The Billing Period of an account is managed by pressing the "Account" button or Admin > Customers and choosing the customer to edit.

From the drop-down of the Billing area, there are many choices of 1,3 or 6 Months starting the day you update the information. Also one can start scheduling to start data collection for the next 1,3 or 6 months at the start of the next month.

Choosing "Custom Period" will allow you to press the Start and End areas to choose a date to start and then End on for starting and stopping data collection.

Note: It is important to understand, dates cannot be set to any date in the past or edited when expired.

You can only set start and end billing, to dates in the future.

When the previously set start date has expired (and is a date in the past) you will not be able to change the start date - same goes for end date.