During normal gameplay, Phineas is only able to show up in the city from 9 AM to 7 PM. However, the game contains unique versions of his music box theme for the night, late night, and early morning variations of the city's background music. These can only be legitimately heard by staying in the city plaza during the turnover to 7 PM and remaining there indefinitely. Because the game world reloads itself at the start of each new day at 6 AM, Phineas will eventually be forced to leave at that time, preventing the player from hearing his music box cycle back to the daytime variant.
Contributed by game4brains