<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type = "text" id="clock" style="width: 449px" value="点击右侧[begin]按钮开始">
<input type = button value="begin" onclick="begin_click()">
<input type  = button value="End" onclick="End()">

<script>
    function begin() {
        var stime = Date();
        var ret = document.getElementById("clock");
        ret.value = stime;

    }
    var ID;
    function begin_click() {
        begin()
        clearInterval(ID)
        ID = setInterval(begin,1000)

    }
    function End() {
        clearInterval(ID);

    }
</script>

</body>
</html>

　　知识点1：begin_click 里面放个 begin 即时刷新出来当前事件。

　　知识点2：begin_click 里面放个  clearInterval(ID) 即时结束上个id。