But what do you want this date to be? You've mentioned you have a separate logic - where did you put it?
I'm also not clear what exactly is the confusion regarding the copy routines... If it's a header field (VBAK-VDATU) then naturally it's not an item routine. The routine examples available in SAP seem self-explanatory. You put some value in the field, that's all there is...
Also have you tried debugging to check if the field is not getting overwritten?