Welcome to Nikonians! Sorry to hear of your problem. It sounds like the rechargeable clock battery in your camera needs replacing. The clock battery is not user replaceable so a trip to a Nikon Authorized Service Sration or Nikon Service is required. Good Luck and Enjoy your Nikons!
I think I found answer from someelse's posting. Internal clock apparently needs periodic charging from the main battery, which probably isn't "on" long enough to do the job well. The suggestion I tried seemed to work, at least in short term. And that is: Insert a fresh main battery and power up the camera. Let it stay powered up for 2 days straight, i.e. long enought to really give the internal battery a good charge.
After a 2-day charge, the internal battery seems to work fine, not defaulting to 2006 when the battery is removed.
Keeping a charged battery in the camera should be enough to charge the. Lock battery. The camera doesn't have to be powered on for the clock battery to charge. Two days of charging should power the clock for about one month. See page 103 of the D80 Users Manual. Good Luck and Enjoy your Nikons!
I agree that one shouldn't have to keep the D80 powered on to charge the clock battery. But then what am I to conclude from the fact that the clock battery appeared dead and each time I changed the main battery the clock defaulted to 2006? This has been the case for some time. But when I left the camera powered on for 2 days, that procedure appeared to finally charge the internal battery again. Yes, page 103 of the manual does state that 2 days of charging should power the internal clock for about a month, but how does that occur if not by keeping the camera powered on? It certainly wasn't happening with the camera powered off.
Perhaps I'm not understanding something pretty basic here.
Many thanks if you have the answer to this conundrum.