Moment.js Sandbox

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/moment.js/2.2.1/moment.min.js"></script>
<script>
function TheorieFunction() {
    var startDate = new Date(document.getElementById('DateTheorie').value);
    var today = new Date();

    if (startDate.getTime() > today.getTime()) {
        alert("Error: Given date is in the future");
        return;
    }            
    if(startDate.valueOf() !== NaN) {
        var a = moment(startDate);
        var b = moment(today);
        var difference = b.diff(a, 'years');
        if (difference < 3) {
            document.getElementById("Theorietxt").innerHTML="Your theorie is still valid for 3+ years";
        }
        else {
            document.getElementById("Theorietxt").innerHTML="Your theorie is invalid";
        }
    } else {
        alert('In Valid Date selected');
    }
}
</script>

Date theorie exam: <input type="date" id="DateTheorie" value="2020-01-01" />
<span class="validity"></span>
<button id="Btntheorie" onclick="TheorieFunction()">Check</button>
<p id="Theorietxt"></p>