I used an Azure free student account and attach a credit card with it. I want to detach the card and then delete the account now. However, it keeps saying I cannot do so because it is in the middle of the billing period. I already delete the subscription, but I'm not sure what the billing period is. Is it the end of the month or I can only delete the card by the end of trial? According to the invoices, the latest billing period is from 8/1-8/31 and the invoice date is 9/9, I tried to delete it on 9/1 and 9/10 and it is still in the middle of the billing period.