The time difference between the human world and monster world are 12 hours apart. (The children go to bed at 9pm, the monsters start their scaring at 9am). I'm reading a little to into the plot because it would change the whole movie if it wasn't ignored. Boo spends the night in monster city for at least one night making it daytime in her world...when she's finally back home its nighttime and her parents didn't notice she was missing for the whole day?