SSRS default date expression I have FROM and TO date parameters. I need to assign default values for them.

For example, if we run the report today (12 September), the default values for FROM and TO should be 10/01/2016 and 03/31/2017. as the current month (9) satisfies in the second condition below:

Based on the report run date (current date):

IF Month                       then From Date               and To Date
&gt;=1 and &lt;=3                    4/1/prior yr                 9/30/prior yr
&gt;=4 and &lt;= 9                   10/1/prior yr                3/31/curr yr
&gt;=10 and &lt;=12                  4/1/curr yr                  9/30/curr yr


Could you tell me the SSRS 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