Проблема с условиями для вывода разници между датами

function timeFromFirst(form) { 
  var first = new Date(form.firstDate.value); 
  var second = new Date(form.secondDate.value); 
  var year = second.getFullYear() - first.getFullYear(); 
  var month = second.getMonth() - first.getMonth(); 
  var day = (second.getDate() + 1) - (first.getDate() + 1); 
  var hours = (second.getHours() + 1) - (first.getHours() + 1); 
  var minutes = (second.getMinutes() + 1) - (first.getMinutes() + 1); 
 
}

<form> 
  <p> 
    <h3>Input date like "October 13, 2014 11:13:00"</h3> 
    <input type="datetime-local" name="firstDate"> 
    <input type="datetime-local" name="secondDate"> 
  </p> 
  <p> 
    <input type="button" value="Click" onclick="timeFromFirst(form)"> 
  </p> 
</form>