Safe as Milk

Alice and Bob

Andy: So I get the feeling the problem isn’t a real-life problem, but anyway. Since Alice and Bob are overlooking the same street, couldn’t Alice just take a picture of Bob sitting at the window showing her his watch?

Or Alice & Bob agree a protocol where they both take pictures of a given event every day (say, the number 7 bus going past, or nightfall, or the coffee-shop closing up), and use those to interpolate the series of photos for the day. That assumes both watches are linear, though. But that seems like a fair assumption.