I need a boost.

I need to calculate a start date based on an end date and hours and wonder
how to do it.

For example, If the end date is Friday and the number of hours is 8 the
start date needs to be Friday too. If the end date is Friday and the number
of hours is between 8 and 16 the start date needs to be Thursday. And so
on. I need to take into account weekends but I'm going to ignore holidays
for now.

I'm having a hard time figuring out how to subtract hours from a date. I
need to consider each date as being a 8 hour day too.

Your help is appreciated.

--
Rick