SQL Server 2017

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CREATE FUNCTION [dbo].[DATEDIFFWD](@interval varchar(2), @ini datetime, @end datetime)
RETURNS int
AS
BEGIN
    DECLARE @output int
    SET @output = DATEDIFF(@interval, @ini, @end)
    IF (@interval = 'hh') SET @output = @output - (DATEDIFF(WK, @ini, @end) * 48)
    IF (@interval = 'dd') SET @output = @output - (DATEDIFF(WK, @ini, @end) * 2)
    RETURN @output
END
Msg 1023 Level 15 State 1 Line 6
Invalid parameter 1 specified for datediff.

CREATE FUNCTION [dbo].[DATEDIFFWD](@interval varchar(2), @ini datetime, @end datetime)
RETURNS int
AS
BEGIN
    RETURN (SELECT CASE @interval 
            WHEN 'hh' THEN DATEDIFF(hh, @ini, @end) - ((DATEDIFF(WK, @ini, @end) * 48))
            WHEN 'dd' THEN DATEDIFF(dd, @ini, @end) - ((DATEDIFF(WK, @ini, @end) * 2))
            END
    );
END
DROP FUNCTION dbo.datediffwd;
CREATE FUNCTION [dbo].[DATEDIFFWD](@interval varchar(2), @ini datetime, @end datetime)
RETURNS TABLE
AS
RETURN (
    SELECT CASE @interval 
       WHEN 'hh' THEN DATEDIFF(hh, @ini, @end) - ((DATEDIFF(WK, @ini, @end) * 48))
       WHEN 'dd' THEN DATEDIFF(dd, @ini, @end) - ((DATEDIFF(WK, @ini, @end) * 2))
    END AS r
);