need with date input?

[eluser]newbie boy[/eluser]
i'm sorry aken, i dont know how to do the tagz, but here's my code

my model:
public function addMember($email,$birthday) {

$this->email = $email;
$this->birthday = $birthday;
$this->save(); }

my controller:
public function registerMember() {

$email = $this->input->post('email');
$birthday = $this->input->post('year')."-".$this->input->post('month')."-".$this->input->post('day');

$UserModel = new UserModel();
$UserModel->addMember($email,$birthday);

echo $birthday; }

thanks for the time.