Processing options MRP - number of planning weeks and planning months

MRP R3482: we work with 2 planning weeks and 12 planning months.

My question now:

Szenario: Today is months end; we have 2 planning weeks, that means in middle of the next month (for example 2015/04/15), the month periods start.

I have to figure out if the first month period is from 2015/04/16  until 2015/04/30 (rest of the current month) or until 2015/05/16. (one complete month period)

This is very important for planning with BF and I cannot find any explanation in the know ledge garden about this.