Leap Second Time Adjustment Day

Celebrated on 12/31/2026

Quick Fact

Leap seconds are added to Coordinated Universal Time (UTC) to keep it in sync with Earth's rotation, which is gradually slowing down. Since 1972, 27 leap seconds have been inserted, typically on June 30 or December 31. The most recent leap second was added on December 31, 2016.

Origins and Establishment of Leap Second Time Adjustment Day

The concept of leap seconds was introduced by the International Earth Rotation and Reference Systems Service (IERS) in 1972 to reconcile atomic time (which is extremely precise) with astronomical time based on Earth's rotation. The need arose because Earth's rotation is gradually slowing due to tidal friction, causing a discrepancy between atomic clocks and solar time. The IERS monitors Earth's rotation and decides when to add a leap second, usually every few years. The first leap second was added on June 30, 1972.

Leap Second Time Adjustment Day is not an official public holiday but a term used to mark the days when leap seconds are inserted. These days are typically June 30 or December 31, chosen to minimize disruption to global systems.

Evolution and Modern Commemoration

Over time, the process of adding leap seconds has remained largely unchanged, but its impact has grown with the proliferation of digital systems. Early leap seconds caused few issues, but as reliance on precise timekeeping for GPS, financial transactions, and computer networks increased, concerns arose. In 2012, a leap second caused widespread outages on websites like Reddit and LinkedIn due to software bugs. This led to the development of techniques like "smearing" (gradually adjusting time over a longer period) to avoid sudden changes.

Today, leap seconds are still added by the IERS, but there is ongoing debate about abolishing them. In 2022, the General Conference on Weights and Measures voted to eliminate leap seconds by 2035, replacing them with a larger adjustment (a "leap minute") every 50 years or so. Until then, leap seconds continue to be observed, often with minimal fanfare but significant behind-the-scenes coordination.

Facts & General Observations

• Since 1972, 27 leap seconds have been added, all positive (adding a second). No negative leap seconds have ever been needed.
• The longest gap between leap seconds was 7 years (1999 to 2005), while the shortest was 6 months (1972 had two leap seconds).
• A leap second can cause significant glitches: during the 2012 leap second, approximately 1,000 websites experienced outages due to software handling errors.

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