GMT is an abbreviation for Greenwich Mean Time. Official time used to be kept at at the Royal Observatory in Greenwich Park, overlooking the River Thames, in England. The Royal Observatory is also the location of the prime meridian, which officially demarcates East from West. Since 1920, nobody is supposed to use "GMT". Instead, you are supposed to use "UTC" which stands for Universal Time Coordinated, which is such a stupid name that nobody uses it, except the time nerds, ofc.
Here is a pic of the Royal Observatory and the "Time Ball", which is real, and not something from Dr. Who.
![](http://upload.wikimedia.org/wikipedia/commons/e/e8/Royalobs.jpg)
As a shorthand notation for American time zones we use GMT-5 for Eastern Standard Time (EST), which means GMT minus 5 hours, since EST is 5 hours behind GMT.
iMav does not live in the Royal Observatory, btw, no matter what he claims.