the increment to be applied to Coordinated Universal Time UTC to give Terrestrial Time
D = sla_DTT (DJU)
UTC date as a modified JD
The UTC is specified to be a date rather than a time to indicate that care needs to be
taken not to specify an instant which lies within a leap second. Though in most cases
UTC can include the fractional part, correct behaviour on the day of a leap second
can be guaranteed only up to the end of the second .
Pre 1972 January 1 a fixed value of 10 + ETTAI
TT is one interpretation of the defunct time scale Ephemeris Time, ET.
See also the routine sla_DT, which roughly estimates ETUT
for historical epochs.