You need to make small modifications in the way use define the interval. One possibility is to use structure like the following:
DECLARE @intervaltype int;
DECLARE @datetouse date
SET @datetouse = GETDATE()
SET @intervaltype = 1;
SELECT CASE @intervaltype
WHEN 1 THEN DATEADD( day, 7, @datetouse)
WHEN 2 THEN DATEADD( month, 7, @datetouse)
END
In the real situation you would replace the variable with some columns from some table.