We know that book 2 takes place in 1992 because Nearly Headless Nick holds his 500th Deathday Party in CS8. There’s a cake on the table of “food” that bears this inscription:
SIR NICHOLAS DE MIMSY-PORPINGTON
DIED 31ST OCTOBER, 1492
Obviously, if you add five hundred years to 1492, you can easily see that the party is taking place in 1992. So far, so good. This date has now been verified by Rowling herself. According to Warner Home Video, she personally edited and approved the timeline that they included on the DVD of Chamber of Secrets. As a result, that timeline is canon and we can “officially” state that Harry Potter was born in 1980, that he started Hogwarts in 1991, and so on.
So now we should be able to make everything in the books fit these dates, right? Unfortunately, it’s not that simple. For one thing, Nick himself at one point in the first edition of Philosopher’s Stone stated that he hadn’t eaten in almost four hundred years (PS7). Now it’s pretty clear that ghosts can’t eat in JKR’s universe (CS8), so that would call the 500 years into question. (UPDATE: As of 2004, this line was officially corrected to read five hundred years for later editions, thus eliminating this discrepancy.)
It gets trickier. There are a number of dates given (without years) that also give the day of the week. Harry’s birthday, for example, is July 31. We know that because the newspaper article telling about the break-in at Gringotts gives July 31 as the date and Harry exclaims that he and Hagrid were at the bank that very day (PS8). The only day they were at Gringotts was on Harry’s eleventh birthday. It’s also clear that his birthday that year was on a Tuesday (PS3) and since we know that the second book took place in 1992-3, his birthday in book one must be in 1991. But July 31, 1991, was not a Tuesday, it was a Wednesday.
There are other specific days mentioned, and they don’t really fit either. For example, PS1 takes place on November 1, the day after Harry’s parents were killed. This November 1 is a Tuesday, according to the book. But there is no way that November 1 of any year and July 31 ten years later can both be Tuesdays. It’s impossible. In 1993, Valentine’s Day is on a Sunday, but that’s the day that Lockhart sent his dwarves around handing out valentine cards between classes—but on Sunday there are no classes! The most glaring inconsistency comes in book four, though. In that book, both September 1 and September 2 are Mondays. (UPDATE: As of 2004, this was officially corrected for future editions, thus eliminating this discrepancy.)
So how do we make it all make sense? We can’t, obviously. There is simply no way to fit every reference to a day of the week, a full moon, the birth year of Nicolas Flamel, or Nearly Headless Nick’s 400 year fast with any “real” calendar. And we don’t have to. It’s a story, and it takes place in a different reality, a different world, a world created by Rowling. What a marvelous, magical world it is. The fact that days and dates slide around a little, not unlike the staircases at Hogwarts, doesn’t take anything away from it at all.