They Both Die at the End by Adam Silvera #1

On September 5th, a little after midnight, Death-Cast calls Mateo Torrez and Rufus Emeterio to give them some bad news: they're going to die today. Mateo and Rufus are total strangers, but, for different reasons, they're both looking to make a new friend on their End Day. The good news: there's an app for that. It's called the Last Friend, and through it, Rufus and Mateo are about to meet up for one last great adventure - to live a lifetime in a single day.