atom:updated is an objective machine readable date which can be used by an author to signal that the changes they've made to an entry are significant.






Add a new sub-section to draft-ietf-atompub-format-01 in section 5 (atom:entry), near any other 'date' sections.

    === 5.x "atom:updated" Element ===

    The "atom:updated" element is a Date Construct indicating the most
    recent date and time when a change was made to the entry which the
    publisher wishes to bring to the attention of subscribers. Such
    changes will typically not include minor adjustments like spelling
    and grammatical corrections, content reformatting, etc.

    The content of this element MUST conform to the specifications of
    the Date Construct defined in section 3.3.

    atom:entry elements MAY contain exactly one atom:updated element.
    If an Atom entry is published with an atom:updated element, 
    all future instances should also, carrying forward the last value.

    The value of this element SHOULD be the actual date of the
    update, and not some date subjectively chosen by the publisher.



atom:updated is OPTIONAL, with the understanding that subsequent minor modifications SHOULD carry forward the last atom:updated. That is SHOULD, not MUST, because it's not a world-ending error and we need to allow for situations where someone migrates from one CMS which supports atom:updated to another CMS which doesn't. Also, rather difficult to test any given feed instance for conformance to a MUST requirement.

Feed-reading software can use this date field to detect "significant" changes and signal this to the end user (via flags, marked as unread, a sort option, etc)

Format rules have been punted, leaving it to section 3.3 Date Constructs to define. For simplicity's sake it is preferable that each specific date element NOT have differing format rules.