Caculator date time on wordpress

$date_time = date_i18n( 'Y-m-d H:i:s' );
$date = date_i18n( 'Y-m-d' );
$diff = strtotime($date_time) - strtotime($date);

var_dump($diff);

$date_expiry = '2022-07-30';
$date_time_ex = date('Y-m-d H:i:s', strtotime('+' . $diff . 'second', strtotime($date_expiry)) );

echo '<br>';
echo $date_time_ex;