Folks , the concept of time and the reference frameworks to understand it have changed a lot over the ages...to give an example - let us take an example of some upcoming event on this 10th of June 2021 and then imagine how the exact time of this event could be validated say - 5,000 years from now , by a successor civilisation to ours :-
Now , we know that a Solar eclipse is due on 10th June , so recording a human social event occurring on 10th June , with reference to this upcoming Solar eclipse , would be more easy to validate 5,000 years from now, than saying this event occurred on 10th June of 2021 A.D. What if , the next civilisation is unable to understand what frame of reference A.D. was referring to ?? In that case , assuming they became at least as advanced as this present day civilisation , they would have tools to validate the timeframe of this 5,000 year old event (from their own perspective) , basis of this Solar eclipse .
These were the frames of reference , by which the Egyptians used to record the time periods of reigns of their Pharaohs , this is also the way the Vedics and the people who built the Stonehenge used to measure time .
it is necessary therefore to look at the field of Archaeo-Astronomy , to better understand how :-