Concatenating Date Values

Hello!  I'm using the following java script to check the validity of dates that are entered.  I need to concatenate the validated fields: startday, startmonth, startyear, etc. in the start_dt.  The format should be a date startmonth/startday/startyear.  I tried just using cfset start_dt=#startmont# + #startday# + #startyear#, but that gives me an error.  I'll give 500 points and I definite A for answer.

                        <cfform name="Insertform" method="post" action="./report_vacation.cfm">
                          <script language="JavaScript"><!--
function padout(number) { return (number < 10) ? '0' + number : number; }

function y2k(number) { return (number < 1000) ? number + 1900 : number; }

function validate(what) {
    var startday = what.startday.options[what.startday.selectedIndex].value;
    var startmonth = what.startmonth.options[what.startmonth.selectedIndex].value;
    var startyear = what.startyear.options[what.startyear.selectedIndex].text;
    var endday = what.endday.options[what.endday.selectedIndex].value;
    var endmonth = what.endmonth.options[what.endmonth.selectedIndex].value;
    var endyear = what.endyear.options[what.endyear.selectedIndex].text;

    unvalidstartdate = startday + '/' + startmonth + '/' + startyear;
    unvalidenddate = endday + '/' + endmonth + '/' + endyear;

    var startdate = new Date(startyear-0,startmonth-1,startday-0);
    var enddate = new Date(endyear-0,endmonth-1,endday-0);

    var validstartdate = padout(startdate.getDate()) + '/' + padout(startdate.getMonth()+1) + '/' + y2k(startdate.getYear())
    var validenddate = padout(enddate.getDate()) + '/' + padout(enddate.getMonth()+1) + '/' + y2k(enddate.getYear())
   
    if (unvalidstartdate != validstartdate) {
        alert('Start Date: ' + what.startday.options[what.startday.selectedIndex].text +
                         ' ' + what.startmonth.options[what.startmonth.selectedIndex].text +
                         ' ' + what.startyear.options[what.startyear.selectedIndex].text + ' is invalid');
        return false;
    }

    if (unvalidenddate != validenddate) {
        alert('End Date: ' + what.endday.options[what.endday.selectedIndex].text +
                       ' ' + what.endmonth.options[what.endmonth.selectedIndex].text +
                       ' ' + what.endyear.options[what.endyear.selectedIndex].text + ' is invalid');
        return false;
    }

    starttime = Date.UTC(y2k(startdate.getYear()),startdate.getMonth(),startdate.getDate(),0,0,0);
    endtime = Date.UTC(y2k(enddate.getYear()),enddate.getMonth(),enddate.getDate(),0,0,0);
   
    if (starttime < endtime) {
        // valid
    }
    else {
        alert('Start Date is not less than End Date');
        return false
    }
   
    currentdate = new Date();
    currenttime = Date.UTC(y2k(currentdate.getYear()),currentdate.getMonth(),currentdate.getDate(),0,0,0);
   
    if (starttime > currenttime) {
        // valid
    }
    else {
        alert('Start Date cannot be less then todays date');
        return false;
    }
   
    what.startdate.value = validstartdate;
    what.enddate.value = validenddate;
      
    return true;
      
                                                      
}
//--></script>