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HowTo: Changing the available display formats for Evernote's "Insert Date" command
I had to restart Evernote for it to recognize the change, but afterward the new formats were available.
Tip of the hat to Martin Maiers in the Evernote forums, who explained this technique: http://discussion.evernote.com/topic/29812-insert-date-formats-please-add-iso-8601/#entry160872