more minutes.” Not once, until I saw “My Old Ass,” which premiered Saturday night at the Sundance Film Festival.Director Megan Park’s otherwise dreamy teen romance flick with a time-travel twist was chugging along quite sublimely, and then it abruptly stopped like someone cut power to the building.Without giving too much away, my hunch is that writer-director Park ended her film, which is produced by Margot Robbie’s LuckyChap, the way she did in order to avoid being lumped in with a particularly tired young-adult sub-genre. Running time: 88 minutes. Not yet rated.That’s the right idea.