While millennia and centuries are numbered ordinally (e.g. the 18th century or 2nd millennium), in modern speech decades are not numbered ordinally, they're dec

While millennia and centuries are numbered ordinally (e.g. the 18th century or 2nd millennium), in modern speech decades are not numbered ordinally, they're declared based upon the tens digit (e.g. 60s, 80s, or 90s), so your point is false.