A thought experiment: as far as I am concerned, the statement "ISO 8601 mandates that you write dates as YYYY-MM-DD" is nothing more than a hearsay 
I haven't read the document, I don't know of anyone I trust who have, and so from my perspective it is practically just a 62890-hand information which may or may not be true.
(YYYY-MM-DD is still a useful format regardless of whether it's actually an International Standard™ or not, though)
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ISO 8601 also defines lots of totally crazy formats (for weeks, seasons, quarters, ...). RFC 3339 defines what, I think, you want.
