Run the below function in the background script to calculate total Working days using Schedule.

function getDiff(date1, date2) {
        /*
        @param1- Date value
        @parm2- Date value
        @return- woking days value in second 
        */
        var updatedDate = new GlideDateTime(date1); //old date
        var currDate = new GlideDateTime(date2); //latest date
        var schedID = 'Pass the sys_id'; //Parent sys_id of the schedule
        var durationCalculator = new DurationCalculator();
        durationCalculator.setSchedule(schedID);
        var diff;
        if (currDate > updatedDate) {
            diff = durationCalculator.calcScheduleDuration(updatedDate, currDate);
        } else {
            diff = durationCalculator.calcScheduleDuration(currDate, updatedDate);
        }
        if (diff != '' || diff != 0 || diff != '0' || diff != undefined || diff != null)
            return diff; //diff / 60 / 60 / 24 - To convert into days
        return 0;
    }
gs.info(getDiff('2022-02-11 15:23:25','2022-02-18 15:23:25'));
gs.info(getDiff('2022-02-11','2022-02-18'));

Please Mark it helpful if it helps you.