SSRS default date expression
Ok, it seems I finally understood what you want.
Create a dataset date_parameters with the following query:
select case when month(getdate()) &lt;= 3 then '4/1/' + cast (year(getdate()) - 1 as char(4))
            when month(getdate()) &lt;= 9 then '10/1/' + cast (year(getdate()) - 1 as char(4))
            when month(getdate()) &lt;= 12 then '4/1/' + cast (year(getdate()) as char(4))
       end as from_date,
       case when month(getdate()) &lt;= 3 then '9/30/' + cast (year(getdate()) - 1 as char(4))
            when month(getdate()) &lt;= 9 then '3/31/' + cast (year(getdate()) as char(4));
            when month(getdate()) &lt;= 12 then '9/30/' + cast (year(getdate()) as char(4))
       end as to_date 


Then in your default parameter values choose the option Get values from a query
Specify your date_parameters dataset and choose from_date for your FROM DATE parameter and the same dataset but to_date as value for TO DATE parameter

And here is the result:

P.S. Adjust case constants as you need (with padding zeros), only now I noticed that in the question you wrote 10/1/2017 but in the comment it became 10/01/2017. I used your constants from the question, i.e. without zeros