in an org file, with no other "#+" lines set.
In my configuration file, I have set
"<p>By <a href="" href="https://pavpanchekha.com">https://pavpanchekha.com' rel='author'>%a</a> on <time>%d</time>.\n")))
(setf org-html-metadata-timestamp-format "%d %B %Y")
When I publish to HTML, the date is printed directly as "<2016-10-20 Thu>".
I am using Emacs 25.1.1 and Org 9.0.1.
I have looked through the code, and think the error is likely on line 1882 of ox-html.el, where the date format ought to be passed to org-export-get-date as the optional second argument.
As a workaround, I am currently setting org-export-date-timestamp-format, but it seems like this ought to be HTML-specific.