On June 30, 2012, at 23:59:60 UTC, time stood still for one second. Servers at Reddit, Foursquare, LinkedIn, Mozilla, and hundreds of other sites crashed. The culprit wasn't a cyberattack — it was a single extra second added to the world's clocks.
What Is a Leap Second?
Atomic clocks are extraordinarily precise — they'd lose less than one second over 300 million years. Earth's rotation, by contrast, is irregular, gradually slowing due to tidal friction from the Moon.
Since 1972, International Earth Rotation and Reference Systems Service (IERS) has inserted leap seconds into Coordinated Universal Time (UTC) to keep it within 0.9 seconds of UT1 (solar time). By 2024, 27 leap seconds had been added.
Why Leap Seconds Break Software
The problem: almost no software expects a minute to have 61 seconds. The POSIX standard, which underlies Linux, Unix, and macOS, defines a day as exactly 86,400 seconds. A leap second creates a 23:59:60 timestamp that most systems simply don't know how to handle.
In 2012, Linux kernels hit a lock contention bug triggered by the unusual timestamp. In 2016, Cloudflare's Go-based DNS resolver subtracted times and got a negative duration — causing it to stop responding for ~90 minutes.
The End of the Leap Second
In November 2022, the General Conference on Weights and Measures voted to stop inserting leap seconds by 2035. After that, UTC will simply drift from solar time — allowed to accumulate up to a minute of difference over decades, with a mechanism to correct it only if the gap becomes unacceptably large (likely not until the 2100s).
What Replaces It?
Nothing, immediately. Tech giants (Google, AWS, Meta) already "smear" leap seconds across 24 hours, spreading the extra second invisibly. After 2035, UTC and TAI (International Atomic Time) will remain 37 seconds apart indefinitely, and the smear approach will no longer be necessary.
For everyday timekeeping — checking what time it is or converting timezones — you'll never notice the difference.