The International Earth Rotation and Reference Systems Service (IERS) is the organization that monitors the differences between these two time scales. It calls for the insertion of leap seconds or the removal of leap seconds from UTC when it is necessary to keep the difference within 0.9 seconds.

According to foreign media reports, on December 31, 2016, a "leap second" would be added to the world clock at 23:59:59 of the Coordinated Universal Time (UTC). When the extra second was inserted into the master clock facility at the United States Naval Observatory in Washington, D.C., it was equivalent to 6:59:59 p.m. Eastern Standard Time. Historically, time was based on the average rotation of the Earth relative to celestial bodies, and the second was defined within this reference frame. However, the invention of the atomic clock defined a more precise "atomic" time scale, and the second was no longer related to the Earth's rotation. In 1970, an international agreement established a procedure to maintain the relationship between the Coordinated Universal Time (UTC) and UT1, where UT1 is a measure of the Earth's rotation angle in space.

The International Earth Rotation and Reference Systems Service (IERS) is the organization that monitors the differences between these two time scales. It calls for the insertion of leap seconds or the removal of leap seconds from UTC when it is necessary to keep the difference within 0.9 seconds. To create UTC, a second time scale, the International Atomic Time (TAI), was first generated. It consists of UTC but without leap seconds. When the system was established in 1972, a 10-second difference between TAI and UTC was determined. Since 1972, an additional 26 leap seconds have been added at intervals ranging from six months to seven years. The latest insertion was on June 30, 2015. After the leap second was inserted in December, the cumulative difference between UTC and TAI was 37 seconds.

The misunderstanding that the occasional insertion of a few seconds every few years indicates that the Earth should stop rotating within a few thousand years sometimes causes confusion. This is because some people wrongly believe that the leap second is an indicator that the Earth is slowing down. However, the addition of one second is a sign of the accumulated time difference between the two systems. As for when to add a leap second, it is determined by the IERS, with the United States Naval Observatory (USNO) serving as a rapid service/prediction center. Measurement results show that, on average, compared with atomic time, the Earth rotates slowly, approximately 1.5 to 2 milliseconds per day. These data are generated by the USNO using the Very Long Baseline Interferometry (VLBI) technique.

VLBI measures the Earth's rotation by observing the apparent positions of distant objects near the edge of the observable universe. These observations show that after approximately 500 to 750 days, the difference between the Earth's rotation time and atomic time is approximately one second. To prevent this from happening, a leap second needs to be inserted to bring these two time scales closer. We can easily change the time on an atomic clock, but it is impossible to change the Earth's rotation speed to match the atomic clock.