UTC stands for Coordinated Universal Time. UTC is the most commonly used time system in the world and is the one used by the Network Time Protocol (NTP). Leap second events are scheduled every few years in order to keep UTC in alignment with the earth's rotation.
Leap second events occur on either June 30th or December 31st and do not happen very often - about every two-three years. The International Earth Rotation and Reference Systems Service (IERS) is responsible for notifying the public when a leap second will be inserted. This information is summarized below:
The next possible leap second event is June 30, 2021.
There will NOT be a leap second introduced into UTC on that date.
December 31, 2016 was the most recent leap second event.
Our GPS-synchronized products precisely adjust for leap seconds to keep them accurately synchronized to UTC. The next time there is a leap second event there is no special action you need to take - it will all be handled automatically.
Our CDMA-synchronized products need some user intervention in order to precisely handle the leap second insertion. This is because the timing information transmitted by the CDMA base stations is guaranteed to be very accurate, except for the leap seconds. Some base stations handle the leap seconds properly and some do not, so it is best to do it yourself. The EndRun Leap Second Bulletin has instructions.
Leap seconds are inserted in order to keep UTC, which is derived from atomic time (TAI), in agreement with the Earth's rotation rate. Relative to TAI, the Earth's rotation rate is slowing down. This means that UTC must be retarded periodically in order to maintain agreement between UTC and the apparent daylength. If this were not done, eventually UTC would drift out-of-sync with Earth's day and many astronomical and navigational problems would ensue.
The IERS is the organization that measures the relationship between UTC and the rotation rate of the Earth. When the difference between UTC and apparent Earth time has exceeded a certain threshold, the IERS coordinates with the International Bureau of Weights and Measures (BIPM) to schedule the insertion of a leap second into the UTC time scale. The IERS publishes Bulletin C about 6 months in advance of each possible leap second insertion event. Leap seconds may only be inserted after 23:59:59 UTC on June 30 or December 31. Bulletin C confirms whether a leap second will or will not be inserted at the next possible insertion point. The IERS website is: https://www.iers.org.
The leap second information at the IERS website is actually the difference between atomic time (TAI) and UTC. The leap seconds we are interested in are the difference between the Global Positioning System (GPS) time and UTC and is called the GPS-UTC Offset. The GPS time scale began on January 6, 1980. At that time, the UTC timescale had undergone 19 leap second events (TAI-UTC). So, if you are obtaining your leap second information from the IERS website, you will need to subtract 19 from the TAI-UTC leap second values published there to obtain the GPS-UTC Offset. Here is a history of the leap second events since 1997:
|Leap Second Insertion Date||TAI-UTC||GPS-UTC Offset||Period Since Previous Leap|
|December 31, 2016||37 seconds||18 seconds||1.5 years|
|June 30, 2015||36 seconds||17 seconds||3 years|
|June 30, 2012||35 seconds||16 seconds||3.5 years|
|December 31, 2008||34 seconds||15 seconds||3 years|
|December 31, 2005||33 seconds||14 seconds||7 years|
|December 31, 1998||32 seconds||13 seconds||1.5 years|
|June 30, 1997||31 seconds||12 seconds||1.5 years|
EndRun clocks do NOT use smear techniques, which are inconsistent with the definition of UTC. Here is an excerpt from the U.S. National Institute of Standards & Technology (NIST) regarding these techniques:
"Some systems implement the leap second by a frequency adjustment that smears the leap second out over some longer interval. This has the advantage that the clock never stops or appears to run backward. However, it has both a time error and a frequency error with respect to legal UTC time during the adjustment period. To make matters worse, different systems that use this method may disagree during the adjustment period."
For more information on smearing, see the paragraph called "The Internet Time Service and Leap Seconds" at this link: NIST Internet Time Services.
The U.S. Dept. of Homeland Security has a paper on "Best Practices for the Leap Second Event" at this link: DHS Best Practices.