What I'm saying is, if you first say that the deadline is August 16th, but then say that the penalty applies if you cancel "after this deadline", doesn't that imply that the penalty applies beginning on August 17th?

I'm just saying that I think there is a lack of clarity in the language used by Marriott. It would be helpful if it said something like, you have until 4 PM on August 16th (or whatever time they want). That would eliminate any potential confusion.