Date/Time: Simplify `mysql_to_rfc3339()`.

Erasing timezone with a regular expression is redundant, the date could be just formatted in the respective format instead.

Props Rarst.
Fixes #42542.

git-svn-id: https://develop.svn.wordpress.org/trunk@43384 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -5863,10 +5863,7 @@ function wp_post_preview_js() {
  * @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 );
 }
 
 /**