That seems a little complicated for a Fiscal Year function.

The Fiscal Year Logic can be pared down to

select YEAR(DATEADD(m,3,RegistrationDate)) as Fyear

,MONTH(DATEADD(m,3,RegistrationDate)) as Fmonth

-- OR A Table Function

CREATE FUNCTION dbo.FiscalYear

(@SomeDate DateTime)

RETURNS TABLE WITH SCHEMABINDING AS

RETURN SELECT Fyear = YEAR(DATEADD(m,3,@SomeDate))

,Fmonth = MONTH(DATEADD(m,3,@SomeDate))

;

GO

SELECT

fy.Fyear

,fy.Fmonth

from YourTable

CROSS APPLY dbo.FiscalYear(RegistrationDate) fy

Or if you prefer to do it in SSRS create a calculated field with vb.net

=Year(DateAdd(DateInterval.Month,3,Fields!RegistrationDate))

=Month(DateAdd(DateInterval.Month,3,Fields!RegistrationDate))

As stated earlier, you can also build a calendar table in memory and do a join, provided that the Registration Date is just the date value and doesn't have time values. I've been burned on calendar tables with fiscal data I find it easier and usually faster to do the DATEADD function