Index: wp-includes/functions.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- wp-includes/functions.php	(date 1505860031000)
+++ wp-includes/functions.php	(revision )
@@ -5482,21 +5482,22 @@
 }
 
 /**
- * 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 output does not conform to RFC3339 format,
+ * which must contain time zone.
+ *
  * @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 );
 
-	// Strip timezone information
-	return preg_replace( '/(?:Z|[+-]\d{2}(?::\d{2})?)$/', '', $formatted );
+	return mysql2date( 'Y-m-d\TH:i:s', $date_string, false );
 }
 
 /**