Usual timezones.

A time zone is an area of the surface land where, initially, the time taken must be the same everywhere.

This system was proposed by the engineer and geographer Montreal Sir Sandford Fleming in 1876 , with the meridian of Greenwich as the origin of time, the change of date line meridian 180 (east and west), and dividing the globe 24 time zones of equal size.

The area covered by a zone bounded by two meridians 15 degrees apart, extending from pole to pole north south and is centered on a meridian whose longitude is a multiple of 15 degrees. The first zone is centered on the meridian of Greenwich. In passing from one time to another time increases or decreases by one hour.

States define the legal time on their territory with a fixed offset relative to Coordinated Universal Time (UTC). This shift is most often equal to a whole number of hours, but some countries use an offset half-hour or quarter hour.

In general, countries tend to use a time zone so that the mean solar time on their territory is not too far from the standard time (that is, for example, for the twelve o'clock Solar is not too far away lunch legal). This principle has many exceptions, however:

• A country usually has a single legal time, for obvious reasons of unification of the hour. He can ignore the limits of a zone if its territory encroaches slightly on the time neighbor is the case of mainland France just east of the country ( Alsace , Corsica ). In extreme cases, it may even, though it covers several time, adopt a single legal time: the case of China and of India.
• Some major countries ( Australia , Brazil , Canada , Congo DR , United States , Indonesia , Russia , etc..) are arbitrarily divided into several areas to avoid too much difference between standard time and solar time: the limit the application of legal time limits generally follows local area or state and can thus be positively offset from the edge of the zone of origin. This is also true of some distant dependencies, such as DOM-TOM French, Azores Portugal, etc..
• A country may adopt a time other than that which would be designed a priori. This is the case of the Spanish mainland or mainland France who are in the era of central Europe, thus one hour ahead of time on their origin.
• Some countries have chosen a time does not correspond to an integer offset from UTC ( Iran to UTC +3:30 , Afghanistan in UTC +4:30 , India UTC +5:30 or even Nepal UTC +5:45 ).
• A part of the world uses a system of summer time and winter time , created to save energy (and sometimes criticized). Legal time can, for several months of the year, largely be offset from the original time zone. Note that two different hemispheres of countries observing daylight saving time shift is also summer in the southern hemisphere corresponding to winter in the Northern Hemisphere.

All these changes from the initial system time zones are losing their original shape, swollen in the middle, tapered at the ends, in favor of cutting areas.

A simple, one time zone can be written as UTC or UTC + X-Y, where X and Y represent the time offset from UTC. The following examples provide examples of variation of local time along several time zones when it is 12:00 UTC

• Los Angeles , USA ( UTC-8 ): 04:00
• New York , USA ( UTC-5 ): 07:00
• Halifax , Canada ( UTC-4 ): 08:00
• London , UK ( UTC +0 ): 12:00
• Stockholm , Sweden ( GMT +1 ): 13:00
• Cape Town , South Africa ( GMT +2 ): 14:00
• Mysore , India ( UTC +5:30 ): 17:30
• Kathmandu , Nepal ( UTC +5:45 ): 17:45
• Seoul , South Korea ( UTC +9 ): 21:00
• Melbourne , Australia ( UTC +10 ): 22:00

In total, all countries of the world currently uses 43 time zones or time zones (counting UTC +8:45 who is not an official time), 37 all year, but not necessarily the same countries: only two of the boreal winter time ( UTC-3: 30 and UTC +3:30 ), a solely southern winter time ( UTC +12:45 ), one exclusively for 'boreal summer time ( UTC-2: 30 ), only a last DST Southern ( UTC +13:45 ).

History

The Greenwich Mean Time (Greenwich mean time in English, abbreviated GMT) was established in 1675 with the construction of the Royal Greenwich Observatory , to help mariners determine their longitude at sea with the first part of the world a uniform time was introduced by British Railways on ¹December 1847 , using stopwatches synchronized and transported by hand. To the 23 August 1852 , time signals were transmitted by telegraph from the Royal Observatory. Around 1855 , approximately 98% of public clocks in Britain used GMT, but it was not adopted as the legal time that the 2 August 1880. Some clocks of this period have two minute hands: one for local time, one for GMT . The boundaries of these areas went through the stations, often in major cities. For example, the boundary between the eastern and central crossing Detroit , Buffalo , Pittsburgh , Atlanta and Charleston. The system was adopted on Sunday 18 November 1883 , also called "day of two noons, when the clock of each station was given to the time zone where it reaches noon. The five zones were named Intercolonial, Eastern, Central, Mountain and Pacific. After a year, 95% of all cities over 10,000 population (200 cities) were using the system.

In 1876 , the Canadian Sandford Fleming proposed to generalize the principle to the world, in addition to its proposal of a standard 24-clock hours would be linked to any meridian. In 1879 , he specified that his universal day would begin at the anti-meridian of Greenwich (now 180 ^th meridian ) while conceding that the time zones spaced an hour could have a limited local interest. He continued promoting his system at subsequent international conferences. In October 1884 , the International Meridian Conference did not adopt his time, who were not in his agenda, but adopted a universal time of 24 hours beginning at midnight in Greenwich.

However, before 1929 , most countries adopted time zone offset in a whole number of hours behind GMT.

Alphabetic Names

Every time integer between -12 and +12 is associated with a letter A to M without the J for time of 1-12, from N to Y for time from -1 to -12 and Z for the time reference. These letters are replaced by a phonetic code , which gives the following correspondence:

Polar Regions

Scientific stations in the Arctic and Antarctic generally use the time zone of their supply bases. Thus, the station Amundsen-Scott , located at the south pole, use the Time Zone of New Zealand : UTC +12 southern winter, UTC +13 during the austral summer: this station therefore respects the time of summer, advancing or retreating the full schedule Antarctic night.

Areas that do not have science facility have no official time zone. Near the North Pole , it is possible to use the Time Zone UTC 0 by convention .

International waters

Vessels navigating in international waters generally observe changes in the whole time crossing meridians that limit the time zones as they were originally defined. There is however no fixed convention on the subject, vessels using what they find most convenient.

Amendments

Some areas of the world have adopted different time zones during the story. The reasons may be diverse, such as:

• Closer to an integer offset with UTC (or GMT when it was the reference): the case of Malaysia in 1905, which passed +6:46:48 GMT to GMT +7.
• The military occupation of one country by another: France passed GMT +0 to GMT +1 and the Netherlands GMT +0:20 to GMT +1 in 1940 following the German invasion, the Malaysia GMT +7:30 GMT +9 in following the Japanese invasion in 1942.
• Obtain a more uniform standard time throughout a country. In 1949, the western areas of China went from GMT +8 to GMT +6 in order to obtain the same time across the country. In late 1994, the eastern parts of Kiribati jumped UTC and UTC-11-10 to UTC +13 and UTC +14, so that the country is crossed by the international date line.
• Ad hoc adjustments, usually related to sun time ( Kyrgyzstan UTC +5 to UTC +6 in 2005, Georgia UTC +4 to UTC +3 in 2004, etc.).

References

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