Thanks to a leap second being added to the clock at the end of the year, 2016 will be a touch longer than most years. A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time, or UT1. The bonus “leap second” will be added on December 31 at 11:59pm.
This leap second is necessary because the Earth’s rotation around its own axis is gradually slowing down, albeit at an extremely slow rate. Time, as it relates to that rotation, can technically vary too; the speed of rotation differs on a daily basis.
A normal day has 86,400 seconds, but in the atomic time scale 1 second is not defined as one 86,400th of the time it takes Earth to rotate around its axis but rather as the time it takes a Cesium-133 atom at the ground state to oscillate precisely 9,192,631,770 times.
Because of irregularities in the Earth’s rate of rotation, a system of correcting atomic time was launched in 1972. Since then, 26 leap seconds have been added to the clock over the years. The UTC time standard uses the international system (SI) definition of the second, based on atomic clocks. Like most time standards, UTC defines a grouping of seconds into minutes, hours, days, months, and years. However, the duration of one mean solar day is now slightly longer than 24 hours (86,400 SI seconds) because the rotation of the Earth has slowed down. Therefore, if the UTC day were defined as precisely 86,400 SI seconds, the UTC time-of-day would slowly drift apart from that of solar-based standards, such as Greenwich Mean Time (GMT) and its successor UT1. The purpose of a leap second is to compensate for this drift, by occasionally scheduling some UTC days with 86,401 or (in principle) 86,399 SI seconds.
The scientific community isn’t sure the way we measure time and play with that time with leap seconds is the right thing to do. In 2003, a meeting named “ITU-R SRG 7A Colloquium on the UTC timescale” took place in Torino, Italy, where it was suggested that time be decoupled from the Earth’s rotation and leap seconds be abolished. No decision was reached. Two years later, US scientists proposed eliminating leap seconds and replacing them with leap hours. That proposal went nowhere. In 2012, delegates of the World Radiocommunication Assembly in Geneva, Switzerland, decided once more to postpone the decision to abolish leap seconds and scheduled a new vote for 2015; that vote was deferred last year to not take place earlier than 2023.