Base calculated due date off start date, not current date

This ensures that the due date will be after the start date, in order to
prevent TII returning error 2309.  Fixes issue danmarsden#59

Author: pauln

lib.php

@@ -2188,7 +2188,7 @@ function turnitin_get_module_dates($module, $tii, &$timestart) {
     if (!empty($timedue) && ($timedue > strtotime('+10 minutes')) && $timedue > $timestart) {
         $tii['dtdue'] = rawurlencode(date($turnitindateformat, $timedue));
     } else {
-        $dtdue = strtotime('+6 months');
+        $dtdue = strtotime('+6 months', $timestart); // Needs to be after the start date, which can be in the far future.
         $tii['dtdue'] = rawurlencode(date($turnitindateformat, $dtdue));
     }
     return $tii;