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.
National Football League on television
Since 1998, early games have the precise, official start time of 1:01 p.m. ET, which allows for one network commercial and the NFL broadcast copyright teaser animation. However, game times are generally advertised simply as 1:00 p.m. starts. In addition, the league revised the late games to start at 4:05 p.m. ET if it was the only game televised by the network that week and to start at 4:15 p.m. ET if it was part of a doubleheader. The additional 15 minutes for doubleheaders allowed the early games extra time to be shown to completion, and avoid continuing past the late game's scheduled kickoff. For single games, only 5 minutes were added to allow the network time for a short introduction (as three hours had passed since the pre-game show has aired) and one commercial break before kickoff. In those cases there is no need to avoid early-game overlap as there is no early game shown. In addition, it allows those games to end earlier.