var c=0
var t
function timedCount()
{
document.getElementById('txt').value=c
c=c+1
t=setTimeout("timedCount()",1000)
}
<form> <input type="button" value="开始计时！" onclick="timedCount()"> <input type="text" id="txt"> </form> <p>请点击上面的按钮。输入框会从 0 开始一直进行计时。</p>