As I noted in my Harry Potter Timeline (no longer on the web), choosing the 1980 birthday can lead to problems. For example, the Sony Playstation console was introduced in Europe in Sept. 1995, but Dudley has one noticeably earlier than that.

Lady Rowling has fallen into the same trap Tom Clancy did, of having a reasonably internally consistent timeline and then trying to overlay on contemporary life and keep up with the audience, leading to oopses.