In American English, a period or duration of time (as a unit) is singular. The 1960s was a decade (note the singular unit of time) of revolution in art and music. If taken as separate individual years, it can be plural: The early 1990s were years (note the plural idea of separate years) of political turmoil in Eastern Europe.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.