--- a/date/date_elements.inc
+++ b/date/date_elements.inc
@@ -515,6 +515,9 @@
         $errors[] = t('The To date is invalid.');
       }
       if (empty($errors)) {
+        if ($to_date_empty) {
+          $item[$to_field] = null;
+        }
         form_set_value($element, $item, $form_state);
       }
     }