A colleague of mine has a spreadsheet with various dates in them in the
format dd/mm/yyyy.

Some of the dates where the day is less than 13 keep toggling between
displaying dd/mm/yyyy and mm/dd/yyyy, however, if they are formatted as
dd-mmm-yyyy they display the correct date and month.

This only seems to happen on the pc at work and not at home which implies
some configuration issue.

I have seen this problem before myself but never got to the root cause, does
anyone out there have a reason and hopefully a resolution to this issue?

Thanks
David