I understand that time travel actually
is theoretically possible, but the energy and cost involved are prohibitive.
It would present any number of problems - such as: if you went back and killed your grandfather, would you cease to exist? And if you did, then you wouldn't exist to go back in time to kill your grandfather, so you would still exist!
If Einstein was right that space and time are coexistent, then God created time when he created space - therefore He exists outside of time. Therefore He knows the end from the beginning and he Himself never changes.
The general theory of relativity also states that there is no constant, universal time throughout the universe. Time may be different in another galaxy than here. In fact, it may move at a different rate here today than it did yesterday. As there is no standard to compare it against, there is no way of knowing.
The only thing we do know for sure is the older you get the faster time goes by
