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Comment by ianbicking | Hacker News Reader

"If we had a cellular modem, we could get the time from the cell towers, which is broadcast through a protocol called NITZ. This is how phones auto-update when you enter a different timezone, but it's somewhat unreliable depending on the carrier, and overall worse than using GPS. And I really don't want my clock to have a sim card."

Just out of curiosity, what can you do with a cellular modem but no sim card? Can you get the time?

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CDMA networks broadcasted the time (from GPS) in the clear as part of the base station advertisements, so you used to see CDMA used as a precision time source without any kind of subscription required. CDMA equipment required accurate time for TDM coordination. Unfortunately GSM uses a different architecture for synchronization and does not require accurate time at all, so you have to be a subscriber to request time information and even then it is not all that reliable.

No. You have to have access to the network to get the time. You get timing from the network “for free” but it won’t give you time.

How can you access this timing I run a little mobile proxy service for myself and a few others, could add some value there. Mobile networks are quite interesting.

I'm not exactly clear on the difference between "time" and "timing"...? Like an accurate incrementing time offset from some unknown start point?

I assume they are referring to the channel timing? Cellular frequencies are segmented into time segments where each channel is allowed to be used by only some devices when it is their "turn" to transmit (this allows multiple phones to share the same frequency at the same time).

Timing is all about periodicity; if something beeps every second, you can measure intervals between two beeps but have no other information. It's often the case that timing is also synchronized to, say, second boundaries too, and most time sources would do this. Time would then be giving some indices to those beeps; the time source would beep and say that it was the N-th beep so that you can work the actual time out from N.

Is there any difference between a phone with no sim, and a phone with an old sim that's not linked to an active account?

Yes, on newer networks, the 5G NR System Information Block 9 (SIB9) provides UTC time.

However, it seems that extra SIBs aren't necessarily broadcast but may be available on demand... and it wasn't clear to me whether making a on-demand SIB request can be done without SIM card.

One possibility is to call 911 and ask what time is it.

I know this wasn't meant literally, but it does make for an interesting thought experiment - under what circumstances might it be valid to dial 911 to ask for the time?

My first thought was something like a nuclear operator - "we need to shut down the core at exactly 19:00, but our clocks are down!" so they call and wait for the operator to advise when the time is reached. Obviously contrived and not realistic, but interesting to think about.

In the UK they are fine with you calling 999 to test your newly-configured office phone routing. They don't want a chat though.

That's a bit different than asking for the time. That's testing a newly configured emergency system to make sure it works.

> under what circumstances might it be valid to dial 911 to ask for the time?

Maybe, if all of your clocks don't work and you went to somewhere else and their clock isn't working either, and it is raining and you cannot use a sundial, and you tried to call everyone else already and they also cannot give you the time for whatever reason, then you might try to call 911 and ask them, because you tried everything else and it didn't work. (I once heard a (fictional) story where this happened. This is an unlikely scenario, but some of the things mentioned here (and other things) might happen, e.g. bad weather so you cannot go out, the television and computers are not working (and maybe the power is out but the telephone uses a separate power), and there are some problems with the telephone too (I have had problems before where some telephone numbers worked and some didn't), etc.)

Well given that virtually everyone's "telephones" are cell phones which would be incapable of making calls unless they connected to the network, and the network would be incapable of handling their calls without also telling the phone what time it is, this situation becomes far less tenable in 2025 vs the 20th century when land lines were king. :)

That's what the US Naval Observatory Master Clock phone service is for: 719-567-6742

It's not a good idea to call 911 with non-emergencies.

But until a few decades ago, the primary way most of us to set our clocks was to call a number the phone company provided, which in our case was TI4-1212. "At the tone, the time will be ..."

You can still call NIST and get the time via "at the tone...": https://www.nist.gov/pml/time-and-frequency-division/time-di...

If a response packet contains a good timestamp, you could initiate a 911 call, get a reply packet, and cancel the call before it reaches any actual operators.

At least in many European countries, the landline phone companies offered a short number for obtaining the time from an answering robot.

Many, many years ago, I have designed a piece of equipment that was integrated in a phone exchange and it provided vocal messages to be sent to a caller, for errors like non-existent phone number, but also for replying to the dedicated number for the time service. The messages were something like "The time at the next beep will be ... hours ... minutes ... seconds".

I have not heard about a similar service for mobile phones, because here the phone gets the time automatically and it displays it.

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