In uk.d-i-y Christopher Tidy wrote:
Hi all,

I'm thinking of building an electronic clock control circuit which uses
the 50 Hz mains frequency for time keeping. The reason for this is that
the clock dial is rather large, so probably wouldn't run for long on
battery power, and I don't fancy spending ?40 buying a programmable
quartz oscillator chip.

300s/30Ms = 10ppm.
300s/15Ms = 20ppm.
That's not too taxing.
IIRC, maxim/dallas do some chips that may suit, for way, way less than
40 quid.