the leap second
loreops cultureprogramming
The extra second occasionally inserted to keep atomic time aligned with Earth's rotation, and the outages it caused.
Because the planet's spin drifts against atomic clocks, timekeepers have irregularly added a 61st second to a chosen minute since 1972; software that assumed minutes have 60 seconds disagreed. The June 30, 2012 insertion famously took down Reddit, Mozilla services, and airline check-in systems, and later events pushed operators toward 'leap smearing', spreading the second invisibly across hours. In 2022 the world's metrology body voted to abolish leap seconds by 2035, a rare case of reality being patched to match the software.
A leap second is an extra second inserted into UTC to keep atomic time aligned with the earth's slightly irregular rotation. It is announced months in advance, it happens at most twice a year, and it has repeatedly broken production systems at large companies.
The reason it breaks things is that a great deal of software assumes every minute has sixty seconds and that time moves forward monotonically. A repeated or stretched second violates both, and code that computes an interval by subtracting timestamps can produce zero or a negative number, which then propagates into loops, timeouts and schedulers that were never written to consider it. Several notable outages were exactly this.
The pragmatic answer that emerged is smearing: rather than inserting a discontinuity, spread the adjustment across many hours so clocks stay monotonic and no second is ever repeated. It makes systems disagree slightly with UTC during the smear, which almost nothing cares about, in exchange for never presenting an impossible timestamp. The international bodies have since agreed to stop inserting leap seconds by the 2030s, which resolves the problem by removing it.