Fascinating. I thought mains frequency was variable, which is why mains hum can be used to date recordings? It seems that there's some drift (why?) but they try

Fascinating. I thought mains frequency was variable, which is why mains hum can be used to date recordings? It seems that there's some drift (why?) but they try to keep the average constant, over some window? The article mentions the device prefers to use the mains frequency for timekeeping, using the crystal oscillator only as a fallback. I found this surprising, but I don't know much about either of those things.