datetime - DeliveryDate in Order Item Grid not sorting correctly -

i have created sales order item grid in admin panel in magento. grid displays display date correctly. when try sort based on delivery date not work. include delivery date follows:

$this->addcolumn('proptions', array(         'header'    => mage::helper('sales')->__('delivery date'),         'index'     => 'proptions',         'renderer'  =>  new bakery_core_block_adminhtml_renderer_data(),         'type'      => 'datetime',     )); 

in renderer_data following:

$val = substr($_option['value'],0,10);  //returns format dd/mm/yyyy                                     $val = str_replace("/","-",$val);  //returns dd-mm-yyyy                 $datetimestamp = mage::getmodel('core/date')->timestamp(strtotime($val));                 $format_val = date('d m y', $datetimestamp);  // returns e.g. 13 aug 2013 

i have set type in grid column datetime. still doesn't sort date in correct order. how can fix issue?

thanks, neet

i checked grid in promo , seems use 'date' 'type'

example

    $this->addcolumn('to_date', array(         'header'    => mage::helper('catalogrule')->__('date expire'),         'align'     => 'left',         'width'     => '120px',         'type'      => 'date',         'default'   => '--',         'index'     => 'to_date',     ));