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    Re: Time on iPhones
    From: Frank Reed
    Date: 2013 Nov 2, 14:54 -0700

    Lu, you wrote:
    "Does anyone know how iPhones calculate time?"

    Yes. Apple does. :) But they won't tell you the details.

    In fact, there is some variability among models of various smartphones. An original iPhone certainly will show a distinct, consistent error that does not match a recent iPhone/iPad. There are four ways that these devices can get time (that I can think of at the moment):
    1) Manual Time. This is rare now, but it still happens occasionally. For example, if your device is frequently in airplane mode, you may need to update the time manually. It used to happen now and then that phone users would switch their phones by accident to manual time setting and then wonder why their phone showed steadily increasing clock error. I'll speculate that the manual option has been removed and/or hidden more deeply in options in more recent devices to avoid customer complaints from this source. Also, continuous connectivity is now much more common so the need for manual setting is reduced.
    2) Internet Time. This seems to be the most common source of time on most smartphones today. When the device has Internet access, it syncs up with a time server, adjusting for network transit time, just as an ordinary computer usually does today.
    3) GPS Time. I don't believe that many smartphone/tablet devices use this for primary time, however there are many apps that can get time from the GPS signals. There are various apps for iOS and also for Android that will compare Internet time and GPS time against the device's "system" time. It's not uncommon to see an error of a second or two.
    4) Tower Time. This used to be the overwhelmingly most common time source in phones. The devices would power on and as soon as they could communicate with one of the cell phone towers (even in phones without subscriptions), they would acquire the current time accurate to a fraction of a second. Ultimately this is presumably "GPS time" re-broadcast by the cell network towers.

    You added:
    "my pre-iPhone cell phone running on Verizon Wireless seemed to keep perfect time."

    Yes, I've seen the same thing. Traditional phones, pre-smartphone, were often dead-on accurate chronometers. When I tested the time on an old flip phone against WWV time ticks back around 2004, I could see no difference implying an accuracy of a tenth of a second. I am nearly certain that these early phones were displaying "Tower Time". When real smartphones took over, starting around 2005 and accelerating rapidly following the introduction of the iPhone in June 2007, designers and engineers had more options available (and greater freedom to prioritize other activities ahead of accurate time, too). I have a first-generation iPhone (out of service, but fully-functional as an Internet browser) and it displays the time consistently 15 seconds in error. Early Android phones also were consistently 15 seconds off. Exact time was not important to early users of these devices. There has been speculation on this 15 second error in Android phones suggesting a link to leap seconds. I haven't found "smoking gun" evidence on this. Another option is that the Android engineers simply stole the system code from early iPhones. More recently, both iPhones/iPads and Android phones and tablets appear to have adopted Internet Time syncing systems that keep the error down to one or two seconds. For greater accuracy, I always use an app rather than the system time, and some apps will compare both Internet Time and GPS Time against the device's internal system time. It's easy to find apps that will display the time to a fraction of a second. When I take celestial sights, I usually activate one or another of these apps for my clock.

    PS: If you want a UT/GMT clock on a phone or other device, and it doesn't have a direct available option for this, use "Accra, Ghana". Accra time is always UT year-round.

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