常用倒计时，用在做题的时候，用来记录做题的时间。

<script type="text/javascript">
  var intDiff = parseInt(60);//倒计时总秒数量
  function timer(intDiff){
      window.setInterval(function(){
      var day=0,
          hour=0,
          minute=0,
          second=0;//时间默认值        
      if(intDiff > 0){
          day = Math.floor(intDiff / (60 * 60 * 24));
          hour = Math.floor(intDiff / (60 * 60)) - (day * 24);
          minute = Math.floor(intDiff / 60) - (day * 24 * 60) - (hour * 60);
          second = Math.floor(intDiff) - (day * 24 * 60 * 60) - (hour * 60 * 60) - (minute * 60);
      }
      if (minute <= 9) minute = '0' + minute;
      if (second <= 9) second = '0' + second;
      $('#day_show').html(day+"天");
      $('#hour_show').html('<s id="h"></s>'+hour+'时');
      $('#minute_show').html('<s></s>'+minute+'分');
      $('#second_show').html('<s></s>'+second+'秒');
      intDiff--;
      }, 1000);
  } 
  $(function(){
      timer(intDiff);
  }); 
</script>

only 只有s秒倒计时的时候

<script>
        var timerindex;
        function handleTimeDown(intDiff) {
            //倒计时
            intDiff--;
            clearInterval(timerindex); // 清除定时器
            timerindex = setInterval(function () {
                console.log(intDiff);    
                if (intDiff >= 0) {
                    if (intDiff <= 9) {
                        intDiff = '0' + intDiff;
                    }
                    $('#ai-question time').text(intDiff);
                    intDiff--;
                } else {
                    clearInterval(timerindex); // 清除定时器
                    setTimeout(function () {
                        //弹窗阶段的时候需要注意
                        console.log('3s后');
                        $('.ai-question .question-btn').trigger('click');
                    }, 3000)
                    return false;
                }              
            }, 1000);
        }
        $('.js_do').off('click').on('click',function(){
            handleTimeDown(90);
        })
    </script>