119 lines
4.8 KiB
XML
119 lines
4.8 KiB
XML
<?xml version="1.0" encoding="iso-8859-1"?>
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<!DOCTYPE article PUBLIC "-//FreeBSD//DTD DocBook XML V5.0-Based Extension//EN"
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"http://www.FreeBSD.org/XML/share/xml/freebsd50.dtd">
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<article xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink" version="5.0"
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xml:id="leapseconds">
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<info>
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<title>&os; Support for Leap Seconds</title>
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<pubdate>$FreeBSD$</pubdate>
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</info>
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<sect1 xml:id="leapseconds-definition">
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<title>Introduction</title>
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<para>A <emphasis>leap second</emphasis> is an ad-hoc one-second
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correction to synchronize atomic timescales with Earth rotation.
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This article describes how &os; interacts with leap
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seconds.</para>
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<para>As of this writing, the next leap second will occur at
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2015-Jun-30 23:59:60 UTC. This leap second will occur during a
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business day for North and South America and the Asia/Pacific
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region.</para>
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<para>Leap seconds are announced by <link
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xlink:href="http://datacenter.iers.org/"><acronym>IERS</acronym></link>
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on <link
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xlink:href="http://datacenter.iers.org/web/guest/bulletins/-/somos/5Rgv/product/16">Bulletin
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C</link>.</para>
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<para>Standard leap second behavior is described in <link
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xlink:href="https://tools.ietf.org/html/rfc7164#section-3">RFC
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7164</link>. Also see &man.time2posix.3;.</para>
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</sect1>
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<sect1 xml:id="leapseconds-posix">
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<title>Default Leap Second Handling on &os;</title>
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<para>The easiest way to handle leap seconds is with the
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<acronym>POSIX</acronym> time rules &os; uses by default,
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combined with <link
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xlink:href="&url.books.handbook;/network-ntp.html"><acronym>NTP</acronym></link>.
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When &man.ntpd.8; is running and the time is synchronized with
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upstream <acronym>NTP</acronym> servers that handle leap seconds
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correctly, the leap second will cause the system time to
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automatically repeat the last second of the day. No other
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adjustments are necessary.</para>
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<para>If the upstream <acronym>NTP</acronym> servers do not handle
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leap seconds correctly, &man.ntpd.8; will step the time by one
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second after the errant upstream server has noticed and stepped
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itself.</para>
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<para>If <acronym>NTP</acronym> is not being used, manual
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adjustment of the system clock will be required after the leap
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second has passed.</para>
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</sect1>
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<sect1 xml:id="leapseconds-cautions">
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<title>Cautions</title>
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<para>Leap seconds are inserted at the same instant all over the
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world: <acronym>UTC</acronym> midnight. In Japan that is
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mid-morning, in the Pacific mid-day, in the Americas late
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afternoon, and in Europe at night.</para>
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<para>We believe and expect that &os;, if provided correct and
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stable <acronym>NTP</acronym> service, will work as designed
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during this leap second, as it did during the previous
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ones.</para>
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<para>However, we caution that practically no applications have
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ever asked the kernel about leap seconds. Our experience is
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that, as designed, leap seconds are essentially a replay of the
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second before the leap second, and this is a surprise to most
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application programmers.</para>
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<para>Other operating systems and other computers may or may not
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handle the leap-second the same way as &os;, and systems without
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correct and stable <acronym>NTP</acronym> service will not know
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anything about leap seconds at all.</para>
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<para>It is not unheard of for computers to crash because of
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leap seconds, and experience has shown that a large fraction
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of all public <acronym>NTP</acronym> servers might handle and
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announce the leap second incorrectly.</para>
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<para>Please try to make sure nothing horrible happens because of
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the leap second.</para>
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</sect1>
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<sect1 xml:id="leapseconds-testing">
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<title>Testing</title>
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<para>It is possible to test whether a leap second will be used.
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Due to the nature of <acronym>NTP</acronym>, the test might work
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up to 24 hours before the leap second. Some major reference
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clock sources only announce leap seconds one hour ahead of the
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event. Query the <acronym>NTP</acronym> daemon:</para>
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<screen>&prompt.user; <userinput>ntpq -c 'rv 0 leap'</userinput></screen>
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<para>Output that includes <literal>leap_add_sec</literal>
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indicates proper support of the leap second. Before the 24
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hours leading up to the leap second, or after the leap second
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has passed, <literal>leap_none</literal> will be shown.</para>
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</sect1>
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<sect1 xml:id="leapseconds-conclusion">
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<title>Conclusion</title>
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<para>In practice, leap seconds are usually not a problem on &os;.
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We hope that this overview helps clarify what to expect and how
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to make the leap second event proceed more smoothly.</para>
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</sect1>
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</article>
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