HMRC require that dates are entered in the format of dd/mm/yyyy.

Ensure via the Control Panel on your computer and within your source file that the date formats are set as dd/mm/yyyy.

To check the cells in the source file if you right mouse click over the cell and select format cells and then set the cell as dd/mm/yyyy. (you will have an option with an asterisk in front of dd/mm/yyyy, ignore this one).