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Diffstat (limited to 'tz-link.html')
| -rw-r--r-- | tz-link.html | 19 |
1 files changed, 16 insertions, 3 deletions
diff --git a/tz-link.html b/tz-link.html index 10c40b5..56ef9e4 100644 --- a/tz-link.html +++ b/tz-link.html @@ -1034,8 +1034,21 @@ title="International Earth Rotation and Reference Systems Service">IERS</abbr> Bulletins</a> contains official publications of the International Earth Rotation and Reference Systems Service, which decides when leap seconds occur. The <code><abbr>tz</abbr></code> code and data support leap seconds -via an optional "<code>right</code>" configuration, as opposed to the -default "<code>posix</code>" configuration.</li> +via an optional "<code>right</code>" configuration where a computer's internal +<code>time_t</code> integer clock counts every <abbr>TAI</abbr> second, +as opposed to the default "<code>posix</code>" configuration +where the internal clock ignores leap seconds. +The two configurations agree for timestamps starting with 1972-01-01 00:00:00 +<abbr>UTC</abbr> (<code>time_t</code> 63 072 000) and diverge for +timestamps starting with <code>time_t</code> 78 796 800, +which corresponds to the first leap second +1972-06-30 23:59:60 <abbr>UTC</abbr> in the "<code>right</code>" configuration, +and to +1972-07-01 00:00:00 <abbr>UTC</abbr> in the "<code>posix</code>" configuration. +In practice the two configurations also agree for timestamps before +1972 even though the historical situation is messy, partly because +neither <abbr>UTC</abbr> nor <abbr>TAI</abbr> +is well-defined for sufficiently-old timestamps.</li> <li><a href="https://developers.google.com/time/smear">Leap Smear</a> discusses how to gradually adjust <abbr>POSIX</abbr> clocks near a leap second so that they disagree with <abbr>UTC</abbr> by at most a @@ -1048,7 +1061,7 @@ cloud service providers. However, according to <a href="https://tools.ietf.org/html/rfc8633#section-3.7.1">§3.7.1 of Network Time Protocol Best Current Practices</a> (Internet <abbr>RFC</abbr> 8633), leap smearing is not suitable for -applications requiring accurate UTC or civil time, +applications requiring accurate <abbr>UTC</abbr> or civil time, and is intended for use only in single, well-controlled environments.</li> <li>The <a href="https://pairlist6.pair.net/mailman/listinfo/leapsecs">Leap |