An instant: an ISO 8601 string or a Date, read as UTC on the engine's time
basis, as positions() and natalChart() read one; or a Julian date on a
named time scale. deltaT fixes ΔT = TT − UT1, seconds, in place of the
time basis's: UT1 still comes from the instant (UTC plus IERS UT1 − UTC
from 1972), and TT = UT1 + ΔT.
An instant: an ISO 8601 string or a Date, read as UTC on the engine's time basis, as positions() and natalChart() read one; or a Julian date on a named time scale.
deltaTfixes ΔT = TT − UT1, seconds, in place of the time basis's: UT1 still comes from the instant (UTC plus IERS UT1 − UTC from 1972), and TT = UT1 + ΔT.