The Lexicon typically uses the most generally-accepted designation for years in the Harry Potter stories. Using the Deathday Party cake and the Black Family Tree as references, the year of Harry's birth is listed as 1980, with other years derived from that one. However, in some instances (such as the Reader's Guides), we will use a less specific designation. This allows us to indicate when events occurred in books in relation to each other while avoiding all the arguments about days and dates, Playstations and wide-screen television, and the like.
In this system of dating, the year of Harry's birth is given as Year Zero, abbreviated as Y0. This corresponds to 1980 in the canon timeline, and therefore is fairly easy to interpret, as the final digit is the same. Harry, then, entered Hogwarts in Y11, he encountered Voldemort in the graveyard in Y15, and Sirius died in Y16. It's important to note that the year so indicated starts on January 1 and ends on December 31 of the year Harry turns that age, so Y12 includes the second half of Harry's first year at Hogwarts and the first half of his second year.
This alternative method of dating events is intended to offer an alternative to those who would rather not tie the stories to any particular years but to read them as happening "now," regardless of when "now" is. It is also intended to aid those researching events who have chosen alternative timelines, using the birth date of Nicolas Flamel, for example, or choosing the best possible fit to the given days and dates.
Of course, when we use generic years, we can't always identify the exact day of the week or month when something happens. For example, we know that Dudley's birthday is a Saturday toward the end of June in Y11. If that year is 1991, we can narrow that down to June 22, but if we don't identify a particular year, we can't be that specific.
For quick reference, here are some well-known events in the stories and when they occur:
We will start including these dates along with the official dates whenever possible.