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

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

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;
}
}};

@charset "utf-8";
/* CSS Document */
body{margin: 0;background-color: #3f055f;}/* 日历 clockWrap */
#clockWrap{width: 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{width: 25px;}
#clockWrap img.day{width: 38px;}