$userColumns[] = array('call_back_fun' => array(&$this,'get_days'), 'title' => 'Remaining Days');
		$this->Editor->setConfig('userColumns',$userColumns); 

and then 
	function get_days($row)
	{
        $whois = "<a href=\"http://whois.domaintools.com/".$row['domain']."\" target=_blank>whois</a> ";
		$diff = strtotime($row['expiry']) - strtotime(date('Y-m-d'));
		$diff_days = floor($diff/(60*60*24));
		$html = abs($diff_days)<10000 ? '<td>'.$whois.$diff_days.'</td>' : '<td> </td>';
		return $html;
	}