I need help with the following calculation. I cannot seem to get my
mind around the If function in a way that would help me with creating a
conditional 'Due Date'. Any help would be greatly appreciated.

Due Date (AN):
If AA2 = blank then leave blank without displaying an error message.
If AA2 = "St1" then add 10 Workdays to date in U2 and display revised
date.
If AA2 = "St2" then add 20 Workdays to date in Z2 and display revised
date.
If AA2 = "St3" then add 28 Workdays to date in Z2 and display revised
date.
If AA2 = "PE" then add 10 Workdays to date in U2 and display revised
date.
If AA2 = "FOI" then add 20 Workdays to date in U2 and display revised
date.
If AA2 = anything else then return "Not Required"

I also need to exclude the following holidays:
14/04/2006 Friday
17/04/2006 Monday
01/05/2006 Monday
29/05/2006 Monday
28/08/2006 Monday
25/12/2006 Monday
26/12/2006 Tuesday
01/01/2007 Monday


Thank you
Scoooter