Count hours between 2 days

Hi. I've been working with my monitoring file which tracks leadtimes. We are working 24 hrs a day excluding weekends. But I can allow start date to be on a weekend.I need to count the number of hours between 2 days. I can already do this but I'm having 2 problems.
1. Result only displays whole number.
Ex.
A1: 6/29/2010 7:00:00 AM (start date and time)
A2: 6/29/2010 7:41:00 PM (end date and time)
Result shows: 24 in hours instead of 12.68 hrs

2. I obtain negative results
Ex.
A1: 7/1/2010 8:30:00 (start)
A2: 7/5/2010 16:39:00 (end)
Result shows: -3 days or -72 hrs

Thanks!