diff --git a/date_api/date_api_ical.inc b/date_api/date_api_ical.inc
index dc107c8..df009fd 100644
--- a/date_api/date_api_ical.inc
+++ b/date_api/date_api_ical.inc
@@ -714,7 +714,7 @@ function date_api_ical_build_rrule($form_values) {
     foreach ($form_values['EXDATE'] as $value) {
       if (!empty($value['datetime'])) {
         $date = !is_object($value['datetime']) ? date_ical_date($value, 'UTC') : $value['datetime'];
-        $ex_date = !empty($date) ? $date->format(DATE_FORMAT_ICAL) : '';
+        $ex_date = !empty($date) ? date_format($date, DATE_FORMAT_ICAL) . 'Z' : '';
         if (!empty($ex_date)) {
           $ex_dates[] = $ex_date;
         }
@@ -734,7 +734,7 @@ function date_api_ical_build_rrule($form_values) {
     $ex_dates = array();
     foreach ($form_values['RDATE'] as $value) {
       $date = !is_object($value['datetime']) ? date_ical_date($value, 'UTC') : $value['datetime'];
-      $ex_date = !empty($date) ? $date->format(DATE_FORMAT_ICAL) : '';
+      $ex_date = !empty($date) ? date_format($date, DATE_FORMAT_ICAL) . 'Z' : '';
       if (!empty($ex_date)) {
         $ex_dates[] = $ex_date;
       }