LogonTime.cs

public class LogonTime
{
    public DayOfWeek DayOfWeek { get; set; }
    public DateTime BeginTime { get; set; }
    public DateTime EndTime { get; set; }

    public LogonTime(DayOfWeek dayOfWeek, DateTime beginTime, DateTime endTime)
    {
        DayOfWeek = dayOfWeek;
        BeginTime = beginTime;
        EndTime = endTime;
    }

    public LogonTime(DayOfWeek dayOfWeek, TimeSpan begin, TimeSpan end)
    {
        DayOfWeek = dayOfWeek;
        BeginTime = new DateTime(begin.Ticks);
        EndTime = new DateTime(end.Ticks);
    }
}

PermittedLogonTime.cs

public class PermittedLogonTime
{
    public static byte[] Calculate( List<LogonTime> logonTimes )
    {
        var permittedLogonHours = new byte[21];

        // calculate the byte values for each individual days
        foreach(var lt in logonTimes)
        {
            CalculateByteValue(permittedLogonHours, lt.DayOfWeek, lt.BeginTime, lt.EndTime);
        }

        return permittedLogonHours;
    }

    private static void CalculateByteValue(byte[] permittedLogonHours,DayOfWeek dayOfWeek,DateTime begin, DateTime end)
    {
        var dayHours = new byte[3];

        switch (dayOfWeek)
        {
            case DayOfWeek.Sunday:
                dayHours = CalculateDayHours(begin, end);
                permittedLogonHours[1] = dayHours[0];
                permittedLogonHours[2] = dayHours[1];
                permittedLogonHours[3] = dayHours[2];
                break;
            case DayOfWeek.Monday:
                dayHours = CalculateDayHours(begin, end);
                permittedLogonHours[4] = dayHours[0];
                permittedLogonHours[5] = dayHours[1];
                permittedLogonHours[6] = dayHours[2];
                break;
            case DayOfWeek.Tuesday:
                dayHours = CalculateDayHours(begin, end);
                permittedLogonHours[7] = dayHours[0];
                permittedLogonHours[8] = dayHours[1];
                permittedLogonHours[9] = dayHours[2];
                break;
            case DayOfWeek.Wednesday:
                dayHours = CalculateDayHours(begin, end);
                permittedLogonHours[10] = dayHours[0];
                permittedLogonHours[11] = dayHours[1];
                permittedLogonHours[12] = dayHours[2];
                break;
            case DayOfWeek.Thursday:
                dayHours = CalculateDayHours(begin, end);
                permittedLogonHours[13] = dayHours[0];
                permittedLogonHours[14] = dayHours[1];
                permittedLogonHours[15] = dayHours[2];
                break;
            case DayOfWeek.Friday:
                dayHours = CalculateDayHours(begin, end);
                permittedLogonHours[16] = dayHours[0];
                permittedLogonHours[17] = dayHours[1];
                permittedLogonHours[18] = dayHours[2];
                break;
            case DayOfWeek.Saturday:
                dayHours = CalculateDayHours(begin, end);
                permittedLogonHours[19] = dayHours[0];
                permittedLogonHours[20] = dayHours[1];
                permittedLogonHours[0] = dayHours[2];
                break;
        }
    }

    /// <summary>
    /// Takes the hours that user is allowed to login and calculates the byte mask for it
    /// </summary>
    /// <param name="begin"></param>
    /// <param name="end"></param>
    /// <returns></returns>
    private static byte[] CalculateDayHours(DateTime begin, DateTime end)
    {
        var result = new int[3];

        for (int i = 0; i < 24; i++ )
        {
            if (begin.Hour <= i && i < end.Hour)   // if the hour is in the allowable range
            {
                var multiplier = (int)Math.Floor((decimal)(i)/8);

                result[multiplier] += CalculateHourValue(i);

            }
        }

        return new byte[3] { Convert.ToByte(result[0]), Convert.ToByte(result[1]), Convert.ToByte(result[2]) };
    }

    /// <summary>
    /// Returns the byte mask for the specific hours
    /// </summary>
    /// <param name="hour"></param>
    /// <returns></returns>
    private static int CalculateHourValue(int hour)
    {
        hour = hour%8;
        switch(hour)
        { 
            case 0: return 1;
            case 1: return 2;
            case 2: return 4;
            case 3: return 8;
            case 4: return 16;
            case 5: return 32;
            case 6: return 64;
            case 7: return 128;
        }

        return 0;
    }
}