<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...