Modify the embedded SQL and use a CASE expression to generate the column. Much easier to manage and maintain than trying to embed IIF statements in an expression.
Better yet - convert that embedded SQL to a stored procedure and use a stored procedure to define the dataset. Using a stored procedure would allow you to modify the hard-coded dates without having to redeploy - and you could even dynamically create the dates based on when the procedure is executed - or you can derive the values based on other criteria.