I like CCYY-MM-DD because it's also a valid file name on most systems, and using "CCYY" (century + year) instead of "YYYY" feels fancy.

Except this could get confusing because the year 1976 (for example) is actually in the 20th century.

	▲	accrual 6 days ago \| parent [-]
		That is a good point. The "ordinal" century doesn't exactly line up with the digits in a "YYYY" format, thus "CCYY" creates some ambiguity depending on how one defines "century". I conclude my fanciness of using "CCYY" is not useful. :)