After some searching and trial-and-error, I have identified that the issue relates to the absence of a key file:, 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). Hence this provides the clue to solving the problem. It turns out that the issue manifests itself in any User account set up on the Mac computer EXCEPT the ‘Guest’ account, which mysteriously sets the date to the UK-format. A particular ongoing problem is then generated when new dates are added to the sheet, as entering 1/12/16 is then taken as 12th January not 1st December 2016. Worse, if an existing workbook is opened, which had previously correctly UK-formatted dates, they are all changed to the US-variation. using the formula =TODAY() returns something like 11/22/16 (for 22nd November 2016) instead of 22/11/16.
The problem shows itself when, in a new Excel worksheet, a date is entered, e.g. In theory, there should be no change, but hidden somewhere in the setup of the new computer lurks something preventing use of the UK date format. This occurs when transferring to a new Apple Mac computer with a previously correctly working installation of Microsoft Excel:mac 2008 (from an older computer, but running the same version of OS X). An annoying issue (for UK users) is of the date format defaulting to US (MM:DD:YY) rather than UK (DD:MM:YY).
