static int CalculateBusinessDays(DateTime firstDate, DateTime secondDate)
        {
            TimeSpan interval = (secondDate - firstDate);
            int totalWorkingDays = 0;

            int numberOfWeeks = interval.Days / 7;
            int numberOfWorkingDays = numberOfWeeks * 5;

            int daysLeft = interval.Days % 7;
            for (int i = 0; i < daysLeft; i++)
            {
                DayOfWeek test = (DayOfWeek)(((int)firstDate.DayOfWeek + i) % 7);
                if (test>= DayOfWeek.Monday && test <= DayOfWeek.Friday)
                {
                    totalWorkingDays++;
                }
            }
            return totalWorkingDays + numberOfWorkingDays;