<form name=forms> 
考试时间:<input type=text name=input1 size=9>          
<script language=javascript>
<!--          
var sec=0;min=0;
var hou=0;flag=0;
function update(){
sec++;
if(sec==60){sec=0;min+=1;}
if(min==60){min=0;hou+=1;}
document.forms.input1.value=hou+"时"+min+"分"+sec+"秒";
window.setTimeout("update();",1000);
if((hou==0)&&(min==55)&&(sec==0))
{
alert("还有5分钟考试结束，请您抓紧时间！");
}
}       
update()
//-->
</script>         
</form>