How do I change the date format in Gnome 3 shell?

I want to change the date/time format on the top panel to a format close to RFC 3339 / ISO 8601, like one of these:

%F %T → 2013-06-24 16:13:00

%F %a %T → 2013-06-24 Mon 16:13:00

%A %F %T → Monday 2013-06-24 16:13:00

I know Unity has a preference somewhere hidden away in dconf, this is how I did it in Unity, but I can't find such a preference for Gnome 3 shell. Preferably, I'd also like to set one of these as my system-wide date/time locale preference.

Open a terminal and set

gsettings set org.gnome.desktop.interface clock-show-date true

YMMW - this did work for me on Fedora 19.

In addition to what Martin says, the following worked for me on Ubuntu 14.04, where dconf-editor didn't:

gsettings set org.gnome.desktop.interface clock-show-date true
gsettings set org.gnome.desktop.interface clock-show-seconds true