How to add number of working days to a date field?

In my form I have two date fields. Date field 1 ('Date current') represents the current date. I want to add 20 working days (also excluding public holidays according to Western Australian Calendar) to the date field 2 ('Date new'). Can anyone suggest a fix?

Without coding I can only add 20 calendar days as below, but it's not the fix I am after.