Dates in Excel

My question is regarding dates in Excel which seem to be coming up as "Excel Codes". I asked this question last week and was given an extremely good answer by Jens, except that it did not work on my Spreadsheet. Do you think that this would be because the original spreadsheet was created on another version of Excel, not Excel 2010 ?

I tried this answer, (All dates have a numerical number. If the cell is formatted as general or number it will show the numerical number. To see it as a date the cell must be date formatted. Select the cell go to the HOME tab and in the NUMBER group select SHORTDATE from the formatting list).

This is what happened when I tried to change the date:

Thanks for the input. Unfortunately, it does not seem to want to work on my spereadsheet - I click on HOME, NUMBER and SHORTDATE but in the window it only states DATE and will not change. I have tried adding another column and starting again, but it still won't work.

Any clues ?