Date: Tue, 24 Jun 2003 10:17:17 +0100 From: Bob Bishop <rb@gid.co.uk> To: "Poul-Henning Kamp" <phk@phk.freebsd.dk>, "Greg 'groggy' Lehey" <grog@freebsd.org> Cc: cvs-src@freebsd.org Subject: Re: cvs commit: src/sys/kern kern_tc.c Message-ID: <4.3.2.7.2.20030624101423.02b31508@gid.co.uk> In-Reply-To: <4533.1056445034@critter.freebsd.dk> References: <Your message of "Tue, 24 Jun 2003 08:16:00 %2B0930." <20030623224600.GE93137@wantadilla.lemis.com>
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Hi, At 09:57 24/6/03, Poul-Henning Kamp wrote: >In message <20030623224600.GE93137@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 latter is obsolete.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GMT is obsolete.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@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. >_______________________________________________ >cvs-all@freebsd.org mailing list >http://lists.freebsd.org/mailman/listinfo/cvs-all >To unsubscribe, send any mail to "cvs-all-unsubscribe@freebsd.org" -- Bob Bishop +44 (0)118 977 4017 rb@gid.co.uk fax +44 (0)118 989 4254
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