There is no need for Mr. Resetti. Every time the player does something, it is logged in a journal. "Saving" just involves committing the journal to the town file.
When the game starts, it reads the UNIX time from the RTC using
time() (or other platform's counterpart) and compares it to the timestamp of the last save.
If the current time is before the last save, the time guardian pops up and gives a speech similar to Resetti's, the player awakes from a dream, and then the game starts 90 minutes after the last save.
If the player has an alarm clock and has set it for a time between the last save and the current time, the game starts from the alarm clock.
Otherwise, the game starts from the current time.
It then runs the simulation in fast forward from the last save to the start time.