js写的数码时钟，在“最小化”浏览器 或者 “切换网页”是动画效果好像不对

一、问题

在“最小化”浏览器 或者 “切换网页”是动画效果不对，不知道哪里出了问题？？？是不是”最小化“时网页定时器关掉了，还是其他什么原因啊 ？？？？

二、HTML代码如下

<div id="clockWrap">
        <h2><img src="images/title.png"></h2>
        <div class="year">
            <img class="numDate" src="images/0.png">
            <img class="numDate" src="images/0.png">
            <img class="numDate" src="images/0.png">
            <img class="numDate" src="images/0.png">
            <img src="images/year.png">
            <img class="numDate" src="images/0.png">
            <img class="numDate" src="images/0.png">
            <img src="images/month.png">
            <img class="numDate" src="images/0.png">
            <img class="numDate" src="images/0.png">
            <img src="images/seven.png">
        </div>
        <div class="time">
            <img class="numTime" src="images/0.png">
            <img class="numTime" src="images/0.png">
            <img src="images/sign.png">
            <img class="numTime" src="images/0.png">
            <img class="numTime" src="images/0.png">
            <img src="images/sign.png">
            <img class="numTime" src="images/0.png">
            <img class="numTime" src="images/0.png">
            <img src="images/week.png">
            <img class="day" src="images/seven.png" >
        </div>
    </div>

 三、js代码如下

window.onload = function(){
    
    var arrNum = ["./images/0.png","./images/1.png","./images/2.png","./images/3.png","./images/4.png","./images/5.png","./images/6.png","./images/7.png","./images/8.png","./images/9.png"];
    var arrDay = ["./images/seven.png","./images/one.png","./images/two.png","./images/three.png","./images/four.png","./images/five.png","./images/six.png"];
    var parent = _Id("clockWrap");
    var aNumDate = getByClass( parent,"img","numDate" );
    var aNumTime = getByClass( parent,"img","numTime" );
    var aNumDay = getByClass( parent,"img","day" );
    var myDate = new Date();
    var preDate = myDate.getFullYear() + toZero(myDate.getMonth()+1) + toZero(myDate.getDate());
    var nextDate = "";
    var preStrTime = toZero( myDate.getHours()) + toZero(myDate.getMinutes()) + toZero(myDate.getSeconds() );
    var nextStrTime = "" ;
    var preDay = myDate.getDay();
    var nextDay = 0;
    var arr = [];
    var arr_date = [];
    
    //初始化设置
    for(var i=0,x=aNumTime.length;i<x;i++){ //设置时间 时分秒
        aNumTime[i].src = arrNum[ preStrTime.charAt(i) ];
    }
    aNumDay[0].src = arrDay[preDay];//设置星期
    for(var i=0,x=aNumDate.length;i<x;i++){ //设置年月日
        aNumDate[i].src = arrNum[ preDate.charAt(i) ];
    }


    setInterval(tochange,1000);
    function tochange(){ //每隔一秒执行此函数，获取下一秒时间
        var myDate = new Date();
        nextStrTime = toZero( myDate.getHours()) + toZero(myDate.getMinutes()) + toZero(myDate.getSeconds() );
        toCome(preStrTime,nextStrTime);
        preStrTime = nextStrTime;
        // 设置星期
        nextDay = myDate.getDay();
        if( preDay != nextDay ){
            //aNumDay[0].src = arrDay[nextDay];
            startMoveDay();
            preDay = nextDay;
        }
        //设置年月日
        nextDate = myDate.getFullYear() + toZero(myDate.getMonth()+1) + toZero(myDate.getDate());
        if( preDate != nextDate ){
            toComeDate(preDate,nextDate);
            preDate = nextDate;
        }

