I pretty much don't even know how to spell "SSRS". With that though in mind, here's how I'd do it in T-SQL. I would imagine that it wouldn't be much more difficult to do in SSRS.

--===== Here are the two parameters + a working variable.

DECLARE @StartDate DATETIME

,@EndDate DATETIME

,@Swap DATETIME

;

--===== This simulates passing the parameters

SELECT @StartDate = GETDATE()

,@EndDate = GETDATE() +10 --<<Change this number to test

;

--===== This will put the parameters in the correct order

-- if they're out of order. This is the code I'm talking about.

SELECT @Swap = @StartDate

,@StartDate = @EndDate

,@EndDate = @Swap

WHERE @StartDate > @EndDate

;

--===== This just verifies the result.

SELECT @StartDate, @EndDate

;