Check billing date

Hi all,

I have to check the billing date when the user save the document at creation.

My user wants that the billing date is the actual goods movement date (LIKP-WADAT_IST) when the billing document is immediate (not periodic billing document type ).

I suppose to check the billing date in an user-exit a save document,but I don't be able to find the right user-exit.

Can someone help me. please?

Thanks in advance

Alex