Walk around with a wristwatch. Even if the watch isn't synced to something authoritative, you can still use it to get all the clocks the same as each other (and the watch).
What if the watch runs fast - how do you synchronise the other clocks to an untrustworthy watch? For extra points, assume that the clocks are all in different rooms so you can only see one at a time and it takes an unknown number of seconds to go from one room to another.
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.
