Actually that doesn’t work for decades. The 20s were 192X, not 1930. If you were to say this is the 200th decade, you might have a point. But ti doesn’t work like centuries, mainly because we are referring to the date when we say “the sixties” and not “counting” them.
You must have 10 years in a decade and there is no year zero. For example, The last year of the 70s was 1980, because we cannot have 197010. Mark Davis got it all wrong.
Actually, it does work that way.
A decade is defined as a 10 year period.
According to your incorrect interpretation, the first decade of this, the 21st Century, would only be 9 years.
That's only 90% of a decade, can't be.