[LEAPSECS] POSIX Time (was WP7A)
sla at ucolick.org
Sun Oct 11 11:38:04 EDT 2009
I believe my additions will make it clear why the POSIX
community threw out the folks who did understand time.
On 2009 Oct 11, at 08:08, Poul-Henning Kamp wrote:
> Historically, the origin of UNIX system time was referred to as
> "00:00:00 GMT, January 1, 1970". Greenwich Mean Time is actually
> not a term acknowledged by the international standards community;
> therefore, this term, "Epoch", is used to abbreviate the reference
> to the actual standard, Coordinated Universal Time.
UTC, which at that point did not exist as a standard, but only as
a term informally employed by some employees of the various
national time service bureaus as they described the form of
time they were broadcasting. That was an attempt to follow
the UT2 then specified by the CCIR Recommendation. Further note
that UT2 itself was never described by a standard, only by the
annual equation in current use by the BIH, an equation which
changed at least once.
> Most systems' notion of "time" is that of a continuously increasing
> value, so this value should increase even during leap seconds.
> However, not only do most systems not keep track of leap seconds,
> but most systems are probably not synchronized to any standard time
> reference. Therefore, it is inappropriate to require that a time
> represented as seconds since the Epoch precisely represent the
> number of seconds between the referenced time and the Epoch.
Clearly this was the state of most computer time, and most old
time_t stamps do not correspond accurately to anything.
I doubt that this paragraph can still be taken seriously in
situations where multiple systems must agree with each other.
The "as closely as necessary" in the next paragraph now has
to deal with many systems which can't just ignore leaps.
> The expression is given to ensure a
> consistent interpretation, not to attempt to specify the calendar.
Ultimately it becomes impossible to demand seconds of uniform
length, 86400 in a day, and agreement with a calendar.
> POSIX.1 is more concerned about the
> synchronization of time between applications of astronomically short
I would find it helpful for the preceding sentence to be
more clear about what they think they are favoring.
> It is a practical impossibility to mandate that a conforming
> implementation must have a fixed relationship to any particular
> official clock (consider isolated systems, or systems performing
> "reruns" by setting the clock to some arbitrary time).
Again, POSIX is admitting that some sacred cow is being gored here.
> Note that as a practical consequence of this, the length of a second
> as measured by some external standard is not specified. This
> unspecified second is nominally equal to an International System
> (SI) second in duration. Applications must be matched to a system
> that provides the particular handling of external time in the way
> required by the application.
To me the above combination of 3 sentences means that a lot of
different behaviors can all be POSIX-conformant.
I do not see the number 86400 anywhere in that text.
Steve Allen <sla at ucolick.org> WGS-84 (GPS)
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