The end time is more complicated because for many sports it's very hard to predict when a game ends because of time-outs, extra-time etc. For example, a game of soccer is much more predictable because the clock doesn't stop during play. So a game is likely to be two halves of 45 minutes, 15 minutes of half-time and perhaps 5 minutes of extra time, so 45+45+15+5 = 110 minutes.