js倒计时如火车站买火车票后15分钟必须付款否则取消火车票

$(function(){

     timer();
});

function timer(){

   interval = setInterval("timer1()",1000);
}

function timer1(){
	var leftTime =  new Date().getTime() -  dateTime.getTime();
	leftTime = 15*60*1000  - leftTime;
    var minute =parseInt(leftTime/1000/60%60,10); 
	var secOnd= parseInt(leftTime/1000%60,10);
	var min = checkTime(minute);
	var sec = checkTime(second);
	$("#leftTimeTag").html(min + ":" + sec);
        if(leftTime <1000){
		alert("hello 倒计时结束");
		clearInterval(interval);
      }
}

function checkTime(i){
	if(i <10 ){
		i = "0" + i;
	}
	return i;
}