정해진 시간부터 줄어드는 스탑워치 > script

<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>