The Atom feed currently displays all dates in atom:updated elements. Instead, I propose to use atom:updated for the date of changes to an entity and atom:published for the date on which an entity was published (in line with their definitions in 4.2.9 and 4.2.15 of RFC4287). My interpretation is as follows:

For posts: Publish date is post_time, updated date is post_edit_time, if any.
For topics: Publish date is topic_time, updated date is topic_last_post_time, if any.
For forums: No Publish date, updated date is forum_last_post_time, if any.