Okay so i found a way to do this, if you change the 'format' property from something like 'DateTimeFormat.ShortDate' to just "dd/mm/yyyy" it will change it. I found that even when using TEXT(<field>, "dd/mm/yyyy") if the format property shows shortdate, or longdate or anything like that, it will use the browser defult.
Thank you for this. I have just tried it and low and behold it works, I have a Europe/UK date format in the date picker.
I spent hours looking for complicated solutions and this appears to do the trick.
A slightly different use case here, but it is somewhat relevant to this thread. If you'd like to apply date/time settings to a field in a gallery, you can use: