Report parameter default value do not default correctly

Question

Hello Experts, I have a cascading parameters in my report.
The first parameter called "Project" will allow to select a single Project Name from the list and based on the Project Name, the second parameters which is a "Date" parameter will give out list Months and Year in the Format 'Nov-14'. 
For this Second parameter I defaulted the parameter to show First Month in the retrieved list of Months. But the problem is it will display the default Date correctly only certain times (there is a refresh problem). If i select a different project and it
has a month that was retrieved in the first project it will still show the that Month but not the actual default month (Start Month) for that particular Project.
Example: If I select Brisk from Project parameter, It will give me 18 months of Dates in the Date parameter. In that 18 month range first month is Sep-13 it will default to Sep-13 correctly. Then if I select a different project like "Creek" (example)
as Project , the Date parameter retrieves some dates corresponding to Creek Project and in the retrieved months if  Date parameter has Sep-13 it will still show Sep-13 even though the
Start Date for Creek project is Jun-12.
1. Project Brisk: Defaulted correctly to Sep-13
2. Creek Project : Should default to Jul-12 but still shows Sep-13
Just for default value I created a separate dataset and used it to populate the default value and the SQL I used just for default value is as below:
SELECT DISTINCT TRUNC(DATE_KY, 'MM') AS DEFUALT_DATE
FROM            DATE_D D
WHERE        (DATE_KY = ADD_MONTHS(:PROJECT_DT, - 6))
Date parameter datatype is Date/Time,
Available Values Tab: label field is a String but the Value field and the default field (Default Values tab) are Dates 
Thanks in advance

Answer 1

Thanks Vicky for reply.
Yes you are right but I did not use First function as the query above retrieves only one date.
I used your suggestion but it did not fix it.
In this report I have three parameters.
1. Project Code (Visible)
SELECT DISTINCT PROJECT_CODE
FROM      DIM.PROJECT     
ORDER BY PROJECT_CODE
2. Project Date (Invisible)
SELECT
MAX(CASENA.MILESTONE
WHEN'ABCD'THENTRUNC(PROJECT_DATE,'MM')ELSENULLEND)PROJECT_DATE
FROM
DIM.PROJECTNA
WHERE(NA.PROJECT_CODE
=:PROJECT_CODE
or'ALL'=:PROJECT_CODE)
3. Start Date (Visible):
Available Values Query:
SELECT DISTINCT
--DER_DATE is "Value Field"
TRUNC(D.DATE_KY, 'MM') DER_DATE 
--Display_date is "Label Field"
, TO_CHAR(D.DATE_KY, 'Mon-YY')   Display_Date
FROM DATE_D D
WHERE DATE_KY BETWEEN ADD_MONTHS(:PROJECT_DATE, -6)  AND ADD_MONTHS(:PROJECT_DATE, 6)
ORDER BY 1
Default Values Query:
SELECT DISTINCT TRUNC(DATE_KY, 'MM') AS DEFUALT_DATE
FROM            DATE_D D
WHERE        (DATE_KY = ADD_MONTHS(:PROJECT_DATE, - 6))
When you select a Project from "Project Code", it will give a "Project Date" and based on the "Project Date" it will give a date range in "Start Date". That is why I am using "Project Date" parameter in my
query. I even tried using Project code as you suggested but I still got the same result.
Thanks a lot for your reply.