User Exit for field Modification(Gap Analysis)

USEREXIT_SAVE_DOCUMENT_PREPARE. This user exits becomes active when you save the document.
The requiremento to the ABAPer should be the following: 1.-If Document type "B"(quotations),and if document class "QT"(this if you want to specify an specific type of quotation), 2.-if field VBAK-BNDDT blank , Set the field VBAK-BNDDT to 30 days, else if field VBAK-BNDDT not blank and value <> 30 days, do nothing(this if you want to accept more than 30 days of quotation validity), if not just set the 30 days always. This is if you want it to be set automatically, if not just validate that if its not the value 30 days or more, send a message and force the user to insert the correct value.