When does the new decade start?

Answer 1

According to the calendar, because there is no "0" year, the new decade just started January 1, 2011.

Common use of the word decade, because we refer to the decade of the 80's or 90's, the new decade starts on January 1 of the decade that is being referred to. This is because it is illogical to say the decade of the 80's doesn't include 1980 but does include 1990. So, if you like the common use of the word decade, instead of the calendar use, the new decade will start on January 1, 2020.