Comparison of American football and rugby union
A rugby union game is divided into two halves of 40 minutes (or shorter for lower-grade games, and breaks given halfway through each half if playing conditions are considered to be extreme) separated by a half time period of up to 15 minutes in an international match. Most notably, a rugby union game will continue after the scheduled end of a half (half-time or full-time) until the ball becomes dead - any occurrence that would have play restart with a scrum or lineout, or when a team scores. This has led to some "nail-biting" finishes where teams losing by only a small margin work their way towards scoring, and games can go on several minutes over time. Something similar can occur at the end of either half in American football, though usually for only a few seconds of additional time, because a play that begins before the half expires continues until the ball is dead regardless of whether there is any time left on the clock. In both sports, the clock is also stopped during substitutions and for injuries, so the referee does not need to add stoppage time as is done in soccer/Association football. American football games are made up of four quarters of 15 minutes each (less at the high school and youth levels), but the clock stops and starts according to specific rules, so that a 15-minute quarter lasts much longer.
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.