“Synchronous electric clocks count the oscillations of the mains current to keep time: After 50 or 60 cycles (depending on where you are in the world), one second has elapsed. This works wonderfully as long as the average mains frequency is constant, or—in practice—adjusted to compensate for errors. Things get a little tricky when 113 GWh of energy somehow go missing and all microwave clocks on an entire continent go slow…” — www.metafilter.com.
It’s an interesting article, IMO. Be sure to read the comments and to check out this Wiki item: wikipedia.org/wiki/Electric_clock#Accuracy.