What Year Was Harry Potter Born?

So in the first HP book, it says, '...tomorrow, Tuesday, is Harry's eleventh birthday." tuesday, July 31st doesn't come around very often. Which means that the current year the first novel takes place is limited. 1962,1973, 1979, 1984, 1990, 2001, 2007, and 2012 all have a Tuesday, July 31. The only other assumption we can deduce in that it takes place after 1945, because Dumbledore is famous for his defeat of a Dark Wizard in 1945. (I gathered that he was like Nazi Hitler Wizard or something). Now, normal assumptions would say that the book takes place in current times, and as the book was published in 1997, and there is no Tuesday July 31 in 1997, we could infer that the first book is set in 1990, because this is the year closest to the publishing date that has a Tuesday July 31. This puts Harry being born in 1979.

Most people would want to infer that the book takes place the year it is published unless otherwise stated. This book was published in 1997, therefore this theory pegs the date at 1986 for Harry's birth year. However, we know this to be incorrect, because of evidence published in Harry Potter and the Chamber of Secrets. In this book, Harry helps Nearly Headless Nick celebrate his deathday anniversary on October 31, 1992, pegging the date of the first one to be 1991. We also know Harry turns 12 at the beginning of book 2, therefore Harry's birthday is July 31, 1980.

So, we can either ignore this 'fact' that places Harry's birth year as 1980, or we can assume that Harry got the day wrong in the first book, because given the circumstances at the time of his birthday, they were stranded on the cold, wet little hut on the island and maybe Dudley wasn't exactly correct about the days of the week, because, hey, he isn't exactly brilliant.