Summation using OffSet

Hi Friends,

I know you guys would help me out in this. i am pretty new to SAP and need some help on one of the report I am working on.I need to create a formula that sum the next 30 days values.

For example: sum of <b>Distribution Demand</b> for the next 30 days.

What's the best way to get this? Should we write a customer exit or the offset would work easily.

Thanks.