I saw an early draft of the screenplay, which included scenes in a library, where Phil was able to move a book (which somehow didn't get reset overnight) to act as a crude calendar, and the implication from these scenes was that he spent centuries re-living the day. As for the overall A/B argument, it's worth remembering that at the end of the film, Rita accepts Phil, because he actually has changed. A/B testing can achieve shallow local maxima, but it is not limited to that; you can also A/B test larger, more significant changes. The nature of the difference between A and B is up to the creator.