Antsy Does Time, by Neil Shusterman

When Anthony Bonano (aka "Antsy") finds out that his friend only has a limited time to live as a consequence of a grave illness, he is so fueled by friendship and sympathy that he signs a month of his life over to his “dying” classmate Gunnar Umlaut. Soon everyone at school follows suit, giving new meaning to the idea of “living on borrowed time.” But Gunnar seems to have other issues, too: a father who is mysteriously absent and a "hot" sister who likes Antsy. Does Gunnar really have only six months to live, or is news of his imminent death greatly exaggerated? And will Antsy's father ever stop working enough to spend precious time with his family? The juxtaposition of these two families in this darkly comic novel will make every reader look at his own family in a new way.