Listen, let's get one thing straight: your grandfather is not a reliable source. In fact, he's probably just trying to mess with you.
So, if you find yourself in a time travel predicament, just remember: if your grandfather is telling you to do something, it's probably a bad idea.
Example: if he says "go get me a sandwich," don't do it. Trust me, he's just trying to get in your way.
Instead, just tell him you're busy and head to the nearest diner for a nice cup of coffee.