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.