The ISP Column
A column on things Internet

                                                               July 2016
                                                            Geoff Huston

One Second Warning

  Time, as measured according to the Earth’s rotation, is getting slower.
  If we want 86,400 seconds to measure precisely one rotation of the earth
  on its axis then over time each second will have to get longer.

  Instead, we defined "one second" as a fixed unit of time based on atomic
  vibrations (9,192,631,770 periods of the radiation emitted by a
  caesium-133 atom in the transition between the two hyperfine levels of
  its ground state). But as the earth’s rotation slows, the fixed time
  period will mean that the time of day, the solar time, will drift. The
  resolution of this difference between a fixed time base and one that is
  relative to the earth’s rotation on its axis is to add (or subtract) a
  second from the fixed time base from time to time.

  We have reserved two time slots each year to make this periodic
  correction: Midnight UTC on the 30th June, and midnight UTC on the 31st
  December.

  The Earth Orientation Centre is the bureau that looks after this aspect
  of Universal Coordinated Time, and each six months they release a
  bulletin about their intentions for the next correction window. This
  month they announced a leap second to be scheduled for midnight UTC 31
  December 2016.

     INTERNATIONAL EARTH ROTATION AND REFERENCE SYSTEMS SERVICE (IERS) SERVICE

     INTERNATIONAL DE LA ROTATION TERRESTRE ET DES SYSTEMES DE REFERENCE SERVICE 
     DE LA ROTATION TERRESTRE DE L'IERS

     OBSERVATOIRE DE PARIS
     61, Av. de l'Observatoire
     75014 PARIS
     France

     Tel: +33 1 40 51 23 35
     e-mail: services.iers@obspm.fr
     web: http://hpers.obspm.fr/eop-pc

     Paris, 6 July 2016 Bulletin C 52

     To authorities responsible for the measurement and distribution of
     time UTC

     TIME STEP on the 1st of January 2017

     A positive leap second will be introduced at the end of December
     2016. The sequence of dates of the UTC second markers will be:
     - 2016 December 31, 23h 59m 59s
     - 2016 December 31, 23h 59m 60s
     - 2017 January 1, 0h 0m 0s

     The difference between UTC and the International Atomic Time TAI is:
     - from 2015 July 1, 0h UTC, to 2017 January 1 0h UTC : UTC-TAI = -36s
     - from 2017 January 1, 0h UTC, until further notice : UTC-TAI = - 37s

     Leap seconds can be introduced in UTC at the end of the months of
     December or June, depending on the evolution of UT1-TAI.

     Bulletin C is mailed every six months, either to announce a time step
     in UTC or to confirm that there will be no time step at the next
     possible date.


     Christian Bizouard
     Head Earth Orientation Center of IERS
     Observatoire de Paris,
     France


  What could possibly go wrong?

  Previous experience has not been all good news. Many systems assume that
  if you sleep for 60 seconds time will have advanced by 1 minute, or if
  you sleep by 86,400 seconds the day counter will advance by 1 day. There
  have also been problems in high resolution timers, where the addition of
  this extra second into the time record has caused the kernel of
  operating systems to enter a tight loop at elevated priority,
  effectively shutting down any other useful activity in the host.

  But we are accumulating experience with these spasmodic events, and we
  appear to be getting better at coping with this troublesome second. For
  example, one of the more inventive approaches is "leap second smearing",
  where the single leap second is achieved by a thousand leap
  milliseconds, spread across a few hours of even a day.

  I reported on this topic at some length back in 2012
  (http://www.potaroo.net/ispcol/2012-08/leapingseconds.html), so I won’t
  repeat it here. But it may pay to make a note that amongst all the New
  Year revels at the end of this year you should keep an eye on your
  systems to ensure that they comfortably handle a 61 second minute at the
  end of the UTC year.




























Disclaimer

  The above views do not necessarily represent the views of the Asia
  Pacific Network Information Centre.



About the Author

  GEOFF HUSTON is the Chief Scientist at APNIC, the Regional Internet
  Registry serving the Asia Pacific region. He has been closely involved
  with the development of the Internet for many years, particularly within
  Australia, where he was responsible for building the Internet within the
  Australian academic and research sector in the early 1990’s. He is
  author of a number of Internet-related books, and was a member of the
  Internet Architecture Board from 1999 until 2005, and served on the
  Board of Trustees of the Internet Society from 1992 until 2001 and
  chaired a number of IETF Working Groups. He has worked as an Internet
  researcher, as an ISP systems architect and a network operator at
  various times.

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