These shelters might be OK for short term stays (say a week up to a couple of months). But unless you could live underground indefintley, why would anyone even want to survive "The Apocalypse"?
You know, seriously, let’s just relax a minute here. Take into account the fact that this is a movie (not reality), and suspension of disbelief is a given, because in a movie that's a "fantasy” on top of everything else, you can’t possibly expect the laws of physics or logic to apply. Within the framework of a movie like Ground Hog Day, which is complete fiction, anything is possible. He could have been there for a few weeks or a month, which I was thinking was a reasonable guess. Who’s to say that in a fantasy movie the main character can’t learn to play jazz piano in a few days? Aside from that, who cares? But if we’re going to get really anal and insane about this, what do you imagine the residents of the world living outside of Punxatawny, PA were doing for those 30 or 40 years? Did time stand still? Did they relive the same day over and over again until Phil “got it right”? You could obsess about this forever.