[dgacmu]

A cheap watch or clock will use an off-the-shelf crystal oscillator, where you'll be lucky if it's accurate to 20 seconds in a month.

A temperature compensated crystal oscillator (TCXO) can do better than one second per month, but probably not better than a second every 2 months.

An oven controlled crystal oscillator (OCXO) or a double ovenized crystal oscillator can get an order of magnitude better than that, But they start costing between $50 and $1,000. Beyond that you get into funky stuff like rubidium or cesium atomic clocks. There are, notably, miniaturized atomic clocks these days - about 2" x 2" - So you could technically put one into a wall clock, although they cost about $2,000.

These days it's usually cheaper to use GPS to control a temperature compensated oscillator, which we call a GPSDO (gps disciplined oscillator).

[russdill]

The mems temperature controlled oscillators are also very good, 0.05ppm range (about a second every 8 months). They are tiny and low power, but still >$79.

[WalterBright]

I used a wall "atomic" clock for a couple decades. It would synchronize itself with a government radio signal.

[dgacmu]

WWVB is great if you can get the signal -- I have two of those clocks in different rooms, and only one of them synchronizes.

[jrockway]

Newer clocks use the phase modulated signal: https://www.nist.gov/system/files/documents/2017/05/09/NIST-...

If you have AM-based clocks, you might find that these work better. However, nobody advertises which module they're using, so it's kind of a crapshoot. I specifically bought a phase modulation WWVB receiver and it reliably receives the signal every night. I also have a random $7 clock from Amazon that pretty much synchronizes every night. Living in NYC, this is truly amazing to me; I never ever got the AM signal here.

[creeble]

Thanks for that! Didn't know there were two kinds of radios.

I have one clock that always works perfectly, syncs after DST 100% of the time. Another that always requires manual reset twice a year, defeating it's purpose.

[cvoss]

It's an incredible feat to me that a single station can broadcast to the almost the entire US, Mexico, and even parts of South America at the right time of day [0]. (I believe this is why many clocks check for the signal overnight.) It's achieved by using an extremely low frequency, at 60 kHz. The antenna is enormous and suspended between 4 towers.

[0] https://tf.nist.gov/stations/wwvbcoverage.htm

[1] https://www.nist.gov/pml/time-and-frequency-division/time-di...

[ghaff]

Communication with subs uses ELF/SLF (3-300 Hz) when they're underwater--in the US case from some big communications stations. The coastal VLF stations, like in Cutler Maine are more in the 24 kHz range. As I recall, at one point, there were ecological concerns with the ELF stations but apparently the projects ended up going ahead.

[eternityforest]

The rubidium standards show up on eBay a lot for much less, but I don't know if they're still accurate and reliable after being used for years, I would imagine they're replaced for a reason