Implementing If-Modified-Since HTTP query

Ive just re-designed and optimised a couple of sites, but was wanting to know if anybody uses the “If-Modified-Since HTTP” and if so , how did you go about implementing the code into your file ?

All the best

Ben

You should let the web server handle that. If the server is set up correctly it should do so.
In any case, what you are asking is not possible with HTML itself.