Hi Yusuf,
First of all, assuming that createdDate is a date/time field, the date in the query should be entered without quotes, e.g.
FIELD createdDate < 10/3/2011
Your problem might be related to the UNK table (an internal table of all the fields in a database, see http://www-01.ibm.com/support/docview.wss?uid=swg21100238). I had some issues with FT searching in date fields in the past.
If I'm correct, fields (including the field type) are added to that table when the first document is created in a database containing that field. If that document has the date field on it but it has en empty value (""), the field is added as a text field to the UNK table and you won't be able to perform FT searches on it using date values. You can workaround this by never setting an empty string in a date field as a value, although this won't solve the problem for existing databases. The UNK table will probably be re-created on a database copy, but you'll have to test that.
Mark