Before the adoption of standard time, each U.S. city kept the local time of its own meridian. With the growth of railroads, these differences caused difficulties. Railroads that met in the same city sometimes ran on different times. In 1883, the railroads of the United States and Canada adopted a system for standard time. In 1884, an international conference met in Washington, D.C., to consider a worldwide system of standard time. The meridian passing through the English town of Greenwich (now a borough of London) was chosen as the prime meridian.

In 1918, Congress gave the Interstate Commerce Commission authority to establish limits for time zones in the United States. Congress transferred this authority to the Department of Transportation in 1967.

Today, nearly all nations keep standard time. Only a few small countries and some other regions keep time that differs by a fraction of an hour from standard time.

Contributor: Joanne Petrie, J.D., Senior Attorney, Department of Transportation.