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Comment by NelsonMinar | Hacker News Reader

On the flipside, clock sync for civilians has never been easier. Thanks to NTP any device with an Internet connection can pretty easily get time accurate to 1 second, often as little as 10 ms. All major consumer computers are preconfigured to sync time to one of several reliable NTP pools.

This post is about more complicated synchronization for more demanding applications. And it's very good. I'm just marveling at how in my lifetime I from "no clock is ever set right" to assuming most anything was within a second of true time.

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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.

Yeah it can only do so much with varying latency

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.

> clock sync for civilians has never been easier

I don't think civilian clock synchronization was an issue since a long time ago.

DCF77 and WWVB has been around for more than 50 years. You could use some cheap electronics and get well below millisecond accuracy. GPS has been fully operational for 30 years, but it needs more expensive device.

I suspect you could even get below 1 sec accuracy using a watch with a hacking movement and listening to radio broadcast of time beeps / pips.

Both of the WWVB clocks I've owned have been very fickle about how they're placed because RF be that way sometimes, and Colorado isn't exactly nearby to my location in Ohio.

The first manufactured GPS clock I owned (as in: switch it on and time is shown on a dedicated display) was in a 2007 Honda.

But a firmware bug ruined that clock: https://didhondafixtheclocks.com/

And even after it began displaying the right time again, it had the wrong date. It was offset by years and years, which was OK-ish, but also by several months.

Having the date offset by months caused the HVAC to behave in strange incurable ways because it expected the sun to be in positions where it was not.

But NTP? NTP has never been fickle for me, even in the intermittently-connected dialup days I experienced ~30 years ago: If I can get to the network occasionally, then I can connect to a few NTP servers and keep a local clock reasonably-accurate.

NTP has been resolutely awesome for me.

On the one hand, some sloppy GPS units fail on a 20 year schedule. On the other hand, a bunch of things using NTP are going to fail in about ten years. (2036 rather than 2038 because reasons)

Yeah, good point.

If I ever get the chance, I'll try to remember to tell the 1995 version of me to watch out for that pesky overflow bug that they might experience with NTP -- two score and 1 year in their future.

The WWVB clocks are around the AM band, which means they carry a great distance despite their lower transmission power, but only at nighttime. Ohio is nothing; the signal needs to make it to the southern reaches of Florida.

Yeah, I "know" how it is supposed to work. I "knew" that back then when I bought my first WWVB-receiving clock, too.

And the placement was still fickle.

It's simple to observe:

A) Purchase and install clock. Wait (days). Observe failure to chooch.

B) Move clock. Wait (hours). Observe correct operation.

My world is ultimately bounded by reality, not theory: When it works in one position but not another, then that's the only reality I have to work with.

At this point the only clock in my life that doesn't auto set is the one on my stove, and that's because I abhor internet connected kitchen appliances.

Same here. I wish there was an easy way around it (that doesn't require me to play sysadmin in my spare time).

In the 80s my uncle had digital clocks that used an antenna to tune into the atomic clock time signal that (was/is?) broadcast nationwide. I've long wished that it was incorporated into stoves, microwaves, essentially everything that isn't an internet device (yet... sigh)

Sadly I think the actual antenna and hardware were relatively large since it's a long wave signal, but maybe with SDR it'll all fit on the head of a pin these days.

> Sadly I think the actual antenna and hardware were relatively large since it's a long wave signal, but maybe with SDR it'll all fit on the head of a pin these days.

Unfortunately there's no real way to cheat physics as far as shrinking a wavelength goes. With RF antennas about the best you can do is a major dimension 1/10th the frequency of interest.

There are many DCF77 receivers in Germany that are contained in a square box that's barely large enough for a AA battery; the rest of the square contains the motor/gears and the electronics/receiver (incl. a ferrite loopstick antenna).

The wavelength is around 3.8km...

Yeah, that's because it's receiving an extremely narrowband signal accumulated over a long window so it can suffer the trash efficiency I'm talking about.

> atomic clock time signal that (was/is?) broadcast nationwide

Probably DCF77 or WWVB.

> I think the actual antenna and hardware were relatively large since it's a long wave signal

Casio has some normal sized wristwatches that synchronizes to DCF77, it would definitely fit into a stove, microwave, or basically anything.

I believe it was a longwave broadcast so probably WWVB which would apparently imply a 60mm antenna, but it was a standard old school "GE digital clock radio" form factor so size wasn't at a premium.

Good ol' Oven Standard Time (OST).

One of the best features of my microwave is the ability to turn the clock off entirely. If it doesn't set itself I'd rather just not see it!

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