If you can count to ten, you can get this stuff.
A Millennium is a 1000 year period.
A Century is a 100 year period.
A Decade is a specific 10 year period of a Century; there are 10 Decades in a Century.
A Decennary is a 10 year period, any 10 year period.
1980-1989 is a Decennary, but not a Decade. The "Eighties."
1981-1990 is a Decennary, as well as a Decade. The ninth Decade of the 20th Century
and the 99th Decade of the 2nd Millennium.
1990-1999 is a Decennary, but not a Decade. The "Nineties."
1991-2000 is a Decennary, as well as a Decade. The tenth and last Decade of the 20th
Century and the 100th and last Decade of the 2nd Millennium.
2000-2009 is a Decennary, but not a Decade. It and eight other Decennaries span the
20th-21st Centuries and the 2nd-3rd Millenniums.
2001-2010 is a Decennary, as well as a Decade. The first Decade of the 21st Century
and the first Decade of the 3rd Millennium.
And for you Year 0 Fairy Tale Believers: https://www.hermetic.ch/cal_stud/newmill.htm