Both java.sql.Date and java.util.Date return the following when trying to persist them in the datastore.
javax.jdo.JDOUserException: Attempt to handle persistence for object using datastore-identity yet StoreManager for this datastore doesn't support that identity type
What data types do you use for date/time values?
Are you sure the problem is with the date field? It sounds more like your object is not set up right to work in GAE - do you have a primary key field set up on your entity? Is your code top secret or can you show us your entity code?