(Adapted from http://www.omgubuntu.co.uk/2010/10/how-to-customize-the-clock-applet-in-ubuntu/)

# gconf-editor:/apps/panel/applets/clock_screen0/prefs/custom_format
<sup><span rise="0" font_desc="ubuntu 10" weight="normal"><span weight="bold">%H:%M</span> %z</span></sup>%n<sub><span font_desc="ubuntu 7.5">%Y-%m-%d | %Ywk%V</span></sub>

# gconf-editor:/apps/panel/applets/clock_screen0/prefs/format
custom