Customer exit Charecteristic value for date range

Hi Experts,

I wanna know how to get the current and previous month/year for a interval variable.

ex: user input date range: 004.2013 - 004.2014 and I wanna get " 004.2012 - 004.2013"

How to write a customer exits to get the above values?

(am using infoobject 0FICPER for the date variables)

Thanks,

Thilini