BAPI/user exit for determining the billing date of reference billing doc no in VA01

Hi all,

I have a requirement in which while creating a returns sales order in VA01,when the reference billing document is entered,I have to calculate if the reference billing document's billing date is earlier than 180 days or not.If yes,then a message should be displayed to the user.Is there any BAPI/user exit for this?

Kindly help.