I've read some theory about space time and its absolutely mind boggling. I know if you ask most anyone what is time? They will reply with educated authority that time is a social construct. Which to a point is true, we use a socially agreed upon method to tell time whether its the movement of a decaying cesium-133 atom or the heavens above. But our social construct can only tell us what time it is, it does not in anyway tell us 'what' time is. Time can also be subjective, how many times have you felt the end of a work day will never arrive, yet the person next to you thinks the day has gone by fast?
There is some theory out there which indicate there is no division of past present or future but that we could theoretically exist in all three states. Nor is time as Newton suggested absolute throughtout the universe rather it is dependent on how fast we travel through space time and our proximity to mass. The more you have of one the less you will have of the other. As this kewl little app will show
https://www.timewarperapp.org/.
Anyway we can see and feel the effects of time which itself is not observable. what exactly time is I dont have a clue.