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bug#22034: time-utc->date shows bogus zone-dependent leap second

From: Mark H Weaver
Subject: bug#22034: time-utc->date shows bogus zone-dependent leap second
Date: Mon, 29 Oct 2018 03:16:19 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)

Hi John,

John Cowan <address@hidden> writes:

> On Sun, Oct 28, 2018 at 4:40 PM Mark H Weaver <address@hidden> wrote:
>  The difference between the two measuring tapes is that they assign
>  different numbers to the markings, and moreover that the UTC analogue
>  has a small handful of places where two adjacent markings have the same
>  number assigned, and all subsequent numbers are shifted by 1.
> Now I think you are entirely right here, modulo a single term: what you are
> calling "UTC", I call (and I think correctly), "Posix".  It is Posix time that
> has two identical adjacent markings.
>    126230400 |------------
>    126230400 |------------
> The numbers here are not UTC seconds since the Epoch, but Posix seconds
> since the Epoch.

Here's a more detailed display of the same leap second that I chose for
my example, which you quoted above:

|      TAI seconds  UTC seconds                             |
|         since       since                                 |
|     midnight UTC  midnight UTC                            |
|      1 Jan 1970   1 Jan 1970    Result of 'time-tai->date'|
|$2 = ((126230410    126230398    "1973-12-31T23:59:58Z")   |
|      (126230411    126230399    "1973-12-31T23:59:59Z")   |
|      (126230412    126230400    "1973-12-31T23:59:60Z")   |
|      (126230413    126230400    "1974-01-01T00:00:00Z")   |
|      (126230414    126230401    "1974-01-01T00:00:01Z"))  |

The table above illustrates my understanding of the relationship between
"TAI seconds since midnight UTC on 1 Jan 1970", "UTC seconds since
midnight UTC on 1 Jan 1970" and the date object expressed in UTC.  See
my previous email for the code to produce the table above using SRFI-19.

If I understand correctly, based on your last sentence that I quoted
above, you believe that the numbers I've given in the second column are
not UTC seconds since the epoch, and that there is confusion here
between UTC seconds and Posix seconds.

Can you please be more concrete and tell me what numbers you think
should be in the second column, to properly reflect the column heading?
I'm not asking for a prose description, but for the actual numbers.

If the table above is incorrect, then it would be good to report the
problem to the SRFI-19 mailing list, because the current SRFI-19
reference implementation produces the same table.


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