public static function firstWeekOfYear(int $y) : \Datetime
    {
        $date = (string)$y . "-01-01 00:00:00";
        $format = "Y-m-d H:i:s";
        $o = \DateTime::createFromFormat($format, $date);

        $wnDay = date("N", strtotime($date));
        $addD = (integer)((11 - $wnDay) % 7);

        $o->add(new \DateInterval("P" . $addD . "D"));
        $o->sub(new \DateInterval("P3D"));

        return $o;
    }