Date and time on the form yyyy-MM-dd, yyyyMMdd, yyyy-MM-ddTHH:mm and yyyy-MM-ddTHH:mm:ss from which the subscription is eligible to schedule first invoice. If no time part is given start of day will be used. A start date in the past can be used, but no more than one period length in the past. A start date in the past can result in an instant invoice for a past billing period start. Default value is current date and time.