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function $(id){return document.getElementById(id);}

function createtime(){

var miao=parseInt($("Times").innerHTML)-1; 

if(miao==0){doSelect(1);$("Times").innerHTML=30;}else{$("Times").innerHTML=miao;}

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onload=function(){$("Times").innerHTML=30;setInterval("createtime()",900);}

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<span id="Times"></span>