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Time zone

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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), resulting in time differing slightly from town to town. As telecommunications improved and with the expansion of the railways this became increasingly awkward. Time zones partially rectified the problem by setting the clocks of a region to the same mean solar time. Time zones are generally centered on meridians of a longitude that is a multiple of 15? thus making neighboring time zones one hour apart. 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.

Standard Time Zones of the World by the CIA
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Standard Time Zones of the World by the CIA

All time zones are defined relative to Coordinated Universal Time (UTC). The reference point for Time Zones is the Prime Meridian (longitude 0?) which passes through the Royal Greenwich Observatory in Greenwich, London, United Kingdom [1] (http://greenwichmeantime.com/). For this reason the term Greenwich Mean Time (GMT) is still often used (by the BBC, for example, amongst others) to denote the "base time" to which all other time zones are relative. UTC is, nevertheless, the official term for today's atomically measured time as distinct from time determined by astronomical observation as formerly carried out at Greenwich.

GMT (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).

The time for a location is given relative to UTC. 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 11:00 UTC, then it is 06:00 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 03:00 UTC, then it is 05:00 in Istanbul)
  • Mumbai, India: UTC + 5.5 (e.g. if it is 13:00 UTC, then it is 18:30 in Mumbai)
  • Tokyo, Japan: UTC + 9 (e.g. if it is 11:00 UTC, then it is 20:00 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)
  • Auckland, New Zealand: UTC + 12 (e.g. if it is 21:00 UTC on Wednesday 30 June, then the time in Auckland is 09:00, Thursday 1 July)
  • 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, Hawai'i, 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.

See also: Sidereal time Calculating local time

Contents

History

The first time zone in the world was established by British railways on December 1, 1847Greenwich Mean Time (GMT) being hand carried on chronometers. About August 23, 1852, time signals were first transmitted by telegraph from the Royal Greenwich Observatory. Even though 98% of the Great Britain's public clocks were using GMT by 1855, it was not made Britain's legal time until August 2, 1880. This paragraph only applies to the island of Great Britain, 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.

Multiple time zones were first proposed by Charles F. Dowd about 1863 for American railroads as a teacher to his students. In 1870, after consulting railroad officials in 1869, he proposed four ideal time zones, the first centered on Washington, DC, but by 1872 the first was centered 75?W of Greenwich with geographic borders. American 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, 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.

Time on 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.

These nautical letters have been added to some time zone maps, like the map of Standard Time Zones (http://aa.usno.navy.mil/faq/docs/world_tzones.html) 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—Britain 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.

List of time zones and contained areas

Regions marked with asterisks (* or **) observe Daylight Saving Time: add 1 hour in summer (* for Northern Hemisphere summer; ** for Southern Hemisphere).

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) [2] (http://www.timeanddate.com/worldclock/full.html?sort=2).

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). 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?.

For Kwajalein, Eniwetok, and Bikini atolls, see note at UTC + 12 M.

UTC − 11 X

UTC − 10 W

UTC − 9:30 V*

UTC − 9 V

UTC − 8 U

UTC − 7 T

UTC − 6 S

UTC − 5 R

UTC − 4 Q

UTC − 3:30 P*

UTC − 3 P

UTC − 2 O

UTC − 1 N

UTC Z

UTC + 1 A

UTC + 2 B

UTC + 3 C

UTC + 3:30 C*

UTC + 4 D

UTC + 4:30 D*

UTC + 5 E

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 [4] (http://www.sptimes.com/Travel97/10198/To_Every_Times__There.html). 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.

UTC + 6 F

UTC + 6:30 F*

UTC + 7 G

UTC + 8 H

Note that the whole of mainland China has the same time, which makes this time zone exceptionally wide. In the extreme west of the PRC 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.

UTC + 8:45 H‡

UTC + 9 I

UTC + 9:30 I*

UTC + 10 K

UTC + 10:30 K*

UTC + 11 L

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. [5] (http://pqasb.pqarchiver.com/nytimes/116416799.html?did=116416799&FMT=ABS&FMTS=AI&date=Aug+22%2C+1993&desc=In+Marshall+Islands%2C+Friday+Is+Followed+by+Sunday)

UTC + 12:45 M‡

UTC + 13 M*

UTC + 14 M†

See also

Bibliography

  • 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.

External links

  • Time Genie (http://www.timegenie.com/) Find the current time and date for more than 6,600 cities in 245 countries. Also includes tools for converting and comparing times.
  • World Time Server (http://www.worldtimeserver.com/)
  • The E Time zone page (http://timezones.tk) - Clock, Time and Zones - View the day and night timezone clock around the world at the E Time zone page
  • Watch Time Zone Database (http://www.watchtimezone.com/)
  • Virtual Travelog (http://www.virtualtravelog.net/entries/000031.html) A Solution for Managing Time Zones, and Dates in International Internet Systems
  • The tz database (http://www.twinsun.com/tz/tz-link.htm) provides specific information on the beginning and ending dates of daylight saving time for each zone and tracks time zones over the years.
  • Time around the World  (http://time.world-stay.com/en/) information about the time in locations all over the world, including daylight maps, sunrise and sunset times and free clocks that can be added to a web page
  • US Official Time Clock (http://www.time.gov/timezone.cgi?UTC/s/0/java) is java enabled to graphically display night and day around the globe.
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