Compute TT, the increment to be applied to
Coordinated Universal Time UTC to give
Terrestrial Time TT.
D = sla_DTT (DJU)
UTC date as a modified JD (JD-2400000.5)
TT-UTC in seconds
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 + ET-TAI is returned.
TT is one interpretation of the defunct timescale
Ephemeris Time, ET.
See also the routine sla_DT, which roughly estimates ET-UT for