That first answer is actually 24 hours difference, which wraps as 1
day and this is not shown because of the way the cell is formatted.
Highlight column J, and then apply a Custom format to those cells of:

[h]:mm

The square brackets around the h prevents the wrapping of 24 hours
into days, so you should see 24:00.

Hope this helps.

Pete

On Mar 24, 3:36*pm, Chi wrote:
Hi,

I would like to calculate the hours between two columns: First Contact (G)
and Appointment (H) with (m/d/yyyy) h:m AM/PM) format.

First Contact (G) * * * * *Appointment (H) * * * * * * * * * * *(J) *(= H-G)
1/2/2010 10:00 AM * * * 1/3/2010 10:00 AM * * * * * * * 0:00
1/1/2010 8:00 AM * * * *1/1/2010 9:00 AM * * * * * * * *1:00
1/1/2010 8:15 AM * * * *1/1/2010 8:30 AM * * * * * * * *0:15

I added the formula (=H-G) on column J to calculate the difference between
(H) and (G) However, I don't understand why (1/2/2010 10:00 AM) (1/3/2010
10:00 AM)=0:00. It should be 12 hours.( the first line of my example). The
last two lines are correct, but the first line is wrong. Please help.
Thanks
Chi

On the column