Well, there are many opinions on this issue so it could become an interesting discussion. :)
The ISO-8601 date format is very useful when ease of sorting is needed. For example, I often append it to the file name to indicate an older version.
The problem with any all-numeric date format is potential day-month ambiguity. Even ISO can be read wrong by non-computer people. My parents, for example, would likely guess it wrong since they are used to mm/dd/yy from U.S. friends and dd-mm-yy (with dashes or periods) from overseas friends.
I don't know the origin of the format [d]d-Mmm-yyyy, but it was the date standard for DEC's VMS in the 1990s. Because I started Metamath development on this system, I chose that format for set.mm
and the date functions in the metamath program.
A 3-letter month abbreviation does have the disadvantage of being specific to the English language. However, the abbreviation is nearly identical in most Romance and Germanic languages, as well as Esperanto, so it should be recognizable to almost anyone with minimal exposure to one of those languages.
From my personal perspective, even though the ISO format is completely unambiguous and I know what it means, there there still is that ever-so-slight moment of distraction to register that 2000-01-02 is January and not February. Basically I see ISO as computer friendly and an alphabetic month as human friendly.
The dates in almost all legal documents have the month spelled out to eliminate potential ambiguity. Strunk and White's Elements of Style says, "6 April 1958...is an excellent way to write a date; the figures are separated by a word and are, for that reason, quickly grasped."
As for 2-digit years, the only place they occur in set.mm
is in the "Recent label changes" list, in order to provide two extra columns on the line. As its name suggests, it is primarily intended to identify "recent" changes to help people update their local work. In 100 years, ambiguity can be resolved by looking at the date tag in a referenced theorem.