The Billing Date is the date on which the entire Billable Amount is to be accounted for. It is set differently depending on the billable object:
Task – By default, the Billing Date is identical to the task Finish date. The user can then modify it as needed.
Project Transaction – The Billing Date (read-only) is identical to the Transaction date.
Task Transaction – The Billing Date (read-only) is identical to the task Billing Date.