javascript毫秒字符串转日期函数

function millisecondsStrToDate(str){
        var   startyear   =   1970; 
        var   startmonth   =   1; 
        var   startday   =   1; 
        var   d,   s; 
        var   sep   =   ":"; 
        d   =   new   Date(); 
        d.setFullYear(startyear,   startmonth,   startday); 
        d.setTime(0); 
        d.setMilliseconds(str); 
        s   =   d.getYear()   +   "-"   +   (d.getMonth()+1)   +   "-"   +   d.getDate()   +   "   "   +   
                d.getHours()   +   ":"   +   d.getMinutes()   +   ":"   +   d.getSeconds(); 
        return d.toLocaleString();
	}