<script language="javascript">
var mm=0, ss=0;
function startExam(obj, state){
if(state==0){
if(mm=="0"){
setmin = Number(5);
setsed = Number(0);
}else{
setmin = Number(mm);
setsed = Number(ss);
}
tid = setInterval("showTime()",1000);
}else{
clearInterval(tid);
}
}
function showTime(){
if(setmin != 0 || setsed != 0){
if (setsed == 0){
setmin--;
setsed = 60;
}
setsed--;
if (setmin<10){
setmin="0"+Number(setmin);
}
mm = setmin;
if (setsed<10) {
setsed="0"+setsed;
}
ss = setsed;
var mytime = setmin+':'+setsed;
if (document.layers){
document.clock.document.write(mytime);
document.clock.document.close();
}else if (document.all){
clock.innerHTML=mytime;
}
}else{
}
}
</script>
<span ID="clock">05:00</span>

<a href="" onClick="javascript:startExam(this, 0);return false;" style="cursor:hand">start</a>
<a href="" onClick="javascript:startExam(this, 1);return false;" style="cursor:hand">stop</a>