The time correction DUT1 is the difference between the Universal Time scale UT1 (which corresponds to Earth's rotation) and the Coordinated Universal Time (UTC) (which corresponds, except for a constant offset, to International Atomic Time (TAI)).

DUT1 = UT1 - UTC

DUT1 is maintained in the range -0.9 s < DUT1 < +0.9 s by introducing a leap second into UTC roughly every 18 months. Forecast values of DUT1 are published in IERS Bulletin A.

The reason for this correction is that the rate of rotation of the Earth is not constant, due to tidal braking and small effects from earthquakes.

DUT1