Re: Predictable Internet Time

Joe Touch <touch@isi.edu> Tue, 28 March 2017 20:28 UTC

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Subject: Re: Predictable Internet Time
To: Tony Finch <dot@dotat.at>
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From: Joe Touch <touch@isi.edu>
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Date: Tue, 28 Mar 2017 13:28:03 -0700
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On 3/28/2017 1:24 PM, Tony Finch wrote:
> Joe Touch <touch@isi.edu>; wrote:
>> When UTC adds a leap second, nothing different happens to POSIX time.
> Clearly not. Either it jumps backwards at the start of the leap second, or
> at the end of the leap second, or it stops for a second, or it runs slow
> for some  period of time in a controlled manner (e.g. leap smear) or an
> uncontrolled manner (NTP swinging around in a wild effort to resync).

POSIX time just keeps ticking.

It is only in relation to UTC that you would notice anything. And it
isn't POSIX that is jumping - it is UTC.

> The POSIX formula that specifies the translation from UTC to time_t
> implies that it jumps back at the end of the leap second.
> http://pubs.opengroup.org/onlinepubs/9699919799.2016edition/basedefs/V1_chap04.html#tag_04_16
That equation is clearly introduced as "A value that approximates..."

Leap seconds and the difference between 86400 seconds/day and SI
definitions are where the approximation errs.

Joe