Once you are Medicare certified, you will be able to bill for all dates of service as long as you tell Medicare "We will begin seeing Medicare patients on 4/1/12". Your application has a place to state when you will begin treating patients. Once you are certified you can bill all, from that date forward. BTW April to October seems like an awfully long time. Make sure your application is perfect the 1st time! Any mistake just slows down the process.