1. Right-click your Gnome panel, and select 'Add to panel...'
  2. Find the 'Clock' applet and either click "Add" or drag-and-drop it to the panel bar.
  3. Press 'Alt' + F2 to open the 'Run' dialog, and start the 'gconf-editor' application.
  4. Scroll down through the tree under apps > panel > applets. You need to find the folder in here that corresponds to your clock applet. If it is the most recently added applet, then it will likely be the highest numbered folder. It should have an key called "bonobo_iid" with a value of "OAFIID:GNOME_ClockApplet".
  5. Open this folder and highlight the 'prefs' folder, then change the value of the 'format' key to 'internet'