Check-in/out time is for access to the property and amenities. It is definitely not a guarantee that you will be allowed to stay in your room that entire time. We were offered access to a different (i.e. inferior) room for freshening up on arrival, but we mistakenly declined on the very incorrect assumption that it should not take to long for our room (which is what they told up upon arrival). I would assume that the same courtesy may be granted if you are staying late on your check-out day, but they need to clear your room for an arriving guest.