In a paradoxical situation that is similar to some of the more bizarre quantum effects, ‘Leap seconds’ are needed because atomic clocks are more accurate than the motion of the earth, against which time needs to be calibrated.
“The problem arises because the Earth cannot keep time as accurately as modern atomic clocks, which count the steady shaking of atoms. These atomic clocks replaced the motion of the Earth as the world’s official timekeeper in 1967. The pull of the moon is gradually slowing our planet down, so every now and then our clocks are halted for a second to let it catch up.”
“GPS time is now running 13 seconds ahead of coordinated universal time – which includes all added leap seconds and to which most clocks on Earth are set – but is some 19 seconds behind international atomic time, which is based on atomic clocks and ignores leap seconds.”
Guardian Unlimited | The Guardian | What time is it? Well, no one knows for sure