I was reading about this somewhere just the other day - I think it was
rec.antiques.radio+phono, but I will confirm and get back if that is wrong.

The topic of the thread was remote-reading electrical power meters - you
could Google it. The gist of the topic was that there are many ways in
which a remote-reading power meter can work. Most cause no harm or
interference of any kind. But one of them involves "modulating the
zero-crossings" in the AC waveform, a signalling method which can be
triggered and read from a remote location. Apparently, this technique
can cause extra zero-crossings to show up, and a clock which counts time
by counting zero-crossings will gain time. There is any easy fix, IIRC.

So the question becomes, has the power company retrofitted the power
meters in your neighborhood recently?

Bill