Standards
What is a leap second?
A one-second adjustment that has kept atomic civil time close to Earth’s irregular rotation.
Atomic clocks and Earth do not tick alike
Atomic time advances with highly stable SI seconds, while Earth’s rotation varies slightly. The astronomical scale UT1 therefore drifts relative to atomic time.
UTC has used occasional adjustments
Since 1972, leap seconds have occasionally been inserted into UTC so that it remains close to UT1. A positive leap second labels an additional second 23:59:60 before the next day begins.
The system is scheduled to change
International metrology bodies have agreed to increase the allowed difference between UTC and Earth-rotation time by or before 2035, reducing or eliminating the need for frequent leap seconds. The exact implementation remains a standards matter rather than a normal time-zone change.