe DST),
- Nebraska (western)*,
- New Mexico*,
- North Dakota (southwestern)*,
- northern 4/5 of Malheur County*,
- South Dakota (western)*,
UTC − 6, S
- Canada (CST—Central Standard Time)
- between 85° West and 102° West (except eastern Kitikmeot Region and western Southampton Island)*,
- Northwestern Ontario west of 90° West (except Atikokan area, New Osnaburgh and Pickle Lake area, and Shebandowan and Upsala area), and Big Trout Lake area east of 90° West,
- Saskatchewan (most of province), except
- Creighton and Denare Beach area, which does observe DST (unofficially, as Saskatchewan has a law prohibiting the use of DST)
- Lloydminster and surrounding area, which observes Mountain Standard Time (UTC-7) with DST by special legal exemption
- Costa Rica
- El Salvador
- United States (CST—Central Standard Time)
- Alabama (most of state)*,
- Kansas (most of state)*,
- Kentucky (western)*,
- Nebraska (central and eastern)*,
- North Dakota (most of state)*,
- Oklahoma (most of state)*,
- South Dakota (eastern)*,
- Texas (most of state)*,
UTC − 5, R
- Canada (EST—Eastern Standard Time)
- Cayman Islands
- Turks and Caicos Islands*
- United States (EST—Eastern Standard Time)
- District of Columbia*,
- Indiana (most of state)* (effective April 2, 2006, the entire Eastern Time Zone portion of Indiana will officially begin observing DST),
- Kentucky (eastern)*,
- Michigan (most of state)*,
- New Hampshire*,
- New Jersey*,
- New York*,
- North Carolina*,
- Rhode Island*,
- South Carolina*,
- West Virginia*
UTC − 4, Q
- Antigua and Barbuda
- Canada (AST— Atlantic Standard Time)
- Dominican Republic
- Falkland Islands**
- Greenland - Kalaallit Nunaat
- Pituffik area* (observes United States DST rules)
- Netherlands Antilles
- Puerto Rico (AST—Atlantic Standard Time)
- Saint Kitts and Nevis
- Saint Lucia
- Saint Vincent and the Grenadines
- Trinidad and Tobago
- Virgin Islands
UTC − 3:30, P*
UTC − 3, P
- Brazil - official time
- French Guiana
- Greenland - Kalaallit Nunaat
- south coast and southwest coast* (observes European Union DST rules)
- Saint-Pierre and Miquelon*
UTC − 2, O
- Brazil - Ocean Islands
- Fernando de Noronha, Trindade, Martim Vaz, etc.
- South Georgia and the South Sandwich Islands
UTC − 1, N
- Cape Verde
- Greenland - Kalaallit Nunaat
- Ittoqqortoormiit and surrounding area* (observes European Union DST rules)
- Burkina Faso
- Bouvet Island
- Côte d'Ivoire
- Faroe Islands*
- The Gambia
- Greenland - Kalaallit Nunaat
- Danmarkshavn and surrounding area
- Saint Helena
- São Tomé and Príncipe
- Sierra Leone
- Spain (not mainland)
- United Kingdom* (GMT / BST)
- Western Sahara
UTC + 1, A
- See UTC1 for the names of time zones
- Bosnia and Herzegovina*
- Central African Republic
- Congo, Republic of the
- Congo, Democratic Republic of the
- Czech Republic*
- Equatorial Guinea
- Macedonia, Republic of*
- San Marino*
- Spain* (except Canary Islands).
- Svalbard and Jan Mayen*
- Vatican City*
UTC + 2, B
- Congo, Democratic Republic of the
- Gaza Strip*
- Russia (Kaliningrad)*
- South Africa
- West Bank*
UTC + 3, C
- Russia* (most of European portion, including Moscow, St. Petersburg, Rostov on Don, Novaya Zemlya, Franz Josef Land, and all railroads throughout Russia)
- Saudi Arabia
UTC + 3:30, C*
UTC + 4, D
- Georgia (  Georgia moved from zone UTC+4 to UTC+3 on June 27, 2004, then back to UTC+4 on March 27, 2005)
- United Arab Emirates
UTC + 4:30, D*
UTC + 5, E
- British Indian Ocean Territory (NAO)
- Heard Island and McDonald Islands
- Kazakhstan (Western)
UTC + 5:30, E*
- India (IST—Indian Standard Time)
UTC + 5:45, E‡
- Nepal's time zone of UTC + 5:45 was adopted in 1986 . This is the nearest quarter-hour from Greenwich to the local mean time of Nepal's capital Kathmandu, which is at 85°19'E or 5:41:16. Old CIA maps, 1995 and earlier, have Nepal at 5:40, which may be their approximation of Kathmandu's local mean time. See Nepal Time.
