Use Atom
Andreas, use atom rather than RSS. It has a <updated/> element for the last time the entry was updated and a <published/> element for the date the entry was published.
If you want to stick with RSS, you can use the <dcterms:issued/> element for the initial published date and one of <pubdate/>, <dc:date/> or <dcterms:modified/>. Don't forget to include the xml namespace for dc and dcterms.



