向数据库添加出生日期

我试图在注册表中添加日，月和年字段，并将其添加到数据库中的用户记录。所以我输入了这个形式:

<div class="form-group"> 
   <div class="form-inline">
      <div class="form-group pull-right">
         <select name="year" id="year" class="form-control">
            <option value="--" selected>Year</option>                       
            <?php
                for($i=date('Y'); $i>1899; $i--) {
                    $birthdayYear = '';
                    $selected = '';
                    if ($birthdayYear == $i) $selected = ' selected="selected"';
                    print('<option value="'.$i.'"'.$selected.'>'.$i.'</option>'."'n");
                }
            ?>                          
          </select>     
      </div>                    
      <div class="form-group pull-right">
        <select name="month" id="month" onchange="" class="form-control" size="1">
            <option value="--" selected>Month</option>
            <option value="01">Jan</option>
               ...
            <option value="12">Dec</option>
        </select>            
      </div>                                             
      <div class="form-group pull-right">
        <select name="day" id="day" onchange="" class="form-control" size="1">
        <option value="--" selected>Day</option>
        <option value="01">01</option>
                ...
        <option value="31">31</option>
      </select>
    </div>                                              
</div>
</div>  

然后在php部分

if(!isset($error)){
    //hash the password
    $hashedpassword = $user->password_hash($_POST['password'], PASSWORD_BCRYPT);
    //create the activasion code
    $activasion = md5(uniqid(rand(),true));
    $dateOfBirth = $_POST['day']."-". $_POST['month']."-".$_POST['year'];
    try {
        $stmt = $pdo->prepare('INSERT INTO users (username,password,email,created,active,user_birthday) VALUES (:username, :password, :email, NOW(), :active, :user_birthday)');
        $stmt->execute(array(
            ':username' => $_POST['username'],
            ':password' => $hashedpassword,
            ':email' => $_POST['email'],                
            ':active' => $activasion,
            ':user_birthday' => $dateOfBirth
        ));
    ....

当我点击寄存器时，所有内容都插入到数据库中，但日期是0000-00-00。我在下拉菜单中选择什么并不重要。

数据库中user_birthday的字段为DATE。为什么不保存选中的内容?