Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

<input type="week" id="myWeek" value="2014-W15">

<p>Click the button to decrement the value of the week field by 10 weeks (each time you click).</p>

<button onclick="myFunction()">Try it</button>

<p><strong>Note:</strong> input elements with type="week" are not supported in Firefox.</p>

<script>
function myFunction() {
    document.getElementById("myWeek").stepDown(10);
}
</script>

</body>
</html>