Running out the clock
In American football, each quarter of a game is measured with a 15-minute game clock, or 12-minute clock in many high school football codes and the German Football League. A team in possession of the lead and the ball will attempt to use up as much of the game clock as possible in order to bring the game to an end more quickly, thus denying the opposition another chance on offense.
Play clock
In all levels of Canadian football, the offensive team must run a play within 20 seconds of the referee whistling the play in; in amateur American football, teams have 25 seconds from the time the ball is declared ready for play. In the NFL, teams have 40 seconds timed from the end of the previous down. Before 2008, in college football, the play clock was 25 seconds after the ball was set, but the clock was not stopped for the ball to be set unless the previous play resulted in a stoppage of the clock. Now, the same intervals as the NFL are used, with minor differences for the final two minutes of each half. Various professional leagues have used their own standards; the XFL, for instance, used a 35-second play clock to encourage faster play. Arena football uses a 32-second play clock.