http://en.wikipedia.org/wiki/ISO_8601

• Display the configuration editor in the system menu by following these steps:
  Click on System → Preferences → Main Menu
  Enable: Applications → System Tools → Configuration Editor (gconf-editor)
• Now open the Configuration Editor (gconf-editor) and do the following:
  • Navigate to / → apps → panel → applets → clock_screen0 → prefs
    and set the value of custom_format to %Y-%m-%d %H:%M:%S. (note: this is the php strftime() format.)
  • Under "format" in the same location change 24-hour to custom