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View source for If-Modified-Since header does work

Just check the results in the Web developer tools -> Network analysis of your browser. There you can see the correct results.

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file:/Forum/Discussion/If-Modified-SinceHeaderDoesNotWorks/403_network_analysis.png?direct
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file:/Forum/Discussion/If-Modified-SinceHeaderDoesNotWorks/403_network_analysis_2.png?direct

Please read the ((source:master/wacko/class/http.php http class)) for more details, see ##function check_http_request()##.

Can't say why the mentioned service don't recognize it.

In the http class there is the comment:
%%if ($_SERVER['REQUEST_METHOD'] == 'GET')  // may be we need HEAD support ??? %%
Do we need ((https://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html HEAD support))?

<[9.4 HEAD

The HEAD method is identical to GET except that the server MUST NOT return a message-body in the response. The metainformation contained in the HTTP headers in response to a HEAD request SHOULD be identical to the information sent in response to a GET request. This method can be used for obtaining metainformation about the entity implied by the request without transferring the entity-body itself. This method is often used for testing hypertext links for validity, accessibility, and recent modification.

The response to a HEAD request MAY be cacheable in the sense that the information contained in the response MAY be used to update a previously cached entity from that resource. If the new field values indicate that the cached entity differs from the current entity (as would be indicated by a change in Content-Length, Content-MD5, ETag or Last-Modified), then the cache MUST treat the cache entry as stale.]> 

Patch for HEAD support: ((revision:441dadd))
%%
diff --git a/wacko/class/http.php b/wacko/class/http.php
--- a/wacko/class/http.php
+++ b/wacko/class/http.php
@@ -191,7 +191,7 @@
 				$lastm = substr($lastm, 0, $p);
 			}
 
-			if ($_SERVER['REQUEST_METHOD'] == 'GET') // may be we need HEAD support ???
+			if ($_SERVER['REQUEST_METHOD'] == 'GET' || $_SERVER['REQUEST_METHOD'] == 'HEAD')
 			{
 				if (!$lastm && !$etag);
 				else if ($lastm && $gmt != $lastm);
%% 

But we can't send body for HEAD requests -> Apache will do all the heavy lifting (stripping the response body).
https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/HEAD