i did not like the default date time format

Default date/time in tray

to make it more readable like so

Custom date/time in tray

i enabled all options in the general tab of the clocks preferences

Clock system preferences

started up gnome configuration editor by running

gconf-editor

there navigated to

/apps/panel/applets/clock/prefs

and changed the key format to

custom

and entered the following into the key custom_format

%Y-%m-%d %H:%M:%S

Gconf custom date format