You need to use JavaScript for this.
Ref
http://www.w3schools.com/js/tryit.asp?filename=tryjs_timing_clock
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<script type="text/javascript">
function startTime() {
var today = new Date();
var h = today.getHours();
var m = today.getMinutes();
var s = today.getSeconds();
m = checkTime(m);
s = checkTime(s);
document.getElementById('txt').innerHTML = h + ":" + m + ":" + s;
if (document.getElementById('txt').innerHTML == "12:00:00") {
alert('hi');
}
var t = setTimeout(function () { startTime() }, 500);
}
function checkTime(i) {
if (i < 10) { i = "0" + i }; // add zero in front of numbers < 10
return i;
}
</script>
</head>
<body onload="startTime()">
<div id="txt">
</div>
</body>
</html>
Demo
