Explained: Millenniums, Centuries, Decades, & Decennaries

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.
   
Examples:

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.
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 And for you Year 0 Fairy Tale Believers: https://www.hermetic.ch/cal_stud/newmill.htm