Two Ways of Measuring Time
Modern timekeeping uses two different references:
- Atomic time (TAI): Measured by cesium atomic clocks, extraordinarily stable — losing only one second per ~300 million years.
- Astronomical time (UT1): Based on Earth's actual rotation — which is gradually slowing due to tidal friction from the Moon.
UTC (Coordinated Universal Time) tries to track both: it uses atomic clock precision while staying within 0.9 seconds of UT1.
What Is a Leap Second?
When the difference between UTC and UT1 approaches 0.9 seconds, the International Earth Rotation and Reference Systems Service (IERS) announces a leap second — an extra second inserted at the end of a chosen day (usually June 30 or December 31). The clock reads: 23:59:58 → 23:59:59 → 23:59:60 → 00:00:00.
History
Since the leap second system began in 1972, 27 leap seconds have been added through 2024. All have been positive (adding a second). In theory, a negative leap second (skipping a second) could be needed if Earth spun faster, but this has never occurred.
Why Leap Seconds Are Controversial
In the age of the internet, leap seconds cause significant technical problems:
- Software systems that assume 60-second minutes can crash or produce incorrect timestamps
- Distributed systems must be synchronized carefully to avoid database corruption
- Major incidents: Reddit, Cloudflare, Qantas, and others have experienced outages tied to leap seconds
The Future: Abolishing Leap Seconds
In November 2022, the General Conference on Weights and Measures (CGPM) voted to abolish leap seconds by 2035. UTC will be allowed to drift from UT1 by up to a minute before a larger correction is applied. The exact mechanism is still being debated.
Leap Seconds vs. Leap Years
Leap seconds and leap years address entirely different problems. Leap years correct the calendar's drift against the solar year (365.2422 days). Leap seconds correct the atomic timescale against Earth's variable rotation. They operate at different scales: one affects how we count days, the other affects how we count seconds.