There are quite a few people saying it happens to them too, many using a u50F and windows 7. They have tried bios updates, Best Buy has done battery replacements on the mother board, and they are still having issues. The assumption is there's a bug in the Bios chips, as some have said they can see the new date on the bios screen before windows takes over.

I would put in a support request with Asus and see if they have any updated information.

Several other people mention it in the Best Buy reviews of one of their machines

http://reviews.bestbuy.com/3545/9701922/reviews.htm