finding # hrs between 2 date times

Excel 2007

Situation: I have two separate dates in format MM/DD/YYYY HH:MM...I want to find the number of hours and minutes between the two date times, but I only want it to calculate between 8am and 8pm.

example:

date 1 = 4/16/2010 16:00
date 2 = 4/17/2010 22:00

answer = 16 hours 0 minutes (16.0 is fine)