I have a legacy spreadsheet (meaning I can't really modify it, and have no one to ask). I have it open in OpenOffice but I checked it in Excel and the same thing happens.
There are two date columns that have the user-defined format code [$-409]DD-MMM-YYYY;@ (see screenshot).
The cells in these columns will not accept any date format I've tried. It says 'invalid value'.
I did some research here on Stack overflow: excel-number-format-what-is-409 so now I know the $=409 is a locale code.
So now my question is: when entering data manually, how can I arrange my date input to satisfy that condition?