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There are a few flaws in your logic...

1). How would you convince yourself that you weren't merely dreaming or hallucinating? The simplest explanation for what you are experiencing is that you are dreaming. Ruling that out just for argument, the next most likely explanation is that your brain is broken; you are delusional and what you are perceiving is the result of something wrong in your head.

2). Assuming that it is really happening, there is no proof that it is happening outside of the portion of the world that you are directly interacting with. In Phil's case, the time reset need only encompass the town he is in and everything within one day's driving distance (remember that he steals a pickup truck). The rest of the universe could be cruising forward through time at its normal pace. There could even be another instance of the town and surrounds which are traveling forward through time. Phil could have fallen into a pocket universe with an extremely short, circular timeline.

3). Even if you somehow rule out every possible explanation except for virtual reality, there is still nor proof that you are alone in the simulation. There is only proof that, so far, the software running the simulation has not allowed you to interact with any other real people.

There is nothing wrong with your conclusion that you are trapped in a simulation, but it is far from an exclusive and thus proven certainty, given the available facts.

It's a possible theory, and maybe even a strong one but to be your only other conclusion, it has to be the only theory. Another possible theory, albeit a bit sci-fi'ish (but probably no more than the virtual reality), is that person may be slipping through alternate universes, each one exactly like the previous except it was 24 hours behind and the universe he left carries on as normal as for the person he replaces, he could have ended up anywhere.

I'm not saying that's a scientifically possible answer or the only answer but it does make the point that no hard conclusion for it being a virtual reality is possible (without other information).

Thinking about it, I remember that in a specific scene he got hit by a train and died. Maybe that's a place to start if you want to convince a story character that it was a simulation. His physical body is being reset, injuries healed. This would at least suggest his body isn't traveling between universes. And if the person in question is versed in virtual reality, it may be enough to remove reasonable doubt.

You couldn't know that time was resetting flawlessly each time, because your own senses and memory aren't perfect, and if you used any kind of external instrument (like, trying to memorise patterns from the cosmic background radiation to see if they repeat) then someone could be manipulating that.

All you know for sure is that you remember this day happening, and external events appear to be happening the same way you remember.

The Occam's-razor hypothesis would be that it's all in your head. The fact that you seem to know the future does not disprove this, because you could just be misremembering your previous memories. You say "the next card will be a three of spades", and then the card is drawn, and you think "my god, the jack of diamonds – just as I predicted!".

If you discount that possibility regardless, then you might think a simulation (VR, Truman Show, etc.) is more likely than the whole universe, outside of your mind, being in a loop. I am not sure that follows. Any of these explanations would require phenomena or technology well beyond your experience, so it's pure speculation to say that a flawless simulation of the whole universe is "more likely" than a phenomenon that teleports a human mind back through time.

In fact, I'm not sure there is even any difference from your point of view. If the simulation is so good that you could live in it forever, and have great-grandchildren who colonise the stars for a billion years and so on, then the only practical difference between that and the "real" universe is the label. All the time loop tells you is that, whatever the nature of the universe you live in, something is happening that is outside your previous understanding of how that universe works.

Speaking as a scientist, it would not preserve my sanity to believe the world and all the people in it were a simulation for only me. It would quite likely make ME a simulation too! (Particularly if, by investigation and/or observation, I cannot find any notable physical difference between my own body and others -- why should I believe they are simulations and I am not?)

If not, I'd be the only sentient in the world, and that would be the ultimate loneliness, and make me a rat treading water forever. Since I couldn't kill myself, I think I'd go insane.

Also, speaking as a scientist, there is no telling how much energy it would take to reset the universe to 24 hours ago, with only my brain and memories changed. It is entirely possible every moment of the universe exists simultaneously, so no "energy" is required at all. More generally, if you have no idea how something is accomplished, you have no idea how easy or difficult it is.

For example, before the existence of gunpowder or other explosives, somebody trying to compute how to accelerate a quarter-ounce stone (about the weight of a 9mm bullet) to supersonic speeds with a handheld device may conclude it is impossible to do with any combination of springs or levers. Does that make a handgun impossible? Of course not, they are just too uninformed to know that there is a pretty cheap, safe and easy way to do it; in fact it is so cheap and safe some people will shoot a few hundred bullets in a day for the fun of it, and never notice the expense.

As a scientist, if I can't figure out how something is done, I can't estimate the cost of doing it. Also, I will believe my own eyes, ears and senses: If everything looks like I traveled back in time with my memories intact, I will believe that is what I am doing, and that neither me (or my senses) or the people are simulations. Also as a research scientist, I am perfectly fine saying "I don't know", and I don't need even a possible explanation for a phenomenon, I am comfortable with no explanation. Most of the things science is investigating have no answer, nobody knows why they happen or how to predict them or how they work. It is our job to come up with testable ideas to give us clues to that.

The real question is the question of proof. If a person was in a scenario where everyday was an absolutely flawless repetition of Groundhog Day, the question is really one of how many possible solutions are there for such a situation.

As a standard of proof this answer will use Ockham's Razor (please note the spelling of "Ockham" does vary). Namely, establishing what is the minimal explanation for any phenomenon.

One possible explanation is that the recursive world is the result of a simulation. Certainly the person is the only one aware that each is exact repeat and for him it is Groundhog Day every day.

But it is not sole explanation. Also, as an explanation the simulation hypothesis has to make a number of assumptions. (1) The world is a simulation, (2) whoever or whatever is in charge of the simulation has taken the person and allowed them to continue advance through the simulation gaining new memories and experience while everybody else does not or they lose that in the daily reset, and (3) what is the purpose of subjecting only one person to the Groundhog Day recursion.

