Every couple of months or so, when I start up my Mac I get an error saying the clock is set to a date before 1 Jan 2008 and some applications may behave erratically. 1. Why does it do this? 2. How can I stop it doing this? 3. If it doesn't know the correct date as a reference, how does my computer know that the date it's set to is incorrect? 4. If it knows it's incorrect, why can't it therefore correct itself?