UTC + 6, F
- British Indian Ocean Territory (CIA)
- Kazakhstan (Eastern)
- Kyrgyzstan ( Kyrgyzstan abolished DST on August 12, 2005 and moved from zone UTC+5 to UTC+6)
- Sri Lanka
UTC + 6:30, F*
UTC + 7, G
- Christmas Island (Australia)
- Indonesia (Western)
- Mongolia* (Hovd, Uvs, and Bayan-Ölgiy)
UTC + 8, H
- Australia (AWST—Australian Western Standard Time)
- Western Australia (most of state)
- China, including the People's Republic of China, the Republic of China (Taiwan), Hong Kong and Macau
- Note that the whole of China has the same time, which makes this time zone exceptionally wide. In the extreme west of China the sun is at its highest at 15:00, in the extreme east at 11:00. It also means that on the short (76 km) frontier with Afghanistan, the official time change is 3 hours and 30 minutes. The two western autonomous regions of China, Xinjiang and Tibet, were in UTC+6 during the Republic era (1912–1949), but were moved to UTC+8 after the founding of the People's Republic of China in 1949. Today, residents of the two autonomous regions do everything 2 hours late. For example, lunch is at 14:00 and business hour ends around 19:00.
- Indonesia (Central)
- Mongolia* (most of country)
- Singapore (geographically in UTC+7, but for historical reasons uses UTC+8. See Why is Singapore in the "Wrong" Time Zone?)
UTC + 8:45, H‡
UTC + 9, I
- East Timor
- Indonesia (Eastern)
- Japan (JST—Japan Standard Time)
- North Korea
- South Korea (KST—Korea Standard Time)
UTC + 9:30, I*
- Australia (ACST—Australian Central Standard Time)
UTC + 10, K
- Australia (AEST—Australian Eastern Standard Time)
- Guam (Chamorro Standard Time via U.S. Law)
- Federated States of Micronesia
- Northern Mariana Islands (Chamorro Standard Time via U.S. law)
- Papua New Guinea
UTC + 10:30, K*
UTC + 11, L
- Federated States of Micronesia
- New Caledonia
- Solomon Islands
UTC + 11:30, L*
UTC + 12, M
- Kwajalein, Eniwetok, and Bikini atolls in the Marshall Islands used to be on UTC−12. Kwajalein advanced 24 hours to the eastern hemisphere side of the International Date Line by skipping 21 August 1993. Eniwetok and Bikini probably advanced even earlier, when the U.S. military relinquished its control of them. 
- Amundsen-Scott South Pole Station uses New Zealand time.
UTC + 12:45, M‡
UTC + 13, M*
UTC + 14, M†
- Bowditch, Nathaniel. American Practical Navigator. Washington: Government Printing Office, 1925, 1939, 1975.
- Hill, John C., Thomas F. Utegaard, Gerard Riordan. Dutton's Navigation and Piloting. Annapolis: United States Naval Institute, 1958.
- Howse, Derek. Greenwich Time and the Discovery of the Longitude. Oxford: Oxford University Press, 1980. ISBN 0192159488.
- Palin, Michael. Pole to Pole. Bay Books, 1995. ISBN 0912333413.
- It's about time Canadian time zone exceptions.
- Saskatchewan time system (156KB pdf file)
- Canadian time zone maps
- The tz database - Provide specific information on the beginning and ending dates of daylight saving time for each zone and tracks time zones over the years. Often called tz or zoneinfo, this database is used by several implementations, including the GNU C library used by many Unix variants.
- Animated time zones - Show the relationship between arbitrary time zones and "natural" zones, from www.radicalcartography.net
- US Official Time Clock - Java enabled clock to graphically display night and day around the globe.
- The World Clock - Time Zones
- Personal World Clock Display only the time zones you pick, and save them for when you come back lateraf:Tydsone
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