The game clock keeps the official time for the game. It is set to 15:00 at the beginning of each quarter, of which there are four per game. It is also used to time halftime period, which lasts 12 minutes during the regular season.
The game clock stops during breaks in the game, after incomplete passes, during time-outs, and while plays are being reviewed by the officials. It also stops at the 2-minute warning at the tail end of each half (the 2nd and 4th quarters). The clock can only be reset by requests of the Head Umpire on the field, or if during playoff games, by the one off-field official sent by the League office to serve as an extra set of eyes on the field.
The game clock is wholly separate from the play clock, which counts down from 45 after the completion of each down to ensure the game moves along at a reasonable pace.