$old_date = '2022-01-15 23:59:59';
$date = new DateTime($old_date);
$date->modify('+10 day');
$new_date = $date->format('Y-m-d H:m:s');
print($new_date);
// Выводит 2022-01-25 23:01:59
$timestamp = strtotime($old_date);
$newTimestamp = $timestamp + 60*60*24*10;
$new_date = date('Y-m-d H:i:s',$newTimestamp);