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    Re: Atomic clock and local time
    From: Frank Reed
    Date: 2014 Nov 26, 14:41 -0800

    Don, you asked:
    "Do cell phones and other devices which derive time from GPS satellites automatically compensate for these 16 seconds?"

    Apparently. Even "obviously"? The offset is part of the GPS signal, and all devices are supposed to make that correction in normal operation. I suspect that this correction is done at a very low-level on modern smartphones. The so-called "GPS Time" is an "internal time" that has no use to typical end users. The former 15-second offset between Android phone time and standard time disappeared starting about three years ago. I still don't know where that offset originated. One theory claimed that it was caused by a failure to correct internal "GPS Time" for leap seconds. That suggestion was no more than a speculation, and all references to it that I could find led back to one single source. The author did not have any evidence, let alone proof. Maybe it was true. Maybe not. I was intrigued when I powered up a first-generation iPhone a few months ago and noticed that it, too, had the 15-second offset, so I suspect the problem with Android system time resulted from simple "borrowing" (well, theft) of libraries from the early iPhone. Maybe some Apple engineer back in 2005 added a 15-second offset as a hack to display the correct time on an early prototype implementation of the iPhone system for reasons which are now long lost and irrelevant. No matter the source, it was simply a bug in early versions of the software on those devices. It's fixed now.

    Frank Reed
    Conanicut Island  USA


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