[.htaccess] - Last Modified & If-modified-since

Hi all,
i'm having trouble in making the server to give out the Last-Modified and
If-Modified-Since inquiry.
I need to do it only by the server (php script isn't what i need) and the
only way i can access to the server config is by .htaccess.
I found that is possible to use Header set Last-Modified "something" or
Header set If-Modified-Since "something" to solve my problem, but i haven't
found nothing about it on net.
If somebody can help or give me an hint i would really apprecciate it.
Thanks all.