Dates are surprisingly tricky. First, there's the definition of the date (in this case, the date the content was first published to site visitors) in what may be a variety of dates per content item. Then there's the fact that the systems may or may not display the content in a useful way. If the date is directly available (such as from a CMS or exposed in the HTML) then this is easy. In complex situations you may have to rely on a variety of mechanisms (if the straightforward source has a date, use that, but if that isn't available for a particular piece of content then use another means such as scraping).
Content Chimera automatically pulls the date field from metadata, but this date is often not useful for content analysis since it may actually be the last time the system wrote the physical file (in which case it is not a meaningful publish date). If that's the case, then Chimera can import dates from CMS exports or via scraping (either text that is visible to users or that is in some other metatadata or HTML fields).