If you are a new customer you can't change the date until after you have paid your first bill.
Also the payment date would change from
the following bill. This is because changing the payment date completely recalculated your bill, so the next bill would likely cost more due to it covering longer than a one month period. Bills are generated 14 days before the billing date therefore if you have a billing date of 28th, you would need to change the payment date before 14tj of that month in order for it to take effect for that month.
