Last-modified date not set in server header, xbithack ?

Does anyone know how to do set the last-modified date in the server header for word press 2.01? I used a server header checker tool on my blog but the last-modified datetime was empty.

My site uses server side includes and php. My files are .shtml files and I had to use xbithack and mod_rewrite to get the Last-modified date to be set on them.

thank you very much.