I'm always getting these confused (silly humans) so here's a little cheat sheet
(watch for summer Daylight Savings Time):

EST (GMT - 5) = Eastern Standard Time from New York City to Indianapolis and Louisville and Tallahassee.

CST (GMT - 6) = Central Standard Time from Chicago, Nashville, Pensacola to Bismark, Pierre and Texas.

MST (GMT - 7) = Mountain Standard Time from Rapid City and New Mexico to Idaho, Utah and Arizona.

PST (GMT - 8 ) = Pacific Standard Time from Nevada and Lewiston to California.

AKST (GMT - 9) = Alaskan Standard Time.

HST (GMT - 10) = Hawaiian Standard Time (also includes the Aleutian Islands in Alaska, then it's referred to as HAST).

GMT = Greenwich Meridian Time for London, Morocco, and Portugal.

GMT + 1 for Spain and France to Sweden and Poland.

GMT + 2 for Finland, Turkey, Romania and Egypt.

GMT + 8 for Hong Kong and Philippines and Western Australia.

GMT + 9 for Japan.

GMT + 10 for Eastern Australia.

Some examples

9pm EST = 8pm CST = 6pm PST = 2am next day GMT (Phillipines/Japan/Australia are 10am-12noon the next day)
12 midnight EST = 11pm CST = 9pm PST = 5am next day GMT.
8am EST = 7am CST = 5am PST = 1pm GMT.
And for a twist:
12 midnight PST = 2 am CST = 3 am EST = 8am GMT (all am for the "morning after" PST).