Each auction has a set time when the auction starts to close. Each individual item in the auction has its own close time. Generally the close times are spaced out in 20 second increments (although sellers can choose different spacing).
That means that if an auction starts to close at 8:00PM, the first item will close at 8:00:00PM, the next item at 8:00:20PM, then 8:00:40PM, 8:01:00PM and so on.