Due Date Calc

Hi Experts,

I am an ABAPer. I am doing an interface to block/unblock a customer based on his credit limit (KNKK table) for a specific Co Code and block him on

KNB1-SPERR.

My Requiremnt : I need to calculate due date for a customer whose payment is in arrears or overdues?

Please tell me how to do this?

Thanks

Dany