What's probably happening is that the date is being populated through the Btrieve interface with spaces instead of a valid value. The value 00-00-0000 is invalid according to the ODBC Specifications.
The proper solution to this problem is to get the software vendor to put a valid date or null for this non-transferred field.
You might try converting the date to a string and seeing if you can restrict that way. For example, SELECT * FROM TABLE WHERE CONVERT(FIELD, SQL_CHAR) = ' ' where the value between single quotes is 6 spaces. You might have to try different values to see what works.