Regarding Disney time (which is not atomic time as current atomic time is 37 seconds ahead of
UT time), almost all computer and smartphones synchronize from one universal time server. That is why when you look at your computer time and your smartphone and your smartwatch they are all changing at the same time.
I realized that Disney time was slightly different. Or else there is a slight lag between when the time changes and the backend software recognizes the time window is open. This happened when I made 60 day fastpass selections. There was about a 3 second lag. This also seems to happen with Southwest Airlines 24 hour checkin. It is not right on the dot. Let the time change and then count 1001..1002..1003. Speculation but it happens for a variety of time-based online systems so there is time window lag in real world applications.