I agree with your maths
The Third Millennium started on 1 January 2001. So did the 21st Century. I get annoyed at people who don't realise this.

So did the 201st decade.

BUT... people very rarely talk about the "Nth decade", but instead refer to decades (groups of ten years) by the first 3 digits (1920s, 1980s etc.).

So, 1 January 2010 is the start of the "Twenty Tens", and 1 January 2011 will be the start of the less famous 202nd decade.
(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, Sat 2 Jan 2010, 16:49, Reply)