From MSNBC: Power-grid experiment could confuse clocks

A yearlong experiment with America’s electric grid could mess up traffic lights, security systems and some computers — and make plug-in clocks and appliances like programmable coffeemakers run up to 20 minutes fast.
“A lot of people are going to have things break and they’re not going to know why,” said Demetrios Matsakis, head of the time service department at the U.S. Naval Observatory, one of two official timekeeping agencies in the federal government.
Since 1930, electric clocks have kept time based on the rate of the electrical current that powers them. If the current slips off its usual rate, clocks run a little fast or slow. Power companies now take steps to correct it and keep the frequency of the current — and the time — as precise as possible.
The group that oversees the U.S. power grid is proposing an experiment that would allow more frequency variation than it does now without corrections, according to a company presentation obtained by The Associated Press.

I have long thought there should be a time stamp on the electric grid power signal, something quite small, but that could be read as embedded information (some highly non-random sequence) from modulation of the phase or frequency of the AC cycle. Other means for synchronizing clocks rely on other networks (internet, GPS, radio, etc.), some old discussion here. This is similar to the idea of powerline modems, but not nearly as sophisticated (i.e. I just want a time signal).