This attribute is used when calculating the Local Apparent Sidereal
Time corresponding to SkyFrameSkyFrame's EpochEpoch value (used when converting
positions to or from the "AzEl" system). It should be set to the
difference, in seconds, between the UT1 and UTC timescales at the
moment in time represented by the SkyFrame's Epoch attribute. The
value to use is unpredictable and depends on changes in the earth's
rotation speed. Values for UT1-UTC can be obtained from the
International Earth Rotation and Reference Systems Service
(IERS) at http://www.iers.org/.
Currently, the correction is always less than 1 second. This is
ensured by the occasional introduction of leap seconds into the UTC
timescale. Therefore no great error will usually result if no value
is assigned to this attribute (in which case a default value of
zero is used). However, it is possible that a decision may be taken
at some time in the future to abandon the introduction of leap
seconds, in which case the DUT correction could grow to significant
sizes.