[рони]

ArtyomZaitsev,

function getLastDayOfMonth(year, month){
        var date = new Date (year, month);
        var dateM=date.getMonth();
        var i = 28;
        while (dateM==month){
            date.setDate(++i);
            dateM=date.getMonth();
        }
        return --i;
    };

    alert(getLastDayOfMonth(2012, 1));