<!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>