show a clock on the desktop

You want a desktop clock... try cairo-clock Homepage: http://macslow.thepimp.net/index.php?page_id=23

Cairo-clock is listed into Universe repositories so you must enable them from menu:
System->Administration->Software source: Ubuntu software

Then install it, open a terminal and type:

sudo apt-get update
sudo apt-get install cairo-clock

give your user password when requested, you don't see nothing when you type it, then press enter.

You will find cairo-clock under Applications->Accessories-> MacSlow's Cairo clock

Then if you want this clock start at session startup you need to add from System->Preferences->Sessions ...
And add the /usr/bin/cairo-clock program

Hope this helps

Thanks jacobS, that solved my question.