We have 4 time zones within the continetal boundaries of the U.S. Eastern Standard Time (ET) is East Coast time , which is 5 hours after Greenwich Mean Time (GMT), Central Time (CT) is 6, hours later, Mountain Time (MT) is 7 hours and Pacific Time (PT) is 8 hours.
Those, of course are for Standard time; non-Daylight Savings Time (DST). For DST times subtract 1 hour from each of the listings.
There are a few States that are split between two time zones which makes for really interesting time keeping between two nearby cities in the differing zones. Additionally, some States and even local communities, choose not to recognize Daylihgt Savings Time...
Don't even think about asking about Alaskan or Hawiaan time...