The Public Function is a customized function that you can use, similar to those included with Excel. I just wrote it myself. If you insert it into the workbook (explained below) it will be available in all of the sheets in that book. One thing to note, you may be prompted to Disable/Enable Macros when you first open the workbook. You will need to Enable the macros for the function to work.
From the worksheet:
1. Tools > Macro > Visual Basic Editor
2. Insert > Module
3. Copy and Paste the code above (Public Function ... End Function)
4. File > Close and Return to Microsoft Excel
5. Insert the function call in the desired cell
About the function:
Code:
Public Function FindDelta(dStart As Date, dEnd As Date) As String
Dim lVal As Long
lVal = DateDiff("n", dStart, dEnd)
FindDelta = Format(lVal / 60, "##.##")
End Function
FindDelta needs to be passed 2 dates and returns the calculated difference. The DateDiff function (VBA) returns the difference between the two values in minutes ("n"

. I just convert that to decimal hours (Format() function) so that it will return a value in hours, to 2 decimal places.
Now, when you call this function from a cell, you just have to specify the cells that contain the start and end dates.
Example:
cell F23 is the start date/time
cell M192 is the end date/time
If you want the difference displayed in cell B8, then insert the following formula into cell B8 (General format):
=finddelta(F23,M192)
Let me know if you have any more problems. I can e-mail you a workbook with this function so you can review it for yourself.