A second added to civil time to keep clocks with the earth's rotation — and it is being retired.
A leap second is an extra second inserted into UTC to keep atomic time in step with the earth's rotation, which is neither constant nor perfectly predictable. Twenty-seven have been added since 1972, always at the end of June or December.
Unlike a leap day, a leap second cannot be scheduled far ahead: it is announced a few months in advance because the rotation has to be measured. That makes it genuinely awkward for systems, which is why several large operators smear it across a day rather than insert it.
In 2022 the international bodies responsible resolved to stop inserting them by 2035. It is the rare case of a timekeeping practice being retired because keeping it was more trouble than the drift it corrected.
A leap day corrects the calendar against the orbit and is fixed by arithmetic. A leap second corrects the clock against the rotation and is announced only months ahead.
See also leap year · intercalation · utc offset
We hold this as not held — the corpus records days, not seconds. See what we hold.