Date & Time preferences won't stick

Having read recently that sitting on one's fat arse all day is the worst
thing you can do in terms of mortality, I tried to set my Date & Time
preferences to announce the time on the hour.

But the preference never sticks. Every time I set it, I go back in to
find it's not set.

I've tried deleting /etc/localtime as per one article on Apple
discussions, but no joy.
I've also tried deleting in my
~/library/preferences, since that seemed to be the last thing modified
when I tried setting the announcement.

Any other suggestions? (Snow Leopard 10.6.8)