Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
That most likely means that a small internal battery is in need of replacing. The time-date battery or real-time clock battery, to be specific.
Would you like to try to replace it yourself? if so, let me know and I will give you all details.
If you would rather hire out the replacement of that small, internal, battery then please let me know, and I will refer you to someone in your locality or a good mail-order servicing house of good repute, that can do the work for you.
(I regret that I am not able to offer Live Phone Call service at this time; However, your request is still open to other Experts in this category, and one of them may pick up on it at any time.)
I am preparing the instructions in a written format to send to you shortly.
Information is hard to find, that is specific to your camera model.
Some models of Nikon camera require the camera be serviced! in a case like this one (since the 'battery' for the clock is really a capacitor or similar component soldered onto the main board of the camera, and very hard to replace.)
Others, not so hard to do.
But I have found no information specific to your camera, so with that, I must Opt Out and leave the problem to other, more well-informed Experts to solve. Regrets, and my apologies for not doing better.