Determining Basic Dates 
The system carries out a planning run that uses the requirements date to determine the basic order dates. The basic order dates are calculated to the exact day.
The system carries out backward scheduling to determine the basic dates. It starts from the finish date and calculates backwards using the goods receipt processing time and the in-house processing time maintained in the material master and thus calculates the basic start date.
If backward scheduling generates a basic date in the past then the system automatically switches to forward scheduling and sets the current date as the basic start date. You can deactivate the automatic switch to forward scheduling for each plant in Customizing for MRP. Then the system executes backward scheduling even if a basic start date was calculated that starts in the past.