What is the leap second?

A leap year occurs every four years when one day is added to the final day of February. However, a leap second is added or extracted and the timing is not fixed.

A leap second is a one-second step adjustment that is carried out through the insertion of one second after the final second (Coordinated Universal Time (UTC)) or the extraction of the final second at the end of December and June (first preference), March and September (second preference), or, if necessary, an arbitrary month.

In Japan, this one-second adjustment is carried out the day after the above-mentioned times due to time difference issues. The insertion (extraction) of one second at these times is called a positive (negative) leap second. As a result, only the insertions of leap seconds are carried out at the moment.

The movements of the earth (rotation, revolution) undergo various changes and are not exact. The time system based on the time of the Greenwich meridian, which is determined by astronomically measuring the rotation of the earth, is called Universal Time (UT).
The time system based on an extremely accurate cesium atomic clock, which is determined by measuring the oscillation cycle of atoms, is called atomic time (TAI). The time system that corrects and manages the measurement error of both times using the above-mentioned methods and synchronizes with real life is called Coordinated Universal Time (UTC).