Re: [Cbor] draft-bormann-cbor-time-tag: POSIX and UTC are not the same

[Henk Birkholz <[e-mail]>]

+1 on adding some references and expositional text in an Appendix. While 
this document most certainly is also "only the message of the 
messenger", the underlying principles result in confusion, occasionally. 
I think it is a good idea to mitigate some of these occasions, pro-actively.

On 03.04.21 09:34, Carsten Bormann wrote:
> On 3. Apr 2021, at 04:00, Emile Cormier <[e-mail] 
> <mailto:[e-mail]>> wrote:
>>
>> In draft-bormann-cbor-time-tag (tags 1001-1003), UTC and POSIX are 
>> confounded together under the same value of 0, under section 3.4. Key 
>> -1: Time Scale.
>>
>> UTC and POSIX are not the same! Per 
>> https://en.wikipedia.org/wiki/Unix_time:
>>
>>     ... leap seconds are ignored, with a leap second having the same
>>     Unix time as the second before it, and every day is treated as if
>>     it contains exactly 86400 seconds. Due to this treatment Unix time
>>     is not a true representation of UTC.
> 
> Well, that text is slightly confused, as it is a true representation of 
> UTC, except it can’t represent the 27 leap seconds we have had so far or 
> any of the future ones.
> 
>> This is reinforced by the fact that C++ has distinct clocks for system 
>> time and UTC:
>>
>> https://en.cppreference.com/w/cpp/chrono/system_clock
>> https://en.cppreference.com/w/cpp/chrono/utc_clock
>>
>> There should therefore be distinct values for POSIX and UTC.
> 
> POSIX and UTC (and thus NTP) are shifted by leap seconds (which is 
> sometimes called “ignoring leap seconds”, a term that should never be 
> used; cppreference says “not counting leap seconds”, which means that 
> you have to be aware of leap seconds to actively not count them).
> 
> TAI, GPS are true monotonic time scales.  Leap seconds have no influence 
> on them (what cppreference calls “including leap seconds”).
> As C++ utc_clock "includes leap seconds”, it appears to be a TAI 
> referenced time scale (with a constant offset of 10 seconds from TAI).
> 
> I’m sorry, I didn’t invent this, I’m just the messenger :-)
> 
> Maybe adding an appendix on platform time representations would be worth it…
> (We could also add more lines to figure 1, pointing out that C++ 
> utc_clock is TAI minus 10 seconds.)
> 
> Grüße, Carsten
> 
> 
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