src/Peach/Http/Header/CookieOptions.php
private function formatExpires()
    {
        $format = CookieExpiresFormat::getInstance();
        $offset = Util::cleanTimeZoneOffset($this->timeZoneOffset);
        $date   = $format->format($this->expires, $offset);
        return "expires={$date}";
    }
private function formatExpires()
    {
        $format = CookieExpiresFormat::getInstance();
        $offset = Util::cleanTimeZoneOffset($this->timeZoneOffset);
        $date   = $format->format($this->expires, $offset);
        return "expires={$date}";
    }
expires 属性を書式化します.

@return string "expires=Wdy, DD-Mon-YY HH:MM:SS GMT" 形式の文字列