Cannot Get Date format from input field

I am trying insert birth day from codeigniter form to mysql database as mysql date format, But i am getting like this format "06/27/2018". My model as below

PHP Code:

       
        $birth_day = date('Y-m-d');
        $birth_day = $this->input->post("b_day");

        $config['upload_path']          = './uploads/member/';
        $config['allowed_types']        = 'gif|jpg|png';
        $config['max_size']             = 5000000000;
        $config['max_width']            = 50000;
        $config['max_height']           = 50000;

        $this->load->library('upload', $config); //Enable upload library and pass data as $config variable
        //upload image
        if ($this->upload->do_upload('photo'))
        {
            echo "File 1 Upload Success!";
            $upload_data = $this->upload->data();
            $data['photo'] = "uploads/member/".$upload_data['file_name'];  //.$upload_data['file_name']; = Dispaly file name & upload path
        }else{
            echo $this->upload->display_errors();
        }

        $data['custom_member_id'] = $this->input->post("custom_member_id");
        $data['surname'] = $this->input->post("surname");
        $data['other_name'] = $this->input->post("other_name");
        $data['initials'] = $this->input->post("initials");
        $data['title'] = $this->input->post("title");
        $data['gender'] = $this->input->post("gender");
        $data['b_day'] = $birth_day;
        $data['category'] = $this->input->post("category");
        $data['nic'] = $this->input->post("nic");
        $data['occupation'] = $this->input->post("occupation");
        $data['address_01'] = $this->input->post("address_01");
        $data['address_02'] = $this->input->post("address_02");
        $data['city'] = $this->input->post("city");
        $data['district'] = $this->input->post("district");
        $data['zip_code'] = $this->input->post("zip_code");
        $data['h_phone'] = $this->input->post("h_phone");
        $data['email'] = $this->input->post("email");
        $data['of_phone'] = $this->input->post("of_phone");
        $data['mobile'] = $this->input->post("mobile");
        $data['religion'] = $this->input->post("religion");
        $data['age_group'] = $this->input->post("age_group");
        $data['join_date'] = $this->input->post("join_date");
        $data['last_renewal'] = $this->input->post("last_renewal");
        $data['status'] = $this->input->post("status");
        $data['created_by'] = $this->session->userdata("NAME");
        

        $this->db->insert('membership', $data);

        $b_d['member_id'] = $this->db->insert_id();
        $b_d['name'] = $this->input->post("other_name");
        $b_d['b_date'] =   DATE($this->input->post("b_day"));

        $this->db->insert('birth_day', $b_d); 


After that I try to get birth day to reminder. But its not work with this date format. I am using bootstrap datetimepicker.