Hi,

If you check the hr_payroll module you can see similar computations. Open the payslip form and change the Period From and To dates, then you can see days get computed in the One2many field. You can reuse the same function for your purpose.

The function name is get_worked_day_lines in this file : https://github.com/odoo/odoo/blob/12.0/addons/hr_payroll/models/hr_payslip.py

You can see this line,

work_data = contract.employee_id.get_work_days_data(day_from, day_to, calendar=contract.resource_calendar_id)

Thanks