Period Units in Updating: Purchasing 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 of the update for every standard information structure.

In the SAP standard system, the default period unit for updating all the information structures in the Purchasing Information System is month. To update the information structure S012 for long-term planning, refer to Updating Standard Analyses for Long-Term Planning: Purchasing Information System.

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