Standard time was introduced in 1884; it was the outcome of an international conference held in Washington, D.C., to consider a worldwide system of time. By international agreement, Earth was divided into 24 different “standard” time zones; within each time zone, all clocks are to be set to the same time. The device of standardized time zones was necessitated by the expansion of industry: businesses, particularly those in the transportation industry, could not coordinate schedules when each community used its own solar time (the local time as determined by the position of the sun). Railroad schedules had been extremely complicated before the establishment of standard time zones, which the railroads readily adopted.

Each time zone spans 15 degrees of longitude, beginning at zero longitude (called the “prime meridian”), which passes through the observatory at Greenwich (a borough of London), England. Time kept at the observatory is called Greenwich mean time (GMT). Time zones are described by their distance east or west of Greenwich. The model also dictates that each time zone is one hour apart from the next. However, the borders of the time zones have been adjusted throughout the world to accommodate national, state, and provincial boundaries.

The contiguous United States has four time zones: Eastern, Central, Mountain, and Pacific. Waters off the eastern seaboard are in the Atlantic time zone; Alaska, Hawaii, Samoa, Wake Island, and Guam each have their own time zones. Congress gave the Interstate Commerce Commission (ICC) authority to establish limits for U.S. time zones in 1918. This authority was transferred to the Department of Transportation (DOT) in 1967.