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Tonight there will be an extra second, called a leap second. It will be added just before midnight GMT. The reason for this extra second is that the earth’s rotation and our system of atomic timing aren't properly aligned so every couple of years a leap second has to be added to stop a gradual time shift. But as we are more and more reliant on technology these leap seconds can cause a lot of problems. Felicitas Arias, Director of Time at the International Bureau for Weights and Measures in Paris, told us more. (Photo: Clock face from a clock in Bern, Switzerland. Credit: Pulpitis/Thinkstock)