Greenwich Mean Time

The time zone that contains the "Prime Meridian", the line of longitude that passes through the Royal Observatory in Greenwich, England. 

Local time in all other time zones are specified as a number of minutes ahead of, or behind, GMT time. For example, during the winter months, the local time along the East coast of the United States (the Eastern Time Zone) is always five hours earlier than GMT. This is often written as: GMT -5:00.

Note: GMT does not observe Daylight Saving Time. When clocks are shifted during the summer in the UK, the shifted time zone is called British Summer Time.  GMT remains unchanged.

Note: GMT is also called UTC (Universal Coordinated Time, following the inside-out French word order).