Add x days from input number to calendar date.. and get end date

I had this form fields.. 

One with the date Mar 4, 2021 and I want to add in this case 10 days but I want this be a free input and been able to put any number and at the want automatically get the date, in this case Mar 14, 2021.. and I'm not at all good a JS.. 

So I found a code where I cand add a X number but will be just that number.. how can I change that o get to work this formulas.. 

Attached a picture with forms and code that need to fix/change..

var strStart = this.getField("DateStart").value;
if(strStart.length)
{
var dateStart = util.scand("mmm d, yyyy",strStart);
var oneDay = 24 * 60 * 60 * 1000;
var dueMillis = dateStart.getTime() + 5 * oneDay;   <--- in this line the +5 are the day added, but

var dueDate = new Date(dueMillis);                                    w ant this be an input number..
event.value = util.printd("mmm d, yyyy",dueDate);
}
else
event.value = "NA";