Most of the world may have celebrated the new Millenium last year, but most of the world was 365 days early. The third Millenium actually starts on January 1st 2001. Jeff Detray edits the website astronomy boy.com, which devotes a page to explaining why the new millenium starts when it does. He says the confusion can be traced back to Dionysis, who created the calendar we use beginning with the year one, instead of zero: