app\models\Invoice::getNextSendDate PHP Method

    public function getNextSendDate()
    {
        if ($this->start_date && !$this->last_sent_date) {
            $startDate = $this->getOriginal('start_date') . ' ' . $this->account->recurring_hour . ':00:00';
            return $this->account->getDateTime($startDate);
        }
        if (!($schedule = $this->getSchedule())) {
            return null;
        }
        if (count($schedule) < 2) {
            return null;
        }
        return $schedule[1]->getStart();
    }