public static function vacation_interval(&$vacation)
    {
        $rcube = rcube::get_instance();

        if (isset($vacation['seconds'])) {
            $interval = $vacation['seconds'];
        }
        else if (isset($vacation['days'])) {
            $interval = $vacation['days'];
        }
        else if ($interval_cfg = $rcube->config->get('managesieve_vacation_interval')) {
            if (preg_match('/^([0-9]+)s$/', $interval_cfg, $m)) {
                if ($seconds_extension) {
                    $vacation['seconds'] = ($interval = intval($m[1])) ? $interval : null;
                }
                else {
                    $vacation['days'] = $interval = ceil(intval($m[1])/86400);
                }
            }
            else {
                $vacation['days'] = $interval = intval($interval_cfg);
            }
        }

        return $interval ?: '';
    }