diff --git a/date_api/date_api.module b/date_api/date_api.module
index 160bc9f..b506faa 100644
--- a/date_api/date_api.module
+++ b/date_api/date_api.module
@@ -1706,9 +1706,14 @@ function date_timezone_abbr($refresh = FALSE) {
  * post-2038 dates and accept a date object instead of a timestamp as input.
  * Translates formatted date results, unlike PHP function date_format().
  * Should only be used for display, not input, because it can't be parsed.
- *
- * @param object $date
- *   A date object.
+ * post-2038 dates, and accept a \DateObject (from date_api.module) instead of
+ * a timestamp as input. Translates formatted date results, unlike PHP function
+ * date_format(). Should only be used for display, not input, because it can't
+ * be parsed.
+ *
+ * @param \DateObject $date
+ *   The date object to format. Note this cannot be an instance of the PHP
+ *   \DateTime class: you must use date_api's wrapper class, \DateObject.
  * @param string $type
  *   (optional) The date format to use. Can be 'small', 'medium' or 'large' for
  *   the preconfigured date formats. If 'custom' is specified, then $format is
@@ -1725,7 +1730,7 @@ function date_timezone_abbr($refresh = FALSE) {
  *
  * @see format_date()
  */
-function date_format_date($date, $type = 'medium', $format = '', $langcode = NULL) {
+function date_format_date(DateObject $date, $type = 'medium', $format = '', $langcode = NULL) {
   if (empty($date)) {
     return '';
   }