如何在一半时间使用javascript完成在线考试时显示警告框

我正在进行在线考试。我使用javascript设置了考试计时器。这太完美了。我对javascript知之甚少。我不知道考试过半时如何显示警告框。请把解决方案给我。

我的计时器代码：

function countDown() {
      sec--;
  if (sec == -01) {
    sec = 59;
    min = min - 1;
    if(min==-01)
        {
            min=59;
            hour=hour-1;
        }else
            {
                hour=hour;
            }
  } else {
   min = min;
  }
if (sec<=9) { sec = "0" + sec; }
  time = (hour<=9 ? "0" + hour : hour) + " : " + (min<=9 ? "0" + min : min) + " : " + sec + "";
if (document.getElementById) { document.getElementById('timer').innerHTML = time; }
  SD=window.setTimeout("countDown();", 1000);
if (hour=='00'&& min == '00' && sec == '00') { sec = "00";min="00"; window.clearTimeout(SD); window.location=document.forms[0].action+"?fs=yes";}
}
function addLoadEvent(func) {
  var oldonload = window.onload;
  if (typeof window.onload != 'function') {
    window.onload = func;
  } else {
    window.onload = function() {
      if (oldonload) {
        oldonload();
      }
      func();`enter code here`
    }
  }
}