用js实现倒记时

<%@ page contentType="text/html;charset=GB2312" %>
<%@ include file="javabean/useBean.jsp"%>

<!--控制二次提交的js start-->
<script language="JavaScript" type="text/javascript">

function submitQuest(frm){
var allselected=0;
var needSelectCount=1

if (frm.refer_flag.value=="Y"){
alert("您已经提交过一次答案!请重新开始一组题!");
return false;
}else{
frm.timeout_flag.value=1;
if(frm.refer_flag.value!="Y"){
if(frm.remain_time.value<=0){
frm.timeout_flag.value=0;
}else{
frm.timeout_flag.value=1;
}
frm.refer_flag.value="Y";
frm.submit();
return true;
}
}
}
</script>
<!--控制二次提交的js end-->
<%
  
String classId=request.getParameter("classId");

%>

<form name="actionfrm" id="actionfrm" method="post" action="doquest.jsp">
<p>
  <input type="hidden" name="timeout_flag">
  <input type="hidden" name="refer_flag" >
  <script language="JavaScript" type="text/javascript">
function reflash_page(){
if (document.actionfrm.refer_flag.value=="Y")
window.location.href = window.location.href;
}
reflash_page();
</script>

<!--控制时间的（到计时）开始-->
  <input type="text" name="remain_time" readonly size="3" value="120" style="color:#AC1605;height:18px;border:0px;font-size:14px">
    秒</p>
<p>
<script language="JavaScript" type="text/javascript">
function time_step(){
var frm = document.actionfrm;
var r = frm.remain_time;
if(r.value<=1){
r.value=0;
alert("用户您好！规定完成答题的时间已到！系统将自动提交您的答案。");
submitQuest(frm);
}
if (frm.refer_flag.value!="Y")
r.value=(r.value!=0)?r.value-1:0;

window.setTimeout("time_step()",1000);
}
time_step();
</script>
<!--控制时间的（到计时）结束-->
    
<img src="img/dti2.jpg" width="115" height="28" onClick="submitQuest(document.actionfrm);"/></p>