Add no. of days in a date to get next date(excluding weekends)

Try this

var startDate = "9-DEC-2011";
startDate = new Date(startDate.replace(/-/g, "/"));
var endDate = "", noOfDaysToAdd = 13, count = 0;
while(count < noOfDaysToAdd){
    endDate = new Date(startDate.setDate(startDate.getDate() + 1));
    if(endDate.getDay() != 0 && endDate.getDay() != 6){
       //Date.getDay() gives weekday starting from 0(Sunday) to 6(Saturday)
       count++;
    }
}
alert(endDate);//You can format this date as per your requirement

Working Demo

@ShankarSangoli

Here's a newer version which avoid recreating a Date object on each loop, note that it's wrapped in a function now.

function calcWorkingDays(fromDate, days) {
    var count = 0;
    while (count < days) {
        fromDate.setDate(fromDate.getDate() + 1);
        if (fromDate.getDay() != 0 && fromDate.getDay() != 6) // Skip weekends
            count++;
    }
    return fromDate;
}
alert(calcWorkingDays(new Date("9/DEC/2011"), 13));

Here is an elegant solution without any looping or external library:

function addBusinessDaysToDate(date, days) {
  var day = date.getDay();

  date = new Date(date.getTime());
  date.setDate(date.getDate() + days + (day === 6 ? 2 : +!day) + (Math.floor((days - 1 + (day % 6 || 1)) / 5) * 2));
  return date;
}

var date = "9-DEC-2011";
var newDate = addBusinessDaysToDate(new Date(date.replace(/-/g, "/")), 13);
alert(newDate.toString().replace(/\S+\s(\S+)\s(\d+)\s(\d+)\s.*/, '$2-$1-$3')); // alerts "28-Dec-2011"