[time-nuts] How are iPhones' clocks set under LTE?

Brian Garrett garrettbrian1960 at gmail.com
Mon Aug 4 14:24:29 EDT 2014

Emerald Time is a very nifty little app, and yes, I've used it a number of 
times to check my phone's displayed time.  It's often off by more than two 
seconds but never more than three, so far.

There are several other apps that can check the time with NTP, and display 
it in different formats, some more attractive/useful than others. 
However,they will not alter the phone's internal clock.  I used to think 
this was due to the communication protocol needing accurate time in the 
mobile unit, but it's more likely just some kind of legal thing with Apple 
that they cannot mess with the clock settings (which _maybe_ could be 
construed as tampering with the OS).


-----Original Message----- 
From: Bill Dailey
Sent: Monday, August 04, 2014 6:09 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] How are iPhones' clocks set under LTE?

For informational purposes I will show what I use to compare with my stock 
Iphone.  It is an app called emerald time.  Screenshot at: 

You can set it up to sync with you own ntp server.  I think. You can just 
spot check it.

I have never done a rigorous analysis but it appears to be within 1.5s or 
better most of the time.


Sent from mobile

> On Aug 4, 2014, at 7:38 AM, BIll Ezell <wje at quackers.net> wrote:
> LTE does support the long-standard NITZ (network information and time 
> zone) service. It's an easy way to find out just where you are without 
> having to change your TZ settings constantly. In fact, if you go to time 
> settings on HTC Android phones, the 'automatic time update(NITZ)' setting 
> turns on NITZ syncing. iPhones also use NITZ, as do most 3G or LTE phones. 
> But, not necessarily for time.
> NITZ implementation is carrier-optional, although almost all do support 
> it. I know that Vodafone-Austrailia and a handful of other carriers at 
> least at one point didn't support it. Additionally, the standard doesn't 
> specify how accurate the time has to be, and it varies widely across 
> providers. It's usually within a few seconds, but this isn't a 
> high-precision time reference and can be off by minutes. But, a phone can 
> use the timezone information to then localize time from some other time 
> service.
> An alternative to determine what your physical location location is uses 
> lower-level information such as the ECGI (extended cell group identity) or 
> location information from the MME (Mobile Management Entity). Don't you 
> just love telecom? Everything's an acronym and frequently an acronym^2 or 
> ^3.
> Anyway, the phone then looks up the physical location from whatever id it 
> uses, then uses a time service to get the actual time, then localizes it 
> based on the physical location.
> Clearly, just using something like NTP directly isn't all that useful 
> because you have to know your physical location to know what timezone 
> correction to apply.
> I work on cell infrastructure, mostly 3G and LTE (Ericsson), and it just 
> amazes me that phones work at all. It is incredibly complicated and 
> convoluted.
>> Unlike CDMA (where time distribution was an automatic part of the
>> low-level protocol) I suspect the time displayed on many modern phones is
>> not set by the telephony synchronous protocol but rather by IP-over-Wifi
>> packets.
>> And the packets don't seem to do a very effective job keeping the clock
>> ont he phone correct. My employer gave me a Nokia Lumia 630 "Windows 
>> Phone"
>> and its clock has always been off by at least a minute.
>> There was a few years ago, a very nice article about the effort to repair
>> the clocks in clock towers in many cities. What rang most true to me was
>> "if you visit a town they can't even keep the clock correct, who else 
>> knows
>> what else is wrong there?".
>> Tim N3QE
> -- 
> Bill Ezell
> ----------
> The day Microsoft makes something that doesn't suck
> will be the day they make vacuum cleaners.
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