The cells can be set to any date format, if the cell doesn't contain a date you just see the text. The biggest clue from your screenshots that some of these are unlikely to be dates is that youv'e chosen a date format that should show the full month name yet the
Sample field still shows a 3-letter month - a sure sign that Excel hasn't recognised it as a date. This is what it should look like:
To check if a cell has an Excel date (a number) in it, open the cell formatting dialogue box and choose
General in the
Category list and you should see a number in the
Sample box:
(You don't need to close the box with
OK, you can just
Esc out of it without changing anything.)
In your case, I'd select all the dates in the top row, open the cell formatting dialogue box and convert them all to
General. All the non-dates will show up as text, so you just need to re-enter those dates in an Excel-recognisable format. Excel will try to be helpful and if it recognises a newly-entered value as a date it will format it for you in some date format so to double check they're all dates again you'll have to convert them all to
General again. Once they're all dates, format them as you will and your charts should be OK.