“输出距离建党日期总天数(1921.7.1)”这一步怎么写？为什么我写的是错的？

<script>
        var year=prompt("请输入年份：");
        var month=prompt("请输入月份：");
        var allDay=0;

        //计算年份的总天数
        for(var i=1900;i<=year;i++){
            if((i%4==0&&i%100!=0)||i%400==0){
                allDay+=366;
            }else{
                allDay+=365;
            }
        }

        //计算月份的天数
        var monthDay=0;
        for(var i=1;i<=month;i++){
            switch(i){
                case 1:
                case 3:
                case 5:
                case 7:
                case 8:
                case 10:
                case 12:
                    monthDay=31;
                    break;
                case 2:
                    if((year%4==0&&year%100!=0)||year%400==0){
                        monthDay=29;
                    }else{
                        monthDay=28;
                    }
                    break;
                case 4:
                case 6:
                case 9:
                case 11:
                    monthDay=30;
            }
            //allDay+=monthDay;
            //allDay++;
        }

        //计算1号的星期数
        //var day=allDay%7;
        var day=new Date(year+'/'+month+'/'+'01').getDay();
        if(day===0){
            day=7;
        }
        console.log(day);

        //画日历
        document.write("一&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;二&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;三&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;四&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;五&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;六&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;日<br/>");
        for(var i=1;i<day;i++){
            document.write("&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;");
        }
        for(var i=1;i<=monthDay;i++){
            document.write(i+"&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;");
            if((i - 1 + day)%7==0){
                document.write("<br/>");
            }
        }

        //输出距离建党日期总天数(1921.7.1)
        var allday=0;
        for(var i=1921;i<=year;i++){
            if((i%4==0&&i%100!=0)||i%400==0){
                allday+=366;
            }else{
                allday+=365;
            }
            console.log(allday);
        }
    </script>

“输出距离建党日期总天数(1921.7.1)”这一步怎么写？为什么我写的是错的？

js的话这么麻烦吗？
java的话可以用 Duration.between()