I assume you are formating your time as a normal number (not as time) so that 1 hour and 15 minutes show up as 1.25 hours. Assuming start time in A1 and end time in B1, use
=(B1-A1)*24
Barrie
If you are using Excel "Time" values, you can just subtract them.
First format columns A, B and C as "Time" as follows...
Select columns A & B then go to "Format -> Cells," select "Time" from the Number tab and pick a format type you like. Select column C and format it as time with a format like 37:30:55 (which is hours:minutes:seconds).
After that, enter a time in A1 like "10:00 am" and another in B1 like "5:00 pm" (include the space between the number and the "am" or "pm" otherwise Excel will think it is just text). If you put =A1-B1 in C1 it will display "7:00:00"
Jay