The time scale used prior to 1984 as an independent variable in gravitational theories of the solar system.
After 1984, ET become Dynamical Time (Barycentric and Terrestrial Time).
In practice, Dynamical Time (TD) is a prolongation of the Ephemeris Time (ET).
Barycentric Dynamical Time (TDB) and Terrestrial Dynamical Time (TDT) differ
by at most 0.0017 seconds.
Because of this very small difference, TDB and TDT are supposed to be the same
and named TD (Dynamical Time) or ET or TT.
To get the planet positions, we start with the UT time, the new name for GMT
Time.
UT time is the time of our clocks, corrected by an amount of hours that rely
on the Time Zone of the birth place.
UT time is then translated to ET time as the main variable of ephemeris computation.
Difference between TD (ET) and UT (GMT) can only be deducted from observations.
However, one can use an approximative value given be Meeus:
Delta T= TD-UT = -15 + 0.00325 (year-1810)^2
(This formula is from Morrison and Stephenson)
Explanatory Supplement to the Astronomical Almanac gives another one
and the French IMCCE
yet another one.
So you have to keep in mind that, since we don't know
exactly Delta T, the planetary positions for a historical horoscope
past may be imprecise.
For 4000 BC, the error can reach 2 hours (Meus), or even 4 hours according to NASA.
The new algorithm used by Proastro is described here:
https://eclipse.gsfc.nasa.gov/SEcat5/deltatpoly.html
For accuracy, see this page:
https://eclipse.gsfc.nasa.gov/SEcat5/uncertainty.html