This is definitely something you can do in Airtable. Let’s use dueDate, beforeAfter, and payDate as field names. DueDate is the date it’s due, beforeAfter is the info from the T&Cs, payDate is the day it gets paid. Because I’m on mobile, this particular formula says says that Before payments happen on Thursday or Friday and After payments happen on Monday or Tuesday. Typing long formulas on mobile is the worst.
IF(AND(beforeAfter =“Before”, OR(WEEKDAY(dueDate)=6, WEEKDAY(dueDate)=0)), DATEADD(dueDate, -2, ‘days’), IF(AND(beforeAfter=“After”, OR(WEEKDAY(dueDate)=6, WEEKDAY(dueDate)=0)), DATEADD(dueDate, 2, ‘days’), dueDate))