With a check in time of 10:30am and a return time of 9:30am it is almost certain you will have to pay for an additional 24 hours for just a short time of extra parking. For a check in of 10:30 you cannot realistically park later than 10am (and even that’s cutting it a bit fine if you‘re parking anywhere other than the valet parking adjacent to the terminal). If your flight lands on time at 9:30am it is most unlikely that you will make it back to the car park before 10:30am and this takes you into the next 24 hours for which you will have to pay.
So, it does not really matter what time you say you will arrive and depart !