Here is my problem.
I have an Excel file that I call my Summary file. In it is a worksheet that
takes key data from a number of other separate excel files ("Data Files"). I
need to calculate the amount of time in hours and minutes that has passed
between now and when the Data File was last saved. I want to ensure that the
information in the Data File is not more than 72 hours old.

Here is what I tried:
In the Data File I have used the NOW() function in a cell to record the date
and time the file was last opened/updated.

I reference the cell containing that information in my Summary file. The
problem is I do not know how to calculate the difference in hours and minutes
between that information and the current date and time.

Maybe there is another way to do what I need to do. If someone can help me
I would be most grateful. Thank you.