Two events made September 1st a memorable day for Jesse Cullum. First, he lost a pair of Oakley sunglasses. Second, he saved the life of President Ulysses S. Grant. It's the near future, and the technology exists to open doorways into the past but not our past, not exactly. Each 'past' is effectively an alternate world, identical to ours but only up to the date on which we access it. And a given 'past' can only be reached once. After a passageway is open, it's the only road to that particular past; once closed, it can't be reopened.