If the times in the Date are not needed, then you can simply use the date object's methods to extract the Month,Year and Day and add "n" number of days to the Day part.
var n=5; //number of days to add.
var today=new Date(); //Today's Date
var requiredDate=new Date(today.getFullYear(),today.getMonth(),today.getDate()+n)
Ref:Mozilla Javascript GetDate
Edit: Ref: Mozilla JavaScript Date