--- date/date_elements.inc	2010-01-19 19:24:42.000000000 +0100
+++ date/date_elements.inc.new	2010-01-19 19:24:38.000000000 +0100
@@ -519,6 +519,7 @@ function date_combo_validate($element, &
         $errors[] = t('The To date is invalid.');
       }
       if (empty($errors)) {
+        if ($to_date_empty) $item[$to_field] = '';
         form_set_value($element, $item, $form_state);
       }
     }