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.