Как проверить тело ответа в почтальоне?

Лучшее решение

Игнорирование временной части

возвращает 0, если обе даты одинаковы.

function dayDiff(firstDate, secondDate) {
  firstDate = new Date(firstDate);
  secondDate = new Date(secondDate);
  if (!isNaN(firstDate) && !isNaN(secondDate)) {
    firstDate.setHours(0, 0, 0, 0); //ignore time part
    secondDate.setHours(0, 0, 0, 0); //ignore time part
    var dayDiff = secondDate - firstDate;
    dayDiff = dayDiff / 86400000; // divide by milisec in one day
    console.log(dayDiff);
  } else {
    console.log("Enter valid date.");
  }
}

$(document).ready(function() {
  $('input[type=datetime]').datepicker({
    dateFormat: "mm/dd/yy",
    changeMonth: true,
    changeYear: true
  });
  $("#button").click(function() {
    dayDiff($('#first').val(), $('#second').val());
  });
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<link rel="stylesheet" href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
<script src="//code.jquery.com/ui/1.12.1/jquery-ui.js"></script>

<input type="datetime" id="first" value="12/28/2016" />
<input type="datetime" id="second" value="12/28/2017" />
<input type="button" id="button" value="Calculate">