How do I fix Excel 2008 Date Formats in Mavericks
After some searching and trial-and-error, I have identified that the issue relates to the absence of a key file: com.apple.HIToolbox.plist, which for some reason is not created when setting up a new User account (and may not be copied across if importing from another computer). Luckily this file is created when opening the Guest account, which can then be copied across to all required User accounts.

Here is a detailed summary of the procedure I eventually used to fix this date format issue:


Create and open a Guest Account (if not already enabled, go to Users
&amp; Group Preferences)
Go to the Library folder (it’s a hidden folder,
so to find it, use Finder to go to the home folder, then select from
the menu ‘Go:Go to Folder’ and type in ‘Library’)
Go to the
Preferences folder 
Make a local copy of the file:
com.apple.HIToolbox.plist (e.g. onto an SD card, or external/cloud
drive) 
Now open your desired User account 
Go to your Library folder
(again it will be hidden, so find it as above) 
Go to the Preferences
folder 
Copy the file: com.apple.HIToolbox.plist (from local storage)
into this folder 
Quit Excel and then log out of your user account,
and then log back in 
Next time Excel starts, UK date format will be
correct!


(Source: 
http://www.yellowsbest.com/solved-uk...excelmac-2008/)