Two kinds of time keep different rhythms
Atomic clocks measure time by counting highly regular vibrations inside atoms. Earth’s rotation is less steady: tides raised by the Moon gradually slow the planet, while shifts in the atmosphere, oceans, and Earth’s interior can make its rotation speed up or slow down temporarily.
That creates a small but accumulating gap between atomic time and time based on Earth’s rotation. The leap second is a correction to the official clock, not a change to the length of an atomic second.
How a clock makes room for the correction
When a positive leap second is scheduled, Coordinated Universal Time (UTC)—the international time standard used by clocks around the world—adds an extra second at the end of a day. Instead of moving directly from 23:59:59 to 00:00:00, the clock briefly shows 23:59:60. The additional tick gives Earth’s rotation a little more room to catch up with the time kept by atomic clocks.
Without such adjustments, the sun’s apparent position would slowly drift away from what clock time suggests. The difference grows gradually, rather than making everyday clocks suddenly fall out of step.
A tiny adjustment with a technical cost
People can usually absorb a one-second change without noticing it. Computer systems are less forgiving: they may need to coordinate timestamps across networks and databases, and not every system handles an extra second in the same way. Some systems repeat or stretch a second instead of displaying 23:59:60, which can create mismatches if connected clocks use different methods.
The leap second is therefore a compromise: atomic clocks provide a stable measure of duration, while occasional adjustments keep civil time close to the turning planet beneath them.
