I've been looking for a resolution to this problem and have so far been unlucky so I'm hoping one of you can help. In my Date & Time Preferences, the 'Show AM/PM' option is permanently grayed-out - regardless of what other options I select. I am in the UK, so have my time set to London BST. I am running a 1.83ghz Intel Macbook with Leopard 10.5.5. This problem also affects my iMac at work. Thanks!