On: Falsehoods Programmers Believe About Time (2012)

[Angostura]

Hmmm? In the UK I'm sure the frequency changes with load on the grid and is actually a way of measuring grid load.

[anexprogrammer → Angostura]

Indeed, and they have to remain within +-1%. They have a realtime graph:

http://www2.nationalgrid.com/uk/Industry-information/electri...

Before quartz clocks were around, synchronous mains clocks were common, and accurate.

[dfox → Angostura]

The idea is that while the instantaneous frequency varies with load, the power grid is actively regulated such that frequency is constant on average over longer periods (typically such that from midnight to midnight there are 24 * 3600 * 50 or 60 cycles).