The billing schedule period is linked to the project's Contract details. If you only change the dates inside the billing schedule, the change may not take effect. So change the From and To dates in Contract details on the project, and the schedule updates with the new period.
How to change the period
Click the organization or region at the top of the left menu.
Hover over the project in the list, and click the pencil that appears.
Select Contract details in the left margin.
Change From date and To date.
Save and go back to Billing Schedule Client. The schedule is updated with the new period.
Which dates should I choose?
Start: Make sure the first invoicing is included. The project start may be before the first invoice.
End: Set the date far enough ahead to include the final settlement and withheld amount, for example a couple of months after handover.
Good to know
The dates can be changed later.
These are contract dates, not the base month for index regulation. The base month is set separately in the LPS setup on the billing schedule.