Add / subtract number of months from given date (original date in param stays immutable).

add_months.js

function addMonths(date, months) {
  var d = new Date(date.getTime());
  var m = d.getMonth();
  var y = d.getFullYear();
  var ym = y * 12 + m;
  ym = ym + months;
  m = ym % 12;
  y = (ym - m) / 12;
  d.setYear(y);
  d.setMonth(m);
  return d;
}