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.
Conversion (gridiron football)
In American football, the game clock does not run during an extra-point attempt, except for some rare circumstances at the high-school level (some state associations allow for the clock to run continuously in the second half if one team is leading the other by a large margin) and for arena football, which runs the clock continuously except during the final minute of each half and overtime. In Canadian football, the clock runs during a conversion attempt except during the last three minutes of each half.