could it be that you are using webservices ? I believe in that case andromda uses the Calendar, and bpm4struts is probably not mapping it to the same type (could be a matter of setting properties in the correct namespace in andromda.xml)
Yes, I am using webservices
However, meanwhile I switched to Timestamp.
By the way what is the real difference between Timestamp and DateTime? From conceptable view it seems to be the same. I did not found a docu on that