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

[Jan Hudec]

On Tue, Sep 07, 2004 at 11:52:35 -0400, Phil Frost wrote:
> time_t is an integer. Leap seconds are irrelevant to time_t. Leap
> seconds become relevant only when determining the natural date and time,
> such as "Tue Sep  7 11:04:35 EDT 2004".
> 
> The date counter can be easilly corrected for leap seconds, and in fact,
> it is. POSIX defines the unix epoch as a count of non-leap SI seconds.

Yes. And by this it diverges from both UTC standard and UT1 standard.
From the first by relying on reseting the clock on each leap second and
from the other by using SI seconds. 

> Here are some exerpts from man pages:

Man pages are not the most relevant thing :-(. Excerpts from the POSIX
standard would be more useful, but it does not seem to be publicaly
available anywhere.

> [...]
> "time returns the time since the Epoch (00:00:00 UTC, January 1, 1970),
> measured in seconds." --time(2)

That's the problem. It does not. That is, it does not represent the
number of SI seconds since 1970-01-01 00:00:00 UTC. It however does
represent the number of mean solar seconds since that point.

And there is another one. The UTC definition has changed in 1971, which
was after the start of the epoch. And there are no UTC dates whatsoever
before 1960, but POSIX time claims to go much before that (remember it's
signed).

> [...]
> In another thread it was suggested that UTC does not allow times in the
> future to be described. This is not true. UTC is by definition never
> more than 0.9s in error of GMT; this is the purpose of leap seconds. In
> fact, unless the earth's rotation is more important than the observed
> rate of change in everything else on the Earth's surface, GMT does not
> allow specification of time in the future either. Those who require
> accurate future times measure in SI seconds from an epoch.

Yes. There is TAI for that purpose.
 
> Given that POSIX defines times with an SI second, this is the
> overwhelmingly common practice, and that not only computers but every
> time service in the world uses UTC (the natural time based on SI
> seconds), I see no reason the historical and inacurate "GMT" should be
> prefered over "UTC".

Unlike computers, the time services do actualy use UTC. Because they
inform about the leap seconds.

> > The article at:
> > http://www.ucolick.org/~sla/leapsecs/onlinebib.html
> > contains some discussion of how POSIX time and NTP time should be viewed
> > and concludes, that it's appropriate to consider them UT, that is GMT,
> > rather than UTC.

-------------------------------------------------------------------------------
                                                 Jan 'Bulb' Hudec 
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