&os; Support for Leap Seconds

+ + + &os; Support for Leap Seconds + + $FreeBSD$ + + + + Introduction + + A leap second is an ad-hoc one-second + correction to synchronize atomic timescales with Earth rotation. + This article describes how &os; interacts with leap + seconds. + + As of this writing, the next leap second will occur at + 2015-Jun-30 23:59:60. This particular leap second is unusual, + being in the middle of a business day for North and South + America and the Asia/Pacific region. + + Leap seconds are announced by IERS + on Bulletin + C. + + Standard leap second behavior is described in RFC + 7164. Also see &man.time2posix.3;. + + + + Default Leap Second Handling on &os; + + The easiest way to handle leap seconds is with the + POSIX time rules &os; uses by default, + combined with NTP. + When &man.ntpd.8; is running and the time is synchronized with + upstream NTP servers that handle leap seconds + correctly, the leap second will cause the system time to + automatically repeat the last second of the day. No other + adjustments are necessary. + + If the upstream NTP servers do not handle + leap seconds correctly, &man.ntpd.8; will step the time by one + second after the errant upstream server has noticed and stepped + itself. + + If NTP is not being used, manual + adjustment of the system clock will be required after the leap + second has passed. + + + + Cautions + + Leap seconds are inserted at the same instant all over the + world: UTC midnight. In Japan that is + mid-morning, in the Pacific mid-day, in the Americas late + afternoon, and in Europe at night. + + We believe and expect that &os;, if provided correct and + stable NTP service, will work as designed + during this leap second, as it did during the previous + ones. + + However, we caution that practically no applications have + ever asked the kernel about leap seconds. Our experience is + that, as designed, leap seconds are essentially a replay of the + second before the leap second, and this is a surprise to most + application programmers. + + Other operating systems and other computers may or may not + handle the leap-second the same way as &os;, and systems without + correct and stable NTP service will not know + anything about leap seconds at all. + + It is not unheard of for computers to crash because of + leap seconds, and experience has shown that a large fraction + of all public NTP servers might handle and + announce the leap second incorrectly. + + Please try to make sure nothing horrible happens because of + the leap second. + + + + Testing + + It is possible to test whether a leap second will be used. + Due to the nature of NTP, the test might work + up to 24 hours before the leap second. Some major reference + clock sources only announce leap seconds one hour ahead of the + event. Query the NTP daemon: + + &prompt.user; ntpq -c 'rv 0 leap' + + Output that includes leap_add_sec + indicates proper support of the leap second. Before the 24 hour + period before the leap second, or after the leap second has + passed, leap_none will be reported. + + + + Conclusion + + We hope this discussion of leap seconds on &os; has been of + use. Now let us never speak of it again. + +

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