

Year is the most significant (read: big) unit in the list, but it is the least significant (pertinent to daily life) unless you’re a time traveler. Of month and day, month is more significant than day in both size and pertinence, so it gets ordered first. But when sorting things into folders or file naming conventions, biggest category descending down to smaller categories is always the best.
After reading the lack of consensus in the comments, I’ll just be over here using decimal time, confusing everyone around me. ;)