JSP计时器,该怎么解决

JSP计时器
<html>
    <head>
        <title>6-9</title>
        <script type="text/javascript">
            var c = 0;
            var t;
            function timedCount() {
                document.getElementById('txt').value=c;
                c = c+1;
                t = setTimeout("timedCount()", 1000);
            }
            function stopCount() {
                clearTimeout(t);
            }
        </script>
    </head>
    <body>
        <form>
            <input type="button" value="开始计时!" onclick="timedCount()">
            <input type="text" id="txt">
            <input type="button" value="停止计时!" onclick="stopCount()">
        </form>
        <p>请点击上面的“开始计时”按钮。输入框会从 0 开始一直进行计时。点击“停止计时”可停止计时。</p>
    </body>
</html>
怎样使时间是自己输入进去的开始,新手求解
------解决思路----------------------

<html>
     <head>
         <title>6-9</title>
         <script type="text/javascript">
             var c = 0;
             var t;
             function timedCount() {
                 document.getElementById('txt').value=c;
                 c = c+1;
                 t = setTimeout("timedCount()", 1000);
             }
             function startCount() {
                 c=document.getElementById('txt').value*1;
                 timedCount();
 }
             function stopCount() {
                 clearTimeout(t);
             }
         </script>
     </head>
     <body>
         <form>
             <input type="button" value="开始计时!" onclick="startCount()">
             <input type="text" id="txt" value="0">
             <input type="button" value="停止计时!" onclick="stopCount()">
         </form>
         <p>请点击上面的“开始计时”按钮。输入框会从 0 开始一直进行计时。点击“停止计时”可停止计时。</p>
     </body>
 </html>


------解决思路----------------------
引用:
谢谢,功能实现了。
有一个小BUG,就是多按几次”开始计时“的话,停止和时间的跳动也会变化。请问怎么解决?

判断下,如果已经在计时了就跳过