ΔT

In precise timekeeping, ΔT (Delta T, delta-T, deltaT, or DT) is the time difference obtained by subtracting Universal Time (UT) from Terrestrial Time (TT): ΔT = TT − UT.

The Earth's rotational speed is ν = 1/2π dθ/dt, and a day corresponds to one period T = 1/ν. A rotational acceleration dν/dt gives a rate of change of the period of dT/dt = −1/ν² dν/dt, which is usually expressed as α = ν dT/dt = −1/ν dν/dt. This has units of 1/time, and is commonly quoted as milliseconds per day per century (ms/day/cy).

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