The time zone is irrelevant. It might be hard to explain exactly why, but I'll try.
2 hours or 1 day or whatever is the same everywhere. Don't think about what time it is where you are, just think in terms of how long since X happened.
So if you register at 17th Nov 23:55 UK time, then on 18th Nov 23:55 UK time your account would be exactly one day old. If the server was using American time (e.g. EST) then the two times you registered would both be 5 hours earlier i.e. 17th Nov 18:55 and 18th Nov 18:55. But that's still one day.