The Last Infinity

Cal Hunter is a cryo, a person who was cryogenically frozen after his death in the 22nd century. Resurrected five hundred years later, he now lives in the 26th century and works as a time traveler for the time agency on Mars. After a mission goes disastrously wrong, Cal returns to his present to find that he's seriously altered the timeline. All life on Earth and the Moon has been wiped out, leaving only a failing settlement on Mars of fewer than two thousand people.
Even worse, Cal can no longer time travel due to his injuries. The only person who can fix the timeline is Renata Carr, a teenager from the 20th century. She's brought out of cryogenic suspension, and a reluctant Cal is put in charge not only of her training, but also her well-being. Meanwhile, Renata grapples with the grief of waking up in a strange century on another world while also coming of age-and falling in love with the daughter of one of Cal's friends.
When disaster strikes the settlement, the timeline for humanity's demise rapidly speeds up. While Cal knows he must send Renata on a mission she isn't ready for, and which will likely kill her, he is torn between saving humanity's past, present, and future, or the girl he's come to see as a daughter.