Without an attached workbook its hard to know what has gone wrong. I created an excel file based on your picture and then used that as my source in Tableau. Below are the steps I followed.
1) Convert the string dimensions to date dimensions. I think this is where you might have gone wrong since you can't work with string dimensions that way.
2) Create a calculated field: IF [Date 1] > [Date 2] then 1 else 0 END
3) Line them up next to each other and add a column grand total
Let me know if you have any questions!
tableau test.twbx 40.7 KB
The calculation field should be IF [Date 2] < MIN([Date 1]) ... because I need that every cell verify if it's less then the minimum of the date 1.
I attached the easy resolution in Excel
Example1.xlsx 84.6 KB