For everybody, the 70s ran from 1970 to 1979, and the 1900s ran from 1900 to 1999. But the 198th decade ran from 1971 to 1980, and the twentieth century from 1901 to 2000.
(Except, of course, that when one refers to a decade, one is often referring to a cultural decade. The most extreme instance would be the classic line from How I Met Your Mother, “The 80s didn’t get to Canada until about 1993.”
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John “Let’s go to the mall, everybody!” Kennedy