Most people say today marks a new decade. A few say it doesn't. What matters isn't who's right. What matters is that we understand there will always be two ways of counting these things. And that's really no big deal.
As it so happens, this is a perfect example of what we have to do in New Testament Chronology. Ancient methods of counting time could be just as confused. Were Luke, John and Paul counting with strict chronological reckoning or inclusively, by years? Were they referring to Roman or Hebrew calendar conventions? For that matter, were they always consistent? Naturally, we ask these same questions about Josephus, Dio, Tacitus, etc.
Mathematically, decades should end with years that have zeros on the end. Culturally and linguistically, it makes much more sense to discus the 80's, the 90's, the 00's, etc. Technically, by mathematical standards, the decade isn't over yet. Technically, by other standards, it very much is. Normal folks will take the convenient method. Educated folks know both methods, and that should be enough. Base-Ten numeric counting is arbitrary anyway, and terminology is merely what we make it.
All that's left to say is that - whichever counting method you prefer - don't get yourself all in a tizzy about it. Please.