$_SERVER['REQUEST_TIME']), but knows nothing about the time of submission. How could it be calculated precisely?
So as suggested, you'd want to add the timestamp to your XHR request as an additional URL parameter:
"?sent_at=" + (new Date()).getTime()
$date = intVal($_REQUEST['sent_at']) / 1000;
If you really want millisecond precision, then you can use the submitted value as is - depends what you want to do with it.
Hope this helps!