为啥我的不显示，求指点
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>暑假倒计时</title>
<script language="javascript" type="text/javascript">
window.onload = function(){
var currentTime = new Date();
var endTime = new Date("2016,8,28");
var leftTme =Math.ceil((endTime.getTime() - currentTime.getTime() )/(24*60*60*1000));
document.getElementById('showTime').innerHTML = leftTime;
}
</script>
</head>
<body>
<div>距离暑假结束还有<span id="showTime"></span>天</div>
</body>
</html>
