It has happened twice. The date and time suddenly changes during a brief field trip, so the date (Day, Month and Year) changes from correct to totally wrong. Today is 5th of July 2016. When I came back home I had three dates, correct, and to other: 2016 and 2061 and 2068. Clock time was however correct. Anyone that has observed something similar? My camera is Nikon D750.
Solution found: I use GPS, and the camera was set to get "clock from satelite" - and there were obviously som bad data out there. Sorry for posting!