SagePay Module & Shop Date Format

[jaimeweb]

Hi Everyone,
I have purchased the Sagepay module from Presto Chango. Have it installed on 1.4.
However when i try to change the shop date format to UK format using this method: http://www.prestashop.com/forums/viewthread/108477/#442430

But for some reason, when i make this change my SagePay payments fail.
My shop says the order has been acsepted correctly but when i login to sagepay they fail, giving a error message of:
Transaction completed but Vendor systems returned INVALID or ERROR in response to notification POST. Transaction CANCELLED by the Vendor.

I know its this thats causing the problem as i have checked a few times now.

Can anyone help please?

Thanks