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    Re: NIST website time accuracy
    From: Gary LaPook
    Date: 2010 Aug 05, 10:40 +0200

    I now have three clocks running on my computer, NIST (
    http://nist.time.gov/timezone.cgi?Pacific/d/-8/java# ), Navy celnav
    website (
    http://www.usno.navy.mil/USNO/astronomical-applications/data-services/cel-nav-data
    ) and the USNO time website that you pointed out (
    http://www.usno.navy.mil/USNO/time/display-clocks/simpletime )
    
     When I refresh both of the Navy clocks they start off one second behind
    NIST and drift slower. The celnav website updates every 5 minutes but
    always starts one second behind NIST. The USNO website updates every 3
    minutes and also starts one second behind NIST. The two Navy sites stay
    in lockstep but there may me a several second difference between them
    depending how much the first site has drifted when the the second site
    updates. After an hour I noticed that the NIST time had advanced a
    second compared to my watch which I am sure did not loose a second
    during that period. I refreshed the NIST site and it now agrees with my
    watch.
    
    So, who do you trust?
    
    I am only looking at this to make sure my watch is set properly which I
    usually use WWV to do but I can't receive that here in Europe.
    
    gl
    
    Frank Reed wrote:
    >
    > Gary, you wrote:
    > "it displays the time from the USNO master clock. If you also have
    > open the NIST site you can compare the two times. When you first go
    > the the Navy site the two times are in agreement but after a short
    > while the USNO master clock time runs slow, it is 39 seconds slow
    > right now on my computer. If I refresh the Navy site the time is again
    > in synchronization.
    > Curiouser and curiouser."
    >
    > The time display on the USNO site uses a simple bit of javascript to
    > display the time. It also, most likely, does not do any of the tricks
    > like checking round-trip travel time or using UDP which, I think, are
    > employed in the java app that you see on the NIST site (despite the
    > similarity in name, javascript and java are radically different things).
    >
    > So why does the USNO clock fall behind? Probably because the coder of
    > that javascript tool made a common error (also known as a "less than
    > optimal design choice") in creating the clock display on this specific
    > web page. There are coding objects usually called "timers." They are
    > used in most applications like this that have to update themselves on
    > a regular basis. For example, you can set up a timer to "fire" once
    > every second to update a display. When this occurs with a clock
    > application, you have two choices: take the previously known time and
    > add one second to it, OR you can read the system clock (your
    > computer's clock in other words) and add to that some offset in
    > seconds which was generated when the software first polled the really
    > accurate online source. Those might sound like they would yield
    > identical results, but the first depends on those timer events
    > occurring at exact one second intervals. But they don't do that. A
    > software clock programmed to update that way will drift rather
    > quickly. Instead you have to rely on the local built-in clock to get
    > some time with an error determined on launch. A third approach would
    > be to re-poll the accurate online source at some short interval, maybe
    > once a minute. I suspect that the USNO javascript on that web page
    > does this eventually but the interval may be over an hour.
    >
    > Update since I wrote the above: I've just discovered another USNO
    > online clock, probably implemented by a different programmer or maybe
    > by the same folks a few years later, that does exactly this third
    > option. It's located here:
    > http://www.usno.navy.mil/USNO/time/display-clocks/simpletime
    > and it re-polls the exact time once every three minutes thus avoiding
    > the drift problem.
    >
    > With modern GPS-equipped smartphones and similar small computers,
    > there's still another option. It's possible to write applications (and
    > they do exist) which request the GPS fix time from the GPS hardware
    > along with the position data. That can then be displayed with an
    > accuracy of some small fraction of a second depending on the
    > processing time of the code. I've experimented with this on an Android
    > phone (Android is an operating system developed primarily by Google
    > and used on huge numbers of new smartphones ...as Windows is to
    > Macintosh, Android is to iPhone). The standard system time displayed
    > through the usual clock applications on these devices is accurate to
    > about 15 seconds which is good enough for most phone users, but you
    > can bypass that close-enough time and get at the more accurate GPS
    > time using various freely-available apps. The same should be true on
    > iPhones and other iOS devices, but I haven't actually yet.
    >
    > -FER
    >
    >
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