Kubuntu (KDE) comes with a clock plasma, but in Ubuntu you need to add that
capability. In GNOME you can install Screenlets to do the same kind of
thing. It can be set to run as a daemon, so once it is setup it requires no
work. There is also Cairo clock which I have never used and you can even get
Screenlets to work with Google Gadgets which I have used. There is also a
nifty creenlet that will change your wallpaper to display the time called
Wallpaper Clock. You can add lots of Screenlets to the original application
which comes with a basic clock and calendar. You can adjust the time to GMT
or any other time using an offset from GMT.