Don't you mind to post a package workbook with sample data? That will facilitate us to find a solution for you.
You'll need to reshape your data to do this in order to get the dimension to use to build the calculated fields (which is step 2 in the answer marked Correct on the post you linked). You can use the Tableau data reshaper for Excel at Installing the Tableau Add-In for Reshaping Data in Excel | Tableau … , or a set of UNION queries or potentially a cross-product query.
The reason why is that to draw the grouped bars, you need a dimension in the data that can split up the measures so the bar measures can have dates that are slightly ahead&behind the date aggregation, while the line is drawn on the exact date aggregation. Tableau doesn't give us that capability inside of Tableau, so we have to feed data the right way.