Perhaps my memory fails me, but when I began my engineering studies in 1975, we were issued an SI document which had this standard.

Anyway, it's easier to file and search starting with the broadest definition and narrowing it down from there, so I have, since 1975.09.??, used the ymd convention. Of course, I don't care what anyone else uses - makes no nevermind to me. Just don't call me late for dinner.