What I want you to do is boot to the BIOS and check the system time. If it's correct, then it's not the motherboard battery. The next thing to check in the list of drives in the BIOS. Your hard drive should be SATA port 0. If that port displays no drive, then your drive's data or power cable might be loose. You'll need to open the system to check.
Let me know what you find. There are a couple other things you can try.