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The U.S. Department of Transportation (DOT) oversees the Nation's time zones and the uniform observance of Daylight Saving Time. The oversight of time zones was assigned to DOT because time standards are important for many modes of transportation.
In 1883, U.S. and Canadian railroads adopted a four-zone system to govern their operations and reduce the confusion resulting from some 100 conflicting locally established “sun times” observed in terminals across the country. States and municipalities then adopted one of the four zones, which were the eastern, central, mountain, and Pacific Time zones. Local decisions on which time zone to adopt were usually influenced by the time used by the railroads.
Federal oversight of time zones began in 1918 with the enactment of the Standard Time Act, which vested the Interstate Commerce Commission with the responsibility for establishing boundaries between the standard time zones in the continental United States. This responsibility was transferred from the Interstate Commerce Commission to DOT when Congress created DOT in 1966.
Today, the Uniform Time Act of 1966 (15 U.S.C. §§ 260-64) establishes a system of uniform Daylight Saving Time throughout the Nation and its possessions, and provides that either Congress or the Secretary of Transportation can change a time-zone boundary.
The time zones established by the Standard Time Act, as amended by the Uniform Time Act, are Atlantic, eastern, central, mountain, Pacific, Hawaii–Aleutian, Samoa, and Chamorro.