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Comment by joha4270 | Hacker News Reader

Phone networks can already do this. And its true broadcast, you can decode it passively without ever connecting to the network, so no SIM/transmission required.

I have no idea this is actually enabled in networks in practice, but the specification exists.

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And in my experience, it's often wrong on some towers. I can't enable the setting on my phone to automatically set the time from the network, because if I do, it gets several minutes out of sync with the real time, which breaks TOTP generation.

Yeah, a large German cell provider had their time off by a couple of minutes some years back, which made the news as it resulted in people's alarms ringing late and them being late to work/school etc.

Hopefully modern phones prioritize GPS or NTP derived time and timezone information over NITZ (which is how networks provide it). At least Android has the capability to do so, but it's ultimately up to device manufacturers, as far as I understand [1].

[1] https://source.android.com/docs/core/connect/time

Could the detection and reporting of this inaccuracy be crowdsourced, either with mobile devices or perhaps SDRs? Fetch tower time, fetch real time over GNSS or NTP, report delta with tower ID and drift to a central server? Perhaps even include a map, like the map of GPS error rates derived from ADS-B data feeds.

I'm almost certain that at least Apple already has such a database on the operator level, i.e. a list of MNCs and the (non-)trustworthiness of NITZ. I vaguely remember seeing some carrier settings keys to that extent somewhere.

Why bother? If you can download data from a central server you have an internet connection, so you can also retrieve time via NTP instead.

Distributed monitoring and centralized reporting would be used to correct anomalous towers providing bad time and can provide independent verification of precision. It is not enough to provide a service; you must also have a system to validate it, and correct when needed.

Then you'd need Internet access to get the data, and if you had that, you could just use NTP.

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