Well, it isn't time travel. Even though he goes back to the morning of the same day, there is only one Bill Murray. If it was time travel, he would have had encountered multiples of himself, first the original, then every time after an additional Bill Murray would exist. Plus you can't travel though time when you are dead. So that's not the case.

He could have gone though some sort of worm hole or transport to another dimension, a dimension where it is that day and morning again, but that's not exactly right either. No matter what happens to Bill Murray, he always wakes up in the same health, and age that he woke up to the on the very first morning of Groundhog Day. When traveling to another dimension, if you spill coffee on your self, the coffee stain would come with you to the new dimension. Again, the dead can't travel to other Dimensions.

So what happens? Well, when Bill Murray goes to sleep he actually does wake up and continues with the following day. Bill Murray is unaware of this life just as YOU are unaware of your other existences in other dimensions. Bill Murray continues with his entire life, and time goes on until the universe collapses on itself again only to start over in another big bang. Everything that has happened happens again, including Bill Murray's trip to Punxsutawney to cover the Groundhog story.

What happens to Bill Murray is that his original "existence" or "consciousness" is picked out of the universe when he falls asleep. Time passes to the end of time and the universe; and start of it again to the point of Groundhog Day. At that point the consciousness of Bill Murray is re-inserted into the same but started over universe. This occurs in a blink of an eye. But in that blink of an eye, everything and everyone everywhere has been destroyed. Bill Murray's existence is in a sense asleep in the meantime.

The only sort of "time travel" that occurs in the film would be the same time travel that occurs when we fall asleep at night and wake up in the morning only moments later. All the time is accounted for; it just passes really fast, almost instantaneously. Bill Murray is unaware of this time as is the viewer of the Groundhog Day movie.

To put it another way, imagine a video tape being a universe. When you start the video tape, that universe begins, when the tape is over, that universe has been destroyed. Every time the video is played, the same thing happens. After watching the video tape again, a viewer would know what to expect. Anybody that is in the video tape will still do what they do every time because in a sense, they don't know any better. Bill Murray is just replaying in his video tape universe, except he is aware that the tape is being played again. However, he is unaware that the video tape has been played all the way though to the end, rewinded, and started over regaining and continuing his conscious existence again on the morning of Groundhog Day.

Note: I suppose it could be said that the universe was just "rewinded" back to the morning of Ground Hog Day and the reason Bill Murray knows it has been rewinded is for the same reason of his "consciousness" being plucked out of the universe and moved back to that point. There is nothing that I know of in science that could explain how time can magically rewind. Rewinding time to me is more fantasy than sci-fiction.

MURRAY: ""I have to be leaving, but I won't let that come between us, OK?"

JOHANSSON: "Okay."

And let's be honest: the only reason that this movie did as well as it did, and generated such buzz, is because Scarlett Johannson is a "hottie." Nothing wrong with her being pretty at all...but how many men would have even bothered to watch the movie at all, let alone profess to their wives or girlfriends that it was a great movie, if the female lead had been played by, say, Anna Paquin or Heather Matarazzo? These young ladies are not unattractive, but they aren't "hotties" in the same sense that Johannson is.