document.addEventListener("DOMContentLoaded", function(event) {
  var input1 = document.querySelector("#a");
  var input2 = document.querySelector(".b");
  
  input1.addEventListener("input", function(e) {
  document.querySelector(".result").innerHTML = "";
  if(parseFloat(input1.value)) {
      document.querySelector(".result").innerHTML = parseFloat(input1.value) + parseFloat(input2.value);
  }
  });                         
});

$start    = (new DateTime('08-10-2019'))->modify('first day of this month');
$end      = (new DateTime('10-11-2019'))->modify('first day of next month');
$interval = DateInterval::createFromDateString('1 month');
$period   = new DatePeriod($start, $interval, $end);

$countMonth = 0;

foreach ($period as $dt) {
    echo $dt->format("Y-m") . "<br>\n";
    ++$countMonth;
}

echo $countMonth;