Qfyilyi

I'm making a personal identity number program in javascript,
this is how it looks like :

        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

it says invalid birth year, anyone got any idea why?
* This system is based on the system in Finland, so if youre born in 1900, your number consists of a "-".
https://en.wikipedia.org/wiki/National_identification_number
arhundrade stands for Century.