Historically and theoretically speaking, a decade, a century, or a millennium starts when the clock turns midnight on January 1 of the year one of its decimal order. For example, the 20th century started on January 1, 1901, not on January 1, 1900. The reason for this is simple: there is no year 0 in our modern calendar, so these periods of time start on year 1 (using the Gregorian calendar). But that’s not how we refer to time periods colloquially and culturally; for example, when we mention ’80s music or movies, we’re talking about the period from 1980 to 1989.
