When you have a Java Date field in the AppEngine JDO and try to enter the new value using Google Web interface:
Data Viewer -> Edit Entity or Create an Entity
you may be getting this error: "Could not instantiate int: invalid literal"
I think this is a bug, as this field should be a Date not an int, but there is a simple work-around:
1) change the int to null, then save
2) change the null to gd:when and you can enter a date in the format