International Earth Rotation and Reference Systems Service
The International Earth Rotation Service is the body responsible for deciding when to issue leap seconds. They recently changed their name to The International Earth Rotation and Reference Systems ServiceThe Sub-bureau for Rapid Service and Predictions of Earth Orientation Parameters of the IERS, located at the United States Naval Observatory, monitors the Earth's rotation. Part of its mission involves the determination of a time scale based on the current rate of the rotation of the Earth. UT1 is the non-uniform time based on the Earth's rotation.
Related topics: International Atomic Time, Coordinated Universal Time.