Try this:
A1 B1 C1 9:00 AM 12:30PM =(B1-A1)*24
Should see:
A1 B1 C1 9:00 AM 12:30PM 3.50 Hrs
Make sure you format C1 as a Number, with whatever desired decimals (I usually use 2).
To get the "Hrs" to show up, make the format code, which is the 0.00 shown in the bottom of the formatting numbers box, into "0.00 Hrs" (quotes included) It'll change the format to "User-Defined".
That should do that you want.
time is stored internally as
So 1 day (24 hours) = 1.00
12 hours = 0.50
3.5 hours = 0.15 (actually: .14583333 = 3.5/24 = 7/48)
Hope this helps.