I want to be able to calculate total hours using three Excel columns. 
Column one is "date and time in", column two is "date and time out,"
and column three is total time in hours.  "Date" must be used because
the time period between "time in" and "time out" can cover more than
24 hours.  I need the data and time format and formula to create this
calculation.

This worked for me:

1. Create a new Excel 2000 spreadsheet. 
2. Select columns A and B.
3. From the Format menu, select Cells.
4. On the Number Tab, choose Date from the list of Categories. From
the list of types, choose one that displays both Date and Time. The
one I used looks like this: 3/14/98 1:30 PM
5. Click OK.
6. Select column C.
7. From the Format menu, select Cells.
8. On the Number Tab, choose Number from the list of Categories. Set
the decimal places to 2.
9. Type dates into A1 and B1. You can edit both the date and time
portion.
10. Enter this formula into C1:
=(B1-A1)*24
11. Press Enter.

The elapsed time in hours should appear in C1. It spans any number of
days.

Good luck with your Excel project!

- Hammer


*** Search strategy ***
Experimentation with Excel 2000.

This not only answered my question but now I see what I was doing
wrong so that I can fix the issues raised by using times in Excel.
Great answer