Scientists think that the earth is spinning faster than it used to.
Evidence suggests that each day is a smidge shorter than 24 hours because the planet is rotating faster than it has in 50 years.
July 19, 2020, was the shortest day since scientists began keeping records in the 1960s — 1.4602 milliseconds shorter than the full 24 hours.
Due to this the International Earth Rotation and Reference Systems Service (IERS) announced last year that no “leap second” would be added to the world’s official timekeeping in December 2020.
Like leap years, leap seconds are time adjustments. Timekeepers at the Paris-based IERS have added leap seconds to 27 days since the 1970s, with the most recent on Dec. 31, 2016.
Since leap seconds are always added on the last day of June or December, the next possible date for a leap second is June 30, 2021.