public function ceilWeek($weekStartsAt = null)
    {
        if ($this->isMutable()) {
            $startOfWeek = $this->copy()->startOfWeek($weekStartsAt);

            return $startOfWeek != $this ?
                $this->startOfWeek($weekStartsAt)->addWeek() :
                $this;
        }

        $startOfWeek = $this->startOfWeek($weekStartsAt);

        return $startOfWeek != $this ?
            $startOfWeek->addWeek() :
            $this->copy();
    }