• The writer, Danny Rubin, had originally conceived of the Groundhog Day timespan loop to be about 1,000 years. Basically, Phil would read hundreds and hundreds of books just by reading them one page per day. Director Harold Ramis didn't put a time span on the movie's conception of the loop, but later said he approximated it to be about 10 years. (That makes sense. I mean, the man does learn ice sculpture and how to play crazy jazz piano.)
• The movie was mostly filmed in Woodstock, IL, not Punxsutawney, PA.
• Groundhog Day has been a big hit with various spiritual communities. I even remember it being featured in a big Spiritual Film Festival at the Met a couple years back. It's popular for its concepts of rebirth (Buddhism), the importance of a life of good deeds (Christianity) and continuing work until God determines its perfection (Judaism).
• The studio wanted some kind of explanation, by way of science or witchcraft, for the time loop. Writer Danny Rubin very reluctantly wrote a scene in which an ex-lover puts a curse on Phil. Thankfully, it was never filmed. Harold Ramis said it would have detracted from the mundane nature of the film, which was important to the story.
• The studio felt that the homeless man's death was too much of a downer. But Ramis insisted on it-- he felt that the movie needed to address the idea of mortality, so that Phil understood that he was not, in fact, a god.

Overall, great movie. There is a lot about it that feels dated (Andie McDowell, I'm looking at you), but it is still one of my faves (and one that I still watched last night even though we already have the DVD!) It seems like at the time it was released, it was marketed as a run-of-the-mill comedy, but over time, critical opinion has changed. It seems that the balance of philosophical questions it raises (in line with other more serious movies like Eternal Sunshine), and great comedy via Bill Murray have raised it into what critics regard as a timeless (no pun intended!) movie. It's a NY Times Critics' Pick (as well as being on their list of Top 1,000 Movies), and is also on Roger Ebert's 100 greatest movies. Yay Punxsutawney Phil!

Phil kidnaps Punxsutawney Phil

Roger Ebert: Groundhog Day (a re-review, 2005)