Re: simple time server - NTP

This is a discussion on Re: simple time server - NTP ; >>> I don't mean "local time" as in 12:00 means solar noon. I mean the time > >Most people, asking here, who use the term, mean wall clock time, I hate to descend further into this terminology abyss, but I've ...

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Thread: Re: simple time server

  1. Re: simple time server

    >>> I don't mean "local time" as in 12:00 means solar noon. I mean the time
    >
    >Most people, asking here, who use the term, mean wall clock time,


    I hate to descend further into this terminology abyss, but I've never
    seen "wall clock time" used that way. I've seen it used plenty to
    refer to passage of time such as a wall clock would measure, in
    contrast to subset time scales such as CPU time. E.g. "This program
    takes 5 seconds of CPU time but 1 minute of wall clock time to run" or
    "the lawyer will take 15 minutes of billable time but four hours of
    wall clock time to get us the answer."

    Incidentally, I use the same definition of "local time" as others; what
    I really said in my original post was "local system time," which I hoped
    would prevent confusion with local time. Didn't work.

    >>> 1) Philosophically, the Linux kernel has no business messing with the
    >>> hardware clock. That's something that makes more sense done by a

    >
    >In engineering terms, it is the most sensible place to do it, because
    >the kernel is in the position to do this at the most accurate time
    >without excessive overheads.


    That argues for a system call that says "set the hardware clock to the
    system time," not an 11 minute clock maintenance policy inside the
    kernel. The automatic update is so imprecise anyway that a few
    microseconds of instruction execution is not noticeable. The hardware
    clock can be set only to a full second. The kernel rounds the system
    time to the nearest full second to do it.

    There are (Ntpd-less) people who want a very accurate hardware clock,
    who use 'hwclock --systohc' to do much better than this (Hwclock waits
    for the system time to reach the top of a second, sometimes holding up
    the whole system while it waits, and sets the hardware clock at that
    moment -- using estimations of the time it takes for the kernel part
    of the processing. Hwclock could do even better with a system call
    designed for that purpose.

    >>> The system clock on a Unix system doesn't operate in a time zone at
    >>> all. It keeps absolute time. The OS doesn't convert to local time;

    >
    >I don't know what you mean by absolute time, but I think it would have
    >to be TAI, not UTC.


    There's probaby a better word could have used, but I just meant
    timezoneless time -- time by itself without any concept of where the
    sun is or what people in some place customarily call a certain moment.

    UTC and TAI are both standards for specifying that. In fact so are
    EDT, PST, etc. taken individually. That's the kind of time that a
    Unix system clock measures. By constrast, a Windows clock keeps local
    time. If you transport a Windows computer from Los Angeles to New
    York, you normally tell the kernel to change its clock by 3 hours. If
    you transport a Unix system, you don't. You just tell the various
    programs that report the kernel's time that you'd like to see it in
    EST now.

    >>> it generates it -- from two pieces of information: 1) absolute time;
    >>> 2) time zone.

    >
    >That's what you were being told.


    No, I don't think it is. Many people think the Unix kernel keeps time
    in the Greenwhich time zone as opposed to the New York time zone.
    Maybe because the number it uses to represent the time is based on a
    moment that is nicely at the beginning of a year in that time zone
    (1970). Whereas on the contrary, the kernel knows nothing of time
    zones. The wording of that posting sounded a lot like that
    misconception.

    But then, in retrospect, if one thought I meant local time when I said
    "local system time," one would have thought I was pretty much an idiot
    (to want to an alternative to Ntpd in order to have local time), so maybe
    he was trying to come down to my level!

    >One other point, anything that uses the NTP wire formats, but doesn't
    >use the NTP algorithms from the reference implementation, isn't NTP,
    >it is only SNTP.


    Thanks for that. I had started to get that impression by reading the
    introduction to the SNTP spec, but I haven't actually read either yet.
    I will -- to make sure I don't create more confusion in terms here.

