js獲取前一天的日期

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);
    }