9-12 means that they arrive on the 9th, sleep there the 9th, 10th. and 11th, and vacate on the morning of the 12th. It is a 3 night reservation. Luckily you did not bother them and they don't know about your confusion. I just had a woman ask for a refund since she did not want to stay on the last day of her booking. What she didn't understand is that she never paid for the last date, since that is when she leaves and guests are only charged for the nights when they sleep at the property.