The Official Discontinuation of Leap Seconds
The debate surrounding leap seconds has persisted within the metrological community for three decades, but it has recently reached a definitive conclusion: leap seconds have been officially abolished. For many, the removal of a measure that has largely gone unnoticed may seem like a novel development. However, this decision carries significant implications for the technology we rely on daily. Moreover, it helps avert a potentially precarious situation—namely, the prospect of the first negative leap second in history.
The International Bureau of Weights and Measures (BIPM) has announced that, starting in May 2027, it will cease the introduction of leap seconds, of which 32 have been added since 1972 to adjust what is known as Coordinated Universal Time (UTC).
The Importance of Coordinated Universal Time
The implementation of leap seconds in 1972 was necessitated by an increasing disparity between time measured by atomic clocks and time based on the Earth’s rotation. This discrepancy arises because the Earth’s rotation is gradually slowing down due to tidal friction caused by the Moon and the inherent irregularities in its rotation. Influences such as the molten core, oceans, and atmosphere also play a role in this variation.
Leap seconds serve to maintain UTC—anchored in atomic clock measurements—in alignment with Universal Time (UT1), which is based on astronomical observations. One might liken this process to adding an extra track for a train to ensure it continues running parallel to another. Unlike leap years, which occur at fixed intervals, the BIPM assesses an average from hundreds of atomic clocks worldwide and compares their time calculations with astronomical time to determine when a leap second is warranted. The last leap second was added in 2016.
While the addition of a leap second may seem like a minor inconvenience, the implications for UTC are profound. Given the multitude of APIs and databases that rely on UTC, the introduction of leap seconds has created alignment challenges between UTC and UT1, causing disruptions in various systems that operate on UTC. Companies have had to devise strategies to manage these adjustments, which occur every few years.
Several tech giants, including Google and Meta, have implemented a “smoothing” method to manage leap seconds, as noted by Judah Levine, a researcher at the National Institute of Standards and Technology in the United States. Essentially, this involves distributing the leap second across a single day in small increments. However, leap seconds have caused significant issues for other systems; for instance, Qantas experienced a collapse of its reservation system due to a leap second in 2012.
The Risk of Negative Leap Seconds
The complexities surrounding the addition of leap seconds have only escalated over time. The likelihood of needing to introduce a negative leap second has increased, as the Earth’s rotation has accelerated in recent years due to climate change. The melting of polar ice has prompted such significant shifts in the planet’s gravity that, according to a 2024 study published in Nature, a negative leap second could have been necessary as early as 2029. A key consideration in the decision to eliminate leap seconds entirely was the risk associated with transitioning to negative values.
Levine explains that challenges persist even with positive leap seconds, and the concern was that a negative leap second could present far more severe complications, as its effects had never been tested.
Concerns Over Negative Leap Seconds Confirmed
Patrizia Tavella, director of the time department at BIPM, has corroborated these apprehensions in correspondence: “The probability of a negative leap second prompted us to set the implementation date for 2027.” She added that urgency was heightened following tests conducted by a standardisation group that revealed various failures associated with a negative leap second.
Tavella indicated that the group intends to wait until May 20, 2027, for the implementation, marking the anniversary of the signing of the Metre Convention in 1875, which led to the creation of the BIPM, as well as World Metrology Day. In essence, it is a fitting moment to commemorate the passage of time—or the absence of it.
The discontinuation of leap seconds now simplifies life for everyone. “The good news is that we don’t have to do anything! We can simply continue accumulating regular seconds and minutes of 60 seconds each,” Tavella notes. Levine concurs, stating, “One less concern.”
The Future of Timekeeping
However, the postponement of leap seconds does not imply that atomic time and astronomical time will cease to diverge. Thus, the BIPM has also resolved to establish an intercalary hour. Given the time required for leap seconds to accumulate, the prospect of an intercalary hour remains millennia away, considering there are 3,600 seconds in an hour.
“We are not yet discussing an intercalary hour. If systems are based on UTC, they have nothing to worry about. If they rely on UT1, they must be able to utilise UT1-UTC information, which in the future will exceed one second,” Tavella concludes.
In other words, this change is not an immediate concern for us, leaving future generations to navigate the complexities of timekeeping.