CMOS Checksum Error + System Clock Shows Wrong Time/Date After Every Boot

Few days ago when I used to boot the system (before the windows load) there was error message saying "CMOS Checksum Error...." and then windows load.

Now that error doesn't appear but now the system date/time always are wrong. The date stuck on 7th of the month and time is 1 hour and few minutes behind too.

First it used to stuck on 7th Jan, now it shows 7th Feb.

My CMOS battery has expired or what? Do I need to replace only the battery or the entire motherboard will get replace?

If it is a desktop, it's easy. Just replace the coin shaped battery on the motherboard. It is usually clamped inside a battery holder / socket on the motherboard. Be careful while removing the battery, you may end up breaking your battery holder.

If laptop, it's bit difficult depending upon make/model.

Yes, its desktop PC.

What's the price of a CMOS battery? any idea?

It's cheap. Probably PKR15-50 depending on brand and quality.