Just as leap years keep our calendars lined up with Earth’s revolution around the sun, leap seconds adjust for Earth’s rotation.
Cesium-based clocks, one kind of atomic clock, measure time more precisely than those based on the rotation of our planet, so adding a leap second allows astronomical time to catch up to atomic time.
Most of us will not notice the addition, which happens on June 30 at 23:59:59 coordinated universal time (UTC) - 05:59:59 local time on July 1 - unless we deal in timescales shorter than a second or use a computer programme that cannot handle the leap second.
It has happened before: The 2012 leap second brought down Reddit, Gawker Media and Mozilla.
Not everyone recognises the leap second, although Apple does on its devices. Google devices sync with internet time usually tied to atomic clocks.
Financial markets like the New York Stock Exchange will take the leap second into account, closing the market a half hour earlier than normal on June 30, to help their systems deal with the addition.