SilverStripe page create error Class 'DateField' not found

Im creating new SilverStripe project based on the tutorial of SilverStripe.

After adding this code, the CMS field dont show and gives error.

class ArticlePage extends Page {
private static $db = array(
    'Date' => 'Date',
    'Author' => 'Text',
);

// .....

public function getCMSFields() {
    $fields = parent::getCMSFields();

    $dateField = new DateField('Date', 'Article Date (for example: 20/12/2010)');
    $dateField->setConfig('showcalendar', true);
    $dateField->setConfig('dateformat', 'dd/MM/YYYY');

    $fields->addFieldToTab('Root.Main', $dateField, 'Content');
    $fields->addFieldToTab('Root.Main', new TextField('Author', 'Author Name'), 'Content');

    return $fields;
}

}

the error is:

Fatal error: Class 'DateField' not found in sample/sample/ line*

I don't see any problem with the code as I copy it from the tutorial itself. please help