if you only want to get todays orders on admin order history page, something like this might work for you

wppizza v2.x only , for v3 see here http://docs.wp-pizza.com/developers/?section=get-only-todays-orders-on-order-history-page

add_filter('wppizza_filter_orderhistory_ordersquery', 'myprefix_admin_orderhistory_query_filter');
function myprefix_admin_orderhistory_query_filter($query){
$today = date('Y-m-d',current_time('timestamp'));
$today_start = $today.' 00:00:00';
$today_end = $today.' 23:59:59';
/* edit/alter the query adding condition after where*/
$query = str_ireplace('WHERE', 'WHERE order_date>="'.$today_start.'" AND order_date<="'.$today_end.'" AND ');
return $query;
}