There are 3 ways this should be done.
Automatic, hidden form item. For example, if this is a request for X, and processing that request shouldn’t take more than 2 days, then this field should be set to Today + 2 days. And that’s the due date that’s automatically applied.
Allow the submitter to set the due date.
By logic. Here the due date could have a default date, unless some other parameter was set. For example:
Default Due Date = Today + 2 Days
IF “Rush” field = “Rush Today” THEN = Today
IF “Rush” field = “Rush Tomorrow” THEN = Today + 1 Day