API Reference / deltaT · function
Function: deltaT()
function deltaT(jdUt): number;
ΔT (TT − UT1) in seconds at a given instant — the gap between Terrestrial Time and universal time. Observed IERS values 1955–2025, Espenak–Meeus polynomials before, and a gentle extrapolation after (Earth's rotation sped up post-2016). The engine applies this for you; call it directly only for timescale work.
Parameters
| Parameter | Type | Description |
|---|---|---|
jdUt | number | Julian Day (UT). |
Returns
number
ΔT in seconds.