I'm bringing this up to the top because I have the exact same issue. I can't understand why it's not working. I am definitely using the correct fieldname. Is there something else in the calculated field I should be doing? Is there another way to ask for a custom date field that is 3 days earlier than another date field in the same form?
This is the calculated field, chose "date" as the format and I get an error "Custom Expression Invalid"
ADDDAYS(Video - first edit due date,-3)