A State shall specify in its plan for AFDC the time period covered by the payment (payment month) and the time period used to determine that payment (budget month) and whether it adopts (a) a one-month or two-month retrospective system; and (b) a one-month or two-month prospective system for the initial payment months. If a State elects to have a two-month retrospective system it must also elect a two-month prospective system.