If you want to be able to do searches using greater than and less than on two dates you have two options:
- perform the query on actual date fields
- perform the query on fields that contain a number representation of the dates (e.g. 20111001)
I've had some problems in the past with performing FT queries on date fields that were related to an incorrect field datatype stored in the database's UNK table (see also http://www-10.lotus.com/ldd/nd6forum.nsf/0/6976b1cd5642d70e8525718500749106?OpenDocument). You might want to check into that.
Mark