Wordpress | Output copyright with creation date

function yoolk_copyright() {
    global $wpdb;
    $copyright_dates = $wpdb->get_results("SELECT YEAR(min(post_date_gmt)) AS firstdate, YEAR(max(post_date_gmt)) AS lastdate FROM $wpdb->posts WHERE post_status = 'publish'");
    $output = '';
    if($copyright_dates) {
        $copyright = "© " . $copyright_dates[0]->firstdate;
        if($copyright_dates[0]->firstdate != $copyright_dates[0]->lastdate) {
            $copyright .= '-' . $copyright_dates[0]->lastdate;
        }
        $output = $copyright.' - Website made by Yoolk';
    }
    return $output;
}