The time difference between Greenwich Mean Time and Eastern Standard Time is five hours. A time of 1 a.m. GMT translates to 8 p.m. EST.
The time difference decreases to minus four hours during Eastern Daylight Savings Time. GMT is also referred to as Coordinated Universal Time, or UTC, an international standard commonly used in computing. Aviators refer to GMT as Zulu Time. GMT is measured at zero degrees longitude, which runs through the Royal Observatory in Greenwich, a suburb of London. GMT is identical to the United Kingdom time zone when the U.K. is not observing British Summer Time.