add days to a specific date with java script

Separate the problem into 2 functions, 1 call parseDate through which you pass your date with the format you indicate and it returns an object instance of type Date valid, and the second sumDays is responsible for add n days to a date that you pass as the first parameter. Since your format is day-month-year and to parse it correctly to Date it must be at least one of the variants month / day / year, I convert the entry so it has that format and then pause the date.

var parseDate = function(date){
  var parts = date.split('-');
  var temp = parts[0];
  parts[0] = parts[1];
  parts[1] = temp;
  return new Date(Date.parse(parts.join('/')));  
}

var sumDays = function(date, days){
  var fdate = parseDate(date);
  fdate.setDate(fdate.getDate()+days);
  return fdate;
}

And to prove it really works:

sumDays("15-02-1981", 3);
//Te muestra en la consola
//Date 1981-02-18T05:00:00.000Z

Y:

sumDays("15-02-1981", 20);
//Date 1981-03-07T05:00:00.000Z

To add days you can use this method:

dias = 3;
fecha = new Date(); // ó new Date(valor);
fecha.setDate(fecha.getDate() + dias);

Edit: If you need to enter the date in a specific format, you can use the library moment.js

dias = 3;
fecha = '15-02-1981'; // string
fechaMoment = moment(fecha,'DD-MM-YYYY').add(3, 'day');

Then you can transform the moment object to a Date object:

fechaMoment.toDate(); // Wed Feb 18 1981 ...

Or print it with format:

fechaMoment.format('DD-MM-YYYY'); // 18-02-1981

You can read more about how to manipulate dates with momentum in your documentation .