Longitude is measured east and west of the prime meridian passing through Greenwich , England. Since the continental United States lies to the west of Greenwich, the time everywhere in the United Sates. Nowadays it is obviously impractical for everyone to keep their own local time. The meridian passing through the center of each zone is called the standard time meridian . The corresponding time zones are Eastern Standard, Central Standard, Mountain Standard, and Pacific Standard.
Standard time is defined as the local mean time of the standard time meridian.
Within each zone everyone agrees to keep the LMT of the zone's standard time meridian. For example, in the EST zone each watch is set to read the LMT of an observer at 5 hours west longitude. Within each zone some people will be located to the west of the standard meridian and some people to the east. The LMT of an individual living at the western edge of the zone is 30 minutes earlier than the standard time, while at the eastern edge of the zone the LMT is 30 minutes later. In addition, the LAT differs from the LMT by the equation of time.