diff --git a/includes/common.inc b/includes/common.inc
index 5ecdcddf6c4..a8964a630ec 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -2279,6 +2279,22 @@ function format_date($timestamp, $type = 'medium', $format = '', $timezone = NUL
   return preg_replace_callback('/\xEF([AaeDlMTF]?)(.*?)\xFF/', '_format_date_callback', $format);
 }
 
+/**
+ * Returns an ISO8601 formatted date based on the given date.
+ *
+ * Can be used as a callback for RDF mappings.
+ *
+ * @param $date
+ *   A UNIX timestamp.
+ * @return string
+ *   An ISO8601 formatted date.
+ */
+function date_iso8601($date) {
+  // The DATE_ISO8601 constant cannot be used here because it does not match
+  // date('c') and produces invalid RDF markup.
+  return date('c', $date);
+}
+
 /**
  * Callback function for preg_replace_callback().
  */