diff -Naur standard_moodle/server/moodle/mod/assignment/lang/en/assignment.php my_moodle/server/moodle/mod/assignment/lang/en/assignment.php
--- standard_moodle/server/moodle/mod/assignment/lang/en/assignment.php	2013-01-12 02:48:31.000000000 +0530
+++ my_moodle/server/moodle/mod/assignment/lang/en/assignment.php	2013-04-10 19:11:27.691045455 +0530
@@ -71,6 +71,7 @@
 $string['draft'] = 'Draft';
 $string['due'] = 'Assignment due';
 $string['duedate'] = 'Due date';
+$string['duedateinvalid'] = 'Due date must be after the allow submission date.';
 $string['duedateno'] = 'No due date';
 $string['early'] = '{$a} early';
 $string['editmysubmission'] = 'Edit my submission';
diff -Naur standard_moodle/server/moodle/mod/assignment/mod_form.php my_moodle/server/moodle/mod/assignment/mod_form.php
--- standard_moodle/server/moodle/mod/assignment/mod_form.php	2013-01-12 02:48:31.000000000 +0530
+++ my_moodle/server/moodle/mod/assignment/mod_form.php	2013-04-10 19:12:54.399045479 +0530
@@ -120,6 +120,11 @@
         // Allow plugin assignment types to do any extra validation after the form has been submitted
         $errors = parent::validation($data, $files);
         $errors = array_merge($errors, $this->get_assignment_instance()->form_validation($data, $files));
+if ($data['allowsubmissionsfromdate'] && $data['duedate']) {
+            if ($data['allowsubmissionsfromdate'] > $data['duedate']) {
+                $errors['duedate'] = get_string('duedateidation', 'assignment');
+		print "<script type=\"text/javascript\">"; 
+		print "alert('Due date must be after the allow submission date.')"; 
+		print "</script>";
+            }
+        }
         return $errors;
     }
 }