Limited Wish / Mark Lawrence.
"One choice. Two possible timelines. And a world hanging in the balance. It's the summer of 1986 and reluctant prodigy Nick Hayes is a student at Cambridge University, working with world-renowned mathematician Professor Halligan. He just wants to be a regular student, but regular isn't really an option for a boy-genius cancer survivor who's already dabbled in time travel. When he crosses paths with a mysterious yet curiously familiar girl, Nick discovers that creases have appeared in the fabric of time, and that he is at the centre of the disruption. Only Nick can resolve this time paradox before the damage becomes catastrophic for both him and the future of the world. Time is running out - literally...