1 A.D. is part of the First Century, which is 1 A.D. to 10 A.D. This being the 21st Century (2001 to 2100) this is the last year of the first decade. Decade is derived from the Latin decas meaning ten. Other words for spans of years also come from Latin: lustrum (5 years), century (100 years), millennium (1000 years).
Since the common calendar starts from the year 1, its first full decade contained the years from 1 to 10, the second decade from 11 to 20, and so on. The interval from the year 2001 to 2010 could then be called the 201st decade, or 21st Century, using ordinal numbers.
Elegantly stated. Love your tagline too:)