You need to Pivot your 2 columns start date and due date using tableau pivot functionality. Afetr that your data will look like this:
number startdate/duedate(pivot value) pivot name
55 7-15-2018 start date
55 8-7-2018 due date
Then you can just drag month and day from startdate/duedate(pivot value) this field to show on your viz.
Please find my solution attached.
Below is the steps.
1st, join your data to a calendar data by using the key 1=1
2nd, create a filter to filter out the unnecessary data
Hope this helps
Book2_v10.2.twbx 12.1 KB
Is this calendar table present in Tableau by default? or have you created one?