It still takes its cue from the alleged year of the birth of Jesus of Nazareth, as it is merely the (late) Late Antique and Medieval West Roman BC/AD system papered over with non-Christian language.Even the East Roman/Byzantine and Slavic Christians had their own calendar, and did not employ the BC/AD system of dating, but a calendar similar in spirit to the Jewish one, based on the presumed age of the world (in light of this it is amusing ever to see a contemporary Orthodox Christian griping about the replacement of AD with CE).
Given the fusion of Romanity and Christianity, it makes sense that the public calendar would take its cue from Jesus of Nazareth, hailed as victorious on the cross over evil, sin and death as emperors were over their enemies, and as having acceded to the throne of of the Father as the emperors acceded to the imperial throne. It may be that dating conventions are like constellations — time, perhaps, can be cut up in numberless ways, just as one might connect the dots between stars in numberless ways to form numberless images.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.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.At Nearly Headless Nick's deathday party in that book, his death is stated to have been on 31 October, 1492.Since the celebration was commemorating the five-hundredth anniversary of Nick's death, this seems to say the scene takes place on 31 October, 1992. Rowling confirmed, via her Twitter account that that was the day the Epilogue was set.