International Atomic Time
The current version of UTC is defined by International Telecommunication Union Recommendation (ITU-R TF.460-6), "Standard-frequency and time-signal emissions", and is based on International Atomic Time (TAI) with leap seconds added at irregular intervals to compensate for the accumulated difference between TAI and time measured by Earth's rotation. Leap seconds are inserted as necessary to keep UTC within 0.9 second of the UT1 variant of universal time. See the "Current number of leap seconds" section for the number of leap seconds inserted to date.