A decade is 10 years.
The first decade of the 21st century was January 1, 2000 through Dec. 31, 2009.
The second decade will be Jan. 1, 2010 through Dec. 31, 2019, and so on.
If you consider the year 2010 to be part of the first decade, that would make the "decade" 11 years long (Jan. 1, 2000 through Dec. 31, 2010). And a decade isn't 11 years long, regardless of when (or if) Jesus was born!