public function sub_length($access_token, $channel, $target) {
        $curl = curl_init();
        curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
        curl_setopt($curl, CURLOPT_HTTPHEADER, array(
            'Authorization: OAuth '.$access_token
        ));
        curl_setopt($curl, CURLOPT_URL, $this->base_url.'channels/' .  $channel . '/subscriptions/' . $target);

        $output = curl_exec($curl);
        $response = json_decode($output, true);
        curl_close($curl);

        if(isset($response['error'])) {
            $error = 'Unauthorized';
            return $error;
        } else {
            $total = $response['created_at'];
            return $total;
        }
    }