Base calculated due date off start date, not current date

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;