Time - Time Zones

In the past, the states followed various standards of time until the railroads of the nation cooperated in establishing a standard time zone system, which was then adopted by federal statutes. Under the standard time zone system, the continental United States is divided into four different zones. The time in each zone is based upon the mean solar time at a specified degree of longitude west from Greenwich, England. Eastern standard time is based on the mean solar time at 75° longitude west; Central standard time, on 90° longitude west; Mountain time, on 105° longitude west; and Pacific time on 120° longitude west.