The easy answer is that the watch doesn't run that fast. If it runs, say, 10 seconds fast a day (which is a lot), then if walking from one room to the other ta

The easy answer is that the watch doesn't run that fast. If it runs, say, 10 seconds fast a day (which is a lot), then if walking from one room to the other takes a minute, the watch is off by... 0.007 seconds. I'm not going to be setting the clock that accurately. But if the clock just runs consistently fast (and if I know how fast it runs), then I can use the watch to measure how long it took to walk from one clock to the other, and from that I can compute the correct time to set the second clock to.