Fans have created a timeline of the Harry Potter series from a single piece of information in Harry Potter and the Chamber of Secrets.
At Nearly Headless Nick's deathday party in that book, his death is stated to have been on 31 October, 1492.
For the sake of convenience, this dating system will be used by the Harry Potter Wiki for all Harry Potter articles.
There are numerous contradictions in the timeline, though it should be noted that, in the FAQ section of her website, Rowling has admitted having difficulty with managing mathematics, so perhaps perfect internal consistency is not to be expected.
In addition, considering the Harry Potter universe is a fantasy world, it's possible that certain dates may differ between it and the "real world".
Despite its problems, this timeline is extensively used by fans and Warner Bros.'s timeline of the series (featured on the DVDs for Harry Potter and the Chamber of Secrets and Harry Potter and the Prisoner of Azkaban) contains dates that were interpolated from this dating system (such as Harry's birthday being on 31 July 1980 and his first defeat of Lord Voldemort being on 31 October 1981.) However, several anachronisms have crept through in the movies, such as featuring the destruction of the Millennium Bridge in the film version of Harry Potter and the Half-Blood Prince, even though the novel (and the film) are supposed to take place two years before the bridge was built.
In Harry Potter and the Order of the Phoenix, the Dursleys are seen to have a flat-screen television, which were not commonly available in the mid-1990s.
In addition the Dursley's car, shown in the same film, bears a 2006 year registration plate.
A short film with the movie series cast, The Queen's Handbag, was produced in 2006 as part of the 80th birthday celebrations for Queen Elizabeth II.
The skit violates the dating convention by having the characters reference the event, even though for them it should still be the mid-1990s.
Often when dates are given, they are given with a day of the week that does not match with that date as it in actual history.
One such example occurs in Harry Potter and the Prisoner of Azkaban when Sybill Trelawney refers to 16 October as Friday, although 16 October 1993 was a Saturday.
This is usually explained as artistic licence on the author's part.