WASHINGTON – Saturday night will stretch longer by a second. A leap second.

International timekeepers are adding a second to the clock at midnight universal time today, going into Sunday. That’s 7 p.m. Central time Saturday. Universal time will be 11:59:59 and then the unusual reading of 11:59:60 before it hits midnight.

A combination of factors, including Earth slowing down a bit from the tidal pull of the moon, and an atomic clock that’s a hair too fast, means that periodically timekeepers have to synchronize the official atomic clocks, said Daniel Gambis, head of the Earth Orientation Service in Paris that coordinates leap seconds.