Some pedantic notes: 1) Transforming from EST to GMT is easy: they always differ 5 hours. But you probably have a time that is either in EST or EDT. 2) EST is an ambiguous name, it could mean Eastern Standard Time, from either North America, or Australia, or it could mean European Summer Time. 3) Without knowing whether the timezone is EST or EDT, there's one hour a year in which you can cannot uniquely map the time to GMT. 4) EDT lasts longer than EST, so most of the time the difference is actually 4 hours. 5) I prefer to use UTC over GMT.