The big difference is that time is obviously not static. It might be possible to store the time the power was disconnected, but is that any use? To maintain the

The big difference is that time is obviously not static. It might be possible to store the time the power was disconnected, but is that any use? To maintain the time power is needed. A solution might be to include a second rechargeable battery to maintain time while the main power is disconnected, but ... extra cost and complexity.