To see if it's a hardware problem, before you turn it off at night, go into your PC's bios, set the correct time, take note of it, save the settings and shut it down. Pull the plug too.
By the next day, don't go into the OS yet. Enter into your PC's bios and check the clock. If it's delayed or has reset to another year, something might be wrong with your motherboard since you changed your CMOS battery already. I hope that's not the case. If the cock is still correct, then maybe it's the OS or some program that's altering your clock.