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Re: [art] Predictable Internet Time

Michael Thornburgh <mthornbu@adobe.com> Tue, 28 March 2017 18:41 UTC

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From: Michael Thornburgh <mthornbu@adobe.com>
To: Philip Homburg <pch-ietf-art@u-1.phicoh.com>, "art@ietf.org" <art@ietf.org>
Thread-Topic: [art] Predictable Internet Time
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Date: Tue, 28 Mar 2017 18:41:12 +0000
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References: Your message of "Mon, 27 Mar 2017 12:34:10 -0700 ." <869e1c74-2e6e-f4cd-4830-50985bab6be8@isi.edu> ,<m1csrkh-0000GYC@stereo.hq.phicoh.net>
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Subject: Re: [art] Predictable Internet Time
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> From: art [art-bounces@ietf.org] on behalf of Philip Homburg [pch-ietf-art@u-1.phicoh.com]
> Sent: Tuesday, March 28, 2017 7:05 AM

[...]
> - It is not clear to me what 'Unix time' is in this context. Typically POSIX
> time is linked to UTC, i.e. every value of time_t corresponds to a specific
> UTC timestamp. The inverse is not true, leapseconds cannot be represented in
> time_t.
[...]

while this has been the case so far, this is not true in general.  there can be negative leap
seconds, for which there will be values of time_t with no corresponding UTC time.
fortunately there have not been any negative leap seconds yet.

as i mentioned on a different mailing list [0], unix/posix time is a (deficient) time representation form,
but people use it as a time keeping form (as if it's actually the "uptime" since the unix
epoch).  i'm sure it was initially envisioned as both, even though that's wrong.

really unfortunately (or at least annoyingly) for timekeeping, "1970-01-01 00:00:00 UTC" isn't
a whole number of seconds offset from TAI.  UTC's ticks weren't synchronized to TAI ticks until
1972-01-01 00:00:10 TAI / 1972-01-01 00:00:00 UTC.  so having 1970-01-01 00:00:00 UTC
as a timekeeping epoch is the wrong choice anyway.  as i indicated in [0], i think the
right choice is 1970-01-01 00:00:10 TAI as the timekeeping epoch.

because solar time is variable, the standardized second is useful, and it is useful for
civil time to be in close agreement with solar time, there will need to be leap seconds or
at least periodic civil time steps.  therefore, the representation of civil/human time in something
like a time_t or a struct timeval is wrong, especially for future times.  the correct choice should
represent civil/human time in something like a struct tm or ISO 8601 or something.

[0] https://lists.w3.org/Archives/Public/ietf-http-wg/2017JanMar/0424.html

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