A decade can be any range of ten years. By common modern convention, we tend to refer to them as things like 80s, 90s, etc. No-one would argue that 1990 was in

A decade can be any range of ten years. By common modern convention, we tend to refer to them as things like 80s, 90s, etc. No-one would argue that 1990 was in the "80s". Wikipedia has a bit more on this convention: https://en.wikipedia.org/wiki/Decade https://en.wikipedia.org/wiki/Decade