When a new service or product is added or removed from an account, partial month's charges or credits are applied, based on the billing date. These partial months' charges appear in the Adjustment section of your bill.
Monthly charges are billed in advance based on a 30-day billing month and those appear in the Local Services section of your bill. The first bill received after placing an order for new service includes partial month's charges, from the date ordered to the bill date, plus the charges for the next full month. This process is reversed when the service is removed. At removal, monthly charges billed in advance are credited to your bill.