I noticed I wasn't seeing a specific feed from the frontend (which used to be there) and after increasing the 'Total items' found that all feeds from this specific source were at the bottom and reporting Dec 31, 1969. After looking at the feed link directly, I noticed there is no <pubDate> on any of the items. The <pubDate> line is totally omitted.
I looked at /components/com_content/views/category/view.feed.php but after comparing the file with a working sites view.feed.php, I found that they were identical.
I realize this indicates a problem with the source feed, which is coming from a Joomla 3.4.8 site. Although I've been searching and am unable to determine why pubDate is missing or how to correct it. Any suggestion would be greatly appreciated.