    --
    Bryan Henderson Phone 408-621-2000
    San Jose, California
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  2. Re: simple time server

    Bryan Henderson wrote:
    >>>>I don't mean "local time" as in 12:00 means solar noon. I mean the time

    >>
    >>Most people, asking here, who use the term, mean wall clock time,

    >
    >
    > I hate to descend further into this terminology abyss, but I've never




    > UTC and TAI are both standards for specifying that. In fact so are
    > EDT, PST, etc. taken individually. That's the kind of time that a
    > Unix system clock measures. By constrast, a Windows clock keeps local
    > time. If you transport a Windows computer from Los Angeles to New
    > York, you normally tell the kernel to change its clock by 3 hours. If
    > you transport a Unix system, you don't. You just tell the various
    > programs that report the kernel's time that you'd like to see it in
    > EST now.
    >


    Maybe YOU reset the Windows clock that way. Windows does have the
    facility to specify a time zone and at least some of us use that
    facility to set the correct time zone for the zone we happen to be in.

    I suspect that a lot of people have just left the default PST time zone
    in place but it doesn't have to be that way.

  3. Re: simple time server

    Richard B. Gilbert wrote:
    >> By constrast, a Windows clock keeps local
    >> time. If you transport a Windows computer from Los Angeles to New
    >> York, you normally tell the kernel to change its clock by 3 hours. If
    >> you transport a Unix system, you don't. You just tell the various
    >> programs that report the kernel's time that you'd like to see it in
    >> EST now.
    >>


    > Maybe YOU reset the Windows clock that way. Windows does have the
    > facility to specify a time zone and at least some of us use that
    > facility to set the correct time zone for the zone we happen to be in.


    I believe "specify a time zone" and "tell the kernel to change its
    clock" are equivalent on Windows. Yes?

    --
    Darren Dunham ddunham@taos.com
    Senior Technical Consultant TAOS http://www.taos.com/
    Got some Dr Pepper? San Francisco, CA bay area
    < This line left intentionally blank to confuse you. >

  4. Re: simple time server

    Darren Dunham wrote:

    > Richard B. Gilbert wrote:
    >
    >>>By constrast, a Windows clock keeps local
    >>>time. If you transport a Windows computer from Los Angeles to New
    >>>York, you normally tell the kernel to change its clock by 3 hours. If
    >>>you transport a Unix system, you don't. You just tell the various
    >>>programs that report the kernel's time that you'd like to see it in
    >>>EST now.
    >>>

    >
    >
    >>Maybe YOU reset the Windows clock that way. Windows does have the
    >>facility to specify a time zone and at least some of us use that
    >>facility to set the correct time zone for the zone we happen to be in.

    >
    >
    > I believe "specify a time zone" and "tell the kernel to change its
    > clock" are equivalent on Windows. Yes?
    >


    Since Windows 2000 and Windows XP both have W32TIME (Microsoft's feeble
    attempt at implementing SNTP) Windows *can* run a UTC clock and display
    local time. I would be the last to assert that everyone set up their
    Windows systems correctly but I know that I did at least three of them
    (two XP and one W2K) and it's just not that difficult!

    Click on your windows clock (in Systray). Mine has three tabs, the
    rightmost labeled "Internet" time. The server defaults to
    time.windows.com but you can pick any other one. My household, Windows
    systems included, synchronizes to my stratum 1 (GPS) server. W32TIME
    appears to be correcting the clock every hour. This does not give me
    microsecond accuracy but on my Windows systems 100ms is "close enough
    for government work".

  5. Re: simple time server

    Richard B. Gilbert wrote:
    > Darren Dunham wrote:
    >> I believe "specify a time zone" and "tell the kernel to change its
    >> clock" are equivalent on Windows. Yes?


    > Since Windows 2000 and Windows XP both have W32TIME (Microsoft's feeble
    > attempt at implementing SNTP) Windows *can* run a UTC clock and display
    > local time.


    That's the bit I wasn't aware of. I thought internally it was still
    keeping local time.

    --
    Darren Dunham ddunham@taos.com
    Senior Technical Consultant TAOS http://www.taos.com/
    Got some Dr Pepper? San Francisco, CA bay area
    < This line left intentionally blank to confuse you. >

  6. Re: simple time server

    Darren Dunham wrote:
    []
    > I believe "specify a time zone" and "tell the kernel to change its
    > clock" are equivalent on Windows. Yes?


