Tempest by Julie Cross

19-year-old Jackson Meyer can time travel.  Not much, usually just back a few hours or days.  And when he does go back, nothing he does actually impacts the future - no one remembers interacting with him if it happened on one of his jumps, if he hurts himself he isn't seriously damaged when he gets back, and if he does something on one of his jumps, like break a window, that thing is undone when he gets back to the present.  It's not really serving a purpose, but he's having fun experimenting, and keeping it a secret from everyone but his science genius friend who is helping him figure out why it's happening and how to use it.  At least it is all fun until some people who also appear to be able to "jump" back in time show up at his girlfriend's college dor, attempt to kidnap Jackson, and kill his girlfriend in the struggle.  Now Jackson has jumped back 2 years, farther than he's jumped before, and no matter how hard he tries, he can't get back to his "real" time.  And the Enemies of Time are still after him.

Prepare to Die by Paul Tobin

Despite being at their mercy, Steve Clarke, aka superhero Reaver, is inexplicably given a boon by arch-nemesis Octagon and his cadre of supervillains, Eleventh Hour - instead of killing him outright, they grant him 2 weeks to take care of his affairs before he reports back to them to fight to the death.  Steve uses the two weeks to come to grips with how he became a superhero in the first place, the loss of superfriends, his own moral ambiguity at times, and to reunite with his high school love, the girl who got away (or was pushed away due to Steve's unanticipated mutation caused by a chemical spill which transformed some of the cars occupants and killed others).