There are two slip-ups here that have to do with time.

One is in the episode where Arnold and Helga are in the greenhouse. They go in at 8 AM, according to Arnold, and have to stay for 24 hours. Later, Arnold says it's been 20 hours, and they've been silent for the past two hours. If they have 6 hours left, that means they got up at 2 AM! It sure didn't look like they got up at 2 AM though, 'cause it was broad daylight.

Another is, the one called 24 Hours to Live. When Arnold goes to bed, Helga says he has 16 hours. Arnold then wakes up at 7:00 AM, and his radio says its 2 hours before he "dies." He must have slept for 14 hours! That also means he went to bed at 5:00 at night!