Calculating Dates

I need to calculate a new date based off another date and exclude weekends.

For example I have a field called "Date Position Closed". If I enter a date of 5/20/2016 it should populate another field called "Target Date QRB" with 9/27/2016. Which is 90 business days after the initial date.

So far this is what I have, but it calculates based off all calendar days:

var strStart = this.getField("DatePositionClosed").value;

if(strStart.length)

{

  var DatePositionClosed = util.scand("m/dd/yyyy",strStart);

  var oneDay = 24 * 60 * 60 * 1000;

  var dueMillis = DatePositionClosed.getTime() + 130 * oneDay;

  var dueDate = new Date(dueMillis);

  event.value = util.printd("m/dd/yyyy",dueDate);

}

else

  event.value = "NA";