JavaScript function for getting the last day in month

function getDaysInMonth(aDate){
   // returns the last day of a given month
    var m = new Number(aDate.getMonth());
    var y = new Number(aDate.getYear());

    var tmpDate = new Date(y, m, 28);
    var checkMonth = tmpDate.getMonth();
    var lastDay = 27;

    while(lastDay <= 31){
        temp = tmpDate.setDate(lastDay + 1);
        if(checkMonth != tmpDate.getMonth())
            break;
        lastDay++
    }
    return lastDay;
}