My daughter called me this evening for help.

She typed 3.1 (decimal number) into a cell and it came out as "3,jan" which
would have been correct had this been a date. But it is not.

Of course I asked her to check the cell format, but whatever she changed the
format to (on my instruction) it displayed the same! Decimal numbers that
did not constitute a valid date (e.g. 3.16 or 35.2) came out as desired.
(This is Norway with default date format DMY)

Any suggestion where to start looking?

regards Sven