He frowns a little, because he will have to call on all of the science fiction (and some fact) references he has to explain this. He shakes his head and finishes his drink. "I don't know exactly how it works, but time is different here," he says. "When you go back - if you go back -" he doesn't do 'false hope' "you'll be walking out the door to go see Donna. It'll be like you never left. That's why you didn't notice I was gone."