cvs commit: src/sys/kern kern_tc.c

[Poul-Henning Kamp]

In message <20030623224600.GE93137 at wantadilla.lemis.com>, "Greg 'groggy' Lehey"
 writes:
>
>--6rK0r/vw6KmQcbJ3
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>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  latter is obsolete.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GMT is obsolete.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. 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.