
It's easy! - count from the beginning:
As I write this, the year is 1999. These years are counted from 1 AD
(otherwise known as 1 CE; the year before that was 1 BC, or 1 BCE.)
To complete the 2nd millennium, we have to complete 2000 years: 1, 2, ... , 2000.
The 2nd millennium therefore ends on Dec 31, 2000, and the 3rd millennium begins on Jan 1, 2001.
Last updated: 04 January 2005