public function getTicketSaleStartDate()
    {
        $saleStart = null;
        if (($tickets = $this->owner->Tickets())) {
            /** @var Ticket $ticket */
            foreach ($tickets as $ticket) {
                if (($date = $ticket->getAvailableFrom()) && strtotime($date) < strtotime($saleStart) || $saleStart === null) {
                    $saleStart = $date;
                }
            }
        }

        return $saleStart;
    }