How does the 24 hours work?

I set the limit to 4 hours on my son’s iPad which worked fine, it cut him off after 4 hours yesterday. This morning he was still not able to get onto the internet. When I looked in my Android app, it said he had 0 of 4 hour left for today. I toggled the time to unlimited and then back to 4 hours and it went to 240 minutes left. I guess one question is, what time does the 24 hours start, I assumed midnight, or is it a rolling 24 hours.