It doesn't make much sense to respond to this header from a dynamic page, such as an FWX script. The purpose of this HTTP header is to save you bandwidth if the requested page has not changed since the last time the user (or in this case Google) asked for it. If the page has not changed since the timestamp included in the If_Modified_Since header, your server is supposed to return an HTTP 304 reply, instead of the whole document. This instructs Google to use its previously downloaded content for that URL.
The only disadvantage of not supporting the If_Modified_Since header is that your page will be re-downloaded whenever Google (or other clients) request it, even if they downloaded it in the past.
Sent by Joe Goldsmith on 04/06/2004 08:55:19 PM:
For page rankings the Google Bot wants if-modified since added to each page. At http://www.google.com/webmasters/guidelines.html it states:
How would I add this to each page using FoxWeb?