One second delay to 2009.
Leap seconds are added to keep the clocks synchronized with the Earth's rotation.
The Earth is rotating slower and slower over time,
while the atomic clocks are not slowing down. On one average day the difference is around 0.002 seconds, which means around 1 second in 500 days.
In order to synchronize the atomic clocks with the Earth's observed rotation,
the atomic clocks are occasionally instructed to add an extra second – the leap second.
