[Const-me]

> A typical PC clock is +/-100ppm. After 1 hour that's 0.36s

Are you confident in these numbers? They add up to 52 minutes of drift/year.

Good modern quartz watches specify 5 seconds/year drift, almost 3 orders of magnitude better.

[eqvinox]

Yes, albeit 100ppm is bad/cheap crystals. 50-30ppm is normal.

The difference with a quartz watch is that it's factory calibrated with the load capacitance on the crystal, and that it's a 32768Hz tuning fork. For a variety of reasons, generating higher frequency clocks off 32768Hz is... "annoying" (huge PLL ratio, very slow feedback loop step), and typical crystals in the 10-100MHz range are just less precise and thermally stable. (Not sure why, I'm not an oscillator manufacturer...)

(NB: you can of course correct for initial deviation in software. The actual problem is stability over temperature.)

Ed.: https://www.digikey.com/en/products/filter/crystals/171 (or, in the hopes the filter on the link works, https://www.digikey.com/en/products/filter/crystals/171?s=N4... ) - look at the options and prevalence for frequency stability & tolerance.

Ed.2: a wristwatch also benefits from being kept at constant-ish body temperature.

[Const-me]

> typical crystals in the 10-100MHz range

I think most quarts watches oscillate at 32 kHz = 2^15 Hz, high precision quartz watches at 8.4 MHz = 2^23 Hz.

> The actual problem is stability over temperature

Apparently, designers of these watches compensating for that somehow: https://en.wikipedia.org/wiki/Quartz_clock#Thermal_compensat...

> benefits from being kept at constant-ish body temperature

Some people take off their watches every day before going to sleep.

These high-end quartz oscillators are probably too expensive to use in commodity computers. Still, the cost shouldn’t look too bad when compared to a price or an airplane, marine vessel, or most military equipment.

[eqvinox]

We're in agreement; 1ppm is 31.5s/yr so this lines up with OCXO performance / keeping the crystal at constant temperature. It's still 1km zone of spoofability per hour without resync.

(GPS sync is a question of nanoseconds.)

[DoctorOetker]

add on top of this that oven controlled crystal oscillators (or any more performant technologies if affordable) would be selected by militaries...

[eqvinox]

I'm unaware of any technology between OCXOs and Rb standards. The latter have gotten smaller but not tiny and also need quite a bit of ongoing maintenance and calibration.

[raron]

Microchip has some "chip-scale atomic clock"s, not much bigger than an OCXO, but a lot more expensive.

https://www.microchip.com/en-us/product/csac-sa65