I got bumped off my question (it being perceived as a duplicate of this one) so I'll leave an answer here as a parting shot. <PERSON> mentions that time flows in a direction at the rate of one second per second. That is fine to say but there are an infinite number of variations of that rate within our Universe alone.
Even for ourselves as individuals the rate appears to change according to how our brain is functioning. Our brains have different gears (alpha, beta, gamma, delta [and maybe more] wave types) and depending on which combination of them is in process time can appear to move more slowly or quickly. When we're multi-tasking we want it to slow down and when we're bored it appears to drag.
But enough of the philosophy already - we're talking the physics of time here which is basically the physics of something that doesn't exist. I can't remember who it is, some celebrity actually but the person experiences a bit of a anomaly, a "medical" condition and they don't have a perception of time like the rest of us (whatever % of human race).
Just like some people have no appetite reflectors which tell them when they have eaten enough (people have died from this) the person I forget who cannot judge time. So are they dysfunctional or more attune to nature than some?
I don't think animals have a perception of time. I think they perceive events and can remember them (actually and strangely experiments have proved that birds can pass "memories" of specific threatening persons to their offspring and the children react accordingly to the persons without previously meeting them) - however only humans have an abstract conception of time as a "thing".
In fact in <PERSON>'s Henry the IV the protagonist mocks a fool because he refers abstractly to time. The kind of moral is that people who have time to wonder about time don't really achieve much. That was written 400 years ago and now (laughing at myself) I am doing the same thing. So I guess I won't after this little answer.
Recently listening to <PERSON> (who has given a solution to the three body problem) he kind of says the same thing. He tells us that time as we see it is a kind of leftover from events that don't match up perfectly. Like if you're supposed to meet a friend and they arrive at the meeting place after you one might begin to notice the passing of time. 283
If you had met up simultaneously your mind might have remained fully engaged in life and time would have been unimportant.
To give a similar example a dude said he once met <PERSON> (the martial artist) in an elevator and they began discussing Kung Fu. They got to the same floor so they tried out a few moves. The next time the dude glanced at his watch it was 7am. He got into the elevator at 10.35pm so time disappears when we don't watch it.
Similar to quantum mechanics if we go to sleep we can depend that our reliable alarm clock will tick (as <PERSON> says) one second per second all night until it wakes us from unconsciousness. BUT we have no way of proving this is what actually happens. Just like an uncollapsed wavefunction what is time actually doing when we aren't watching?
How do we even prove that we are the same person who woke up that went to sleep? The Buddhists say that the ego is a function. It is temporary and sometimes is there, sometimes not. That is the actual meaning of the "if a tree falls in the forest and no-one is around, does anyone hear it?" koan. It translates as "if the ego is not there to watch is there an ego?" - well, obviously not if it is not there for a moment. It might come back though if the Physics Stack Exchange answer you worked hard on gets a lot of downvotes ha ha ha.
So on earth the nearest accurate measure of time is days. Everything else is nit-picking that humans get up to because they want a lot of things so they start calculating how to get them and that means we divide events into portions we call time.
That is <PERSON>'s point too. We need to perceive division in order to function and we possess an innate time-dividing tool just as we possess an identity dividing tool (the ego) but as is seen in people who have anomalies this can be absent.
Even an emotion like sadness is never the same twice and as a famous philosopher said "You never jump into the same river twice." I'm not sure how to end this ramble conclusively. Perhaps I'll just say we agree to believe we all roughly know what an hour is and a second is a very convenient measurement.
No, wait. I'll end the answer with a question. 283