Dating in the 1970s
Another mistake occurs in dealing with the attendance of Bellatrix Lestrange and Severus Snape at Hogwarts.
Sirius Black mentions Snape and Bellatrix's attendance of the school overlapping at a certain point; however, this is impossible, as Bellatrix attended the school from either 1962 to 1969 or 1963 to 1970, while Snape started at Hogwarts in 1971.
According to the timeline, this would be in the summer of 1994, but that sort of gaming console was not launched anywhere until December 1994, and then only in Japan.
However, considering his parents gave him whatever he wanted, they could have gone out of Britain to get it, as well as use special connections.
This timeline is reinforced in Harry Potter and the Deathly Hallows when the graves of James and Lily Potter confirm that they were born in 1960 and died on 31 October 1981, further reinforced in Pottermore when the Death Eater riot on the Quidditch World Cup is stated to have taken place in 1994, and further yet in Harry Potter and the Cursed Child, in which the Triwizard Tournament is mentioned to have taken place in the 1994-1995 school year. For the sake of convenience, this dating system will be used by the Harry Potter Wiki for all Harry Potter articles.
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.
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".