How to publish the literal "<<" or ">>" with planner-calendar-prev-date-href-from-page?

Hi there,

I want use "<<" and ">>" as "prev" and "next" link in my planner
published pages.

The code in my header.html is:
<lisp>
  ;; insert pre day link when in Planner mode
  (let ((page (planner-page-name)))
   (when (and page (string-match planner-date-regexp page))
    (planner-calendar-prev-date-href-from-page "<<")))
</lisp>
<lisp>(muse-wiki-publish-pretty-title)</lisp>
<lisp>
  ;; insert next day link when in Planner mode
  (let ((page (planner-page-name)))
   (when (and page (string-match planner-date-regexp page))
    (planner-calendar-next-date-href-from-page ">>")))
</lisp>	

But after it is published, "<<" is converted to html-encoded
"&lt;&lt;". What should I do to let muse publish it as original "<<"?

Thanks,
Zhao Lei