Codeigniter helper extending

controller/test.php

<?php
class Test extends Controller {
    function __construct() {

    }
    function show_date(){
        $this->load->helper('date');
        echo "current date in mysql format" . date_mysql();
    }
}
?>
application/helpers

<?php
function date_mysql(){
    if(!time){
        $time = time();
    }
    return date('Y-m-d H-i-s', $time);
}
?>
and im gettting error:

Fatal error: Call to a member function helper() on a non-object in F:\Xampp\htdocs\ci_series\application\controllers\test.php on line 12

what can i do??