The first millennium started in year one, not year zero. It ened at the close of year 1000. Therefore the third millennium started on Jan 1st 2001 and the first century of that millennium started on the same date. It follows that the decades are in sync with the centuries. Our society however likes to use the terms "the tens, the twenties, the thirties to designate ten year periods. But society has been wrong before. There was a time when it was obvious to everyone that the sun revolved around the earth.