The first problem is to even think of a way to think about it. In the general context, I don't know. We have to correct our observations of distance events to take account of delays involved in the propagation of information to us, but once that is done, every event can be assigned a unique time, and all events with the same time label belong the same frame of the movie.
Time is measured along the movie reel.
Plausible as that may seem before you think about it logically, reality doesn't work this way. If it did then the speed of a television signal would look different to people moving at various speeds, e.g. almost equally fast alongside it or in the opposite direction. Maxwell's Equations for all forms of electromagnetic propagation show a speed uniquely determined independent of the observer's or the source's speed. Well, the fact is, they work fine. It follows by simple logic that there is no unique time base available to everyone.