... NEVER. Just acknowledging that, for a term of years unrelated to centuries or millenniums, there is a generally accepted convention that makes sense. If the issue was to identify the last decade of the 20th century, then that awkward "1991-2000" would apply.

The decade of "the '90's", however, is one generally accepted way to refer to a ten year period of time that some people find significant. (Overly significant in some respects.)

Anyway, here's another one I like. ----- jim o\-S