In the early 2000s, Harry Potter fans debated the possible range of dates for the saga. Small clues like the reference to a PlayStation in book four were discussed in detail. — Since the PlayStation wasn’t available until December 1994 in Japan and September 1995 in Europe, some argued that the events of Goblet of Fire couldn’t have happened until after that time. Some toyed with the idea that Uncle Vernon might have gotten his hands on a Japanese version while on a business trip, but that would still make book four have to happen no earlier than 1995. Others argued that Rowling’s world doesn’t have to match the real world — and clearly doesn’t in a lot of other ways — so the date of the PlayStation shouldn’t be a determining factor when dating events in the Potter universe. And the debate raged. You can find some of the essays written during that time here on the Lexicon. 
Far more compelling was the fact that Nearly Headless Nick celebrated his 500th Deathday in October of the second book. The cake at the party listed the date of his actual death as being the 31st of October in 1492, which would date the first half of the second book to 1992. I held that opinion, especially since that would mean that the first books take place during the actual years Rowling was writing them in the early 1990s.
The debate went on for years and it wasn’t until the release of the Black Family Tree in 2006 that Rowling finally stated in canon that the year of Draco’s birth, and therefore also Harry’s, was 1980. This settled the arguments once and for all. Harry’s school years therefore were from 1991 through 1998. This was verified in the novels themselves, which many fans consider to be the highest form of canon, when the dates on the gravestone for Lily and James Potter were revealed in the seventh book. 
Before then, Rowling had been particularly cagey about coming right out and giving specific years for things. Even when it would have been easy to slip in a date, she chose not to do so. In Order of the Phoenix, for example, the prophecy that Neville, Harry, Ron, Hermione, Ginny, and Luna discovered in the Department of Mysteries was labeled with the date when it was spoken by Trelawney. However, instead of stating it outright, Rowling describes it this way: “In spidery writing was written a date of some sixteen years previously…” (OP34).
By the time that fifth book was published in 2003, the Lexicon had already included a very detailed timeline of the Wizarding World. I had compiled it over the course of two years, from 2001 to 2003. The amount of information available to be included was enormous, but most of it wasn’t from the Harry Potter novels. Oh no, there were several other amazingly detailed sources of historical information available back then, all written by Rowling herself. 
Back in 2001, Rowling published two little books for charity which we now refer to as the Schoolbooks. They were Quidditch Through the Ages and Fantastic Beasts and Where to Find Them. These two books were filled with Rowling’s quirky humor. They were also filled with historical information about the Wizarding World. 
In 2003, Electronic Arts released a video game based on Chamber of Secrets which featured a series of Famous Wizard cards which players could find and collect. Rowling wrote the information for those cards and once again, the text was loaded with puns on the names and clever humor in the descriptions. And, like the Schoolbooks, the cards were chock-full of historical information about the Wizarding World. 
When the Schoolbooks were published, I immediately began taking notes and making lists. My notes evolved quickly into a detailed timeline, starting in ancient times and running to the present. This timeline caught the eye of Warner Bros who borrowed it as the official timeline to be included as part of the Extras on the DVD of Ch...