I have a similar issue, believe it's due to the main battery not having good contact with the contacts in the camera.
Some have tried to fix it by cleaning the contacts on the battery and in the camera.
Did you remove the main battery when in storage? Over a period of time that internal battery will discharged. I suggest you put in a fully charged battery and leave it for a few days. Test it again in a few days. If the date does not hold when you remove the battery, send it for service. The internal may have discharged so low that it cannot be recharged.
It is not advisable to leave the camera without any battery when it's in storage.