[petercooper]

It's not even a technicality, really. A decade is any span of ten years, and in modern English we refer to decades by their tens digit. If someone wanted to talk about the 202nd decade or something, then sure, that'd end next year, but instead we merely talk about the 60s, 90s, or whatever. Calendar shifts in the interim even make the "technical" approach a bit of a farce, though.