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Since you never mentioned how you want the calculation / computation to work, I'll just use an example form that computes a due date, 7 days after the selected date on the form.
Here's the demo form: https://form.jotform.com/70351509403953
Just pick any date from the DATE FIELD, and the CALCULATED DUE DATE field will automatically add 7 days to it.
I just used conditions to make it happen. Here's how it was setup:
If you wish to adjust the due date (e.g. 20 days after) then just change the calculation from +7 to +20 (and so on).
You may clone my form so you can have a closer look.
Related guide: How-to-clone-an-existing-form-from-a-URL