yeah, mine don't work either, so irritating. I just covered the stock clock with my FC Commander, and use the clock on the (aftermarket) radio to keep time. I went without any clocks for awhile, came to the realization that it's a pretty important piece of kit that you want in your car.
It's a common problem - the 300ZX suffers the same - some of the guys on the Z forums have tried to work out a fix. You'll need to be handy with the soldering iron though.
Failing that it's a new one or a breaker.