If you make an early morning reservation at a time before the park officially opens, you are allowed to enter the park. As for the evening, Disney will know the closing times well in advance, even if the site doesn't display them yet, so it would not show bookings outside of those hours. If you try to book a Halloween party night ADR you are warned you must have a party ticket to enter and dine. If there's a special event on at MK (sometimes it closes at 6pm), again Disney would not show reservation times for after that.
In my experience, AK closes at 6pm and Epcot at 9pm, almost without fail. It's HS and MK whose hours can vary by an hour or so in the evenings.