Calendar alterations are irrelevant
The Romans used to start the year on the first (well, kalends if we're being picky) of March, then changed their minds later. We used to start it on 25 March (Lady Day - nine months before the supposed birth of Christ), which is why the tax year still starts on 6 April (when we switched to the Julian calendar, the tax man didn't want to lose 13 days' revenue).

However, a millennium is a thousand complete years, no matter how short or long some of those years happen to be, and since that monk bloke who came up with the current system decided it would start in AD 1 (the first year of Our Lord), the current millennium therefore began in 2001.

The "Noughties" were still 2000-2009 though. :o)
(Vice Admiral Sir Charles Cockbrush KCB MBE DSO, who is finding it increasingly difficult to log into b3ta.com wants to duck your aunt and kick your puppy, Sun 3 Jan 2010, 9:11, Reply)