Сохранение купленной даты и добавление X времени [дубликата]

function calculateWarranty() {
  // get the date text and convert into date Object
  var bought = new Date(document.getElementById('bought').value);
  // get the number of warranty months and convert to integer (second arguments ensures that the integer is base 10)
  // if you do not convert the number, it will do String concatenation while calling dateObj.setMonth(getMonth_int + warrant_string )
  // for example: 2017-10-21 will give bought.getMonth() === 9 + "1", which will equal 91 instead of 10
  var warrantyMonths = parseInt(document.getElementById('warranty').value, 10);

  // display in console for debugging
  console.log(bought);
  console.log(warrantyMonths);

  var boughtMonth = bought.getMonth();

  // add bought month and warranty month (both should be integer as stated earlier)
  var warrantyMonth = boughtMonth + warrantyMonths;

  // set the month to bought object
  bought.setMonth(warrantyMonth);

  // result
  console.log(bought);
}

// adding event listeners on the input boxes
document.querySelector('#bought').addEventListener('change', calculateWarranty);
document.querySelector('#warranty').addEventListener('change', calculateWarranty);

<input id="bought" name="bought" class="textbox-n form-control" type="text" placeholder="Date Bought" onfocus="(this.type='date')" onblur="(this.type='text')" />

<input id="warranty" name="warranty" class="textbox-n form-control" type="number" value="1" />