Your billing date is determined by the activation of your phone number.
Porting a number to NumberGarge
If you port to NumberGarage, you are only charged when your port completes. On the day your number transfers, you will be billed for your first month and the one time setup fee.
Purchasing a new number
If you purchased a new number with NumberGarage, you will be charged when that number has been activated. On the day your number activates, you will be billed for your first month and the one time setup fee.
This day of the month will now be your monthly billing date. (e.g. if your number transfers on 10/10, then every month on the 10th, you will be billed for that month of service).