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The Gnome Clock has a very limited set of preferences. If you want to customize the way it displays dates, you can change it like this:

gconf-editor /apps/panel/applets/clock_screen0/prefs/format

Change the format key to "custom". Then change the custom_format key to anything you want it to be. You can see the valid variables by running "man date". A good example is the ISO date format: "%Y-%m-%d %H:%M".