last modified set in header, but not shown in sitemap

[richard.rh]

I've set sitemap to use the server's response for last modification, but the generated sitemap shows the current time (time at generation)
The HTTP header (using the viewer on the sitemap site) for a sample page is:
HTTP/1.1 200 OK
Date: Mon, 16 Mar 2009 17:24:40 GMT
Server: Apache/2.2.3 (CentOS)
X-Powered-By: PHP/5.1.6
Set-Cookie: PHPSESSID=7u5qact2qma55i49s747mqpml4; path=/
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: must-revalidate
Pragma: no-cache
Last-Modified: Fri, 25 Apr 2008 00:00:00 GMT
Connection: close
Content-Type: text/html; charset=UTF-8

Any suggestions?

[XML-Sitemaps Support]

Hello,

What is the URL of the page that returns this header? Does it work correctly with our online sitemap generator?