Delete a javascript cookie

I tried this way, but it's not deleted.

This is my code, it is in the function of "close session" I would like to delete the cookie

$(document).ready(function(){

var lascookies = document.cookie.split(';');
var a;
for (var i = 0; i< lascookies.length; i++) {
    var c = lascookies[i]
    var igual = c.indexOf("=");
    var valor = c.substring(igual+1);
    alert(valor);
    if(valor==1){
        a="sesion"
    }   
}

if(a=="sesion"){
        $("#pum-55").hide();
    }else{
        $("#pum-55").show();
}


$("#btn2").click(function() {
    document.cookie = "bandera=1;";
    $('#pum-55').hide();
});

$("#cerrar").click(function() {
     var mensaje = confirm("¿Deseas cerrar la sesión?");

        if (mensaje) {
            document.cookie = "bandera=; expires=Thu, 01 Jan 1970 00:00:00 UTC;";
            alert("adios");
            location.reload(true);
        }
});

$("#wp-submit").click(function() {
    document.cookie = "bandera=1;";
    //alert("ASDASD");
    //alert("hola");
});

});