将datepicker字段添加到Wordpress中的插件设置页面

I'm developing a simple plugin (my first WP plugin development) and I'm trying to add a datepicker field in the plugin settings page using this code:

add_settings_field('example_date_picker', 'Example Date Picker', 'pu_display_date_picker', 'ft_admin.php', '', array());
add_action('admin_enqueue_scripts', 'enqueue_date_picker');

function pu_display_date_picker($args)
{
    extract($args);
    echo '<input type="date" id="datepicker" name="example[datepicker]" value="" class="example-datepicker" />';
}

/**
 * Enqueue the date picker
 */
function enqueue_date_picker()
{
    wp_enqueue_script(
            'field-date-js', 'ft.js', array('jquery', 'jquery-ui-core', 'jquery-ui-datepicker'), time(), true
    );

    wp_enqueue_style('jquery-ui-datepicker');
}

The code come from this post but I get this error:

Fatal error: Call to undefined function add_settings_field() in /var/www/html/arubaair/wp-content/plugins/frequent-traveler/frequent-traveler.php on line 41

And I don't know why. The plugin is installed and active and if I remove the code all works. What I'm doing wrong?

UPDATE

I change the original code to this one:

add_action('admin_enqueue_scripts', 'enqueue_date_picker');

function enqueue_date_picker()
{
    wp_enqueue_script(
            'field-date-js', 'ft.js', array('jquery', 'jquery-ui-core', 'jquery-ui-datepicker'), time(), true
    );

    wp_enqueue_style('jquery-ui-datepicker');
}

The file ft.js is on js plugin directory. Then in the page where I build the form I have this line:

<input type="date" id="frequent_traveler_from_date" name="frequent_traveler_from_date" value="" class="datepicker" />

And ft.js contains this code:

jQuery(document).ready(function() {
    jQuery('.datepicker').datepicker();
});

And it's not working, I check the page source and scripts aren't loaded, why?

As your code is not compilable, I'll post a working example based on the skeleton of your sample code. Required: PHP 5.3, see Lambda functions:

add_action( 'admin_menu', function()
{
    $hook = add_menu_page( 
        'Date Pick', 
        'Date Pick', 
        'manage_options', 
        'sub-page-date-picker', 
        function() { 
            echo '<h1>Date Pick</h1>'; 
            echo '<input type="date" id="datepicker" name="example[datepicker]" value="" class="example-datepicker" />';
        }
    );
    # echo $hook; die(); // get the correct hook slug

    add_action( 'admin_enqueue_scripts', function( $hook )
    {
        if( 'toplevel_page_sub-page-date-picker' !== $hook )
            return;

        wp_enqueue_script(
            'field-date-js', 
            plugins_url( '/ft.js', __FILE__ ), 
            array('jquery', 'jquery-ui-core', 'jquery-ui-datepicker'), 
            time(), 
            true
        );
        wp_enqueue_style('jquery-ui-datepicker');
    });
});  

And ft.js with a small adjustment:

jQuery(document).ready(function($) {
    $('.datepicker').datepicker();
});

For loading bellows script & style add bellows code on theme functions.php file.

Script for front-end use

function add_e2_date_picker(){
//jQuery UI date picker file
wp_enqueue_script('jquery-ui-datepicker');
//jQuery UI theme css file
wp_enqueue_style('e2b-admin-ui-css','http://ajax.googleapis.com/ajax/libs/jqueryui/1.9.0/themes/base/jquery-ui.css',false,"1.9.0",false);
}
add_action('wp_enqueue_scripts', 'add_e2_date_picker'); 

Script for back-end use

function add_e2_date_picker(){
//jQuery UI date picker file
wp_enqueue_script('jquery-ui-datepicker');
//jQuery UI theme css file
wp_enqueue_style('e2b-admin-ui-css','http://ajax.googleapis.com/ajax/libs/jqueryui/1.9.0/themes/base/jquery-ui.css',false,"1.9.0",false);
}
add_action('admin_enqueue_scripts', 'add_e2_date_picker'); 

Just put this code also funtions.php file or bellow those code.

function register_datepiker_submenu() {
        add_submenu_page( 'options-general.php', 'Date Picker', 'Date Picker', 'manage_options', 'date-picker', 'datepiker_submenu_callback' );
    }

    function datepiker_submenu_callback() { ?>
            <div class="wrap">
             <input type="text" class="datepicker" name="datepicker" value=""/>
            </div>
            <script>
            jQuery(function() {
                jQuery( ".datepicker" ).datepicker({
                    dateFormat : "dd-mm-yy"
                });
            });
            </script> 
    <?php }
        add_action('admin_menu', 'register_datepiker_submenu');
    ?>

You’ll got a date picker on Admin Menu->Settigns->Date Picker. See more details Add a jQuery DatePicker to WordPress Theme or Plugin.