So much can be said about this conversation. But let’s stick to the facts.

The situation is explained well on Wikipedia-
“The third millennium of the Gregorian calendar began on 1 January 2001, rather than the popularly-celebrated 1 January 2000. This is a direct consequence of the absence of a year zero in the anno Domini era. Had there been a year zero, which might be considered part of the first millennium, then 1 January 2000 would indeed mark 2000 years since the year numbering datum and be the start of the third millennium. This also applies to centuries and decades. Thus, the 20th century began on 1 January 1901; and the 21st century began on 1 January 2001.”

… And this decade begins on 1 January 2011.

For those who follow the facts: “Happy New Year”.

For those who follow CNN: “Happy New Decade… Sorry, no, you can’t break my ten if you only have nine dollars”.

Links: Wikipedia- http://en.wikipedia.org/wiki/Year_zero
CNN- http://www.cnn.com/2009/HEALTH/12/29/motivation.new.years.resolutions/