codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>Pagina senza titolo</title>

    <script language="javascript" type="text/javascript">
// <!CDATA[
function trova()
{
    var riga = document.getElementById("terzo3");
    if(!riga) return;
    var celle = riga.cells;
    if(celle && celle.length && celle.length <=3)
    {
        var valore = celle[2].innerHTML;
        alert(valore);
    }
    
    

}
// ]]>
    </script>

</head>
<body>
    <table id="tabella1" border="1">
        <tr id="primo1">
            <td>Maggio</td>
            <td>25</td>
            <td>06:05</td>
        </tr>
        <tr id="secondo2">
            <td>Maggio</td>
            <td>25</td>
            <td>06:20</td>
        </tr>
        <tr id="terzo3">
            <td>Maggio</td>
            <td>25</td>
            <td>07:10</td>
        </tr>
        <tr id="quarto4">
            <td>Maggio</td>
            <td>25</td>
            <td>07:25</td>
        </tr>
    </table>
    

    trova
</body>
</html>