The Reason why (unique) Time does not existEinstein says: Time is what your watches are telling you — but ...
According to General Relativity, clocks meeting, separating, and later on meeting again
will usually not agree about the time difference between these two events.
necessarily develop different time concepts (!)
We are not aware of this in our everyday life only because the difference is usually quite small. For space travelers however, or for unmanned space ships being accelerated beyond what humans could endure, this difference will become quite noticeable.
Here is a more drastic example:
Let A and B be persons that have watches showing exactly the same time (we assume these are atomic watches and so are guaranteed to work most precisely). Then assume that while A is living on earth all the time, B is entering a space ship [= event 1] and is travelling with nearly the speed of light away form earth but is later on turning around to meet A again.
When B is back on earth and finding A's watch [= event 2], the time difference between event 1 to 2 recorded on A's watch will be larger by many years than shown on B's clock. Depending on how fast B was travelling and how soon he came back, the difference may have become arbitrarily large (which will explain why B will find A long dead already).
Quite an interesting question to ask might be:
What is their minimal time distance?
More acceleration will always shorten it.