Deriving Working days based on the schedule of the employee

We have a requirement whereby customer wants to derive the Working days based on the assumption that employee was in a schedule from the start of the month to the end of month e.g.

An employee was in a Part Time Schedule(M to Wed) between 01-Sep-2024 to 08-Sep-2024 with Salary of 2000 GBP and from 09-Sep to 30-Sep the emp 's schedule changed to a full time schedule(M to FR) and with a new salary of 3000 GBP.

As per the business requirement, the customer wants the working days for Part Time Schedule(M to Wed) Schedule to be derived assuming emp was in the this schedule from 01-Sep to 30-Sep.