Clock battery on D3x

I recently bought a second hand D3x, Im very happy with it. I read somewhere (?) that I may need to replace my clock battery. This makes sense to me as every time I take the battery out to recharge, it resets to 2007 and resets the counter. Where is the clock battery on this camera and is it something I can replace myself?

If I recall correctly they gave a battery type in the article, which I sensibly didn't write down.

I trust you can assist me with my current quest. Thank you in advance for being here in the first place.

Bebb