Can we measure time? No, not directly. We can measure processes like our planet rotating and going round the sun and oscillating crystals and we call it days, years, and seconds. But we only observe causality. But because we can remember and count how many ticks and rotations have passed, we get a sense of scale and distance and call this number 'time' and we talk about the 'past'.

Is time fundamental? Is there some 'universal clock'? No, if that was the case, there would have to be a 'central clock' somewhere, dictating the speed of all processes in the entire universe if these processes were to run everywhere at the same speed. And they do, we can measure the light of stars billions of light years away (and thus billions of years ago) and see they emit the same frequencies of light as our own star (corrected for expansion). How would this happen? How does the ticking of the 'universal clock' spread in an infinite and expanding universe and be fixed over billions of years? What makes it tick?

So, if elemental particles behave the same everywhere, they do so because of 'something' at their own core, not because of some external 'clock'. And because of the nature of quantum mechanics, it has most likely something to do with particle interaction and the probability of events. But we simply can't probe this deep. If it is true that matter slows down time (i.e. particle interaction), it is logical to assume that the number of interactions between particles define the combined speed of all local processes. A nice explanation of time dilation. In a moving object (and dense masses), every particle has more interactions with all particles on its way, so all processes run slower. And the number of quantum processes and the random intervals at which this happens even out in our classical world and yield a nice average value that we experience as ticking, or…. Time. If a crystal oscillates, we measure the same number of oscillations every second, but to elemental particles a second is like eternity. We won't ever be able to tell is every oscillation is exactly at the same interval, and it probably isn't, but the average over millions is.

Is there a 'flow' or 'arrow' of time? No, only probability. Events that are probable happen often; events that are highly improbable almost never do. Objects always fall down, never up. We don't need time to dictate this, only causality and probability.

Time is only what conscious beings experience since we have a memory that enables us to remember things that happened. We call it the past, but it only exists in our memory. The universe itself is totally unaware of its history, its 'past', it has no memory, nor does it have an inkling about its future.

Thinking about the future is also only something conscious beings do and is essential for survival. I must eat, or else I will die.

Without consciousness there is only 'now'.

But hold on, all our physics is based on time! …And so are our societies based on money and yet we can all agree that money is only emergent and was invented to facilitate complex societies. And so is time invented to facilitate physics. Physics is perfectly fine with time as emergent property, especially classical physics. And quantum mechanics does not care about time, only about probabilities and statistics.

And there we have the answers as to why general relativity won't ever be combined with quantum mechanics, in my humble opinion. Just my two cents.