Given the Start Date and No. of Business days  find ending date, if you pass a negative amount of days then should get Current Date less those business days,

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    public static Boolean IsWeekendDay(Datetime dateParam)
    {
       boolean result     = false;
        
       //Recover the day of the week
       Date startOfWeek   = dateParam.date().toStartOfWeek();
       Integer dayOfWeek  = dateParam.day() - startOfWeek.day();
           
       result = dayOfWeek == 0 || dayOfWeek == 6 ? true : false;
        
       return result;
    } 
    
    
    public static Datetime AddBusinessDays(Datetime StartDate, integer BusinessDaysToAdd )
    {
       //Add or decrease in BusinessDaysToAdd days 
       Datetime finalDate = StartDate;
       
       integer direction = BusinessDaysToAdd < 0 ? -1 : 1;

        while(BusinessDaysToAdd != 0)
        {
            finalDate = finalDate.AddDays(direction);            
            
            if (!isWeekendDay(finalDate))
            {
                BusinessDaysToAdd -= direction;
            }
        }

        return finalDate;
    }