How much work does it take to change on a program, from "Monday to Sunday" to whichever 7 days in a row?

Let’s say you’re trying to buy something using computerized system that only recognizes weekly or 7 days in a a row to mean Monday to Sunday and 5 days in a row (say full time job) to mean Monday to Friday. If you want it changed to whatever x number of days in a row, because one person’s need of 7 days in a row could be Wednesday to Tuesday, etc., or someone else’s full time job could be something other than Monday to Friday. So how easily do-able is it?