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Standard time is the synchronization of clocks within a geographical region to a single time standard, rather than a local mean time standard. Generally, standard time agrees with the local mean time at some meridian that passes through the region, often near the centre of the region.
Time zones of the world. A time zone is an area which observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it is convenient for areas in frequent communication to keep the same time.
The evolution of United States standard time zone boundaries from 1919 to 2024 in five-year increments. Plaque in Chicago marking the creation of the four time zones of the continental US in 1883 Colorized 1913 time zone map of the United States, showing boundaries very different from today Map of U.S. time zones during between April 2, 2006, and March 11, 2007.
It is ultimately the authority of the secretary of transportation, in coordination with the states, to determine which regions will observe which of the standard time zones and if they will observe daylight saving time. [7] As of August 9, 2007, the standard time zones are defined in terms of hourly offsets from UTC. [8]
In theory, ships are supposed to adopt the standard time of a country if they are within its territorial waters within 12 nautical miles (14 mi; 22 km) of land, then revert to international time zones (15° wide pole-to-pole gores) as soon as they leave. In practice, ships use these time zones only for radio communication and similar purposes.
Standard time zones of the world. The number at the bottom of each zone specifies the number of hours to add to UTC to convert it to the local time. As international commerce increased, the need for an international standard of time measurement emerged. Several authors proposed a "universal" or "cosmic" time (see Time zone § Worldwide time zones).
This is a list representing time zones by country. Countries are ranked by total number of time zones on their territory. Time zones of a country include that of dependent territories (except Antarctic claims). France, including its overseas territories, has the most time zones with 12 (13 including its claim in Antarctica and all other counties ).
Map of U.S. time zones with new CST and EST areas displayed. Some U.S. time zones, such as the Samoa Time Zone, are not on this map. This is a list of the time offsets by U.S. states, federal district, and territories. For more about the time zones of the U.S. see time in the United States. Most states are entirely contained within one time zone.