cvs commit: src/sys/kern kern_tc.c
phk at phk.freebsd.dk
Tue Jun 24 01:57:20 PDT 2003
In message <20030623224600.GE93137 at wantadilla.lemis.com>, "Greg 'groggy' Lehey"
>Content-Type: text/plain; charset=us-ascii
>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
>> 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 latter is obsolete.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GMT is obsolete.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:
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
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
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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