Looby it's all down to the American lack of understanding of the number 0 (zero). The ground floor is floor 0 the 1st floor is the first above the ground, etc etc.
Hence the great fuck up about the Millennium. Way back when they started to make the calenders the number zero was not in use. So in fact there was no year zero 0 (except gor the poor souls who had to suffer the Khmere Rouge in Cambodia) so the first year was 1AD. 1+2000 = 2001 so the 3rd Millennium actually started on January 1st 2001. Of course it was the Stupid Yanks who railroaded the rest of the planet into an early Millennium celebration.

Of course looby you are not reading this tosh are you! You are down at the Seri giving and receiving blow jobs.