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Comment by Uehreka | Hacker News Reader

I was doing something at work that involved calculating round trip times from/to Android devices, and learned that although it should be possible for NTP to sync clocks with below-second precision, in practice many of the Android devices I was working with (mostly Pixels 2-7) were off from my server and each other by up to 5 seconds, which blew my mind.

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It's hard to keep a phone's clock closely synchronized because they experience a lot of temperature swings, going between pockets and hands and open air and sometimes in direct sun, and the processor goes between idle and 100% as well.

Once you get to internationa phones, you'll have places where the phone does not include all timezones and specifically is missing the actual local timezone, so automatic sync is typically disabled so that the time can be set so that the displayed time matches local time... even if that means the system time is not correct.

Don't a lot of cellular networks rely on highly synchronized clocks to properly handle TDMA-style transmissions? Shouldn't they be very in sync with the towers' times?

I do believe there are (fairly) tight tolerances for clock synchronization between the network and the user equipment/handsets, but I don't know that it necessarily involves communicating the wallclock time. And the oscillators for signal timing aren't necessarily used for timekeeping.

I get that, but often the towers use GPS disciplined oscillators from what I understand (and have seen in limited circumstances), they inherently know exactly what time it is. Seems trivial to sync that as well, just kind of assumed they did that.

It’s not that hard. You would not expect 5 sec drift on phones that can sync time on the web at least once a day or once a week. A basic quartz crystal can keep time to within seconds per month of drift. High quality phones can do the same or better. Also the phone should keep track of system time as epoch time, and convert to local.

> Also the phone should keep track of system time as epoch time, and convert to local.

Yes, but imagine your local time is US Pacific time, but you have a phone intended to be sold in Mexico, so your phone only has Mexico time zones and MX Pacific Time has no DST. During part of the year, you can use automatic time sync, but during the summer, you disable automatic sync and set the clock so that the time displayed matches local time. Your epoch time is now an hour ahead of properly synched devices, but whatevs, your phone shows the right time and that's what counts.

Depending on carrier-specific configuration and firmware phones may be configured to prefer NITZ (time transmitted by the cellular network) instead of NTP. That time is probably what’s off and would explain your observation.

Yeah it can only do so much with varying latency

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