How To Retcon Doomsday

"This world will end in seven years. How do I know? Well, because it's based on the novel I wrote, of course. I'd save it by myself if I could, but I'm not so great with magic, so..."

When she meets a mysterious boy who claims to be the author of both the world and its destruction, magic academy student Tione finds her uneventful life thrown into chaos. With only her wits and magic, and the boy's vast knowledge and strange "System" power, the two have to protect the world she lives in from an ancient conflict between unfathomable forces. Should be easy enough, right?