微信小程序js倒计时功能

//倒计时方法

var that=this;
this.data.intervarID= setInterval(function () {
var leftTime = (new Date(2017, 6-1, 28, 17, 40, 0)) - (new Date()); //计算剩余的毫秒数 
var days = parseInt(leftTime / 1000 / 60 / 60 / 24, 10); //计算剩余的天数 
var hours = parseInt(leftTime / 1000 / 60 / 60 % 24, 10); //计算剩余的小时 
var minutes = parseInt(leftTime / 1000 / 60 % 60, 10);//计算剩余的分钟 
var seconds = parseInt(leftTime / 1000 % 60, 10);//计算剩余的秒数 
      days = checkTime(days);
      hours = checkTime(hours);
      minutes = checkTime(minutes);
      seconds = checkTime(seconds);
      that.setData({
        clock: days + "天" + hours + "小时" + minutes + "分" + seconds + "秒"
      })
if (days == '00' && hours == '00' && minutes == '00' && seconds=='00')
      {
        clearInterval(that.data.intervarID);

        that.setData({
          btnname: '立即抢购'
        })
      }
    }, 1000
    )