Humans learn about time when they are about five years old from the monster that lives under their bed.
Actually, I think that we piece together our idea of what time is, as we are developing, from the days inexhorably going by, birthdays, christmasses, thanksgivings and so on, corresponding to an opening up of our minds, an expansion of personal possibilities, down to time to get up, eating times etc.
We learn to look forward to, or dread an event by counting down to it, as if it were, because it is, in fact, a kind of prediction.
But what is time? What is time like? When we reach a certain state of education we learn that time is not as straightforward a process as we have, up 'til now, imagined.
We are told that time is relative to movement. For example two clocks set to the same time, if one is accelerated relative to the other one, the time-frame will change, things happen slower for one, or less time goes by for one, depending on the perspective you want to use.
This is a kind of useless fact for most of us. I hardly think that I'd be willing to be accelerated around the Sun in order for less time to go by in comparison to everyone else.
Nevertheless, in thought experiments about this phenomenon, we're inclined to be puzzled about the idea that, if we get in a vehicle that accelerates away from the Earth, we can come back, "in the future".
I think this puzzlement is, while an honest and fair reaction, a mixing of the idea that one is 'travelling' and that one is returning to a planet where more time has passed.
Simply because we ALWAYS live in this one time-frame, it is very difficult to imagine that there are countless time-frames 'out there', each relative to ours, depending on how fast the object in question is accelerating compared to us.
Basically, time is not a thing. Time is not lack of a thing. Time is simply the rate of events going by, in the local environment.
When we treat fairly steady events, such as a clock ticking by or a body ageing as the basis from which we measure events, and we've learned to do this since taking our first breath remember, then we imbue time with an essence that it does not have, universally.
Or to put it another way, time does not have a universal rhythm, just a relative rhythm.
I'm wondering what this means for the ideas that the universe is 'so many' billions of years old, but that's a question for another day, boys and girls. (Won't you be my neighbor?)