AT&T will start charging customers for the full month after they cancel TV or Internet service, ending its customer-friendly practice of providing a prorated credit for the final month.
Even if you cancel on the first day of a new billing period, you'll be charged for the full month and service will continue for the rest of the month whether you want it or not. To avoid paying for a month of service that you don't want, you'd need to cancel by the last day of the previous billing period.
The change will take effect on January 14, 2019 and apply even when a customer is paying on a month-to-basis and no longer under contract.