Add hours to datetime but exclude weekends and should be between working hours

Question

I am trying to add hours to a date but want the new date to exclude weekends and only be between 9.00 to 17.00.

example:

first scenario:

if my date is 23/02/2012 16:00:00.
if I add 4 hours to that my new date should be 24/02/2012 12:00:00 (which will be within working hours)

second scenario:

if my date is 24/02/2012 09:00:00 (which is a friday)
if I add 24 hours then the new date should be 27/02/2012 09:00:00 (which would be monday next week at 9am)

so far I got this but am stuck as this will not count for any date that is in past say date passed was 10/02/2012(Friday last week) :

   private static void ExcludeWeekend(Datetime dt)
    {
        DateTime todaysDate = DateTime.Today;
        DateTime dueDate = null;

                if (dueDate.DayOfWeek == DayOfWeek.Friday)
                {
                    dueDate.AddHours(48);
                }
                else if (dueDate.DayOfWeek == DayOfWeek.Saturday)
                {
                    dueDate.AddHours(72);
                }
    }