title: Using client generated time in the "If-Modified-Since" HTTP header
body: <p>We are using the "Last-Modified" response header from the server and the "If-Modified-Since" request header for the resource validation. HTTP header definitions states that the "If-Modified-Since" SHOULD contain value received from the server (it does not prohibit usage of another value).
  At some point we started using a "last updated time" as value for the "If-Modified-Since" sent from the client. The "last updated time" is the time when client last time received an update from server (either new version of the resource or 304 error). We were told that we must not use any client generated time because of possible time synchronization issues.</p>
  
  <p>Of course all times are expressed in GMT.</p>
  
  <p>I could not find confirmation that this is a requirement. It would be nice to hear espert's opinion on it is a MUST to use a value returned from server as a value for the "If-Modified-Since" or there is some degree of flexibility? In what circumstances it is possible to use client generated time?</p>
  
  <p>Thanks a lot</p>