41 lines
2.0 KiB
Diff
41 lines
2.0 KiB
Diff
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From 48e68fbfd181f4c23a52021dc27cc45fea75fa25 Mon Sep 17 00:00:00 2001
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From: Paul Eggert <eggert@cs.ucla.edu>
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Date: Thu, 12 Nov 2020 12:45:42 -0800
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Subject: [PATCH] Document "right" seconds better
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* tz-link.html (Precision timekeeping):
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Clarify "right" vs "posix" time_t.
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---
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tz-link.html | 15 +++++++++++++--
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1 file changed, 13 insertions(+), 2 deletions(-)
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diff --git a/tz-link.html b/tz-link.html
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index 10c40b5..eb4a235 100644
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--- a/tz-link.html
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+++ b/tz-link.html
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@@ -1034,8 +1034,19 @@ title="International Earth Rotation and Reference Systems Service">IERS</abbr>
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Bulletins</a> contains official publications of the International
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Earth Rotation and Reference Systems Service, which decides when leap
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seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds
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-via an optional "<code>right</code>" configuration, as opposed to the
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-default "<code>posix</code>" configuration.</li>
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+via an optional "<code>right</code>" configuration where a computer's internal
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+<code>time_t</code> integer clock counts every <abbr>TAI</abbr> second,
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+as opposed to the default "<code>posix</code>" configuration
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+where the internal clock ignores leap seconds.
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+The two configurations agree for timestamps starting with 1972-01-01 00:00:00
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+UTC (<code>time_t</code> 63 072 000) and diverge for
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+timestamps starting with <code>time_t</code> 78 796 800,
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+which corresponds to the first leap second
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+1972-06-30 23:59:60 UTC in the "<code>right</code>" configuration, and to
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+1972-07-01 00:00:00 UTC in the "<code>posix</code>" configuration.
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+In practice the two configurations also agree for timestamps before
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+1972 even though the historical situation is messy, partly because
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+neither UTC nor TAI is well-defined for sufficiently-old timestamps.</li>
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<li><a href="https://developers.google.com/time/smear">Leap Smear</a>
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discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a
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leap second so that they disagree with <abbr>UTC</abbr> by at most a
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--
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1.8.3.1
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