[Zope] Use of last_modified

[Don Porter]

I want to state at the bottom of all pages when the page was last
modified.  So, I put this line in my standard_html_footer in the root
folder:

	<P>Last modified: <dtml-var last_modified>

This results in a Zope Error:
  
        Zope Error

        Zope has encountered an error while publishing this resource. 

        KeyError

        Sorry, a Zope error occurred.

What did I do wrong?  Better yet, how does one go about debugging
such things?  Sorry to ask silly questions, but you've got to start
somewhere.  Searches for "last_modified" in web resources and in the
indices of printed documentation come up empty.

-- 
| Don Porter                                       dgporter@erols.com  |  
|           "Some days you just can't get rid of a bomb!"              |
|                       -- Adam West as BATMAN                         | 
|______________________________________________________________________|