Actually of those assumptions itself requires their own explanation. In fact, all three explanations are needed to explain the simulation as such.

Simpler explanations for Groundhog Day can be devised. The person is caught in a time-loop type of phenomenon. When he reaches the end of Groundhog Day he is transported back to its beginning. While the nature of the time-loop itself cannot be explained, its consequences conform to those experienced by someone caught in a time-loop. Basically the person experiences the Groundhog Day effect.

The person has become adrift in the multiverse. At the end of each Groundhog day the person is shunted "sideways" and one day "backwards" in time to the beginning again.

The person is a brain in a vat experiencing an exceptionally fidelity virtual reality which is, of course, Groundhog Day. Simulating the world for one person is much more economical hypothesis than the simulation of the whole world.

Voltaire's demon. This concept was a philosophical proposition that reality as we know it was created by a being, possibly a supernatural entity, by manipulating our senses to generate the impression that we were experiencing an actual world. Anyone could be effectively a sort of brain in a vat experiencing an artificial reality from synthetic perception. This means we could be simply conscious spirits floating in formless void with Voltairean demons fooling us into believing we exist in a physical world.

This makes Voltaire's demon the precursor of both the brain in the vat and the universe as a computer simulation.

In conclusion, with the application of Ockham's Razor and the realization that it is possible to construct simpler counter-examples as explanations for a Groundhog Day than the presumption that the existence of a Groundhog Day would be proof for the universe as a simulation. The answer is no. Groundhog Day does not prove the universe is simulation. Because other simpler explanations are also possible. Also, it is not possible to choose conclusively between the alternative explanations. This makes the suggestion of Groundhog Day proves the simulation hypothesis only one possible explanation among many.

Although there is not enough evidence, nor opportunity, to make conclusive reasoning, a Simulation Hypothesis would be a major cause you should consider

The issue, in a way, with any natural cause is that it is only happening to you.

Furthermore, current scientific thought is based on observations of our physical world. 'Everything is physics' some people say. So how come all around you everything resets, except the atoms in your brain? After all, your memories (which are still intact) are just collections of chemicals and processes, for which if you remember would have to have been continued unaffected for you to even perceive your days are repeating.

This gives credence to the suspicion (and I say that word sparingly) that all is not well, that some alien or other force is artificially interfering with your world.

As you are immortal, one way to test this hypothesis is to conduct brain surgery on yourself. Do removing parts of your brain responsible for various functions also carry over? What if you take a drug such that effects should carry over, do these? If memories are recorded and you wake up the next morning, how come your brain is ok suddenly, retaining memories, when last night you removed those memories? Test the boundaries.

In a way, this is where physics thrives. It is at its most exciting when investigating things that don't line up, and finding the point at which it does or doesn't.

(By the way, I do not condone anyone doing brain surgery on yourself - this is just a theoretical thought experiment - Kids don't do this at home).

No. Consider HBO's Westworld. For the hosts, everyday is "Groundhog Day", but the simulation is not for them, it's for the guests of the park.

While Phil has no way of knowing this, the second conclusion (that it was for Phil's benefit) is demonstrably untrue.

Groundhog day, as a movie, IS a simulation, and it is NOT for Phil's benefit. It is for the benefit of the movie studios, and the actors, and the people who paid to watch the film.

While I cannot rigorously show that a simulation is never for the benefit of that simulation's occupant, no simulation I have ever heard of matches that criteria.

Your premise that resetting a universe is difficult is a baseless assumption. We also have no idea how difficult it is to "reset a universe", it may be happening all the time.

And while a NEARLY resetting universe (after all, his memory is in the universe, and that doesn't reset) does have benefits for Phil in the short term, that benefit is only real INSIDE that universe - it does not automatically carry over to experience OUTSIDE that universe (unless Phil, himself, is outside the simulation of Groundhog Day).

Even if he was "patched in" to the simulation, the things he learned would not be of significant benefit once he came out. His memory is partitioned (he does not recall going into the universe), and the people inside the simulation are presumably NPC (since they don't remember). If he is a player controlling Phil, then it is not for Phil's benefit (and, since he's controlling Phil, Phil wouldn't notice unless the player wanted him to). If the player IS Phil, then Phil would remember going in (or would have his experience partitioned), and the skills Phil learned while in the simulation, could just as easily have been learned outside the simulation.

Whilst that's possible, even perhaps the most rational, sadly it's a matter of the design decisions of God. If one takes on the assumptions of deliberate design and of retro-active fixes, the possible mechanics of the underlying system become as varied as imagination.

In simulations we have the concept of instancing, that is to say that whilst the general system is ongoing and universal, sections of it can be duplicated and isolated (dungeons in mmorpg's, remote user sessions on vm's.) Or in general, a program's continued operation may be delayed or operate using a broad(temporary) assumption whilst a given clackerlation/process/function is completed to derive the data with which the program will operate going forward.(aka local history)

Using an instancing implementation every one of those individuals may have experienced their own closed simulation simultaneously, algorithms may delete the memories of those who 'failed the mission' or any other such criterion if a preferred progression path is accepted.

There's also the possibility of 'timeline' divergence to consider, common in anything dealing with time travel, each individual user may have had their own realities spun off until they came up with the 'correct' solution/series of events, so far as they're concerned I guess then 'this' reality actually is just for their benefit.