function check_for_numeric_page_slug( $query_vars ) {
    if ( ! empty( $query_vars['year'] ) ) {
        $page = get_page_by_path( $query_vars['year'] );
        if ( $page ) {
            $query_vars['pagename'] = $page->post_name;
            unset( $query_vars['year'] );
        }
    }

    return $query_vars;
}
add_filter( 'request', 'check_for_numeric_page_slug' );