Nginx Equivalent to LAST_MODIFIED variable in Apache SSI

[marlowe at antagonism.org]

I am working on transitioning my web sites from Apache to Nginx.  In doing
so, I came across an issue.  Under Apache, I was able to include the
timestamp when a page was last modified through the use of the following
statement,

This document last modified <!--#echo var="LAST_MODIFIED" -->

When I made the conversion to Nginx, this statement returns the following

This document last modified (none)

This was expected as the SSI module documentation states the variable will
display "none" if no value is set.  Reading the documentation, it doesn't
appear there is a builtin variable which is the equivalent of Apache's
LAST_MODIFIED.  I did a google search to attempt to find a solution to
generate the timestamp into a new variable.  Unfortunately the only item I
found was this thread,

http://article.gmane.org/gmane.comp.web.nginx.english/3336/match=last+modified+ssi

Can someone please point me in the proper direction to create an
equivalent to Apache's LAST_MODIFIED variable?

Thanks,
Patrick