Schau dir mal folgenden Schlüssel in der gconf an :
/apps/panel/applets/clock_screen0/prefs/format
This key specifies the hour format used by the clock applet. Possible values are "12-hour", "24-hour", "internet", "unix" and "custom". If set to "internet", the clock will display Internet time. The Internet time system divides the day into 1000 ".beats". There are no time zones in this system, so time is the same all over the world. If set to "unix", the clock will display time in seconds since Epoch, i.e. 1970-01-01. If set to "custom", the clock will display time according to the format specified in the custom_format key.
Der passende Custom-Schlüssel steht darüber :
/apps/panel/applets/clock_screen0/prefs/custom_format
Du müsstest also den format-Schlüssel auf "custom" und den custom_format-Schlüssel nach folgender Dokumentation ändern :
http://de.php.net/strftime
%a, %d.%m.%y, %H:%M
ist ein passender Schlüssel für folgendes Format :
Do, 08.02.07, 23:46