This is just my understanding but I believe it let’s google know when the post was last updated. The date of the post will begin to appear in the excerpts on the search engine. Here are a couple links about it but all the fixes only work with other themes:
http://technotes.khitrenovich.com/fix-missing-required-fields-warnings-rich-snippets-testing-tool/
http://www.ilmigo.com/fix-hfeed-warning-field-set-hatomentry.html
Try running another site with this theme through the tool:
http://www.google.com/webmasters/tools/richsnippets
