Dating conventions

Fans have created a timeline of the Harry Potter series from one shred 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 October 31, 1492 (though in chapter 7 of Harry Potter and the Sorcerer's Stone Nearly Headless Nick states that he has been dead for Nearly 400 years, it is assumed that this is an error on the part of the author). Since the celebration was commemorating the five-hundredth anniversary of Nick's death, this seems to say the scene takes place on October 31, 1992.

Dates

 * Events prior to 1940
 * 1940s
 * 1950s
 * 1960s
 * 1970s
 * 1980
 * 1981
 * 1982
 * 1983
 * 1984
 * 1985
 * 1986
 * 1987
 * 1988
 * 1989
 * 1990
 * 1991
 * 1992
 * 1993
 * 1994
 * 1995
 * 1996
 * 1997
 * Events after 1997

Contradictions
There are numerous contradictions in the timeline, though it should be noted that Rowling herself has admitted having difficulty with managing mathematics in the FAQ section of her website, so perhaps perfect internal consistency is not to be expected.

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 July 31 1980 and his first defeat of Lord Voldemort being on October 31 1981.) For the sake of convenience, this dating system will be used by the Wikipedia for all Harry Potter articles.

Problem with time
Often when dates are given, they are given with a day of the weak that does match with the said date as it in actual history. Just one such example occurs in Harry Potter and the Prisoner of Azkaban when Sybill Trelawney refers to October 16 as Friday, despite October 16, 1993 being a Saturday. This is usually explained on the grounds of it being simply poetic licence on the author's part.

There are also contradictions within the books themselves in this area. For example, in Harry Potter and the Goblet of Fire, both 1 September and 2 September are given as Mondays and, in Harry Potter and the Prisoner of Azkaban, Buckbeak's trial is set on 20 April, but careful parsing of the text reveals that it could have happened no later than February.

Historical problems
In Harry Potter and the Half-Blood Prince, Cornelius Fudge uses a male pronoun to refer to the "predecessor" of the Prime Minister of the United Kingdom. According to the above timeline, this should occur in the summer of 1996, when the Prime Minister was John Major, whose immediate predecessor was a female – Margaret Thatcher.

It is possible, however unlikely, that Fudge could be referring to a more distant predecessor, as Thatcher took office before Fudge supposedly became Minister for Magic. (Although one could imagine a scenario in which Fudge was delegated by the Minister of Magic to meet with the Prime Minister while he, Fudge, was in an inferior office.) Additionally, the personality and history of the Prime Minister portrayed appears to more closely match that of Tony Blair than of John Major. An article in the Daily Mail notes that Rowling is close to the Chancellor of the Exchequer Gordon Brown (often viewed as a rival to Blair's leadership of the Labour Party), and that she might have been attempting to show Blair's worst side. If the books were in sync with publication date of Philosopher's Stone, then this event would occur in 2002 (after Blair became Prime Minister).

A similar problem involves Nicolas Flamel. Flamel was mentioned as being 665 at the time of the first book, but Nicolas Flamel was a real historical figure and wouldn't have reached that age until 1996. The rebuttal for this is that the date the "real" Nicolas Flamel was born does not correspond with the Nicolas Flamel of Harry Potter.

There is at least one anachronism in the books. The occurs in a letter Harry writes in Harry Potter and the Goblet of Fire contains a reference to Dudley Dursley owning a PlayStation. According to the timeline, this would be in the summer of 1994, but the PlayStation was not released until December, 1994 in Japan and September, 1995 in Europe.