adding hours to dates using networkdays formula

I have a problem!!! Please help!!!
I have a chart

--A-- project --B-- Production time needed in hours --C-- start date --D-- end date

1-- Mount R. 2 1/2/05 7:00AM 1/2/05 9:00AM

2-- Mount R. 9 1/2/05 9:00AM 1/3/05 8:00AM

what formula can i put in D2 to get this answer. Our working hours are 7:00AM
to 3:00PM. I need it to calculate only working hours to estimate the fnishing date of this project. can you help!
Thank you!
Melanie