The film itself is a meta-textual comment on the movie-watching experience and the Saw "Star Wars" Twenty-Seven Times trope. Watching a movie multiple times, viewers begin to get used to the story itself, which is the same every time watched. Avid fans will repeat lines as they happen as Phil does In-Universe, and will know exactly when and how the story will evolve, and continue to get new experiences from every successive view. It becomes even more meta with Groundhog Day itself, as viewers who have seen it multiple times will repeat lines even Phil doesn't know will repeat (such as "Ned! Ned Ryerson! Needlenose Ned! Ned the Head!") and so on, and recognize seeming background characters who become central to the story. Viewers literally put themselves in a "Groundhog Day" Loop by re-watching the film.
In a similar sense, the musical version can be seen as a meta-textual comment on the live theater experience. The actors do the same lines over and over for eight shows a week, just as Phil and the people of Punxatawney do the same day over and over.
At the end of Groundhog Day, absolutely everyone in Punxsutawney adores Phil Connors and treats him almost as the town's favorite son despite the fact that his recent outbursts of philanthropy and goodwill aside, from their perspective he should still be a relative stranger who has been in town one day tops. At first, it seems that this is a reflection of his new likeable nature requiring Willing Suspension of Disbelief until you remember that earlier in the movie, one of the selfish things that Phil did with the "Groundhog Day" Loop was use the information he gleamed about the various townspeople he met in order to manipulate them to do what he wanted them to do, often by pretending that he was someone that they knew in their past but had now forgotten. By the end of the movie, he's learned so much about them and the town that it's not inconceivable that he could easily integrate himself into the fabric of the town even over one day by 'reintroducing' himself to a few select people and claiming to be someone who used to live there a long time ago and has now returned, hence their shaky recollection of him - only now, he's doing so in order to enrich their lives rather than merely enriching himself. Also, it's stated that the film's events are the fourth time Phil goes to Punxsutawney to cover Groundhog Day, so locals would have actual memories of encountering him.
In the restaurant when Phil is trying to determine Rita's interests through trial and error, she says at one point, "There is something so familiar about this. Do you ever have déjà vu?" This line supports a theory that Phil isn't the only one who can remember the previous iterations of Groundhog Day; Rita, and perhaps all the townsfolk, have it buried somewhere in the depths of their memory. That could explain a lot of things, including the fact that Rita finally lets Phil sleep with her (even though he declines to do so) at the end, when earlier in the film she had adamantly refused on the grounds that she didn't know him well enough, and that mysterious black bartender possibly knows more than he shows.
During Phil's initial suicide attempt, his first one involved kidnapping the groundhog and planning on killing it with him. It seems crazy at first, but Phil, after so many loops, theorized that when he started his first Groundhog Day, he called the groundhog a rat and must have thought the Groundhog was able to control the loop, and was using it to get back at him.
On Phil's first day in Punxsutawney, he delivers his report on Groundhog Day in a typically sardonic, Deadpan Snarker fashion, and Rita retorts "Okay, let's try it again without the sarcasm". Sounds like a throwaway line, except that it neatly encapsulates the film's entire arc: Phil has to keep trying Groundhog Day over and over again until he learns to lose his sarcastic edge and become a genuinely decent person. It's no coincidence that he commences his final day in the loop by delivering a warm and heartfelt report on the celebrations.
One could argue that the loop was finally broken when Rita entirely voluntarily put her little hand in Phil's one, just as that annoying song had demanded.
From the musical: In "One Day", Rita's hypothetical man she has no interest in becomes "drunk and existential every time the Steelers lose a game." Why the Steelers, out of any sports team? Because they live in Pennsylvania, where Punxatawney is located. The nearest major city is Pittsburgh, where the main characters live and work. Most of the men Rita has been interacting with lately are probably Steelers fans. It's also early February, so the football season recently ended.
From the musical: The mayor is doting over Phil the groundhog before the party, with Rita in proximity, and consistently notes that the smiling groundhog has a "tint of sadness" each repeating day. But on the perfect day, Phil the weatherman's final day in the time loop, the "tint of sadness" line is conspicuously absent.
Notably, the "tint of sadness" line was also applied to Phil the weatherman by the landlady in the loop immediately after Rita changes his perspective, signaling the link between the groundhog and the weatherman.
Why Rita didn't guess just how Phil would know what noises Nancy makes when excited and exactly which means he resorted to to elicit this excitement. One could imagine that Rita would have been outraged to learn it and would not have offered sympathy to him in that case, but she doesn't seem to put two and two together even after having believed in the time loop story. Might look like an academic point since she would have forgotten it on the next iteration anyway, but you have to remember that it was precisely the moment of true intimacy with Rita that made Phil strive for the transformation - without it he might have forever stuck at the suicidal stage.
The piano teacher accepting Phil as her prize student in the end could easily have involved the same Memory Gambit he used on Nancy to casually insert himself into her life, just with a positive basis this time. Learn some details of her life over the course of lessons, then casually bring them up in future loops to present himself as having known her for much longer and back to town for a refresher course, or just to demonstrate how well he uses her teachings into the present.
At one point during the sequence with the old homeless man, on learning that he's died Phil demands to see his charts. This would seemingly be useless for a layperson, but Phil's probably been teaching himself medicine and medical biology.
Phil figures out that he can commit a crime and it will all be wiped out the next day. Imagine if Phil were replaced by someone worse than him and they use the time loop to commit any worse crime possible knowing that the crime will be erased tomorrow.
The exact same thing would happen to the criminal as happens to Phil, though; what does it matter if you can get away with your crimes, no matter how terrible, when you're just doing the same thing over and over again? You never get to spend your ill-gotten loot, because it vanishes the next day and you have to steal it all over again. The people you assaulted or murdered "yesterday" come back to life "today", taunting you with their immortality and invulnerability. Eventually, the thrill of escaping consequences would wear off and it would just become the same kind of hell for them as it became for Phil.
Sure it wouldn't matter as long as the loop keeps repeating itself, but what if you did horrible things and the loop suddenly ended?
After Phil's various suicides, he would be in the unique position of being able to *remember* the experience of committing suicide.
Accounts vary, but Phil must have been in the loop for years. It would have taken that long to become a skilled pianist, for example. After having spent so long living the same day, and coming to rely on knowing everything that would happen, will he be able to cope with returning to a world full of unknowns?
The reason Phil is eventually released from the loop is after spending a day going around town and helping everyone shows that he has become a good person compared to how he used to be. However one of the more dramatic parts of the movie was Phil trying and failing to save the old homeless man. We don't see the old man much after this part, which means eventually Phil just stopped trying to save him in favor of focusing on others instead. He probably never stopped caring, but eventually came to the hard realization that no matter how much you want to, it's impossible to save everyone.
After one of Phil's suicide attempts, Larry & Rita are shown identifying Phil's corpse at the Punxatawney morgue. This implies that the same thing happened each of the (according to one source)12,395 days that Phil relives. In other words, each loop actually generated its own separate and continuing timeline.
Alternatively; the timeline simply resets at the same time every day, regardless of the state of Phil or his body.
A question that the movie thankfully never addresses: all the times Phil kills himself, does it automatically cause the loop to reset, and he wakes up... or does his body lie in state until 6am?
Given that we see Rita and Larry weeping over him, it's almost certainly the latter.
12,395 days that Phil relives. In other words, each loop actually generated its own separate and continuing timeline.