This issue is most commonly caused where you have either a formatting issue on your source spreadsheet, and / or your machine is not set to English UK format.
Please check that the date you have entered is a valid date for example you have entered 31st April instead of 30th April.
If you are happy that the dates are valid, please check, using the control panel on your machine, that the regional settings are set to English UK and date format dd/mm/yyyy. We also recommend that you check the formatting of the date cells in your source spreadsheet. To do this, depending on what version of Excel you are running, click into the cell, right mouse click and select the date option within format cells. You will have two showing as dd/mm/yyyy one of which has an Asterix in front it, select the one that doesn’t as the Asterix causes regional settings to be used from elsewhere on the machine.
After any changes, please close and re-open your source spreadsheet to save the changes and then re-link the dates to the VAT Filer so the values then update and try your submission again