DUT1

DUT1 is the difference between the UT1 and UTC time frames. In short, using the DUT1 allows a better representation of the local sidereal time within hyperdrive.

Since July 2022, MWA metafits files contain a key DUT1 populated by astropy.

If available, uvfits files display the DUT1 with the UT1UTC key in the antenna table HDU. However, the times in hyperdrive-written uvfits files will still be in the UTC frame, as if there was no DUT1 value.

Measurement sets don't appear to have a way of displaying what the DUT1 value is; when writing out measurement sets, hyperdrive will change the time frame of the TIME and TIME_CENTROID columns from UTC to UT1 iff the DUT1 is non zero.

More explanation

A lot of good, easy-to-read information is here.

UTC keeps track with TAI but only through the aid of leap seconds (both are "atomic time frames"). UT1 is the "actual time", but the Earth's rate of rotation is difficult to measure and predict. DUT1 is not allowed to be more than -0.9 or 0.9 seconds; a leap second is introduced before that threshold is reached.