Everyone throwing the "billion" number around, we have absolutely no idea how old earth or the universe is, there's no accurate way to measure and put a number on it, just like how things are "trillions of light years away." That's waaaaaay beyond human comprehension, I don't believe it for a second.
But a paradox. If a man goes back in time and kills his grandfather while he's at a young age, does his grandfather ever actually die?