I really enjoy DeLillo's delinearization of time in Underworld. It seems that he embraces a cyclical vision of history in which the past and future depend on each other. In this vision, the traditionally distinct beginning and end really stand for a single arbitrary point along the circle depending on one's frame of reference. The book seems to function much like a human memory which patches together discrete frames of stopped time which influences what will happen, yet at the same time, what happens will influence the collection, arrangement and interpretation of memory such that the two become inexorably intertwined.
One example of this process occurring in the book happened on page 314 where Marvin's wife, Eleanor, tells Marvin how she justifies his pursuit of the baseball. She asks him "What good's a story without the ending? Although I suppose in this case it's not the ending we need but the beginning." The story ultimately completes itself through becoming a full circle with beginning and end ultimately at the same point. The end of the hunt for the ball's owner reveals both the successive chain of people it impacted and reciprocally, the way the ball impacted its previous owners ultimately creating a new, complete history and understanding of its existence. This might be stretching it a bit, but Eleanor sees Marvin's motivation to finish the quest as "a little childlike spot of logic." DeLillo seems to imply through Eleanor's voice that filling in the spaces of a cyclical history brings us closer to a childlike state of understanding which is simultaneously a state of innocence.
There are other times where DeLillo changes the frame of reference such as on p366 (the end of Manx Martin 2) where Manx is wearing a watch that has stopped running six weeks ago as if the last six weeks have really just been one moment in time and memory. Another instance that struck me happened on page 394 where Acey says "This city is the ticking clock. Makes me panic but I'm ready" as if it has completely altered her perception of the passing of time.