@SavoirFaire Well, I think there’s general agreement that the present decade is the 2000 -teens & the following decade will be the 2000 -twenties, and so on. I suppose a real pedant might claim that the period Jan 1, 1900 through Dec 31, 1900 was the beginning of the nineteen hundreds but not yet the start of the twentieth century. It’s an ancient conundrum : do we reference time by counting or measuring?
It also makes sense that (ordinal) century numbers begin with “first.” (Computer programmers, however, may want to call it the zeroeth.) This makes the 1900s the 20thC and the present century 21C. So far so good.
But we expect the beginning of a new decade, century, or millennium to coincide with the 10 – , 100 – , or 1000-year anniversary of the start of the previous period. Thus whether the initial year is labeled 0 or 1 in the system matters very much, because it will propagate forever into the future. In a zero-based system the second decade begins in year 10 (not 11), the second century begins in 100 (not 101) and so on. (If not, why not?)
Buddhist and other alternative calendars are of great cultural interest, but they don’t represent international standards. I hadn’t previously heard of ISO 8601, but if it represents some kind of new consensus among intelligentsia then maybe present new decade did start with 2010.
The late evolutionary biologist Stephen Jay Gould discussed the historical calendar debate in Questioning the Millennium, published in 1997. My copy is buried somewhere…