代码哪里写错了，为什么stop按钮没有作用？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>计时器</title>
<script type="text/javascript">
function clock(){
var time=new Date();
document.getElementById("clock").value = time;
}
function timestart(){
var timeshow=setInterval("clock()",1000);}
function timestop(){
var timestop=clearInterval("timestart()");
}
</script>
</head>
<body>
<form>
<input type="text" id="clock" size="50" />
<input type="button" value="Stop" onclick="timestop()" />
<input type="button" value="Start" onclick="timestart()" />
</form>
</body>
</html>
