cvs commit: src/sys/kern kern_tc.c

[Bob Bishop]

Hi,

At 09:57 24/6/03, Poul-Henning Kamp wrote:
>In message <20030623224600.GE93137 at wantadilla.lemis.com>, "Greg 'groggy' 
>Lehey"
>  writes:
> >
> >--6rK0r/vw6KmQcbJ3
> >Content-Type: text/plain; charset=us-ascii
> >Content-Disposition: inline
> >
> >On Monday, 23 June 2003 at 13:14:09 -0700, Warner Losh wrote:
> >> imp         2003/06/23 13:14:09 PDT
> >>
> >>   FreeBSD src repository
> >>
> >>   Modified files:
> >>     sys/kern             kern_tc.c
> >>   Log:
> >>   Use UTC rather than GMT to describe time scale.  latter is obsolete.
> >
> >GMT isn't obsolete.  It's the British national time zone (without
> >DST).  But the change looks correct.
>
>GMT is obsolete.
>
>Mostly becuase GMT is a solar time.  While that means that there
>are no leap-seconds in GMT, it also means that you never quite know
>how long a second might be tomorrow:
>
>http://www.npl.co.uk/time/leap_second.html says:
>
>         The rotation of the Earth on its axis and its rotation
>         around the sun have served as the basis for timekeeping
>         since the dawn of history. The day was divided into 24
>         hours, each of 60 minutes, each of 60 seconds. Because the
>         length of the solar day (as shown, for example, by a sundial)
>         varies in a regular way during the year it became necessary
>         to average-out this effect and define a mean solar day.
>         This explains the name Greenwich Mean Time(GMT), a time
>         scale in which the mean position of the sun at noon, averaged
>         over the year, is above the Greenwich meridian (longitude
>         zero).
>
>         [...]
>
>         In 1972 a new Coordinated Universal Time scale was adopted
>         by the scientific community for international use. It is
>         abbreviated in all languages as UTC. It has since been
>         adopted by many countries as the legal basis for time.

But not by the UK. See for instance: http://www.dti.gov.uk/er/sumtimetb.htm

"For 2003-2007 inclusive, the summer-time periods begin and end 
respectively on the following dates at 1.00am Greenwich Mean Time [etc]"

>It
>         combines all the regularity of atomic time with most of the
>         convenience of GMT. The seconds of UTC are of the same
>         length as those of TAI, and they occur at the same instants.
>         UTC is kept always within one second of GMT by the insertion
>         of extra seconds as necessary (positive leap seconds). It
>         could happen that seconds would need to be removed (negative
>         leap seconds), however all leap seconds so far have been
>         positive.
>
>         When a leap second is inserted, it is done in the last
>         minute of a UTC year, or in the last minute of June (at
>         midnight UTC). The decision is taken by the International
>         Earth Rotation Service (IERS), and notices are distributed
>         well in advance whether or not a leap second is required.
>         An example follows (using UTC date and time):
>
>         1998 December 31 23h 59m 58s
>
>         1998 December 31 23h 59m 59s
>
>         1998 December 31 23h 59m 60s *
>
>         1999 January 01 00h 00m 00s
>
>         1999 January 01 00h 00m 01s
>
>
>
>         *... in the UK, where Greenwich Mean Time is in use, the
>         new year begins during the leap second as UTC changes from
>         being ahead of GMT to being behind GMT.
>
>--
>Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
>phk at FreeBSD.ORG         | TCP/IP since RFC 956
>FreeBSD committer       | BSD since 4.3-tahoe
>Never attribute to malice what can adequately be explained by incompetence.
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--
Bob Bishop		    +44 (0)118 977 4017
rb at gid.co.uk		fax +44 (0)118 989 4254