I was recently looking for a way to trick out my clock in the default Ubuntu 10.04 environment, when I found this page:

http://ubuntu.igameilive.com/2009/12/custom-clock-in-ubuntu-910.html

Turns out, the Gnome clock applet uses this function to display time:
http://php.net/manual/en/function.strftime.php

And, you can format it with settings from here:
http://library.gnome.org/devel/pango/stable/PangoMarkupFormat.html

I decided I wanted to go for a retro digital clock look, so I installed this font:
http://www.dafont.com/ds-digital.font

So, without further adieu, here’s my settings string:

<span face="DS-Digital" color="green" background="#003300" size="14336"> %a %m-%d-%Y %l:%M %p %Z </span>

The above settings give this: