problem when add days to date javascript code example

Example 1: javascript add days to date

var myCurrentDate=new Date();
var myFutureDate=new Date(myCurrentDate);
    myFutureDate.setDate(myFutureDate.getDate()+ 8);//myFutureDate is now 8 days in the future

Example 2: javascript add days

Date.prototype.addDays = function(days) {
    var date = new Date(this.valueOf());
    date.setDate(date.getDate() + days);
    return date;
}