The D800 does not have a removeable/replaceable clock battery. Instead it is a fixed internal battery. The D800 clock battery is charged off the main battery or by using a power adapter. If the clock battery is completely discharged (unlikely if the camera is in active use), then you would see a flashing "clock" icon.
Hope this helps. It's in the manual on page 28. But that page num is for the manual as viewed on my iPad so I don't know if those are the page numbers for the actual printed manual? The iPad has a Nikon Manual Viewer app which you can get from the App Store. Then you have to download the specific manuals you want to see. But once downloaded the manuals remain on the iPad. It is rather handy and I've come to use it quite a lot. Easier to search through and so forth.