He's suggesting (rather correctly) that time is relative. He's also suggesting that time is subjective (again, kinda correct).

Everything that you haven't done yet is in your future, no matter when in the rest of the flow of time that occurs. Everything that has already happened to you is in your past and therefore immutable for you. Is what He's trying to tell them. By going "back in time" they will be changing their present, not their own past.