I'm sorry, let me give a full bug report:

I have

#+TITLE: title here
>
#+DATE: <2016-10-20>
>

in an org file, with no other "#+" lines set.

In my configuration file, I have set

(setf org-html-postamble-format
>       (list
>        (list
>         "en"
>          "<p>By <a href='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.

—Pavel Panchekha

On Sat, Nov 19, 2016 at 1:33 AM, Nicolas Goaziou <mail@nicolasgoaziou.fr>
wrote:

> Hello,
>
> Pavel Panchekha <me@pavpanchekha.com> writes:
>
> > I've been trying to set the date format in the HTML postamble. It looks
> > like a date given in #+DATE is not being formatted according to the date
> > format specification, but if no #+DATE is given, the format specification
> > is used.
> >
> > Is this intended behavior—should I write formatted dates in #+DATE?
>
> We need more information to answer this.
>
> What exactly did you try (configured variables, #+DATE keyword ...)?
> What Org version are you using? What did you expect?
>
> If you have some spare time,
> <http://www.chiark.greenend.org.uk/~sgtatham/bugs.html> may be a good
> read.
>
> Regards,
>
> --
> Nicolas Goaziou
>