I think we can all agree that the
perception of time is a personal experience, different from everybody else's, so it cannot be "absolute". A typical example: pining for the whole week until the "time" comes for crossdressing seems to take an eternity; the actual few hours while crossdressing slip past in a hurry
Or, putting it into a different way: spending three hours in the bathroom getting ready for going out is "never enough time", while the same three hours, from the perspective of my wife, seem like an eternity.
Another typical example: an hour having dinner with one's S.O. takes just an instant; an hour lunching with a droning professor of philosophy gets on our nerves because it seems it will never end. A good movie is always shorter than a boring movie — even if both take exactly 90 minutes.
Well, that's fine; but one might wonder if there is indeed something else beyond our own perceptions, and which "marks time" in an absolute, intrinsic way, even if we perceive it differently. If there is, well, then relativity is impossible and Einstein not only was wrong, but we have been deluding ourselves for the past century every time a scientist makes an experiment confirming Einstein's theorems to an ever-increasing accuracy. It makes no sense postulating that time is both absolute and relative at the same time!
However, we certainly have a notion of
conventional time. By this I mean that even though relativistic clocks might disagree on the passage of time, we can still agree that there is entropy (we might just disagree on the precise amount, as this will be slightly different depending on the observer). In simple Buddhist terms, what this means is that we can observe entropy and agree that it exists conventionally — Buddhists might use the word "impermanence" to describe entropy — and we can also see how it happens, moment by moment. Thus, we can measure entropy at one moment in time, and on the next Planck time unit (the smallest possible time subdivision according to quantum mechanics), and see that there is a difference, however slightly — entropy has grown slightly, and we can define an "arrow of time" that way. This is irrefutable.
What Buddhism says about that is that time exists conventionally as long as there are observers (and Buddhism also claims that wherever there is a universe — commonly known as "phenomena" or translated as "appearances" — there are minds to observe it, and vice-versa), but it's not a property that exists on its own, i.e. intrinsically. Quantum mechanics have at least nine (or eleven, according to some) different interpretations, but the most popular ones tend to agree with that definition of conventional time. Einstein's relativistic universe has been proposed in dependence of observers; it makes little sense to talk about "time" (or even "space"!) if there is no way to measure it, and measurements require an observer to make the measurement (even though modern physics accept "mindless observers", something which Buddhism rejects: observing is an act which requires a conscious observer, and it's delusional to believe that a rock can observe other rock).
We humans can certainly speculate (as thought experiments) if a universe devoid of any observing minds can exist and if time would pass on that universe. The convention in this case is to accept that this is a valid thought experiment. However, like many other similar thought experiments, it cannot be proven or disproven — as soon as someone observes that universe to see if it has "time" in it, then there is at least one observer! — and, as such, it might be unscientific to postulate its existence (at least, by a strict interpretation of Popper's definition of a scientific theory: one that can be disproven) beyond a thought experiment. Buddhist thought is a bit more consistent in that regard: whatever universes are out there, they will always have observers with conscious minds, and, for them, time will exist conventionally; there cannot be any universes without any observers whatsoever; so, according to Buddhist thought, time cannot be an absolute, but merely a conventional experience — just like everything else, really.
