[Dilettante_Pro]

<input type="date">
<script>
   document.querySelector('input').addEventListener('change', function(e) {
      var today = new Date(),
           testDate = new Date(this.value),
           todayMonth = today.getMonth(),
           testMonth = testDate.getMonth(),
           todayYear = today.getFullYear(),
           testYear = testDate.getFullYear();
       alert(testMonth - todayMonth + (testYear - todayYear) * 12);
   });
</script>

Не совсем понял рассуждения насчет текущего...