Time zones are areas of the Earth that have adopted the same standard time. Formerly, people used local solar time (originally apparent and then mean), 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 meridianss of a longitude that is a multiple of 15º thus making neighbouring 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.
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:
Multiple time zones were first proposed by Charles F. Dowd about 1863 for American railroads. In 1870, 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, Sun time, and Eastern Standard Time until it settled on EST by ordinance May 1915, ratified by popular vote August 1916. Standard zone time was not made legal in the United States until 1918.
Time zones were first proposed for the entire world by Sir Sandford Fleming in 1876. Although he advocated them at the International Meridian Conference in October 1884, it did not adopt them 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). 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 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. When zone time is specified as an offset from GMT (or UTC), the sign changes to those listed below.
Regions marked with an asterisk (*) observe Daylight Saving Time: add 1 hour in summer. 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). 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.