I'm guessing you haven't watched much football then. I'm not saying that as an insult or anything, just a guess. There are plenty of weird things about the game, but the clock isn't one of them IMHO. The clock always runs while the ball is in play. It stops until the start of the next play when a player goes out of bounds or throws an incomplete pass, but continues running after a normal down. Each quarter is 15 minutes of play time, and it's quite strictly applied, and it is always displayed prominently, so it definitely ends. The length of a game is a lot more predictable/understandable than a game like baseball where time is disconnected from game length.