Add working days to a date in SalesForce, taking into account weekends and company holidays

DateHelper.cls

global class DateHelper {
    public static Date addBussinessDays(Date startDate, Integer iDays)
    {
        Integer businessDaysAdded = 0;
        Date currentDate = startDate;
        while (businessDaysAdded < iDays) {
            currentDate = currentDate.addDays(1);
            Datetime d = datetime.newInstance(currentDate.year(), currentDate.month(),currentDate.day());
            if (d.format('E') != 'Sat' && d.format('E') != 'Sun' && checkifItisWorkingDay(currentDate)) {
                // it's a business day, so add 1 to the counter that works towards the amount of days to add
                businessDaysAdded = businessDaysAdded + 1;
            }
        }
        return currentDate;
    }
    
    public static List<Holiday> holidays {
        get {
            if(holidays == null)
                holidays=[Select h.StartTimeInMinutes, h.Name, h.ActivityDate From Holiday h];
            return holidays;
        }
        private set;
    }
    
    public static boolean checkifItisWorkingDay(Date currentDate){
        Date weekStart = currentDate.toStartofWeek();
        for(Holiday hDay:holidays){
            if(currentDate.daysBetween(hDay.ActivityDate) == 0){
                return false;
            }
        }
        datetime predicatedDateTime = datetime.newInstance(currentDate.year(), currentDate.month(), currentDate.day());
        
        // if date fall in weekend return false
        if(predicatedDateTime.format('E') == 'Sat' | predicatedDateTime.format('E') == 'Sun'){
            return false;
        }
        else
            return true;
    }
}