Time travel is fascinating, complex, and confusing. It is perhaps not surprising then that Mrs. Rowling, who freely admits that she is bad at numbers (as I am myself), only partially fleshes out this aspect of her world and largely confines it to a single book. It is not so much that maths are so necessary for time travel, but really, think about it, Hogwarts always starts on September 1st, which always seems to be a Monday. Does that sound like a promising start to an in depth exploration of time?