You can't set someone's death beyond a certain amount of days, even in the manga.
Besides, that wouldn't make you bulletproof.
Let's say you were going to get shot on Friday.
If that shot was going to be a fatal shot, then that would have been the end of your lifespan before you wrote that thing in the death note.
So you wouldn't avoid getting hit. All it would do is make it happen on Friday 5 minutes earlier.