Creating a 24 hour clock using setInterval function in Javascript

<script>
setInterval("fun1()",1000);
function fun1()
{
	var d=new Date();
	document.getElementById("sp1").innerHTML=d.getHours()+":"+
           d.getMinutes()+":"+d.getSeconds();
}
</script>
<input type="button" value="click" onclick="fun1()">
<br>
<span id="sp1" style="color:red;font-size:30px;">click</span>