Did the new millennium start in 2000 or 2001?

The new millennium officially started on January 1, 2001. This is because the Gregorian calendar does not have a year zero; it counts from 1 BC to AD 1. Therefore, the first millennium spanned from the year 1 to 1000, and the second millennium ran from 1001 to 2000, with the third millennium beginning in 2001.