SAP Note 19208 - Cross-period update of the confirmation

Symptom:

If you confirm services for production orders (with transaction CO11), which have already been rendered in the previous period, in the current period for organizational reasons, the service update and possibly also the backflush is made in the current period even if the start date and finish date are in the previous period.
The current date which is proposed in the field 'Posting date' is set as posting date.

Cause and prerequisites

The confirmed services are updated according to the date that is entered in the field 'Posting date' in the actual data or personal data screen. As a default, the entry date is proposed.

Solution
If you want to update the confirmation in a previous period, you have to overwrite the proposed current date in the field 'Posting date' in the actual data or personal data screen with the required date.