The continental U.S is divided into four time zones. Eastern Standard Time is five hours earlier than Greenwich Mean Time.
Most states change to Daylight Saving Time during the summer months.
This means that clocks are advanced one hour on a given date in April and restored to standard time in October. Arrival and departure times of planes and trains are usually given in the current time of the arrival or departure point. Your telephone directory has a map of the U.S. time zones.