The format is European. I wonder if in upgrading your settings were changed.. I
live in Canada which is metric and we use the dates in Europena format. When
Iinstall Ubuntu I choose the U.S. keyboard, but other than that I want British
dictionaries and metric formats. This means that I must manually make changes
after installation because it defaults to U.S. style for dates and imperial
measure. Since you are attempting to do this in reverse, I wonder if somehow
you ended up with European settings by mistake.

I am not sure how to do it universally in Gnome. You can do it in KDE in the
control panel. However, I would start looking in each program's settings and on
your clock on the taskbar. Set the preferences there and it will probably carry
over into other programs that read the clock.