Nikon D6 Reference Manual page 1135

Replacing the Clock Battery
The camera clock is powered by a CR1616 lithium battery.
CR1616 batteries have a life of about two years. If the B icon is
displayed in the top control panel while the standby timer is on,
the battery is running low and needs to be replaced. When the
battery is exhausted, the B icon will flash while the standby
timer is on, indicating that although photographs can still be
taken, they will not be stamped with the correct time and date.
❚❚ Replacing the Clock Battery
1
Turn the camera off and remove the main battery.
2
Slide the clock battery chamber cover, which is
located on the roof of the main battery chamber,
toward the front of the main battery chamber.
3
Remove the clock battery.
Caring for the Camera
1083