Modify Date without DateAdd or DateDiff

Question

Is it possible to modify the day part of a date without having to use DateAdd or having to calculate the date difference?

SET @ReturnDate = '5/16/2012'

SET @ModifyDay = 20

Modify only 16 to 20

Result should be 5/20/2012

The reason for this is that I am translating vb code into a stored proc:

    Dim InvDate_Day_of_Month As Integer = CInt(Day(InvDate))
    PaymentDate = DateAdd(DateInterval.Month, 1, InvDate)
    PaymentDate = New Date(PaymentDate.Year, PaymentDate.Month, DayofTheMonth)

Answer 1

SET @ReturnDate = @ReturnDate + (@ModifyDay - DATEPART(day, @ReturnDate))

Answer 2

If you are just trying to add 4 days to the date, you can literally just add 4 to it:

DECLARE @ReturnDate DATETIME;
SET @ReturnDate = '5/16/2012'

SELECT @ReturnDate, @ReturnDate + 4

This selects 2012-05-16 00:00:00.000 and 2012-05-20 00:00:00.000