Billing date differ from Created on Date

Hi SD Gurus

I have Order related Billing wherein i have seen that in the Invoice the created on and billing date is different.

In invoice the created on date is : 27th March whereas billing date is 29th march 2010.

The accounting document creation is automaticfor client.. In quality if i run a full cycle the system takes same date i.e. created on and Billing date in invoice is same.

The factory calendar says 27th march is a working day and in Holiday calendar also 27th is working day.

Now how the billing date differ from Created on date ?????

Also how can i get a Error message or Pop-up when in Invoice "created on and Billing date" differ.

Is there any standard functionality in SAP apart from using userexits.

Regards

Abhijeet