diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index 3b8ed27b20..c94096e4c0 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -5849,15 +5849,18 @@ function wp_post_preview_js() {
 }
 
 /**
- * Parses and formats a MySQL datetime (Y-m-d H:i:s) for ISO8601/RFC3339.
+ * Parses and formats a MySQL datetime (Y-m-d H:i:s) for ISO8601 (Y-m-d\TH:i:s).
  *
  * Explicitly strips timezones, as datetimes are not saved with any timezone
  * information. Including any information on the offset could be misleading.
  *
+ * Despite historical function name, the output does not conform to RFC3339 format,
+ * which must contain timezone.
+ *
  * @since 4.4.0
  *
  * @param string $date_string Date string to parse and format.
- * @return string Date formatted for ISO8601/RFC3339.
+ * @return string Date formatted for ISO8601 without time zone.
  */
 function mysql_to_rfc3339( $date_string ) {
 	$formatted = mysql2date( 'c', $date_string, false );