I'm listening for the BILLING.SUBSCRIPTION.ACTIVATED event and everything gets stored in my backend. This event contains a property called next_billing_time, which is all good. Then whenever a user makes a payment to that subscription ID it is shown in the PAYMENT.SALE.COMPLETED event, this event however has no next_billing_time property. My question is, how do I keep track of the new billing time after payment is made? Do I have to use the old next_billing_time and calculate that manually?