This is taken from the Eee User Forum posting http://forum.eeeuser.com/viewtopic.php?id=28055 but with some added details for Synaptic users.
The Eee's Xandros install is missing the Regional Settings Control Panel. It therefore doesn't allow various region customisations, including the Date & Time format option when right clicking the clock.
The following repository is required in your /etc/apt/sources.list file:
deb http://www.geekconnection.org/ xandros4 main
To add the repository edit the /etc/apt/sources.list file. Issue
sudo kwrite /etc/apt/sources.list
and add the above repo, if it's not already there.
Once you have added this run
sudo apt-get update
to update the list of available packages using the terminal; for Synaptic users, click on “Reload” once you have added the repository using Synaptic. Next run
sudo apt-get install xandros-regionalsettings
to install the Regional Settings Control Panel. For Synaptic users, scroll down the list of Sections on the left, and click once on “xandros”. This will load the packages in the right hand panel available in the xandros section. You can then scroll down to 'xandros-regionalsettings' and then install in the usual way.
This will now allow you to right click on the time, then on “Date and Time Format” on the pop up menu and this will then load the control panel. You'll need to restart to see the new settings take effect.
This section is here so that people searching for time & date format, and regional settings will find this article.