Moments in Time

Last night I saw a movie trailer for a film coming out next month called The Time Traveler's Wife. Based on a book by Audrey Niffenegger, it's about a man whose genetic disorder causes him to time travel.
Like economics and the George W. Bush presidency, time travel is a concept I've never been able to get my head around. The problem for me is basic: how can someone exist in two different time periods simultaneously? How can someone go back into a past that's over and done with and subsequently change the future? Over the years, I've watched it happen numerous times. James T. Kirk did it in TV and movies. Marty McFly made a habit of it, as did Terminators in several different forms. In The Lake House, Sandra Bullock and Keanu Reeves lived in the same house but a couple years apart, leaving each other letters in the same mailbox to communicate between the years. HUH??!
Fred, a lover of science fiction, has tried to explain the idea to me over the years. He says it's a matter of parallel universes, where a character can exist in 2009 while a different version of himself resides in 1945. So because of a tear in the fabric of the universe or a lightning strike on the town clock, it's possible for each self to go forward or backward in time and affect the future.
Just to be clear, let me assure you that I know time travel isn't real--my issue is that it can't be real in science fiction, either. Sci fi is supposed to be about things that haven't happened yet but could possibly happen in the future. I don't believe there will ever be parallel dimensions where we can pass ourselves walking across the street 100 years ago. So, it's hard for me to get past this premise in a movie, even when it involves Arnold Schwarzenegger dropping naked from the sky/future. (Some time travelers apparently lose their luggage.)
Maybe I don't have enough imagination. Maybe it's a natural human desire to want to experience the past that we missed or fix a disaster before it happens. All I know is that I'm stuck in the here and now--but I'd love to make a quantum leap back to 1999 and bring my then-120 pound body back to the future.