Period Units in Updating: Sales Information System 
In addition to the type of update, you can also define the period unit to be used in the updating process.
The period unit specifies the intervals in which the statistical data is to be cumulated. You can choose from the following periods:
You can determine the period unit for the updating of every standard information structure.
The SAP default setting for the period unit in all the SIS information structures is posting period.

The prerequisite for changing the period unit is that no actual data has been written to the information structure.
You can set the period unit for the standard information structures in Customizing for the Logistics Information System.