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function my_search_query( $query ) {
	// not an admin page and is the main query
	if ( !is_admin() && $query->is_main_query() ) {
		if ( is_search() ) {
			$query->set( 'orderby', 'date' );
		}
	}
}
add_action( 'pre_get_posts', 'my_search_query' );