Strange expiry headers detected

hi,

I noticed some weird entries in the header response sent by nginx
server.

I found this in the response header sent by the server:

Last-Modified Thu, 31 Jan 2008 02:21:00 GMT

w.r.t the request header sent by the browser:
If-Modified-Since Thu, 31 Jan 2008 02:21:00 GMT

What I noticed was the Last-Modified time sent in the response header is
same as the If-Modified-Since request header.

Can someone throw some light on it.

If you want I can paste my config file here.

Thanks,
Rakesh.