To see when the new millennium actually starts, go to my post of 1 January 2001!
Much of the world celebrates the new Millennium today, but it will actually happen in a year's time.
The year 2000 is the last year of the XX century, not the first year of the XXI, and thus also the last year of the 2nd millennium AD...
This is so, even though it did not have to be, and in fact it is the result of a mistake by medieval monks Exiguus and Bede who established the counting method for our calendar but did not have the number "0" at their disposal: they were still using zero-less Roman numerals.
It was a mistake because they skipped a year between year 1 B.C. and year 1 A.D. As things stand now, you only have 19 years between 10 B.C. and 10 A.D., but you have 20 years between, say, 790 A.D. and 810 A.D., or 1990 and 2010.
But we are stuck with that now, and short of renumbering all of the last 2000 years (astronomer Cassini tried but did not get any traction) every century and millenium in the future will end with a 0 and begin with a 1.
PS: As I retroactively post this message, Google Blogger does not accept 00:00 as a time to indicate midnight! It wanted 12:00!!
Ref: "Millennium Hairsplitters Won't Give Zero a Break" (New York Times, Meanwhile, Nov. 16) by Robert Kaplan and Dick Teresi