DATA FORMAT INCORRECT ON EXCEL

We use an Excel file for our business to track our daily sales and there is a particular cell on the file where a date needs to be entered. When I right click on the cell and click on Format, and then I go to the Number tab and select Date under category, what's selected there is *3/14/2001. However, when I enter a date such as 8/15/2011, I get an error message "DATA FORMAT INCORRECT! Please ensure your date is formatted MM/DD/YYYY".
The guy who created this Excel file already resigned from our company and it is not easy for us to just simply replace this file so I would really appreciate it if anybody can help me correct this.
Error:
"DATA FORMAT INCORRECT! Please ensure your date is formatted MM/DD/YYYY".
