Greenwich Mean Time, or GMT (also called Universal Time, or UT):
An international time-keeping standard based on the local time in Greenwich, England (0 degrees longitude). GMT (UT) enables us to refer to things that happen at the same moment in different time zones. (It would be confusing to use "local time.") At any moment, GMT is the same everywhere on earth. (That's why it's also called universal time.) What time is it now? Check the Greenwich Mean Time clock.
This map shows how many hours you need to add to or subtract from your local time to get the GMT.