Assuming you are using PhpDig 1.8.8 RC1, in search_functions.php find:
Code:
'update_date' => mb_ereg_replace('^([0-9]{4})([0-9]{2})([0-9]{2}).*',PHPDIG_DATE_FORMAT,$content['last_modified']),
And replace with:
Code:
'update_date' => mb_ereg_replace('^([0-9]{4})[-]?([0-9]{2})[-]?([0-9]{2}).*',PHPDIG_DATE_FORMAT,$content['last_modified']),