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.