I recently had the motherboard on my iMac replaced and I've noticed a minor issue with my system preferences settings. I scheduled an automatic shutdown at 11:00PM every evening and noticed there was a 10 minute gap between the 11:00PM shutdown and when it actually shutdown. I have changed the shutdown time six or seven times and there is always a 10 minute difference between the time I set and the time it actually shuts down. Any way to fix this?