How to send offset as a input to customer exit variable

Hi Experts,

I have a  requirement  like,for 0CALWEEK we need to pass offset value 13 then it should return  result =Current+13 weeks .If the week is 52 then it should return 52th week only.Please let me know is this possible through BEx variable as a customer exit?

First of all, if i tried to enter numeric value they is through error like ' 13 value not available in 0CALWEEK' .Please guide how can i overcome this error and

display the desired value.

Thanks & Regards,

Anjan