The Uniform Time Act of 1966 established a uniform Daylight Saving Time system throughout the Nation and its possessions. It provides that either Congress or the Secretary of Transportation can change a time-zone boundary.
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 nationwide.
States and municipalities then adopted one of the four zones: 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.
Today, the U.S. Department of Transportation (DOT) oversees the nation’s time zones.
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.
— Edited from the U.S. Department of Transportation website
