When does the day reset?

That scene does seem odd. During the rest of the movie, it seems as though when Phil falls asleep, the day resets. I would think the same thing would happen when he dies.

It could be that the day resets at a particular time, like 4 in the morning the next day. We never see Phil awake at 4:00 AM. (It doesn't reset at midnight; Rita and Phil are together one midnight, and time just keeps rolling forward.)

Most likely, the director wanted the audience to think that the day resets in a magical way, at the "appropriate" time.

From a film business perspective, it's possible that the scene of Rita and Larry in the morgue was added on after principal shooting. That is, some studio heads or test audiences watched the movie without that scene, and their main question was, "So does Phil really die when he drives the car off the cliff with the groundhog?" That scene could have been added to clear up that yes, he actually does die. However, I couldn't find anything about that online.

The day resets when the alarm goes off. Lifting this directly from an article I read on the movie's symbolism:

Radio/Alarm Clock

The song that plays as the alarm informing Phil it is time to get up on the clock/radio is by Sonny and Cher and the title “I Got You, Babe” seem to mock Phil with the taunt that he is trapped forever by the fates. A clock also figures prominently in the movie’s poster and media art. Not surprising, since clocks have long been symbols of attempts to place artificial construction on the dimension of time which becomes a subversive influence on the capacity of one to enjoy life and a psychological impediment to pursuing dreams.