Time standards · 4 min read
Leap seconds: why a minute can last 61 seconds
The difference between atomic time and Earth’s rotation, and the future of leap seconds.
Atomic clocks provide steady seconds, but Earth does not rotate with perfect regularity. Leap seconds were introduced to keep civil time close to the planet’s turning. The solution is simple to state and surprisingly difficult for software.
Two ways to keep time
UTC is built from atomic timekeeping. UT1 describes time associated with Earth’s rotation and its orientation. Because Earth’s rotation varies, the two would gradually diverge without adjustments. Historically, a leap second has occasionally been inserted into UTC to limit the difference between UTC and UT1.
At an insertion, the sequence can include 23:59:60 before the next day starts. Most daily clocks do not display that label. A normal day still has 24 hours on your calendar, but the precise duration of that particular UTC day has an extra second.
Why software finds it awkward
Many systems assume every minute has exactly 60 seconds. Extra seconds complicate logs, distributed systems and timestamp arithmetic. Different systems may handle an event differently, which makes ordering events across machines harder. This is one reason leap seconds are a specialized infrastructure concern rather than something to manage with a kitchen timer.
Not every year has a leap second; none should be assumed in advance. The International Earth Rotation and Reference Systems Service announces them when needed under the current scheme.
What is changing
In 2022, the international metrology community resolved to increase the permitted difference between UTC and Earth-rotation time by or before 2035, with the aim of avoiding frequent leap-second discontinuities. The exact future implementation remains a standards matter; it is not accurate to say that leap seconds have already disappeared.
For most readers, the practical advice is modest: use maintained time libraries and reliable time synchronization instead of writing calendar arithmetic from scratch. Your everyday event countdown does not need to predict leap seconds.
Sources & further reading
These sources provide the underlying standards or research. This guide is educational and is not medical advice.