> It would be crazy for Google to treat that as authorship date, and I cannot believe that they do.

I'm not sure what Google uses for authorship date, but if you do date-range based web searches, the actual dates of the content no longer have any meaningful relationship to what was set in the earch criteria (news seems mostly better but with some problems, but actual web search is hopeless). In both directions -- searching for recent stuff gets plenty of very old stuff mixed in, but searching for stuff from a period well in the past gets lots of stuff from yesterday, too.

On platforms like Wordpress these headers are settable via SEO plugins. Many sites will roll these headers forward.