This article explains a step by step instructions on how to copy the value of the next due date custom field into the due date field by using COT app.
Instructions
The following is the custom field called "next due date"(custom field type is a date) and its value is set to 18/Dev/19.
Update the post function from the respective workflow to copy the "next due date" value into due date customfield by following the below steps.
You need to use the substitution variable to copy the value from the next due date to the due date field.
...