Automatically populate a date field with value greater than the current date in other date field

I run a communications team at a church and we serve the ministries inside the walls of the church (habitat for humanity, soup kitchen, etc, etc) 

Our goal is to have a jotform in place that assists them in submitting information to be shared with our parish. 

Certain things like bulletins are published and design 3 weeks in advance. How can I set up a formula that does something like show the current date in one field and then automatically populate a date that is not less than three weeks from the current date? At that point I would like for them to be able to punch in that date or another date if possible. 

You can do this by setting up a conditional logic in the form. Here's how: https://www.jotform.com/help/268-How-to-Insert-Text-or-Mathematical-Caculation-into-a-field-using-a-Condition.

Here's a sample form: https://form.jotform.com/60414064044949. Here's how the condition is setup:

Current date + 21 days will be populated in the target date field.