个人身份号码

Question

我正在用javascript制作个人身份号码程序， 它是这样的：

        const kontrollTecken = "0123456789ABCDEFHJKLMNPRSTUVWXY";

        function test(){
            var toTest = document.getElementById("input").value;
            document.getElementById("message").innerHTML="";
            document.getElementById("info").classList.remove("hidden");
            if(toTest.length == 11 ||toTest.length == 10){
                var controlVal = toTest.substring(0,6) + toTest.substring(7,10);

                var control = parseInt(controlVal) % 31;
                var arhundrade;

                switch (toTest[6]) {
                    case "a":
                    case "A":
                    arhundrade= "2000";
                    break;
                    case "-":
                    arhundrade= "1900";
                    break;
                    case "+":
                    arhundrade= "1800";
                    break;
                    default:
                    arhundrade= "Okänt"; //Unknown
                    break;
                }

              if(typeof kontrollTecken[control] !=='undefined'){
                document.getElementById("kontrolltecken").innerHTML = kontrollTecken[control] == toTest[10] ? kontrollTecken[control] : "Borde vara//should be " + kontrollTecken[control];
                document.getElementById("arhundrade")innerHTML = arhundrade;
                document.getElementById("kon").innerHTML = (toTest[9] % 2) == 0 ? "Kvinna" : "Man"; //woman - man

                var dateString = arhundrade.substring(0, 2) + toTest.substring(4, 6) + "-" + toTest.substring(2, 4) + "-" + toTest.substring(0, 2);
                var bornAge = new Date (dateString);
                document.getElementById("alder").innerHTML = isValidDate(bornAge) ? getAge(bornAge) + "år" : "Ogiltig födelsedag!";
            }else{
                document.getElementById("info").classList.add("hidden");
    document.getElementById("message").innerHTML = "Felaktig Personsignum";
}
}else{
    document.getElementById("info").classList.add("hidden");
    document.getElementById("message").innerHTML = toTest.length < 11 ? "För lite tecken" : "För många tecken"; // Too few numbers / too many numbers
            }                            

            }
            function isValidDate (d) {
                return d instanceof Date && !isNaN (d);                 
            }
            function getAge(dateString)
            { 
                var today = new Date();
                var birthDate = new Date(dateString);
                var age = today.getFullYear() - birthDate.getFullYear();
                var m = today.getMonth() - birthDate.getMonth();
                if (m < 0 || (m === 0 && today.getDate() < birthDate.getDate()))
                {
                    age--;
                }
                return age;
            }

        *All of it seems to work but when someone who is born in 1900

说无效的出生年份，有人知道为什么吗？ *此系统基于芬兰的系统，因此，如果您出生于1900年，则您的电话号码由“-”组成。 https://en.wikipedia.org/wiki/National_identification_number arhundrade代表Century。