用javascript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"