Forget about jumping back to the age of the dinosaurs. In Primer, a time- travel movie opening this month, the characters barely backtrack a full workday. When Shane Carruth, the writer, director and co-star, set out to make a film about two friends who construct a time machine in a garage, he wanted to offer an unconventional take on the genre. "In other stories, people seem to 'jump' from one point in time to another," Carruth says, which didn't make sense to him. "If I have to walk to the grocery store, I have to move through each point in space between there and my house," he says. So he decided to treat time the same way: If his time traveler wants to go back one day, he has to pass through all 24 hours. With full knowledge of that day's events, he can right wrongs--or make a bundle on the stock market.