This year is set to be one second longer than expected, simply by adding in a 'leap second' on December 31.
The US Naval Observatory announced at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC), a leap second will be added to the world's clock - which corresponds to 6:59:59pm Eastern Standard Time (EST).
The second will be added at the observatory's Master Clock Facility in Washington DC.
Historically, the observatory says, time was based on the mean rotation of the Earth relative to distant celestial objects such as stars, called UT1.
"But the invention of atomic clocks gave a much more precise timescale, one that is independent of Earth's rotation," the observatory says.
In 1970, a procedure to maintain a relationship between Coordinated Universal Time (UTC) and UT1 was established.
If you can wrap your head around that, it means the sometimes a leap second has to be added or removed to keep UTC and UT1 within 0.9 seconds of each other.
"The International Earth Rotation and Reference Systems Service (IERS) is the organisation which monitors the difference in the two time scales and calls for leap seconds to be inserted in or removed from UTC when necessary."
International Atomic Time (TAI) is a second timescale, needed to create UTC, and consists of UTC without leap seconds.
The difference between TAI and UTC was determined to be 10 seconds when it was established in 1972.
"Since 1972, 26 additional leap seconds have been added at intervals varying from six months to seven years, with the most recent being inserted on June 30, 2015," the observatory says.
"Observations show that after roughly 500 to 750 days, the difference between Earth rotation time and atomic time would be about one second. Instead of allowing this to happen a leap second is inserted to bring the two time-scales closer together."
Systems such as electronic navigation and communications depend on precise time, the observatory says, through mechanisms such as the Internet-based Network Time Protocol (NTP) and the satellite-based Global Positioning System (GPS). Time changes can cause mayhem for computer systems as well.
The US Naval Observatory contributes approximately 30 percent of the weighted average to the international timescale UTC.