I always unplug my microwave because it’s old and makes a soft humming noise. Obviously I don’t bother to set the clock every time, so when it is plugged it usually shows --:–. However occasionally it indicates 12:00 instead, and I have no idea why it is sometimes different…
My guess is that there’s a capacitor in there somewhere. The capacitor stores enough charge to keep the time for short periods (like a power flicker). But it’s unreliable for longer periods of time (beyond a few minutes) and will cause clock drift.
So, when the power goes out for a medium length of time, the microwave resets to 12:00 to indicate that you need to set the time again
However, if the microwave is left unplugged for an extended period of time (a few days to weeks), the capacitor is fully drained. My guess is that this causes the time to be set to --:–
Why it would be designed like that? No idea
