Derby, a pure Java SQL database, doesn't support date substraction!
ij> SELECT DATE('2000-01-10') - DATE('2000-01-01') AS date_diff FROM dict_country;
ERROR 42Y95: The '-' operator with a left operand type of 'DATE' and a right operand type of 'DATE' is not supported.
Derby was originally an IBM product, under name "Cloudscape", released on Apache license by IBM some time ago. It's a very nice database engine, but those dates… what the heck?