--- sites/all/modules/date/date/date_elements.inc	2009-07-27 10:31:29.000000000 -0400
+++ sites/all/modules/date/date/date_elementsNew.inc	2009-07-27 10:31:11.000000000 -0400
@@ -503,7 +503,14 @@ function date_combo_validate($element, &
       date_timezone_set($from_date, timezone_open($timezone_db));
       date_timezone_set($to_date, timezone_open($timezone_db));
       $item[$from_field] = date_format($from_date, date_type_format($field['type']));
-      $item[$to_field] = date_format($to_date, date_type_format($field['type']));
+      
+      // If to and from dates are equal, then $to_date has been set by system and is optional.
+      if ($from_date != $to_date) {
+        $item[$to_field] = date_format($to_date, date_type_format($field['type']));
+      } else {
+        $item[$to_field] = NULL;
+      }
+      
       if (isset($form_values[$field_name]['rrule'])) {
         $item['rrule'] = $form_values[$field['field_name']]['rrule'];
       }