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Re: [Gnu-arch-users] s/GMT/UTC/

From: Jan Hudec
Subject: Re: [Gnu-arch-users] s/GMT/UTC/
Date: Tue, 7 Sep 2004 14:15:05 +0200
User-agent: Mutt/1.5.6+20040818i

On Tue, Sep 07, 2004 at 13:06:39 +0200, Juliusz Chroboczek wrote:
> Folks, you're confusing GMT with TAI.

I don't think so. But anyway, the definitions go roughly like this:

GMT (or UT, UT1) is a clock code of the mean sun in Greenwich. A mean
    sun is an imaginary celestial object, that coincides with Sun on
    spring equinox. Second in GMT is 1/86400 day and since Earth
    rotation is not uniform, neither is GMT second.

TIA is a time measured by by atomic clock. It uses the SI second defined
    as some number of Cs oscilations and is MEASURABLY SHORTER than mean
    solar second (used in GMT). There is no SI minute, SI hour, SI day
    etc. TIA is just plain number of seconds since some point in time.

UTC is a time that uses SI seconds, but it synchronizes with GMT by
    insertion/omiision of a leap second whenever it diverges by more
    than 0.9s.

POSIX and NTP times tick in SI seconds, but they ignore the leap
seconds. Thus while the clock ticking is monotonous, intervals are
computed in mean solar seconds.

                                                 Jan 'Bulb' Hudec 

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