In article ,
Derek ^ wrote:

The traditional approach assuming the outages were brief, was to
use a cheap oscillator (an astable mutivibrator) running from a
back up battery when the mains feed was down.

On big tower clocks (which often had difficult access
for adjustment) one standard solution was to make all
power cuts last a multiple of 12 hours exactly, either
manually or with a battery-powered crystal oscillator.

--
Tony Williams.