10th floor,
i am sure your digital clock has no memory,that is why the display goes blank even it has the battery back up,
in comparison with your microwave ovens clock to ur friend does u have the same brand of oven?if not your ovens digital clock has a memory at it has also a built in cmos battery which powered the digital clock circuit so the circuit is independent to the mains..

when it comes to computer clock of course it is very accurate coz it is powered by a cmos battery,so it is not affected by frequency oscilltion.

buy a new one in radio shock, a lot there with cmos battery powered