This article is from the Calendars FAQ, by Claus Tondering email@example.com with numerous contributions by others.
The first millennium started in AD 1, so the millennia are counted in
1st millennium: 1-1000
2nd millennium: 1001-2000
3rd millennium: 2001-3000
Thus, the 3rd millennium and, similarly, the 21st century started on
1 Jan 2001.
This is the cause of some heated debate, especially since some
dictionaries and encyclopaedias say that a century starts in years
that end in 00. Furthermore, the change 1999/2000 is obviously much
more spectacular than the change 2000/2001.
Let me propose a few compromises:
Any 100-year period is a century. Therefore the period from 23 June 2004
to 22 June 2104 is a century. So please feel free to celebrate the
start of a century any day you like!
Although the 20th century started in 1901, the 1900s started in 1900.
Similarly, the 21st century started in 2001, but the 2000s started in