I do not even know where to begin to with a days elapsed calculation
and a days remaining calculation taking the following into
consideration. Any advice would be greatly appreciated:

If AA2 = blank then leave blank without displaying an error message.
If AA2 = "St1" then calculate Workdays elapsed from U2 and display.
If AA2 = "St2" then calculate Workdays elapsed from Z2 and display.
If AA2 = "St3" then calculate Workdays elapsed from Z2 and display.
If AA2 = "PE" then calculate Workdays elapsed from U2 and display.
If AA2 = "FOI" then calculate Workdays elapsed from U2 and display.
If AA2 = anything else then return "Not Required"
If AO2 = is populated with a date then quit counting days elapsed and
display final figure.

Bob Phillips kindly provided me with the following Due Date
calculcation which works great. Is there anyway to adapt this to
conditionally show the number of elapsed days and the number of days
remaining?

=IF(AA2="","",IF(OR(AA2="ST1",AA2="PE",AA2="ECC
ST1"),WORKDAY(U2,10,Holidays!B7:B14),IF(OR(AA2="C A -
ST2",AA2="FOI"),WORKDAY(Z2,20,Holidays!B7:B14),IF( AA2="ST3",WORKDAY(Z2,28,Holidays!B7:B14),"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