Your billing date always remains the same with Piano Marvel. When you add an account to your grouped accounts in between billing cycles you will be charged a prorated amount retroactively on the next billing cycle date for that account.
This is calculated by the adding the number of days from when you created the account until the billing cycle date and dividing by the number of days of the current month.
Example: (10 Days until billing date) / (30 days in the current month) = 0.3333 days of the month.
(0.3333) * (Payment rate) = Prorated Payment
You will also be charged for the next month on the first billing cycle payment date after the new account is created. All payments for all accounts that you manage will occur on the same date.
Please keep in mind that adding multiple accounts in a given month will cause an influx in the amount due on the next billing cycle.