Ok, you probably have the date set as year 1900 in one file and 1904 in the other.
What you need to do is open that file, and go to Preferences then under the Calculation tab uncheck (or check) the box next to "1904 Date system". Click OK.
Hope this helps. Let me know if you have any questions.
Thanks for the accept.
Yes you need to start Excel then go to the File > Open menu command to select your file and open it. Or you can double click on the file's icon from outside Excel to open it.
The Preferences in Mac's Excel is under the Edit menu. If you need further assistance please let me know the exact version of your Excel application.