js获取前一天的日期

function GetDay()   {    var   today=new   Date();       var   yesterday_milliseconds=today.getTime()-1000*60*60*24;       var   yesterday=new   Date();       yesterday.setTime(yesterday_milliseconds);           var strYear=yesterday.getFullYear();    var strDay=yesterday.getDate();    var strMonth=yesterday.getMonth()+1;    if(strMonth<10)    {     strMonth="0"+strMonth;    }    var strYesterday=strYear+"-"+strMonth+"-"+strDay;    alert(strYesterday);   }