Magento - date as default sort order in Magento admin area

gistfile1.txt

public function __construct()
    {
        parent::__construct();
        $this->setDefaultSort('picking_date');
        $this->setDefaultDir('ASC');

        $today = new DateTime;
        $this->setDefaultFilter(array(
            'picking_date' => array(
                'from' => $today->format('d/m/Y'),
                'to' => $today->format('d/m/Y'),
                'locale' => Mage::getStoreConfig('general/locale/code', Mage::app()->getStore()->getId())
            )
        ));
    }