<!doctype html>
<head>
<meta charset="utf-8">
<script type="text/javascript">
function time(){
var da = new Date();
document.getElementById('time').innerHTML = da.toLocaleTimeString();
}
function start(){
window.setInterval( time, 1000 );
return false;
}
</script>
</head>
<body>
<a href="" onclick="return start()">Старт</a>
<div id="time"></div>
</body>
</html>