Pokud to chcete udělat v PHP, mělo by pomoci toto:
$datetime = '2010-02-11 12:00:00';
$days = '5';
$hours = '3';
$new_datetime = date('Y-m-d H:i:s', strtotime("+$days days $hours hours", strtotime($datetime)));
echo $new_datetime; # Will output '2010-02-16 15:00:00'