    }
    function toCome(str1,str2){ //比较两个字符串，把变化存到数组 arr 里
        arr = [];
        for(var i=0,l=str1.length;i<l;i++){
            if(str1.charAt(i) != str2.charAt(i)){
                //aNumTime[i].src = arrNum[ str2.charAt(i) ];
                arr.push(i);
            }
        }
        startMove();
    }
    function toComeDate(str1,str2){ //比较两个字符串，把变化存到数组 arr 里
        arr_date = [];
        for(var i=0,l=str1.length;i<l;i++){
            if(str1.charAt(i) != str2.charAt(i)){
                //aNumTime[i].src = arrNum[ str2.charAt(i) ];
                arr_date.push(i);
            }
        }
        startMoveDate();
        
    }
    
    function startMove(){//图片翻转效果
        var iSpeed = -4;
        var imgH = aNumTime[0].offsetHeight;
        var timer = null;
        timer = setInterval(function(){
            for(var i=0,x=arr.length;i<x;i++){
                if( aNumTime[arr[i]].offsetHeight == aNumTime[arr[i]].offsetHeight%iSpeed ){
                    iSpeed = 4;
                    aNumTime[arr[i]].src = arrNum[nextStrTime.charAt(arr[i])];
                }

                aNumTime[arr[i]].style.height = aNumTime[arr[i]].offsetHeight + iSpeed +'px';
                // 现在图片高度 + top值*2 = 图片高度  
                aNumTime[arr[i]].style.top = (imgH-aNumTime[arr[i]].offsetHeight)/2 +'px';

                if( aNumTime[arr[i]].offsetHeight == imgH){
                    clearInterval(timer);
                }
                
            }
        },30);
    }
    function startMoveDate(){//图片翻转效果
        var iSpeed = -4;
        var imgH = aNumDate[0].offsetHeight;
        var timer = null;
        timer = setInterval(function(){
            for(var i=0,x=arr_date.length;i<x;i++){
                if( aNumDate[arr_date[i]].offsetHeight == aNumDate[arr_date[i]].offsetHeight%iSpeed ){
                    iSpeed = 4;
                    aNumDate[arr_date[i]].src = arrNum[nextDate.charAt(arr_date[i])];
                }

                aNumDate[arr_date[i]].style.height = aNumDate[arr_date[i]].offsetHeight + iSpeed +'px';
                /* 现在图片高度 + top值*2 = 图片高度  */
                aNumDate[arr_date[i]].style.top = (imgH-aNumDate[arr_date[i]].offsetHeight)/2 +'px';

                if( aNumDate[arr_date[i]].offsetHeight == imgH){
                    clearInterval(timer);
                }
                
            }
        },30);
    }
    function startMoveDay(){//图片翻转效果
        //aNumDay[0].src = arrDay[nextDay];
        var iSpeed = -4;
        var imgH = aNumDay[0].offsetHeight;
        var timer = null;
        timer = setInterval(function(){
            
            if( aNumDay[0].offsetHeight == aNumDay[0].offsetHeight%iSpeed ){
                iSpeed = 4;
                aNumDay[0].src = arrDay[nextDay];
            }

            aNumDay[0].style.height = aNumDay[0].offsetHeight + iSpeed +'px';
            /* 现在图片高度 + top值*2 = 图片高度  */
            aNumDay[0].style.top = (imgH-aNumDay[0].offsetHeight)/2 +'px';

            if( aNumDay[0].offsetHeight == imgH){
                clearInterval(timer);
            }
                
            
        },30);
    }

    function toZero(num){ //数字小于10 在前面加零
        if( num < 10 ){
            return "0" + num;
        }else{
            return "" + num;
        }
    }

    



};

 三、css代码

@charset "utf-8";
/* CSS Document */
body{margin: 0;background-color: #3f055f;}

/* 日历 clockWrap */
#clockWrap{ 520px;height: 520px;margin: 15px auto 0;border: 1px solid #fff;}
#clockWrap h2{text-align: center;margin: 20px auto;}
#clockWrap img{vertical-align: top;height: 37px;}
#clockWrap .year,
#clockWrap .time{height: 37px;text-align: center;position: relative;margin-top: 40px;}
#clockWrap img,
#clockWrap img{position: relative;}
#clockWrap img.numTime,
#clockWrap img.numDate{ 25px;}
#clockWrap img.day{ 38px;}