[LEAPSECS] An example

[Warner Losh]

  On 11/02/2010 13:48, Steve Allen wrote:

> On Tue 2010-11-02T18:55:17 +0000, Poul-Henning Kamp hath writ:

>>> The existing international agreement for the meaning

>>> of "day" is "mean solar day".

>> You mean "one of the existing..." ?

>>

>> The astronomical meaning of the word day may indeed be what you say,

>> but the equally internationally agreed standard for computer operating

>> systems define a day as 86400 SI seconds.

>>

>> The question is which one ITU-R adopts.

> I would like to have the citations for such agreements.

http://www.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html 
section 4.15:

"As represented in seconds since the Epoch, each and every day shall be 
accounted for by exactly 86400 seconds."


> The POSIX standard admits that its "seconds" are not all of the same

> length, and for practical purposes that makes them "mean solar

> seconds", not "SI seconds" nor "seconds of TAI".

>

Where does Posix say this?  At most I could find the following statement 
(just before the one I quoted above):

"The relationship between the actual time of day and the current value 
for seconds since the Epoch is unspecified. How any changes to the value 
of seconds since the Epoch are made to align to a desired relationship 
with the current actual time is implementation-defined."

The above text I've always taken to mean "we won't tell you how to tick 
your clocks around a leap second." not that the length of the second is 
somehow variable.

> I'm not interested in this as "I win"; rather to explore more of how

> the status quo is "We all lose" and where it is that effort is needed

> to clearly document that status quo.

Me too.  POSIX is very clear that each and every day is always exactly 
86400 seconds long, at least for the purpose of determining how to 
compute the time_t value.  This is the basis for phk's statement that 
the day is defined to be 86400 SI seconds long.  There's a reference to 
UTC earlier in 4.15 which is what ties the SI Second into this rather 
than the Mean Solar Second.

Warner