If I use Assign to-do task in my workflow it asks for a "Due Date" which will be different everytime I use the Workflow? How do this work if I don't have a Due Date?
No. But you have no details on what version of nintex you are using, sharepoint version. There was an early bug in SP2013 Nintex workflows 2013 on premise that is now resolved in recent updates where the Due Date was required. But this is no longer the case.
What version of Nintex are you using? What SharePoint version?
SharePoint 2013 and Nintex Workflow 2013 220.127.116.11.
Do you have any issues publishing the workflow when not assigning a due date? Or do you have a question on when to use the due date?
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