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.
Running out the clock
In both professional and college football, the offense has 40 seconds (in the Alliance of American Football, 30) from the end of the previous play to run the next play. A team running out the clock will allow the play clock (which records the time remaining until a play must be run) to drain as much as possible before running its next play. In the NFL, this is particularly noteworthy due to the existence of the two-minute warning. If the trailing team has no timeouts remaining and the leading team is in possession of the ball with a first down at the two-minute warning, they can effectively run out the clock and win the game without running another positive play. With two minutes to go (120 seconds), the offense can take three "knees", one each on 1st, 2nd, and 3rd down (using all 40 seconds from the play clock on each), and allow the game clock to expire before having to run a play for fourth down. A similar situation can be had by also achieving a first down inside the two-minute warning. This practice is commonly known as the "Victory Formation", as the offense lines up in a tightly-protective "V" formation to minimize the chances of a fumble or other turnover. (The AAF lets teams run out the clock on three straight victory formations from 90 seconds left in regulation.)