http header (if-modified-since / last-modified) on IIS

It is not returning the Last-Modified data! what to do?

[chinkchink]

I stumbled upon an old thread [http://www.webmasterworld.com/forum23/2433.htm] concerning this topic and used the Server Headers Checker [http://www.webmasterworld.com/stickymail.cgi?action=headers] tool suggested by jdMorgan to see if my site's HTTP header is returning the last modified date correctly... but turns out that it doesn't even include it!

what do I have to do within IIS (or elsewhere, I have admin control of the server) to make sure the last-modified date is included in my HTTP header?

Please Help
Thank you in advance
chinkchink

This is what the checker returned:
>HTTP/1.1 200 OK
>Server: Microsoft-IIS/5.0
>Date: Wed, 25 Feb 2004 22:58:53 GMT
>Connection: Keep-Alive
>Content-Length: 8489
>Content-Type: text/html
>Set-Cookie: ASPSESSIONIDSATRTDSQ=KNIKOHICICENLFIOFEJIMCNH; path=/
>Cache-control: private

[Xoc]

See the code in this post: [webmasterworld.com...]