I absolutely believe I could have missed something, as I only quickly skimmed this (it looked like it was going to not teach me anything non-obvious from spending a ton of time doing this kind of work), but the premise is the client timestamp is irrelevant: the parameter being passed is the previous most recent timestamp of an update seen from the server, and is merely acting as a monotonically-increasing change identifier (wherein I then agree with the other commenter here: it is weird to tie this to time and it would be better to make it opaque).