NO...Atomic clock is NOT the perfect time measure...What made us to think like that? Can't atomic clock fail and better technology come tomorrow?...In fact I have better clock than atomic clock which is not affected by high velocity... :)
Here is a scenario...two places A & B
A--------- 7.5 LM distance (1349066061 KM) -----------------B
Build a wall @ B, A person starts high speed and he stops @ B as soon as he hit.
Person A starts @ 10:00 am and he travels @ .75C speed
So according to A he took 6.61443 mins to cover...