The default clock in Ubuntu is too big and eat a lot of precious panel space, imo. Especially for netbook users with tiny screen. If you are like me, then you'll like this tip.
Run System Tools -> Configuration Editor, or type gconf_editor in terminal.

configuration editor
The result :

small and space efficient clock
This format may not what you want. If you want to have different format, read here and here for more formats and tags.
I wrote something like this earlier, but this time I found a way to make it smaller i.e. by inserting "small" tag inside <span size="smaller"> and using monospace font ("tt" tag).
keyword : custom clock, ubuntu, gnome
Run System Tools -> Configuration Editor, or type gconf_editor in terminal.
- go to /apps/panel/applets/clock_screen0/prefs/custom_format paste this as value
<span size="smaller"><small><tt>%b%d%a%n</tt><b>%H:%M</b></small></span> - /apps/panel/applets/clock_screen0/prefs/format
type custom as value

configuration editor
The result :

small and space efficient clock
This format may not what you want. If you want to have different format, read here and here for more formats and tags.
I wrote something like this earlier, but this time I found a way to make it smaller i.e. by inserting "small" tag inside <span size="smaller"> and using monospace font ("tt" tag).
keyword : custom clock, ubuntu, gnome



Victor Zamanian
December 1, 2009 5:07 PM
"type gconf_editor in terminal"
That should probably be "gconf-editor", right?
Panji Nushantara
December 1, 2009 6:20 PM
:p right, thanks Victor.
type gconf-editor in terminal.
JOE2005
December 2, 2009 12:42 PM
ubuntu 9.10.I do not see clock screen after applet 5.Very much like to do modification
Panji Nushantara
December 2, 2009 4:11 PM
have you already added clock applet? if you already had, it doesn't have to be named clock_screen0. try to browse /apps/panel/applets/ for applet_? with bonobo_iid : OAFIID:Gnome_ClockApplet.
after i received your comment I installed another clock to see what applet-id it is assigned to. in my case it is assigned as applet_4.
hope it helps,
Colin D
December 4, 2009 1:32 AM
<span size="smaller"><tt>%a %d %b</tt> <big><b>%H:%M:%S</b></big></span>
works well for me. I really like the bold time. It stands out.