Actually, GMT and UTC are not quite the same thing. You probably do not want to bother with the difference, because it is completely irrelevant for most purposes, but if you're feeling pedantic or curious about time zones:
- UTC (Coordinated Universal Time, and yes I know that isn't the right order of the letters) is a time standard by which other time zones are compared. The standard is kept based on atomic measurements, but is periodically adjusted by inserting leap seconds, to keep it close to UT1, with the goal of preventing them from drifting more than a second apart. The correct way to describe the US Eastern time zone, not on Daylight Saving Time, for comparative purposes, is “UTC minus 5 hours”.
- UT1 is another time standard based on the progress of the Earth's motion around the Sun. UT1 is relevant here because for many purposes, like navigation, people consider it, rather than “UTC”, the reference for “GMT”.
- GMT is also the name of a time zone which has an offset of UTC+0. Prior to the introduction of the UTC standard, it was the normal means of measuring time zone offsets.
Confused yet? Good.