Saturday, timekeepers will add an extra second to the last minute of June 30th to compensate for Earth's movements.
The 'extra' second is vital to keep clocks on Earth in sync with 'solar time', so that our days and nights match up to light and darkness.
Time on Earth is measured using atomic clocks, and has to be adjusted to keep pace with Earth's wobbly orbits.
The planet takes just over 86,400 seconds for a 360 degree revolution.
But it wobbles on its axis and is affected by the pull of the Sun, Moon and tides meaning it gets out of step with International Atomic Time (TAI)
To avoid solar time and TAI moving too far apart, Coordinated Universal Time (UTC) is adjusted to give us the odd 86,401 – second day.
Coordinated Universal Time (UTC) has, since the '70s, been the agreed universal standard which dictates what time it is in every time zone on the planet.
The speed of the Earth's rotation varies, however, meaning that while one rotation is one day, some days are a few milliseconds longer or shorter than others.
Scientists allow for these fluctuations by adding leap seconds roughly every year-and-a-half to keep UTC synchronized with our traditional, sun-based concept of time.
They are added once the International Earth Rotation Service, which monitors the planet's activity, finds the two measurements of time have drifted apart by 0.9 seconds.
'Today, time is constructed, defined and measured with atomic clocks' said Noel Dimarcq, director of the SYRTE time-space reference system at the Paris Observatory.
'This allows us to ensure that everyone on Earth is on the exact same time.'