Let's all remember that our calendar conventions are arbitrary. The Millenium commemorates absolutely nothing. Every few hundred years a monk, or the Pope, or, God forbid (if you'll pardon the expression) a scientist, discovered an error, or at least a lack of precision, in the previous formula, and promulgated a revision. In the eighteenth century people rioted because they believed they were going to lose 10 days out of their lives due to a calendar revision, just in case anyone believes that this subject has anything to do with a rational approach.

The current calendar (the Gregorian Calendar, subsequently revised by UTC leap seconds and other adjustments) is dated from the putative birth of Christ, but most current-day scientists and historians believe that that event happened three or four years later. Therefore it's completely absurd to argue about which year is the start of the "Millenium". Most people will find it convenient and comfortable to commemorate the onset of having to write dates beginning with "20-", rather than "19-", and I will be among them, to the appallingly minimal extent that I celebrate at all. If someone else wants to celebrate another, more mathematically rigorous date, a year later, they are perfectly free to do so, and as an added benefit their catering bill will be much lower.