3分钟倒计时

<div class="time-left">
          <div class="time"><span id="minute">03</span>:<span id="second">00</span></div>
          <div class="start">
            <a href="javascript:void(0)" id="start" onclick="start_time()"><img src="paimai/start.png" /></a>
            <a href="paimai2.html"><img src="paimai/next.png" /></a>
          </div>
          <div class="clear"></div>
      </div>

var total_time = 3 * 60; // 3分钟
    var timer;
    function left_time() {
      if(total_time > 0) {
        var minute = Math.floor(total_time / 60);
        var second = Math.floor(total_time % 60);
        minute = str_num(minute);
        second = str_num(second);
        document.getElementById("minute").innerHTML = minute;
        document.getElementById("second").innerHTML = second;
        --total_time;
        console.log(total_time)
      } else {
        clearInterval(timer);
        document.getElementById("minute").innerHTML = "00";
        document.getElementById("second").innerHTML = "00";
        document.getElementById("start").style.display = "none";
        document.getElementById("next").style.display = "block";
      }
    }
    function str_num(num) {
      return num > 9 ? num : "0" + num;
    }
    function start_time() {
      setInterval("left_time()", 1000); 
    }