Kenneth G. Wilson (1923). The Columbia Guide to Standard American English. 1993.
decade (n.)
A decade is technically a ten-year period covering years numbered 1 through 10, such as 19511960, but in general use it is more often the ten-year period from 0 through 9, as in 19501959. In that use a decade is referred to as the 50s or the 1950s, sometimes written out as the ()fifties. See also DATES.