JRR Tolkien worked this way as well. He wrote out all the days for the books and where everyone was, even down to what phase of the lunar cycle the day was. There was actually an exhibit of Tolkien's notes that toured around for a bit, and they looked not entirely unlike what Rowling created. For reference, the collection was from Marquette University. (http://www.marquette.edu/library/archives/tolkien.shtml)