gEDA-user: Using 60Hz mains frequency for timing?

[DJ Delorie]

The 60hz mains are accurate only over long durations, like a day or
so.  From what I've read, they adjust the frequency occasionally to
make the total number of cycles accurate over time, but on a moment to
moment basis they may be off.

Crystals are usually pretty precise (compared to oscillators or
resonators), check the spec.

So - mains are more accurate, crystals are more precise.

Also, my furnace project has a circuit to detect zero crossings in the
60hz mains (through a 24vac transformer) if you need it.