你的函数名和你的表单名一样了,所以报错
还有你的按钮名字最好不要用关键字
<!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>无标题文档</title>
<script  language="javascript">
var timeStr,Refresh;
function clock()
 {
 now=new Date();
  hours=now.getHours();
  minutes=now.getMinutes();
seconds=now.getSeconds();
timeStr=""+hours;
timeStr+=((minutes<10) ? ":0" : ":")+minutes;
timeStr+=((seconds<10) ? ":0" : ":")+seconds;
document.clock1.time.value=timeStr;
Refresh=setTimeout("clock()",1000);
}
</script></head>
<body>
<form name="clock1">
  <p>
    <input type="text" name="time" size="8" value="">
  </p>
  <p>
    
    <input type="button" name="Submit1" value="启动" onclick="clock()">
   
<a href="javascript:clock()">aaa</a>
        
    
     <input type="button" name="Submit2" value="取消"  onclick="javascript:clearTimeout(Refresh);document.clock.time.value=''"/>
    
    <br>
    </p>
</form></body>
</html>