jQuery addClass() to another class

Question

I find a class (used for a multiple td) and I would like to add at all td with this class, another class.

The class for every td is $day-momento-$x ($day is the number of today, $x can be 1, 2 or 3, because I've 3 columns).

This is my javascript code

var ora = new Date();
oo = ora.getHours();
giorno = ora.getDate();

//console.log('oo: '+ oo + ', giorno: ' + giorno);

        if (oo >= 4 && oo < 12) {
            $('.'+giorno+'-giornata-1').addClass('online');

        } else if (oo >= 12 && oo < 20) {
            $('.'+giorno+'-giornata-2').addClass('online');

        } else if (oo >= 20 && oo < 4) {
            $('.'+giorno+'-giornata-3').addClass('online');

        }

php/html code

at top I've connection with DB, and $i from of ajax code

$i = $_POST['contatore_giorni'];

    $numeroGiorno = date("j", strtotime("+".$i." days"));

    $contatore = 0;
        for ($m_p_s = 1; $m_p_s < 4; $m_p_s++) {
            $stampa_punti .= '<td class="'.$numeroGiorno.'-giornata-'.$m_p_s.'">';
        [.....]

Answer 1

The last if says: if number is bigger than 20, but smaller than 4 (this will never happen...)