Customer exit varailble creation on 0FISCPERIOD to get Rolling 12 months

Hi Experts,

I need help with BW Customer Exit code. This is my requirement. We need to fetch last 12 months data by designing a filter on 0FISCPER, the code should go back 12 months back from there and apply that period as the lowest range and the current period as
the highest value range. Can anyone help??

EX: Rolling 12 months data has to be displayed

When user checks the data today (09/2015) he should be able to see the data back to 12 months
data(09/2014) as well.

Thanks in Advance,

Srinivas.