Insertion of LEAP SECOND in the Indian Standard Time

  • Council of Scientific & Industrial Research (CSIR) -National Physical Laboratory (CSIR-NPL) is the custodian of Indian Standard Time (IST) and has the responsibility for realization, establishment, maintenance and dissemination of IST through an act of Parliament.

Details on Leap Second?

  • A “Leap Second” is added every now and then to Coordinated Universal Time (UTC) in order to synchronize clocks worldwide with the Earth’s ever slowing rotation.
  • UTC consists of a time scale that combines the output of more than 300 highly precise Atomic clocks worldwide, including the one at CSIR-NPL.
  • Atomic clocks are very accurate and are stable within 1 second over a period of millions of years.
  • On the other hand, the Astronomical Time known as Universal Time (UT1) refers to the Earth’s rotation around its own axis and determines the length of a day.
  • However, the Earth’s rotation around its own axis is not regular, as sometimes it speeds up and sometimes it slows down, due to various factors including the moon’s gravitational Earth-braking forces that often results in ocean tides.
  • As a result, Astronomical Time (UT1) gradually falls out of synch with Atomic time (UTC), and as and when the difference between UTC and UT1 approaches 0.9 seconds, a “Leap Second” is added to UTC through Atomic clocks worldwide.
  • Since 1972, 36 “Leap Seconds” have been added at intervals varying from six months to seven years.
  • 37th “Leap Second” will be added to UTC at the midnight of December 31, 2016 in the countries within this time zone.
  • However, countries in other time zones will have “Leap Second” inserted according to their longitude. As the “Leap Second” is added simultaneously all over the world at UTC 23:59:59 on December 31, 2016, implying that in India the “Leap Second” will be inserted at IST 05:29:59 on January 1, 2017 (IST being five hours and thirty minutes ahead of UTC). In order to follow IST, the clocks need to be adjusted after the insertion of “Leap Second”.
  • Those utilizing CSIR-NPL time dissemination services (NTP, Teleclock etc.) need not worry as they will receive the corrected time post the insertion of “Leap Second”.
  • The “Leap Second” adjustment is not so relevant for normal everyday life; however this shift is critical for applications requiring of time accuracies in the nanosecond e.g. astronomy, satellite navigation, communication networks etc.
  • A “Leap Second” explanatory meeting will be held on December 31, 2016 at 23:30 Hours at the Main Building Reception area of the CSIR-National Physical Laboratory.


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