Calculate Business Hours

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 public static double CalculateBusinessHours(DateTime dtStart, DateTime dtEnd)
    {
        int StartingHour = 9;
        int EndingHour = 18;
 
        // initialze our return value
        double OverAllMinutes = 0.0;
 
        // start time must be less than end time
        if (dtStart > dtEnd)
        {
            return OverAllMinutes;
        }
        DateTime ctTempEnd = new DateTime(dtEnd.Year, dtEnd.Month, dtEnd.Day, 0, 0, 0);
        DateTime ctTempStart = new DateTime(dtStart.Year, dtStart.Month, dtStart.Day, 0, 0, 0);
 
        // check if startdate and enddate are the same day
        bool bSameDay = (ctTempStart == ctTempEnd);
 
        // calculate the business days between the dates
        int iBusinessDays = GetBusinessDays(ctTempStart, ctTempEnd);
 
        // now add the time values to our temp times
        TimeSpan CTimeSpan = new TimeSpan(0, dtStart.Hour, dtStart.Minute, 0);
        ctTempStart += CTimeSpan;
        CTimeSpan = new TimeSpan(0, dtEnd.Hour, dtEnd.Minute, 0);
        ctTempEnd += CTimeSpan;
 
        // set our workingday time range and correct the first day
        DateTime ctMaxTime = new DateTime(ctTempStart.Year, ctTempStart.Month, ctTempStart.Day, EndingHour, 0, 0);
        DateTime ctMinTime = new DateTime(ctTempStart.Year, ctTempStart.Month, ctTempStart.Day, StartingHour, 0, 0);
        Int32 FirstDaySec = CorrectFirstDayTime(ctTempStart, ctMaxTime, ctMinTime);
 
        // set our workingday time range and correct the last day
        DateTime ctMaxTime1 = new DateTime(ctTempEnd.Year, ctTempEnd.Month, ctTempEnd.Day, EndingHour, 0, 0);
        DateTime ctMinTime1 = new DateTime(ctTempEnd.Year, ctTempEnd.Month, ctTempEnd.Day, StartingHour, 0, 0);
        Int32 LastDaySec = CorrectLastDayTime(ctTempEnd, ctMaxTime1, ctMinTime1);
        Int32 OverAllSec = 0;
 
        // now sum-up all values
        if (bSameDay)
        {
            if (iBusinessDays != 0)
            {
                TimeSpan cts = ctMaxTime - ctMinTime;
                Int32 dwBusinessDaySeconds = (cts.Days * 24 * 60 * 60) + (cts.Hours * 60 * 60) + (cts.Minutes * 60) + cts.Seconds;
                OverAllSec = FirstDaySec + LastDaySec - dwBusinessDaySeconds;
            }
        }
        else
        {
            if (iBusinessDays > 1)
                OverAllSec =
                ((iBusinessDays - 2) * 9 * 60 * 60) + FirstDaySec + LastDaySec;
        }
        OverAllMinutes = OverAllSec / 60;
 
        return OverAllMinutes / 60;
 

    }

 public static DateTime AddBusinessDays(DateTime dt, int nDays)
    {
        int weeks = nDays / 5;
        nDays %= 5;
        while (dt.DayOfWeek == DayOfWeek.Saturday || dt.DayOfWeek == DayOfWeek.Sunday)
            dt = dt.AddDays(1);
 
        while (nDays-- > 0)
        {
            dt = dt.AddDays(1);
            if (dt.DayOfWeek == DayOfWeek.Saturday)
            {
                dt = dt.AddDays(2);
            }
        }
        return dt.AddDays(weeks * 7);
    }
 
    private static int GetBusinessDays(DateTime ctStart, DateTime ctEnd)
    {
        TimeSpan ctp = ctEnd - ctStart;
        int iDays = ctp.Days + 1;
        int iWeeks = iDays / 7;
        int iBusDays = iWeeks * 5;
        int iRem = iDays % 7;
        while (iRem > 0)
        {
            // no sunday, no saturday
            int iStartDay = (Int32)Enum.Parse(typeof(DayOfWeek), ctStart.DayOfWeek.ToString());
            if (iStartDay != 1 && iStartDay != 7)
            {
                iBusDays++;
            }
            TimeSpan time1 = new TimeSpan(1, 0, 0, 0);
            ctStart += time1;
 
            iRem--;
        }
        return iBusDays;
    }
 
    private static Int32 CorrectFirstDayTime(DateTime ctStart, DateTime ctMaxTime, DateTime ctMinTime)
    {
        Int32 daysec = 0;
 
        if (ctMaxTime < ctStart) // start time is after max time
        {
            return 0; // zero seconds for the first day
        }
        int iStartDay = (Int32)Enum.Parse(typeof(DayOfWeek), ctStart.DayOfWeek.ToString());
        if (iStartDay == 1 && iStartDay == 7)
        {
            return 0;
        }
        if (ctStart < ctMinTime) // start time is befor min time
        {
            ctStart = ctMinTime; // set start time to min time
        }
        TimeSpan ctSpan = ctMaxTime - ctStart;
        daysec = (ctSpan.Days * 24 * 60 * 60) + (ctSpan.Hours * 60 * 60) + (ctSpan.Minutes * 60) + ctSpan.Seconds;
        return daysec;
    }
    private static Int32 CorrectLastDayTime(DateTime ctEnd, DateTime ctMaxTime, DateTime ctMinTime)
    {
        Int32 daysec = 0;
 
        if (ctMinTime > ctEnd) // start time is after max time
        {
            return 0; // zero seconds for the first day
        }
        int iEndDay = (Int32)Enum.Parse(typeof(DayOfWeek), ctEnd.DayOfWeek.ToString());
        if (iEndDay == 1 && iEndDay == 7)
        {
            return 0;
        }
        if (ctEnd > ctMaxTime) // start time is befor min time
        {
            ctEnd = ctMaxTime; // set start time to min time
        }
        TimeSpan ctSpan = ctEnd - ctMinTime;
        daysec = (ctSpan.Days * 24 * 60 * 60) + (ctSpan.Hours * 60 * 60) + (ctSpan.Minutes * 60) + ctSpan.Seconds;
        return daysec;
    }