Leap seconds scrapped: BIPM accelerates shift to leap hour system by 2027

The International Bureau of Weights and Measures (BIPM), custodian of the International System of Units (SI), has accelerated the elimination of leap seconds, shifting the deadline from 2035 to May 20, 2027. The move sidesteps a potential crisis for computer systems that rely on a predictable 60-second minute.

Leap seconds exist to keep Coordinated Universal Time (UTC) synchronized with Earth's actual rotation. The planet's spin is not uniform; atmospheric pressure, ocean currents, core dynamics, and lunar gravitational effects all introduce variations. According to BIPM documentation, these forces create a 30% chance that a "negative leap second" would be required by 2035.

A positive leap second inserts an additional second into the year it applies. On December 31, 2016, clocks displayed 23:59:60 before advancing to 00:00:00 on January 1, 2017. Machines unprepared for this anomaly can malfunction. Unlike leap years, which follow a predictable four-year cycle, leap seconds arrive unpredictably, preventing programmers from pre-scheduling accommodations. A negative leap second—removing a second rather than adding one—has never occurred and poses substantially greater risks.

The BIPM stated that "a negative leap second has not previously been applied, and it is considered to pose a high risk of causing anomalies and disruption to critical infrastructures that are largely unprepared." The organization compared the remediation effort required to preparations for the millennium bug. Leap seconds have already caused documented failures: Reddit crashed following the 2012 leap second addition. While desktop computers may escape unscathed if their time servers receive updates, the BIPM fears that transportation networks, power grids, telecommunications systems, and space operations could experience severe disruptions.

Rather than invest a decade in system upgrades and hardware replacement, the standards body chose to eliminate leap seconds entirely. Beginning next year, a leap hour will replace them. This approach preserves UTC's long-term stability, with synchronization with TAI expected to hold for several centuries before any adjustment becomes necessary. The shift removes the burden of periodic manual clock adjustments that currently occur every few years.

The BIPM acknowledged that hourly time shifts are not unprecedented globally. The organization noted that "in many countries, there are differences of more than one hour between local apparent solar time and civil time due to the choice of time zone." It added that "changes of an hour are regularly introduced when daylight-saving time is used." Though a future one-hour time jump may surprise many, the BIPM framed it as consistent with existing timekeeping practices.