[time-nuts] Leap Quirks

Magnus Danielson magnus at rubidium.dyndns.org
Sat Jan 3 19:39:14 EST 2009


Poul-Henning Kamp skrev:
> In message <495FF91C.60401 at rubidium.dyndns.org>, Magnus Danielson writes:
> 
>> We have at least three POSIX interpretations here.
>>
>> One which has UTC rubber seconds from 1970 to 1972 and from then true SI 
>> seconds from 1972.
>> One which has true SI seconds from 1970.
>> One which has UTC tracking in pieces and is slid "sideways" to make 
>> midnight match UTC midnight.
>>
>> The two first ones is interpretations of POSIX over UTC variations. The 
>> third one is a hack of POSIX to make it kind of work anyway with NTP. 
>> Only with the third interpretation POSIX midnight and UTC midnight is 
>> the same.
>>
>> Now, which of them is "right"?
> 
> Strictly speaking none of them.
> 
> Instead of thinking of POSIX "wall-clock" facilities as a timekeeping
> mechanism, think of them as asking the next stranger you meet what
> time it is.
> 
> POSIX only offers you the ability to get an estimate of "wall-clock"
> time, it does not guarantee that the such estimates will not represent
> time going backwards, or that the amount of time between the two
> requests will correspond to the difference between them.
> 
> The trouble is, obviously, that people pressume these properties.
> 

Which is more or less what I also think nowdays. The trouble is that it 
looks like time, feels like time and people try to make it behave like 
time, but isn't that. The monotonicity increasing aspect comes to mind 
as well, as you indicated.

Do you agree with my possible interpretations by the way?

Cheers,
Magnus




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