dUT1

DUT1, short for UT1 minus UTC, is the current difference between UT1, a time scale based on Earth's rotation, and UTC, the international time standard kept in step with atomic time. It is published by the International Earth Rotation and Reference Systems Service (IERS) in its daily bulletins and rapid service releases. DUT1 expresses how far UT1 currently diverges from UTC and is typically a small value, measured in seconds, on the order of tenths of a second. The magnitude and sign reflect the irregularities of Earth's rotation caused by tidal forces, atmospheric winds, core dynamics, and other geophysical effects.

DUT1 plays a central role in leap-second decisions. UTC is kept within about 0.9 seconds of UT1 by inserting or removing leap seconds; the observed DUT1 value is monitored and, when approaching the preset bounds, a leap second is scheduled. By convention, DUT1 is defined as UT1 minus UTC, so a positive DUT1 means UT1 is ahead of UTC and a negative DUT1 means UT1 trails UTC. This parameter enables precise conversions between UT1 and UTC, and, together with other Earth rotation parameters, supports astrometric calculations, satellite tracking, and navigation.

In practical use, scientists and engineers refer to DUT1 to relate observations tied to Earth's rotation to the coordinated time standard. While UT1 reflects the instantaneous rotation of the Earth, UTC remains a continuous atomic timescale with occasional leap adjustments; DUT1 provides the link between the two.