Sometimes if we change the date on linux with date or time function, whenever the computer starting up again, the date setting is back to the BIOS time. It's mean that the date or time function only affect to the linux operating system but not to the BIOS.
So, you can do this :
[root@ppbj root]#hwclock --set --date="2006-07-21 10:15:00"
[root@ppbj root]#hwclock --hctosys
Restart your computer. And then you'll find your BIOS time changed with your new setting.