They Both Die at the End by Adam Silvera

The story is set in a world where a service called Death-Cast informs people of their impending death within 24 hours. Two teenagers, Mateo and Rufus, receive their Death-Cast calls on the same day and connect through an app called Last Friend, which helps people find a companion on their last day. As they spend their final hours together, they form a deep bond, share their fears, and reflect on their lives, finding meaning and love in the face of inevitable death.