Leap year function doesn't work

I have a leap year if inside my function Kalender() but for some reason it crashes at the february month of the leap year (for example 2012) . with debugging i get no errors. for some reason it won't change the monthLength[1] to 28?

    var dayNames = ['Zon', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'];
    //Volledige Dagnamen//
    var dayNamesFull = ['Zondag', 'Maandag', 'Dinsdag', 'Woensdag', 'Donderdag', 'Vrijdag', 'Zaterdag'];
    //Volledige Maandnamen//
    var monthNames = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'Oktober', 'November', 'December'];
    //Maand lengte van 0 - 11//
    var monthLength = [31,28,31,30,31,30,31,31,30,31,30,31]; 
    //Nieuwe datum//
    var today = new Date();
    //Vandaag//
    var day = today.getDay();
    var currentDay = day;
    //vandaag zoekmaand//
    var month = today.getMonth();
    var currentMonth= month;
    //vandaag//
    var vandaag = today.getDate();

    //volledig jaar//
    var Leapyear = today.getYear();
    var year = today.getFullYear();
    var currentYear = year;

    //events in een array//
    var act = ['Opa en Oma 50 jaar getrouwd','Afspraak met de baas', 'Lunchen met Oma','Kleding kopen met mijn moeder','Frank gaat verhuizen', 'School opdracht inleveren','Feestje van Henk', 'Kijken voor een nieuwe auto','Kinderen van school halen','Afspraak doktor 15.00','Zalf halen voor mijn voeten','Onderhoudsbeurt auto','Hond uitlaten','Vaccin krijgen voor reis naar jakarta','Vliegticket boeken', 'Kamer opruimen', 'Hond wegbrengen voor vlooienbehandeling', 'Ov-chipkaart opladen met geld', 'APK keuring Auto', 'Nieuwe velgen voor de auto ophalen in Loenen ad Vecht','Paardenstal opruimen','Golfen met mijn vrienden','Uit eten met mijn schoonfamilie','Werkoverleg overname', ' Bedrijfsbeleid aanpassen'];

//Kalender schrijf functie//    
function Kalender() {
   var buttons = '<button id="vorige" onclick="vorige()">Vorige</button><button id="volgende" onclick="volgende()">Volgende</button>'
   var kalender = "";
 //Leap Year//
    if (month == 1){
        if (year % 4 == 0){
            monthLength = 29;
        }

    }

document.getElementById('Header').innerHTML = monthNames[month]+" "+year+buttons;
    kalender += '<table id="cal"><div id="taken"></div><div id="events"></div>';


        for (var j = 1; j <= monthLength[month]; j++) {
            if ((vandaag == j) && (currentMonth == month) && (currentYear == year)) {
                    kalender += "<td class='tabeldagen' id='dagVandaag'><a id='dag"+j+"' onclick='addEvent (this.id)'>" + j;
            }
                else {
                     kalender += "<td id='"+j+"' class='tabeldagen'><a id='dag"+j+"' onclick='addEvent (this.id)'>"+j;
                }

                    if (j % 7 == 0) {

                kalender += "<tr>";
            }
            kalender += '</td></a>';
        }

    kalender += '</table>';
    document.getElementById('kalen').innerHTML = kalender;
        document.getElementById('taken').innerHTML = dayNamesFull[currentDay]+" "+vandaag+" "+monthNames[currentMonth];

    }

//Volgende Maand functie//
function volgende() {
month = month + 1;
    if(month > 11) {
        month = -1;
        month = month + 1;
        year = year + 1;
    } 
        Kalender();
}
//Vorige maand functie//
function vorige() {
    month = month - 1;
    if(month < 0) {
        month = + 12 ;
        month = month - 1;
        year = year - 1;
    } 
        Kalender();
}
//Voegt event toe aan De desbetreffende dag//
function addEvent (id) {
    if ((year == 2014) && (month == 10)) {
        switch (id) {
            case "dag1":
                document.getElementById("events").innerHTML = act[1];
                break;
            case "dag2":
                document.getElementById("events").innerHTML = act[2];
                break;
            case "dag3":
                document.getElementById("events").innerHTML = act[0];
                break;
            case "dag4":
                document.getElementById("events").innerHTML = act[3];
                break;
            case "dag5":
                document.getElementById("events").innerHTML = act[4];
                break;
            case "dag6":
                document.getElementById("events").innerHTML = act[5];
                break;
            case "dag7":
                document.getElementById("events").innerHTML = act[6];
                break;
            case "dag8":
                document.getElementById("events").innerHTML = act[7];
                break;
            case "dag9":
                document.getElementById("events").innerHTML = act[8];
                break;
            case "dag10":
                document.getElementById("events").innerHTML = act[9];
                break;
            case "dag11":
                document.getElementById("events").innerHTML = act[10];
                break;
            case "dag12":
                document.getElementById("events").innerHTML = act[11];
                break;
            case "dag13":
                document.getElementById("events").innerHTML = act[12];
                break;
            case "dag14":
                document.getElementById("events").innerHTML = act[13];
                break;
            case "dag15":
                document.getElementById("events").innerHTML = act[14];
                break;
            case "dag16":
                document.getElementById("events").innerHTML = act[15];
                break;
            case "dag17":
                document.getElementById("events").innerHTML = act[16];
                break;
            case "dag18":
                document.getElementById("events").innerHTML = act[17];
                break;
            case "dag19":
                document.getElementById("events").innerHTML = act[18];
                break;
            case "dag20":
                document.getElementById("events").innerHTML = act[19];
                break;
            case "dag21":
                document.getElementById("events").innerHTML = act[20];
                break;
            case "dag22":
                document.getElementById("events").innerHTML = act[21];
                break;
            case "dag23":
                document.getElementById("events").innerHTML = act[22];
                break;
            case "dag24":
                document.getElementById("events").innerHTML = act[23];
                break;
            case "dag25":
                document.getElementById("events").innerHTML = act[24];
                break;
            case "dag26":
                document.getElementById("events").innerHTML = act[2];
                break;
            case "dag27":
                document.getElementById("events").innerHTML = act[6];
                break;
            case "dag28":
                document.getElementById("events").innerHTML = act[9];
                break;
            case "dag29":
                document.getElementById("events").innerHTML = act[12];
                break;
            case "dag30":
                document.getElementById("events").innerHTML = act[11];
                break;
            case "dag31":
                document.getElementById("events").innerHTML = act[0];
                break;
        }
    }
}

http://jsfiddle.net/nshd0o47/1/