orderby-postmeta-query.php

<?php

add_action( 'pre_get_posts', 'apply_akce_query_filter' );
function apply_akce_query_filter ( $query )
{
    if ( is_admin() ) {
        return $query;
    }
	
	if ( is_single() ) {
		return $query;
	}
	
		$date = date( 'Ymd' );

		$meta_query = array(
				'date_clause' => array(
					'key'     => 'datum_order',
					'value'	  => $date,
					'compare' => '>='
				 ),
			);
			$query->set('meta_query', $meta_query );
			$orderby = array(
				'date_clause' => 'ASC'
			);
			
		$query->set( 'orderby', $orderby );

    return $query;

}