    No. Internally, all 32-bit Windows (from Windows NT in 1992 and Windows
    95 in 1995) keep time internally in UTC, and have the option via the
    regional settings to display time in "local time". The local time offset
    can change between summer and winter as you wish. All this is already
    built into Windows. The internal timekeeping does not change when a
    time-zone is specified, just the time displayed to the user. Much of my
    software, for example, which deals with satellite tracking, uses UTC
    throughout, and obtains this via a single system call to Windows:
    GetSystemTime

    http://msdn.microsoft.com/library/de...systemtime.asp

    Windows assumes that the RTC on the motherboard is set to local time, as
    it would typically be set from someone's watch when it is first set up by
    an individual user (it may be set to Chinese or Taiwanese time when first
    delivered).

    Cheers,
    David



  7. Re: simple time server

    Darren Dunham wrote:
    > Richard B. Gilbert wrote:
    >>> By constrast, a Windows clock keeps local
    >>> time. If you transport a Windows computer from Los Angeles to New
    >>> York, you normally tell the kernel to change its clock by 3 hours. If
    >>> you transport a Unix system, you don't. You just tell the various
    >>> programs that report the kernel's time that you'd like to see it in
    >>> EST now.
    >>>

    >
    >> Maybe YOU reset the Windows clock that way. Windows does have the
    >> facility to specify a time zone and at least some of us use that
    >> facility to set the correct time zone for the zone we happen to be in.

    >
    > I believe "specify a time zone" and "tell the kernel to change its
    > clock" are equivalent on Windows. Yes?
    >


    No. Windows uses UTC for internal timekeeping. Timezones are only for
    local display of time, just like Unix. Notice that you need to nothing
    to go between daylight savings time and local standard time in any
    timezone. VMS is the only operating system that I am aware of that uses
    local time for internal timekeeping, and that may have changed since I
    was involved with it. I don't know what IBM mainframes do.

    Danny
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  8. Re: simple time server

    Darren Dunham wrote:
    > Richard B. Gilbert wrote:
    >> Darren Dunham wrote:
    >>> I believe "specify a time zone" and "tell the kernel to change its
    >>> clock" are equivalent on Windows. Yes?

    >
    >> Since Windows 2000 and Windows XP both have W32TIME (Microsoft's feeble
    >> attempt at implementing SNTP) Windows *can* run a UTC clock and display
    >> local time.

    >
    > That's the bit I wasn't aware of. I thought internally it was still
    > keeping local time.
    >


    No.

    Danny
    _______________________________________________
    questions mailing list
    questions@lists.ntp.isc.org
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  9. Re: simple time server

    Danny Mayer wrote:

    > Darren Dunham wrote:
    >
    >>Richard B. Gilbert wrote:
    >>
    >>>>By constrast, a Windows clock keeps local
    >>>>time. If you transport a Windows computer from Los Angeles to New
    >>>>York, you normally tell the kernel to change its clock by 3 hours. If
    >>>>you transport a Unix system, you don't. You just tell the various
    >>>>programs that report the kernel's time that you'd like to see it in
    >>>>EST now.
    >>>>

    >>
    >>>Maybe YOU reset the Windows clock that way. Windows does have the
    >>>facility to specify a time zone and at least some of us use that
    >>>facility to set the correct time zone for the zone we happen to be in.

    >>
    >>I believe "specify a time zone" and "tell the kernel to change its
    >>clock" are equivalent on Windows. Yes?
    >>

    >
    >
    > No. Windows uses UTC for internal timekeeping. Timezones are only for
    > local display of time, just like Unix. Notice that you need to nothing
    > to go between daylight savings time and local standard time in any
    > timezone. VMS is the only operating system that I am aware of that uses
    > local time for internal timekeeping, and that may have changed since I
    > was involved with it. I don't know what IBM mainframes do.
    >

    VMS joined the modern era, timewise, back in the 1990's. I believe that
    VMS V6.2-1 supported a UTC clock and local timezones. The UCX TCP/IP
    stack (V4.0, I believe) supported a primitive (V3.x) ntpd. The time
    change from standard to daylight and back is not automatic; you have to
    run a script at 2:00 AM on D-day. If you're running a Database, you
    shut it down while this is going on. Not sure if it's really necessary
    but I've always been told to do it that way.

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