Reg VF01 Billing Date

Hi Sdn frnds,

In Vf01 Delivery Date should not be greater than Billing date. For this What i have to. I have to find out User exit for this. But i am not able to find a particular Userexit.HOw can i find Out. The Billing date field is FKDAT.

Please Give me some suggestion.

Thank you very much.

With regds

Sirisha Reddy.