JavaScript实现Div内容更新后自动滚动到底部

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
</head>
<body>

<script type="text/javascript">
function add()
{
var now = new Date();
var div = document.getElementById('scrolldIV');
div.innerHTML = div.innerHTML + 'time_' + now.getTime() + '<br />';
div.scrollTop = div.scrollHeight;
}
</script>
<div id="scrolldIV" style="overflow:auto; height: 100px; width: 400px; border: 1px solid #999;">
</div>
<input type="button" value="插入一行" onclick="add();">
用js控件div的滚动条，让它在内容更新时自动滚到底部</body>
</html>