<script type="text/javascript">
var lastTime = 3;
function abc() {
var btnReg = document.getElementById("btnReg");
if (!btnReg) { return; }
if (lastTime <= 0) {
btnReg.value = "同意";
btnReg.disabled = "";
clearInterval("TimeId");
}
else {
btnReg.value = "還剩下" + lastTime + "秒";
lastTime--;
}

}
var TimeId = setInterval("abc()", 1000);
</script>