Can you be more specific? Are you seeing wrong dates on www.fuelly.com, m.fuelly.com or the mobile app?
I set my country to the UK and browsed around.
On the car profile page, for Best MPG, I have 21/06/14
On the fuelup log, I also have 21/06/14
Annual Reports and All Fuelups have Jun 21 2014
The only "wrong" date that I see is on the Add Fuelup/Add Event/Add Reminder page where it defaults today's date to 6/21/2014 and I will fix that one. The Edit Fuelup/Edit Event/Add Reminder page also shows the same format but fixing the Add page will also resolve that one. The same date will also be fixed on m.fuelly.com as well as they all share the same source.
Iain, another thing that is confusing for me is that if I pull up old Fuelly, which I can do, and set my country to Jersey, I see US dates there as well. It seems that setting Jersey as your country has never shown UK style dates in Fuelly.