How would you handle a player who wants to play a character from the future?

Basically they want to play a chronurgist who at the hight of their power sent their mind back in time to stop some calamity before it grew out of control. I'm not sure how a DM can even begin to work with this as the character would need to know a lot about the future... Am I overthinking this?

Edit: there seems to be some confusion. The player is not in the distant future. They're from around 80ish years in the future and they lived through the calamity. They didn't only live after it.

Edit 2: Man ya'll really like repeating the same advice over and over huh?