How to store post details and timestamp in database?

<?php

class Post_model extends CI_Model {

public function add_post() {

$data = array(
      'id' => $this->session->userdata('id'),
      'username' => $this->input->post('username'),
      'post' => $this->input->post('post')
);

$this->db->insert($this->db->dbprefix . 'posts');

}

public function edit_post() {

$data = array(
     'username' => $this->input->post('username'),
     'post' => $this->input->post('post')
);

$this->db->where('id', $this->session->userdata('id'));
$this->db->update($this->db->dbprefix . 'posts');

}

}

<?php
class Welcome extends CI_Controller {

   public function __construct() {
      parent::__construct();
      $this->load->model('folder/post_model');
      $this->load->library('form_validation');
      $this->load->helper('url');
      $this->load->helper('form');
   }

   public function index() {
      $this->form_validation->set_rules('post', 'Post');

      if ($this->form_validation->run() == FALSE) {

         $this->load->view('common/header');
         $this->load->view('posts');
         $this->load->view('common/footer');

      } else {

         // $this->post_model->add_post();
         // Or
         // $this->post_model->edit_post();

         redirect('success-controller');

      }
   }

}