Re: Predictable Internet Time

Joe Touch <touch@isi.edu> Tue, 28 March 2017 19:19 UTC

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Subject: Re: Predictable Internet Time
To: Michael Breuer <michael.breuer@ilsf.de>, =?UTF-8?Q?Patrik_F=c3=a4ltstr?= =?UTF-8?B?w7Zt?= <paf@frobbit.se>
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From: Joe Touch <touch@isi.edu>
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Date: Tue, 28 Mar 2017 12:18:37 -0700
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On 3/28/2017 12:09 PM, Michael Breuer wrote:
> ...
>> When UTC adds a leap second, nothing different happens to POSIX
>> time.
>>
> That's a quite complex topic to find precise wording. If the last
> statement would be right, wouldn't this be a contradiction to your
> earlier statement, POSIX time be defined *not counting* leap seconds?

POSIX ignores leap seconds - whether added (typical) or removed (if that
ever happens).

> AFAIK POSIX time is continuous and not counting leap seconds. So when
> UTC adds a leap second, something quite different happens to POSIX time.
POSIX doesn't know or care - it just keeps ticking.

> It will change the leap-second-offset between UTC and Unix time.

That happens because of something UTC did, not something POSIX did.

> To my opinion, any time system utilising leap seconds isn't continuos at
> all. There are continuous time systems like TAI or GPS system time (or
> any other satellite navigation system time) 
GPS reports TAI+19s, but at least one other satellite system jumps with
leaps (GLONASS).

> and POSIX time, which (by
> design) shift away from UTC over time.
TAI does too.

TAI and POSIX also drift away from each other in unpredictable ways
because TAI uses SI seconds and POSIX has no precise definition (it
appears to assume a second derived from a mean solar day, but it isn't
precisly spec'd).

Joe