You thought we were going to say Julia, right? Or Marcus? Or even Mom? Nope, nope, and nope again. We've decided that the best way to come at this question is to think of the novel's antagonist as time. Yup, TIME, as in the thing that clocks measure? Right.
Anyhow, Miranda – and all of the characters really – are trying to figure out how time works, or how to beat time. Miranda has to understand time's relativity to see how Marcus and the Laughing Man are the same person. Marcus has to figure out a way to go back in time to save Sal. And Miranda's Mom is trying to beat the clock and win the money she needs to help her family have a better life.
Everyone in this novel really is up against time.