About If-Modified-Since

I type in the following in command prompt
telnet www.google.com 80

and type the following as the request html header
GET / HTTP/1.1
Host: www.google.com
If-Modified-Since: Mon, 19 Mar 2007 02:43:31 GMT

and sent, but it always return me http/1.1 200 OK and the entire webpage.  

Is my syntax correct? Can somebody give me an example that will return http/1.1 304 modified with the correct syntax?
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william007Asked:
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JamesCsslCommented:
GET / HTTP/1.1
Host: www.cyberpear.com
If-Modified-Since: Sun, 17 Dec 2006 21:52:08 GMT



You should note that you will only get a 304 response from a page that sends a last-modified header or an ETag header.  Google doesn't do this.  Also, your timestamp has to be the exact same as the last-modified timestamp to receive the 304.

To receive a 304 from an ETag, use the If-None-Match header.
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william007Author Commented:
Thanks, Your example is working great:)

But if I try to send
GET / HTTP/1.1
Host: www.cyberpear.com
If-Modified-Since: Wed, 21 Mar 2007 02:43:31 GMT

Again it gives me http/1.1 200 OK. 21 Mar 2007 is later than 17 Dec 2006, but why doesn't give me the same result?
Also, how do you know what date to put there?

I am trying to write a proxy server that check the pages in its cache whether has been modified, hence I need to understand these things in details.
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william007Author Commented:
Also I discover that, for the images, it will always give me the same result, ie 304 not modified even I put different date, again, what is the reason?

GET http://www.new-dream.de/image/wallpaper/computer/apple/apple_14.jpg HTTP/1.1
Host: www.new-dream.de
If-Modified-Since: Wed, 20 Mar 2007 02:43:31 GMT

GET http://www.new-dream.de/image/wallpaper/computer/apple/apple_14.jpg HTTP/1.1
Host: www.new-dream.de
If-Modified-Since: Wed, 20 Feb 2007 02:43:31 GMT
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JamesCsslCommented:
Some implementations of If-Modified-Since only check to see if the date in the If-Modified-Since header is exactly the same as the file modification date.  The W3C does it correctly (checking if the date is the same or later than the modification date:

GET / HTTP/1.1
Host: www.w3.org
If-Modified-Since: Wed, 21 Mar 2007 18:40:11 GMT
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