Leap second

A leap second is a one-second adjustment occasionally added to (or theoretically removed from) UTC to keep atomic time within 0.9 seconds of solar time. Earth’s rotation isn’t perfectly regular — tidal friction is slowly slowing the planet down, and seasonal variations introduce smaller wobbles. Atomic clocks tick at a perfectly steady rate; the planet doesn’t.

Since 1972, 27 leap seconds have been inserted. They always land at midnight on June 30 or December 31 UTC. The minute containing the leap second has 61 seconds (or, theoretically, 59 if a leap second were ever removed — which has never happened).

Leap seconds are deprecated. In 2022 the General Conference on Weights and Measures voted to abandon the practice by 2035, after decades of operational pain in computing systems. After deprecation, UTC will drift slowly away from solar time — at the current rate, by about a second per few decades.

Most software handles leap seconds via “smearing” — gradually adding milliseconds to clock ticks over several hours. Google and Cloudflare publish their smear algorithms. POSIX time pretends leap seconds don’t exist at all.