def setLastModified(when):
    """
        Set the C{Last-Modified} time for the response to this request.

        If I am called more than once, I ignore attempts to set Last-Modified
        earlier, only replacing the Last-Modified time if it is to a later
        value.

        If I am a conditional request, I may modify my response code to
        L{NOT_MODIFIED<http.NOT_MODIFIED>} if appropriate for the time given.

        @param when: The last time the resource being returned was modified, in
            seconds since the epoch.
        @type when: L{int}, L{long} or L{float}

        @return: If I am a C{If-Modified-Since} conditional request and the time
            given is not newer than the condition, I return
            L{CACHED<http.CACHED>} to indicate that you should write no body.
            Otherwise, I return a false value.
        """
