Partial Month Charges happen when you add services to your account. If you add a new service, you are billed from the first day of the new service. This means your next bill may include a full month of the new service, plus each day you had the new service before the new billing cycle began.
Partial Month Charges are determined by taking the total monthly charge and dividing it by 30 days. This is the cost-per-day of service. This figure is then multiplied by the number of days in the partial month. If the monthly charge is $30, the per-day charge is about $1 per day. If you had ten days of partial month service, that would be 10 days, multiplied by $1 (per day) or about $10. (Don't forget, some months have 31 days, and then there's February with 28 days or 29 for leap year.)
For FiOS® customers: FiOS TV is billed one month in advance, so your first statement will include prorated charges for the current month, as well as charges for the following month. Monthly charges are prorated according to the number of days you are billed for during your 30-day billing cycle. When you make changes to your account following your service installation (by adding or deleting Frontier services or packages, for example), prorated charges will apply as well. Some services, such as premium channels like HBO, require a minimum 30-day billing period.