Time Wrong in Leopard

It has been a consistent problem for me. It will be set to 'Set date and time automatically', and still be off by the hour, so I uncheck that, correct the time, and a few days later, somehow change again. The time zone is always correct though, but for some reason, the time keeps changing, and when the box for auto is checked, it doesn't change it to the right time. Anyone know whats wrong?