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There’s a good reason for Atom’s having atom:published as optional but atom:updated as required (which is what I assume Gavin means by the somewhat opaque phrase “encourage[s] the use of last modified”): the date an entry was updated is more useful to sort on than the date it was published.
(From what I can remember, and a quick look through the archive and wiki, dates in Atom were a complex debate that went on for months. At one point there were proposals on the table for up to five different dates associated with an entry; in the end it was decided to keep Atom slim, and allow extensions to carry the weight of further requirements.)
While we’re here, Dublin Core has exactly the right term to cover the date of the subject of an article (eg: ‘some time in 1891’ according to the Flickr page for the photo above). Coverage is “the spatial or temporal topic of the resource”, and can be expressed as a named period, date or date range. For machine readability, ISO 8601:1988(E) is a good choice (you could use one of its profiles, such as RFC 3339, or W3CDTF as mentioned at the start of the article; 8601:1988(E) has the advantage of supporting start-end date pairs and durations as well as single instants in time).