I did it my previous project. I write the logic. It is better you develop your skill in writing PL/SQL procedure and function. Sooner or later it will help you. You do not need to depend upon others.

You need a hoilday table in which all hoildays are stored.

Set the starting date to STDATE
Set a counter to 15 (say Kount=15)
Do while kount > 0
STDATE = STDATE+1
if STDATE is not found in hoilday table (you need to write SQL)
KOUNT = KOUNT - 1
end if
End do while

The new STDATE will be the ending date for the work order.