RFC 4287 4.1.2: The "updated" element is mandatory, while the "published"
element is optional.

RFC 4287 3.3: "A Date construct...MUST conform to the "date-time"
production in [RFC3339]." Specifically, the Django date filter "O" format
yields a timezone offset without a colon, while the "c" format yields a
fully-conformant timestamp.

RFC 4287 4.2.6: "The "atom:id" element...MUST be an IRI, as defined by
[RFC3987]." Prepending the "urn:uid:" scheme to the naked database ID
seems to be sufficient to satisfy the validator.