This is an old one, and derives from numerical confusion. In theory, a decade is any period of 10 years, regardless of start date. It is convenient to label historical periods as such, viz 1820s, 1930s &c, yet Alba is right in saying that the year ending in 1 is the first of the decade and that ending in 0 is the last. This chestnut last had an airing in Y2K which was heralded as the dawn of the third millennium, when in fact it was the last year of the 2nd millennium. The real dawn of the 3rd millennium, Jan 1st 2001, went largely unnoticed.